JP2005521248A - Quantum phase charge coupled device - Google Patents

Abstract

A superconducting structure comprising a mesoscopic phase element and a mesoscopic charge element. The superconducting structure further includes a mechanism for coupling the mesoscopic phase element and the mesoscopic charge element such that the quantum state of the mesoscopic phase element and the quantum state of the mesoscopic charge element interact. In another aspect, the superconducting structure includes a mechanism for reading the quantum state of the mesoscopic charge device.

Description

  This application is a provisional application serial number 60 / 365,490 filed on March 16, 2002, “Quantum Phase-Charge Coupled Device” (Attorney Case No. 11090-005-888). United States patent application serial number 10 / 121,800 filed on April 12, 2002 "Quantum Phase-Charge Coupled Device" (agent case number 11090-005-999); U.S. Patent Application Serial No. 10 / 121,817 filed on May 12, "Quantum Phase-Charge Coupled Device" (Attorney Case Number 11090-006-999); and April 12, 2002 Claims priority based on US Patent Application Serial No. 10 / 121,810, “Quantum Phase-Charge Coupled Device” (Attorney Case No. 11090-007-999) filed on the same day .

  The present invention relates to the field of quantum computation, and more particularly to superconducting quantum computation.

  Mesoscopic superconducting systems have attracted attention for many years as systems that exhibit complex physical phenomena. Recently, it has been understood that these phenomena have practical applications in the field of quantum computing. For example, “Radio-Frequency Single-Electron Transistor as Readout Device for Quoits: Charge Sensitivity and Backaction” by A. Assime, G. Tohansson, G. Wendin, R. Schoelkopf, and P. Delsing. One-Electron Transistor: Charge Sensitivity and Back Action) ”, Phys. Rev. Lett. 86, p.3376 (April 2001) and references cited therein, and the United States, December 1, 1999, by Alexandra Zagoskin. Reference is made to patent application serial number 09/452749 “Permanent Readout Superconducting Qubit”, which is incorporated herein by reference in its entirety.

Quantum computations generally can initialize the state of N qubits (qubits), generate controlled entanglements between them, and develop these states, and after progress, Includes reading status. A qubit is a qubit and corresponds to a binary digit or a classical bit in quantum computation. The bit is exactly the basic unit of information in a classic computer, while the qubit is the basic unit of information in a quantum computer. Normally, a qubit is a system with two degenerate (eg, of equal energy) quantum states, and the qubit's quantum state is in the superposition of two degenerate states. The two degenerate states are also called basic states. Further, two degeneracy or basic states are represented by | 0> and | 1>. A qubit can be a superposition of these two degenerate states, creating a fundamentally different state from that of a normal digital computer. If the predetermined condition is satisfied, the N qubit can define an initial state that is a combination of 2 ^N classical states. This initial state is determined and developed by the interaction of the qubits between themselves and external influences, and provides quantum mechanical operations that do not resemble classical calculations. The evolution of the state of N qubits defines a calculation, ie, a simultaneous classical calculation of substantially 2 ^N (eg, a conventional calculation performed on them using a conventional computer). The result of the calculation is completely determined by reading the state of the qubit after progress.

The principle of superposition and entanglement needs to be introduced to represent the classical state combination 2 ^N and to recognize the necessary states for N qubits. Superposition can be explained by considering qubits as magnetic field particles. The spin of the particles is either arranged along a magnetic field known as the spin-up state or arranged against the magnetic field known as the spin-down state. The change of particle spin from one state to another is achieved using pulse energy such as a laser. If an arbitrary unit of laser energy is required to change the spin of a particle from one state to another, only half of the laser energy is used and the particle is isolated from all external influences The question arises as to what happens. According to the quantum mechanics principle, the particles enter the superposition state at the same time, and behave as if both states exist at the same time. Each qubit is utilized so that both 0 and 1 superpositions can be taken. Because of this property, the number of states that a quantum computer can guarantee is 2 ⁿ , where n is the number of qubits used to perform the calculation. A quantum computer consisting of 500 qubits has the potential to perform 2500 operations in a single step. A conventional digital computer cannot perform a scale calculation close to 2500 operations in a reasonable time. In order to achieve the enormous processing power exhibited by quantum computers, qubits need to interact with each other in what is known as quantum entanglement.

  Qubits that interact with each other at some point maintain their type of association and can be entangled with each other in pairs in a process known as correlation. When the first qubit is intertwined with the second qubit, the first and second quantum states are quantum mechanically correlated. Entanglement is a quantum computation operation that has no similarity in conventional calculations. Once a pair of qubits are intertwined, information from only one qubit necessarily affects the state of the other qubit, and vice versa. For example, once a pair of qubits are intertwined, an operation performed on one of the pair affects both qubits of the pair simultaneously. Quantum entanglement allows qubits that are separated by greater distances to interact with each other instantaneously (not limited by the speed of light). It is not a matter of how large the distance between correlated particles is, the particles remain intertwined as long as they are isolated.

  Combining quantum superposition and entanglement creates a greatly enhanced computational power. A normal computer's 2-bit register can store only one of four binary configurations (00, 01, 10 or 11) at a given time, and a quantum computer's 2-bit register has each qubit Since it represents two values, all four numbers can be stored simultaneously. If more qubits are intertwined, the increase in capacity increases exponentially.

  Various physical systems have been proposed for qubits in quantum computers. Some systems use molecules with degenerate nuclear spin states. See US Pat. No. 5,917,322 “Method and Apparatus for Quantum Information Processing” by N. Gershenfeld and I. Chuang, which is incorporated herein by reference in its entirety. Nuclear magnetic resonance (NMR) technology can read the spin state. These systems perform well in search algorithms, for example, “Implementation of a quantum search algorithm on a quantum computer” by M. Mosca, RH Hansen and JA Jones, Nature 393, 344 (1998) and its references, for example, see `` Experimental realization of order-finding '' by LMK Vandersypen, M. Steffen, G. Breyta, CS Yannoni, R. Cleve, and IL Chuang. See the preprint (quant-ph / 0007017) of “with a quantum computer” and its references, the entire contents of which are incorporated herein by reference. The number ordering algorithm is related to the quantum Fourier transform, and Shor's factorization algorithm (P. Shor, 1994, Proc. 35th Ann. Symp. On Found. Of Comp. Sci., Pp. 124-134, IEEE Comp. Soc. Press, Los Alamitos, CA) and the essence of Grover's algorithm for searching unsorted databases (Grover, 1997, Phys. Rev. Lett. 78, p. 325) Related to general elements, which are incorporated herein by reference in their entirety. However, it is difficult to extend such a system to a commercial number of qubits. More generally, many of the currently proposed ones do not extend from a few qubits to the 102-103 qubits required for the most practical calculations.

  Unfortunately, current methods of intertwining qubits are sensitive to interference losses. The loss of interference is the loss of phase of the quantum superposition of qubits as a result of interaction with the environment. Thus, loss of interference results in loss of superposition of qubit states. For example, Phys. Today 44, 36 (1999) by Zurek, Rev. Mod. Phys. 59, 1 (1987) by Leggett et al., “Quantitative Dissipative Systems” 2nd edition by Weiss, World Scientific, Singapore ( 1999) and Hu et al., ArXiv: cond-mat / 0108339, which are hereby incorporated by reference in their entirety. The entanglement of qubit quantum states is an important step in the application of quantum algorithms. See, for example, P. Shor, SIAM J. of Comput., 26: 5, 1484-1509 (1997), which is incorporated herein by reference in its entirety. The current method of intertwining phase qubits requires the interaction of each flux of the qubit, and “Quantum state engineering with Josephson junction devices” by Yuriy Makhlin, Gerd Schon, Alexandre Shnirman. See LANL Preprint, cond-mat / 0011269 (November 2000), which is incorporated herein by reference in its entirety. This form of entanglement is sensitive to qubits that combine with the surrounding field, which results in loss of coherence and loss of information.

  As discussed above, the currently proposed qubit readout, initialization, and entanglement involves the detection or manipulation of magnetic flux at the qubit's location, and these methods can be used for coherent loss. It is sensitive and limits the overall scalability of the resulting quantum computing element. Therefore, there is a need in the art for a method of entanglement of qubits or otherwise control of qubits. Such a method can be used to create an efficient quantum register where the loss of coherence and other sources of noise is minimized and scalability is improved.

  The present invention addresses what is needed in the art to entangle and control qubits. The method and apparatus of the present invention is based on the unexpected discovery of a method of entanglement of a charge element such as a charge qubit with a phase element such as a phase qubit, in which the charge element reads the quantum state of the phase element. Can be used for In the present invention, the phase element and the charge element are connected by a π / 2 phase shift element. Without being limited to any particular theory, a π / 2 phase shift element includes a phase element and a charge element at the point of operation where the charge element current is most sensitive to changes in the phase element flux. Push in the circuit. Because of this phenomenon, the charge element can read the quantum state of the phase element.

Certain embodiments of the present invention combine mesoscopic phase elements, mesoscopic charge elements, and mesoscopic phase elements and mesoscopic charge elements such that the quantum mechanical state of the mesoscopic phase element interacts with the quantum mechanical state of the mesoscopic charge element. There is provided a superconducting structure having a mechanism for In some embodiments, the mesoscopic phase element includes a superconducting mesoscopic island characterized by charge energy E _c and Josephson coupling energy E _J. Furthermore, the charge energy E _c of the mesoscopic island is larger than the Josephson coupling energy E _J. That is, E _C is at least 10 times to about 100 times larger than E _J.

In one embodiment of the present invention, the mesoscopic phase element comprises a superconducting mesoscopic island, and the mesoscopic island charge energy E _C is on the same order as the island's Josephson coupling energy E _J. In an embodiment of the invention, the mesoscopic phase element is made of a superconducting material that disturbs the time reversal symmetry. Material for disturbing the time reversal symmetry, d wave like _{Yba 2 Cu 3 O 7-x} , Bi 2 Sr 2 Ca n-1 Cu n O 2n + 4, Tl 2 Ba 2 CuO 6 + x and HgBa ₂ CuO ₄ Superconductors and p-wave superconductors such as Sr ₂ RuO ₄ or CeIrIn ₅ .

  In another embodiment of the present invention, the mesoscopic phase element includes an island, a bulk region, and a clean Josephson junction that separates the mesoscopic island from the bulk region. A clean Josephson junction is a Josephson junction with free scattering sites. In yet another embodiment of the invention, the mesoscopic phase element includes an inner superconducting loop that includes one or more Josephson junctions and an outer superconducting loop that includes two or more Josephson junctions. Furthermore, the inner superconducting loop is inductively coupled to the outer superconducting loop. In some embodiments, the inner superconducting loop of the mesoscopic phase element is made of a conventional superconducting material such as aluminum (Al), niobium (Nb) and lead (Pb). In other embodiments, the inner superconducting loop consists of a superconducting material that disturbs the time reversal symmetry. In some embodiments of the invention, the outer superconducting loop is made of a conventional superconducting material such as aluminum (Al), niobium (Nb) and lead (Pb). In some embodiments of the invention, the outer superconducting loop includes a phase shift element, such as a π / 2 phase shift element.

In yet another embodiment of the present invention, the mesoscopic charge device includes a mesoscopic island characterized by charge energy E _C and Josephson coupling energy E _J. The charge energy E _C in the mesoscopic superconducting region is smaller than the Josephson coupling energy E _J.

  In some embodiments of the invention, the mesoscopic charge device includes a device lead, a mesoscopic island, and two Josephson junctions connected to the device lead. The two Josephson junctions are coupled to the mesoscopic island, thereby isolating the mesoscopic island from the device leads. The mesoscopic charge device further includes an electrode capacitively coupled to the mesoscopic island and a mechanism for controlling the charge of the capacitively coupled electrode.

  In some embodiments of the invention, the mechanism of binding includes a mechanism that introduces a phase shift. In some embodiments, the mechanism for introducing the phase shift is a π / 2 phase shift element, such as a phase shift Josephson junction. In some embodiments of the invention, the phase-shifting Josephson junction includes a ferromagnetic material disposed between the leads of the phase-shifting Josephson junction. In certain embodiments, the phase-shifting Josephson junction comprises an unconventional superconducting material (eg, d-wave or p-wave), and the leads of the phase-shifting Josephson junction are connected across the grain boundary Josephson junction, resulting in The phase shift is the accumulation of transitions across the grain boundaries. In an embodiment of the present invention, the angle of crystal anisotropy across the grain boundary is 0 ° to 45 ° with respect to the grain boundary angle. In some embodiments, the mechanism for coupling the mesoscopic phase element and the mesoscopic charge element interferometrically couples the mesoscopic phase element and the mesoscopic charge element. In some embodiments of the present invention, the mechanism for coupling the mesoscopic phase element and the mesoscopic charge element can entangle the mesoscopic phase element and the mesoscopic charge element.

  Another aspect of the invention includes a mesoscopic phase element, a mesoscopic charge element, and a mechanism for coupling the mesoscopic phase element to the mesoscopic charge element such that the state of the mesoscopic phase element and the state of the mesoscopic charge element interact. A superconducting structure is provided. Furthermore, the present invention provides a mechanism for reading the state of a mesoscopic charge device. In some embodiments, the mesoscopic charge element includes an element lead, the mechanism for reading the state of the mesoscopic charge element includes a shunt resistor, a current source, a mechanism for measuring a voltage drop across the lead of the mesoscopic charge element, and a mesoscopic charge element. And a mechanism for grounding the lead. The shunt resistance, the current source, the mechanism for measuring the voltage drop, and the mechanism for grounding are in parallel with the element lead of the mesoscopic charge element.

  Another aspect of the present invention includes a mesoscopic phase element, a mesoscopic charge element, a mechanism for coupling the mesoscopic phase element to the mesoscopic charge element so that the state of the mesoscopic phase element and the state of the mesoscopic charge element interact, and a mesoscopic phase element And a mechanism for reading the state of the phase element. In some embodiments, the mesoscopic charge element includes a lead, and the mechanism for reading the state of the mesoscopic charge element includes a shunt resistor, a current source, a mechanism for measuring a voltage drop across the lead of the mesoscopic charge element, and a mesoscopic charge element And a mechanism for grounding the lead. In such an embodiment, the shunt resistor, the current source, the mechanism for measuring the voltage drop, and the mechanism for grounding are arranged in parallel with the leads of the mesoscopic charge element.

  Yet another aspect of the present invention provides a superconducting structure including a plurality of mesoscopic phase elements. Each mesoscopic phase element of the plurality of mesoscopic phase elements is arranged in parallel. The superconducting structure further includes the first mesoscopic phase element of the plurality of mesoscopic phase elements and the mesoscopic charge element so that the state of the mesoscopic charge element and the state of the first mesoscopic phase element interact with the state of the mesoscopic charge element. And a mechanism for reading out the state of the mesoscopic charge element. In some embodiments, each mesoscopic phase element of the plurality of mesoscopic phase elements is a phase qubit, and the superconducting structure forms a quantum resistor.

  Certain embodiments of the present invention provide a superconducting structure. The structure includes a plurality of mesoscopic phase elements and a plurality of mesoscopic charge elements. Each mesoscopic phase element of the plurality of mesoscopic phase elements is arranged in parallel. The structure further includes a first mesoscopic phase element of the plurality of mesoscopic phase elements and a plurality of mesoscopic charge elements of the plurality of mesoscopic phase elements such that the state of the specific first phase element interacts with the state of the first mesoscopic charge element. A mechanism for coupling the first charge element is provided. The structure further provides a mechanism for reading the state of the first mesoscopic charge element or the mesoscopic phase element. In some embodiments, each mesoscopic phase element of the plurality of mesoscopic phase elements and each mesoscopic charge element of the plurality of mesoscopic charge elements is a qubit. Furthermore, the superconducting structure forms a non-uniform quantum resistor.

  The present invention deals with the existence of a fundamental limitation in the field of superconductivity or quantum computing-qubit entanglement. In order to achieve the full potential of quantum computation, here N qubits define an initial state, ie a combination of 2N states in which 2N computations can be performed simultaneously, and N qubits are appropriately entangled There is a need.

  When the quantum mechanical state of the first qubit is affected by the quantum mechanical state of the second qubit, two or more qubits are intertwined. For example, when the probability that a first qubit is in its first fundamental state is a function of the fundamental state of the second qubit, the quantum mechanical state of the first qubit is affected by the quantum mechanical state of the second qubit. Is done. In order to achieve entanglement between the first and second qubits, the quantum states of each of the first and second qubits are tightly coupled so that they interact with minimal loss of tightness of coupling. There is a need to. A qubit is tightly coupled when a Cooper pair (ie, a pair of electrons with constrained opposing spin and momentum in a superconducting medium) can flow between the first and second qubits. In the case of direct coupling of the first and second qubits, coherent coupling can be achieved by positioning a qubit whose distance between the qubits is shorter than the phase interference length of the Cooper pair. The phase interference length of the Cooper pair depends on the material. For some materials, the phase interference length is on the order of 100 nm or less.

There are known methods for intertwining qubits. The present invention provides an apparatus and method capable of intertwining qubits, the qubits involved in their operation having different characteristics in different regimes. Advantageously, the entanglement method of the present invention can be determined on a rate basis to entangle various qubits without loss of coherency. In some embodiments of the invention, the phase element is interferometrically coupled to a charge element using a π / 2 phase shift element, such as a phase shift Josephson junction. [pi / 2 phase shift element comprises entangled phases and charge qubits are used to move the point of operation of the device to the [Phi _0/4, dependence of switching current for the magnetic flux of the charge element is strongest. Thus, the π / 2 phase shifter places the charge qubit in a state that is most sensitive to changing the state of the phase qubit.

  The charge element can be used to read the state of the phase element. In another embodiment of the invention, the charge elements are coupled to a parallel array of phase elements. The charge element can be used to read any combination of phase elements.

  Superconducting electronics can be divided into two broad categories: “charge devices” and “phase devices”. The charge element includes a charge qubit. Charge qubits include islands that are superconducting. The charge qubit encodes to different states via the charge trapped on the island. See, for example, Makhlin et al., 1999, Nature 398, 305, which is incorporated herein by reference in its entirety. The phase element includes a phase qubit. Similar charge qubits and phase qubits include superconducting islands. In the phase qubit, each state varies greatly depending on the value of the phase Φ of the superconducting island in the low inductance SQUID loop. For example, see Ivanov and Feigel'man, 1988, JETP 87, p. 349, Ioffe et al., 1999, Nature 398, p. 679, which is hereby incorporated by reference in its entirety. A SQUID loop is a small superconducting loop that includes one or more Josephson junctions. SQUID stands for “Superconducting QUantum Interference Device”. SQUID is very sensitive to the total amount of magnetic field that passes through the region of the loop, and the voltage measured across the element correlates very strongly with the total magnetic field around the loop. In mesoscopic phase elements and mesoscopic charge elements, the superconducting island is mesoscopic (intermediate).

  As a result of long range Coulomb forces, charge qubits interact strongly with the environment and other qubits. Conversely, phase qubits are more effectively disconnected from the environment. In fact, a pure phase qubit that is different by the value of Φ has virtually zero interaction with the environment. See arXiv: cond-mat / 9907317v.1 July 21, 1999 by Feigel'man et al., Which is hereby incorporated by reference in its entirety. Thus, an array of phase qubits is less likely to lose coherence than an array of charge qubits.

  A Josephson junction is a junction that can “tunnel” a pair of superconducting electrons (Cooper pair) through a junction from one superconductor to another without an applied voltage. Josephson junctions are named after BD Josephson, which hypothesized that electron pairs might tunnel between nearby superconductors, where the lack of potential difference between the superconductors is comparable. It was. See B.D. Josephson, “Possible new effects in superconducting tunneling,” Phys. Lett, 1, 251-253 (1962). More generally, a link, such as a Josephson junction, that allows a pair of superconducting electrons (Cooper pair) to “tunnel” from one superconductor to another with no applied voltage. Is called a weak link.

  Cooper pairs are the basic current carriers of superconductors. In particular, a Cooper pair is a pair of opposing spin and momentum bound electrons in a superconducting medium. In the superconductor fundamental state, all electrons are bound to Cooper pairs (thus the fundamental state can only contain an even number of electrons). Thus, the extra electrons occupy an excited state such as bogolon and its minimum energy measured from the ground state energy is Δ.

  The present invention provides a mechanism that includes a phase element, a quantum phase charge coupled device including a charge element, and a mechanism that couples two elements that interact with each other. In some embodiments of the present invention, the phase element is a phase qubit (phase qubit) and the charge element is a charge qubit. In other embodiments, the phase element or charge element is a qubit, and other elements can be used to read the state of the qubit. In some embodiments of the invention, the phase element is a mesoscopic phase element and the charge element is a mesoscopic charge element.

The mechanism for coupling the phase element and the charge element includes a mechanism for introducing a phase shift between the elements. In certain embodiments of the invention, the phase shift is a π / 2-phase shift. The purpose of π / 2-phase shift is to provide adequate coupling between the elements. Usually, π / 2 junctions are used to move the point of operation of a SQUID device through the device to the point where the current dependence on flux is strongest. In certain embodiments of phase element 110 and charge element 120 (FIG. 1), a π / 2 junction is used to provide an accurate form of coherent coupling between elements 110 and 120. The π / 2 junction is a strong junction (ie, the junction has a large Josephson binding energy E _J and therefore has a strong critical current).

  It should be recognized that the phase shift element of the present invention has considerable tolerance. That is, they do not follow a complete π / 2 phase shift. In one embodiment, the phase shift element of the present invention allows a π / 2 ± 0.25π phase shift. In a more preferred embodiment, the phase shift element of the present invention follows a π / 2 ± 0.20π phase shift. In yet another embodiment of the present invention, the phase shift of the present invention follows a π / 2 ± 0.10π phase shift. In yet another embodiment of the present invention, the phase shift of the present invention follows a π / 2 ± 0.05π phase shift. Further, although the phase shift element of the present invention is described as a single element, such as element 115 (FIG. 1), it is continuous so that the sum of multiple elements achieves a total phase shift of π / 2. It should be appreciated that multiple elements can be used.

Mesoscopic devices such as mesoscopic phase devices or mesoscopic charge devices have dimensions that are smaller than macroscopic and larger than microscopic (islands, mesoscopic islands, mesoscopic regions, and mesoscopic superconducting regions , And the dimensions of the element such that the element behaves quantum mechanically. Therefore, Heisenberg's uncertainty principle defines the relationship between the charge and phase of the mesoscopic island in the device. This principle can be described as follows:
ΔnΔφ ≧ 1/2 (Formula 1)
Here, Δn represents the charge uncertainty (or charge fluctuation) of the mesoscopic island (mesoscopic region), and Δφ represents the phase uncertainty (or phase fluctuation) of the mesoscopic island. Therefore, the mesoscopic element has a large mesoscopic island phase uncertainty compared to the charge uncertainty, the charge regime, and the mesoscopic island charge uncertainty is large compared to the phase uncertainty, It can have two systems, a phase system.

Typically, the charge and phase regime is determined by the ratio of Josephson coupling energy E _J to Coulomb energy E _{C in} the device conditions. For example, when the ratio E _C / E _J >> 1, the device is in a charge regime, and when the ratio E _C / E _J << 1, the device is in a phase regime. Furthermore, E _C ≈E _J can also be utilized in embodiments of the present invention.

When the mesoscopic device is in the charge regime (E _C >> E _J ), the device is called a mesoscopic charge device. The charge of the mesoscopic charge element represents a good quantum number and has a finite number of charge states. A good quantum number in this case means a small uncertainty in its charge due to the uncertain relationship of Equation 1 above. For example, “Coherent control of macroscopic quantum state in a single-Cooper-pair box” Nature 398 by Y. Nakamura, Yu. Pashkin and J. Tsai. 786 (April 1999) and references referenced therein, incorporated herein by reference. When the mesoscopic element is in the phase regime (E _C << E _J ), the element is called a mesoscopic phase element. The phase of the mesoscopic phase element is a good quantum number (to a degree of uncertainty) with a finite number of phase states. The mesoscopic phase element has a small uncertainty in its phase due to the uncertainty relationship of Equation 1.

  The state of the mesoscopic charge device (charge uncertainty) and the uncertainty in the mesoscopic phase device (phase uncertainty) can be exploited for quantum computation. The small phase uncertainty of the mesoscopic phase element is used to form the fundamental state of the qubit. Furthermore, mesoscopic phase elements have a fixed charge, and the uncertainty in that phase is used as well for quantum computing purposes.

  With the quantum phase charge coupled device according to an embodiment of the present invention, the charge uncertainty of the mesoscopic charge device can affect the phase of the phase device, and the phase uncertainty of the phase device affects the charge of the charge device. Can influence. Thus, embodiments of the present invention interference couple the charge qubit with the phase qubit. That is, the charge qubit and the phase qubit are such that the phase of the electron wave function of the phase qubit and the phase of the electron wave function of the charge qubit are not broken (shifted in phase) by the coupling of the phase qubit and the charge qubit Combined with Interferometric coupling of charge qubits and phase qubits provides a mechanism for applying non-uniform entanglement operations during quantum computation. A non-uniform entanglement operation is an operation performed using a phase element, such as a phase qubit, and a charge element, such as a charge qubit, where the phase element and the charge element are entangled.

  FIG. 1 illustrates a superconducting structure called a quantum phase charge coupled device that includes a phase element 110 that is directly interferometrically connected to a charge element 120. In some embodiments of the invention, phase element 110 is a phase qubit. The manufacture and behavior of phase qubits are well known in the art. See, for example, US Patent Application Serial No. 09/452749 “Permanent Readout Superconducting Qubit” filed Dec. 1, 1999 by Alexandre Zagoskin, which is incorporated herein by reference in its entirety.

  In some embodiments of the invention, phase element 110 is a permanent read superconducting qubit (PRSQ). PRSQ is fully described in the above-mentioned permanent read patent application by Zagoskin. Simply put, PRSQ is a qubit's natural flux that reacts to the effect of shifting the phase of a given type of flux measurement device, excluding thermal fluctuations, when it is in a frozen (base and decay) state. It is a qubit that has the property of not. Thus, the natural magnetic flux of the frozen (basal and collapsed) qubit remains fixed except for thermal fluctuations.

  The phase element 110 includes a bulk region 110-2 and a mesoscopic island region 110-1 (also referred to as a mesoscopic island and / or a mesoscopic region) separated by a Josephson junction 110-3. A mesoscopic island (mesoscopic region) can support a superconducting current with a non-zero phase that is one of two degenerate states (basic states) | 0> and | 1>, In the context of a qubit such as phase element 110, it is an island. In order to support such currents, the mesoscopic islands must have dimensions that make them sensitive to the number of electrons in the islands. Such dimensions vary depending on the precise configuration of the qubit, but are generally in the micrometer range. In some embodiments, the mesoscopic island has a width of 0.2 μm or less, a length of about 0.5 μm or less, and a thickness of about 0.2 μm. The fundamental states | 0> and | 1> of the phase element 110 are represented by two distinct and degenerate ground state phases 110.

  The manufacture of the phase qubit shown in FIG. 1 is accomplished using techniques well known in the art. For example, grain boundary Josephson junctions can be formed using bi-crystal substrates such as strontium titanate (Chemical Technology, Tokyo, Japan) and standard thin film growth. Another grain boundary junction can be formed using the bi-epitaxial method. The mesoscopic islands and bulk superconducting regions of the phase qubit 110 can be patterned using, for example, electron beam lithography. Exemplary manufacturing techniques can be found in Van Zant, 2000, Microchip Fabrication, 4th Edition, McGrawHill, New York. See also Il'ichev et al. ArXiv: cond-mat / 9811017 and US patent application serial number 09/452749 filed Dec. 1, 1999 “Permanent Readout Superconducting Qubit”. The entirety of which is incorporated herein by reference.

Referring to FIG. 1, the charge device 120 represents a superconducting single electron transistor (SSET). SSET includes a mesoscopic island of superconducting material that is insulated from external circuitry by a tunnel barrier (Josephson junction). A normal tunnel barrier resistance (R> h / 4e ^{2 to} 6.5 kΩ) is sufficient to force an integral multiple of e to the excess charge on the mesoscopic island. Adding electrons to an equilibrium, electrically neutral mesoscopic island requires charge energy E _c = e ² / 2C, where C is the total capacity of the mesoscopic island. This “Coulomb blockade” can be lifted by applying a gate voltage to SSET and overcome by applying a sufficient source-drain bias voltage. See, for example, arXiv: cond-mat / 9909442v1 by Dixon et al.

  The charge device (SSET) 120 includes two Josephson junctions 120-1 and 120-2 that insulate the mesoscopic island 120-4 from the lead of the device. As described above, the SSET typically includes two Josephson junctions that insulate the mesoscopic island from the device leads and a mechanism for capacitively charging the mesoscopic island. In operation, the current flow through the device leads can be manipulated by controlling the charge energy of the mesoscopic island 120-4. Typically, SSET 120 provides control over a single Cooper pair. For example, when the charge on the mesoscopic island is in the first regime, no current flow through the device is allowed, and when the charge on the mesoscopic island is in the second regime, the current flow through the device is allowed. The charge of the mesoscopic island is controlled, for example, by a voltage source 125 that is capacitively coupled to the mesoscopic island.

Charged element (SSET) 120 further includes a capacitor 120-3 capacitively coupling the gate voltage V _g in (from a voltage source 125) mesoscopic island 120-4. In operation, the gate voltage V _g is controlled over the charge energy E _c of the mesoscopic island 120-4, and therefore between a state allowing a superconducting current flow and a state not allowing a superconducting current flow. A mechanism for adjusting the element is provided. SSET is an example of a mesoscopic charged element, wherein the phase ambiguity of mesoscopic island 120-4 is larger than its charged uncertainty, charged energy is controlled by the gate voltage V _g. Since SSET is a mesoscopic charged device, it is sensitive to the laws of quantum mechanics. Thus, SSET has an interfering quantum mechanical wave in charge and can itself be used as a charged qubit.

The gate voltage V _g is the ratio of the Coulomb energy is changed to the Josephson energy of the charge element 120, therefore, controls the flow of current over mesoscopic island 120-4. The material forming the intermediate layer of the Josephson junction (tunnel Josephson) 120-1 and 120-2 has a dielectric constant. The capacitance of the intermediate layer of tunnel junctions 120-1 and 120-2 correlates with the dielectric constant, and this thickness can be used to set the ratio between E _c and E _J.

Some materials useful for forming the interlayer of the junction are conventional metals such as aluminum oxide (Al ₂ O ₃ ), gold (Au) or silver (Ag). Tunnel junctions 120-1 and 120-2 and mesoscopic island 120-4 can be fabricated using, for example, electron beam lithography or shadow mask deposition techniques. Methods for forming tunnel junctions such as tunnel junctions 120-1 and 120-2 are well known as described in the art. In some embodiments, the area of the junction 120 is about 0.5 μm ² or less. In other embodiments of the present invention, the area of the tunnel junction 120 is about 0.1 μm ² or less, and in yet another embodiment, the area of the tunnel junction 120 is about 60 nm ² or less.

The behavior of SSET is described by P. Joyez et al., “Observation of Parity-Induced Suppression of Josephson Tunneling in the Superconducting Single Electron Transistor”, Physical Review Letters, Vol. .72, No. 15 (April 11, 1994) and D. Born, T. Wagner, W. Krech, U. Hubner and L. Fritzch, “Fabrication of Ultrasmall Tunnel Junctions by Electron Beam Direct-Writing ( Fabrication of ultra-small tunnel junctions by direct electron beam drawing) ”, defined in IEEE Trans. App. Superconductivity, 11, 373 (March 2001) and references cited therein and discussed in detail. The whole is incorporated here. In an embodiment of the present invention, when E _J ≈E _C , a small fluctuation in the phase of the mesoscopic element affects the charge of the mesoscopic element, and a small change in the charge of the mesoscopic element affects the phase of the mesoscopic element. A charge element can be made.

  FIG. 1 further illustrates a connection 140 between the phase element 110 and the charge element 120. Connection 140 is a mechanism for the phase state of phase element 110 that interacts with the charge state of charge element 120. In the embodiment of the present invention shown in FIG. 1, the connection 140 is connected to the first end connected to the mesoscopic island region 110-1 of the phase qubit 110 and to the mesoscopic island region 120-4 of the charge qubit 120. And a second end.

  Connection 140 includes phase-shifting Josephson junction 115 and, optionally, phase interference switch 130. In some embodiments of the invention, the phase shift Josephson junction 115 provides a π / 2 phase shift between its leads. In some embodiments of the invention, the phase-shifted Josephson junction comprises a ferromagnetic material. This ferromagnetic material is positioned between the leads of the phase-shifting Josephson junction. In certain embodiments of the invention, the phase-shifting Josephson junction comprises a conventional superconducting material such as an s-wave superconductor. Furthermore, the lead of the phase-shifting Josephson junction is connected across the grain boundary Josephson junction, and the phase shift accumulates at the transition across the grain boundary. A grain boundary Josephson junction is a junction between two superconducting regions, and the crystallographic orientation of the two regions differs by an amount sufficient to create a weak link junction. The difference in crystallographic orientation between the two regions is called the crystal misorientation angle across the grain boundary. In some embodiments of the invention, the crystal misorientation across the grain boundary is between 0 and 45 degrees.

  Phase interference switch 130 provides further control beyond the coupling between phase element 110 and charge element 120. Phase interference switch 130 provides a controllable connection and interference between elements 110 and 120. The phase interference switch 130 can be adjusted by controlling the applied external flux Φx (applied magnetic flux) that threads the loop in the switch 130. The switch 130 may be an interference switch such as SSET. Some embodiments of the present invention do not include the switch 130.

  Embodiments of phase shift elements such as phase shift Josephson junction 115 of FIG. 1 are well known in the art. For example, U.S. Patent Application No. 10 / 032,157, `` Phase Shift Device in Superconductor Logic '' filed on December 21, 2001 by G. Rose, M. Amin, T. Duty, A. Zagoskin, A. Omelyanchouk and J. Hilton. (Phase shift element in superconducting theory) "and references cited therein are incorporated herein by reference in their entirety. In certain embodiments of the present invention, phase-shifted Josephson junction 115 is fabricated using a different method than those required to fabricate other portions of connection 140. As described in the above-referenced application, such a problem provides a connection terminal that manufactures 115 with separate manufacturing layers, insulates 115 and forms a connection across element 115 to complete connection 140. This is overcome by using etching. In other embodiments of the invention, the phase-shifting Josephson junction 115 is made of niobium (Nb), aluminum (Al), lead (Pb) or tin (Sn), and an alloy of copper and nickel (Cu: Ni). It is formed using a conventional superconducting material such as a ferromagnetic material.

Referring to FIG. 1, the phase element 110 may be composed of an unconventional superconducting material having pair symmetry with non-zero angular momentum. In certain embodiments, materials useful for forming phase element 110 include materials that disturb time reversal, such as d-wave superconductors or p-wave superconductors. For example, see ArXiv.org/pdf/condmat/0008235. In certain embodiments, materials useful for forming the phase element 110 include the d-wave superconductor YBa ₂ Cu ₃ O _7-x , where x is between 0 and 0.6. Furthermore, it is also possible to use a superconductor as _{Bi 2 Sr 2 Ca n-1} Cu n O 2n + 4, Tl 2 Ba 2 CuO 6 + x and HgBa ₂ CuO _4. In another embodiment, the material used to form device 110 includes a p-wave superconductor such as Sr ₂ RuO ₄ or CeIrIn ₅ .

  In some embodiments of the present invention, phase element 110 is a mesoscopic phase element that includes a mesoscopic island, a bulk region, and a clean Josephson junction that separates the mesoscopic island from the bulk region. A clean Josephson junction is a junction with free scattering sites.

The charging element 120 is typically formed of a conventional superconducting material such as, for example, aluminum or niobium. Materials useful for the substrate of superconducting structure 100 include, but are not limited to, sapphire and SrTiO ₃ (strontium titanate). The substrate may be bi-crystalline, facilitating the formation of grain boundaries during superconducting layer growth. A grain boundary is an interface between two crystalline thin films with weak link junction properties. See, for example, US Pat. No. 5,157,466 by Char et al. Grain boundaries can be used to make Josephson junctions. Therefore, the bi-crystal substrate is useful for forming the phase element 110 and the phase shift Josephson junction 115. Fabrication and characterization methods for grain boundary Josephson junctions based on bi-crystal fabrication methods are well known in the art. For example, `` Degenerate by E.Il'ichev, M.Grajcar, R.Hlubina, R.Ijsselsteijn, H.Hoenig, H.Meyer, A.Golubov, M.Amin, A.Zagoskin, A.Omelyanchouk and M.Kupriyanov _{ground state in a mesoscopic YBa 2 Cu} 3 O 7-x grain boundary Josephson junction ( Chijimitai the ground state of a mesoscopic YBa ₂ Cu ₃ O _7-x grain boundary Josephson junctions) "Phys. Rev. Letters, 86, 5369 (2001 June) and references cited therein, the entire contents of which are hereby incorporated by reference.

  In another embodiment, the substrate of superconducting structure 100 can be formed using a bi-epitaxial manufacturing method. In such a method, the seed layer is formed on a first region of a single crystal substrate. The seed layer is typically not based on a single crystal substrate, but has a different lattice orientation. Subsequent grown superconducting thin films generally have a similar close crystal lattice orientation as a base. If the thin film grows in a highly directional manner, adjacent particles of material in the thin film are oriented in the wrong direction less than 5 degrees and do not degrade the superconducting electron transport properties of the thin film. Therefore, the characteristics of the highly directional superconducting thin film grown on the seed layer (first region) are different from those of the superconducting thin film grown on the original uncovered substrate (second region). Have The interface between the superconducting material grown in the first and second regions forms a grain boundary with weak link characteristics. That is, the grain boundary allows a pair of superconducting electrons (Cooper pair) to “tunnel” from one region without applied voltage to the next through a link. In general, the grain boundary is defined as the boundary between two highly directional regions (the regions with different orientations less than 5 degrees between adjacent particles in each region), the first region And the overall crystallographic orientation of the second region differ by more than about 5 degrees.

  In another embodiment, a first seed layer with a first crystallographic orientation is formed in a first portion based on a substrate and a second seed with a second crystallographic orientation. The layer is formed in a second part based on the substrate, the first part being adjacent to the second part. The portion of the superconducting thin film grown on the first seed layer has a first crystallographic orientation, and the portion of the superconducting thin film grown on the second seed layer has a second crystallographic orientation. Thus, weak link grain boundaries are formed between the interfaces between the first and second regions of the superconducting layer.

The manufacturing method described above provides a controllable formation of grain boundary regions and is therefore useful for forming multiple structures on a chip and requires the use of grain boundaries. Methods for forming bi-epitaxial grain boundary junctions are well known and described in the art. For example, “Bi-epitaxial YBCO grain boundary Josephson junctions on SrTiO ₃ and sapphire substrates (SrTiO ₃ and sapphire) by S. Nicolleti, H. Moriceau, J. Villegier, D. Chateigner, B. Bourgeaux, C. Cabanel and J. Laval. Bi-epitaxial YBCO grain boundary Josephson junctions on a substrate) ”Physics C, 269, 255 (1996) and references cited therein, and F. Tafuri, F. Carillo, F. Lombardi, F. Miletto Granozio, "Feasibility of biepitaxial YBCO Josephson junctions for fundamental studies and potential circuit implementation" by F. Ricci, U. Scotti di Uccio, A. Barone, G. Tests, E. Sarnelli and J. Kirtley. Biepitaxial YBCO Josephson Junction for) ”Phys. Rev. B, 62, 14431 (December 2000) and references cited therein, incorporated herein by reference.

Referring back to FIG. 1, the bulk region 110-2 of the phase element 110 is connected to the circuit ground node 190, the bulk region 110-2 maintains a constant phase Φ, and the mesoscopic island 110-1 is Φ ± φ _{0 has} a fluctuating phase, and φ ₀ is 0.001 flux quantum (0.001Φ ₀ ). Further, the lead of charging element 120 connected to Josephson junctions 120-1 and 120-2, respectively, is similarly connected to ground 190.

  As mentioned above, in some embodiments of the present invention, manufacturing the device of the present invention requires different manufacturing techniques. For example, the manufacturing equipment required to form the conventional superconducting circuit shown in FIG. 1 such as connection 140 and charging element 120 is shown in FIG. 1 such as phase element 110 and phase-shifting Josephson junction 115. In some cases, a tool different from the manufacturing method required to form a circuit different from the conventional one is required. Such manufacturing problems can be solved by manufacturing the device in different manufacturing stages.

  2A-2C illustrate a method for manufacturing an embodiment of the present invention. FIG. 2A illustrates the substrate 250, the phase shift Josephson junction 115, and the phase element 110. In some manufacturing methods used to construct various embodiments of the present invention, the number of elements formed in the first manufacturing stage varies. FIG. 2B illustrates the deposition of the insulating layer 260. The insulating layer 260 forms a barrier between the element formed in the first manufacturing stage and the element formed in the subsequent manufacturing stage. In certain embodiments, the insulating layer 260 is made of a material such as silicon oxide or aluminum oxide and is deposited using a technique such as evaporation. Aluminum oxide and silicon oxide are deposited using, for example, vapor deposition, etched using, for example, carbon tetrafluoride reactive ion etching, or deposited and etched by other suitable deposition and etching techniques. The structure can be patterned using known value techniques such as optics or electron beam lithography. See, for example, R. Stolz et al., Supercond. Sci. Technol. 12, 806 (1999), which is hereby incorporated by reference in its entirety. In some embodiments, the wiring layers are separated from each other by an approximately 800 nm thick insulating layer of silicon oxide to remove stray or parasitic capacitance. FIG. 2C further illustrates the configuration of contacts 115-A, 115-B, 110-A, and 110-B for each element 115 and 110, respectively. The chip 200 illustrated in FIG. 2C constitutes a substrate for further manufacture of the remaining components according to the present invention. Each of the contacts 115-A to 110-B is "Trilayer Heterostructure Junctions" (Attorney Reference Number) filed December 6, 2001 by Alexander Tzalenchuk, Zdravko Ivanov and Miles FHSteiniger. M-12300), which is incorporated herein by reference in its entirety to form interferometric quantum contacts between conventional and non-conventional superconductors. Providing a method for

  FIG. 3 illustrates another embodiment of the present invention where the phase element 310 is a flux qubit. For example, “Josephson Persistent-Current Qubit” by J. Mooij, T. Orlando, L. Levitov, L. Tian, C. van der Wal and S. Lloyd, Science, 285, 1036 (1999). August) and references cited therein, which are incorporated herein by reference in their entirety. The flux qubit 310 includes an outer superconducting loop 310-2 and an inner superconducting loop 310-1. Outer loop 310-2 includes three Josephson junctions. One of the Josephson junctions is a phase shift Josephson junction 115. Inner loop 310-1 also includes three Josephson junctions. The outer loop 310-2 can be formed of a conventional superconducting material, and the inner loop 310-1 can be formed of either a conventional superconducting material or an unconventional superconducting material. Examples of unconventional superconducting materials include d-wave and p-wave superconductors. Examples of conventional superconducting materials include s-wave superconductors such as aluminum (Al), niobium (Nb) and lead (Pb). The phase shift Josephson junction 115 is typically a π / 2-phase shift Josephson junction. In another embodiment, the phase shift element 115 is stable enough that the external flux preserves the interference and can replace the external flux with constraints that do not destroy the measurement process. FIG. 3 further illustrates superconducting reservoir 145 and capacitively coupled ground 190-Q. Superconducting reservoir 145 is also a large superconducting lead that connects outer loop 110-2 of flux qubit 110 to ground 190-Q. The manufacture of the phase element 310 is well known in the art and refers to the Id of Mooij et al.

  In some embodiments of the invention, inner loop 310-1 (FIG. 3) can be replaced with a phase element such as 110 (FIG. 1). The phase element is inductively coupled through outer loop 310-2. Inductive coupling creates a contact preparation step in unnecessary manufacturing methods.

  “Influence of Controlled Quantum-Mechanical Charge and Phase Fluctuations on Josephson Tunneling” by M. Matters, W. Eljion and J. Mooij, Phys. Rev. Lett. 75, 721 (July 1995) and references cited therein (incorporated herein by reference in their entirety) describe the control over the phase or binding energy of SSET mesoscopic islands. Their experimental work involves a large grounded superconductor such as a phase reservoir with a fixed phase connected via a switch to the SSET mesoscopic island. By adjusting the connection between the SSET mesoscopic island and the phase reservoir, they can gather direct evidence of the effect of Josephson coupling energy on quantum tunneling of the superconducting current through the SSET element. it can. The configuration of the phase reservoir consists of “a macroscopic measuring lead that has a large capacity with respect to ground and is otherwise not connected” (page 721, second column, second paragraph, eighth row). As described, the device is therefore not in a mesoscopic regime and has a constant phase. Certain embodiments of the present invention demonstrate that quantum phase fluctuations of mesoscopic phase elements interact with the charge of mesoscopic charge elements. Such a system is neither considered nor expected in the prior art. Furthermore, such regimes are useful, for example, to perform operational readouts on phase qubits or to perform controlled entanglement of qubits, both of which are critical operations that allow quantum operations.

  In accordance with the present invention, an embodiment of a phase coupling element includes a phase qubit, a superconducting single electron transistor (SSET), a mechanism for biasing SSET, and a mechanism for measuring a potential drop across the element. The SSET includes a mesoscopic island, is isolated from the device lead by a Josephson tunnel junction, and further includes a charged electrode that capacitively charges the mesoscopic island of the device. In operation, the charging mechanism of the SSET controls the coulomb energy of the mesoscopic island and thus controls the capacity of the SSET with respect to the current flow between the SSET leads.

  Embodiments of the present invention include a phase element, a charge element, and a mechanism for coupling the charge element and the mesoscopic region of the phase element. Coupling mechanisms include interference switches and phase shift elements and mechanisms that measure current directly or indirectly via SSET. The qubit has two basic states | 0> and | 1>, which are similar to the classical computational bit states 0 and 1. However, quantum computation is significantly different from classical computation because the qubit can be located in the superposition of its fundamental state. In general, a quantum computer includes a quantum register, which consists of one or more qubits to facilitate interaction with the qubit. In embodiments of the present invention, quantum computation operations are performed and operate in an environment where loss of coherency is minimized. The important coherence loss effects considered during quantum computation are thermal excitation and stray fields. Useful temperatures for interferometric quantum computation are on the order of 1 ° K or less or on the order of 10 mK to 50 mK.

FIG. 4 illustrates an embodiment of the present invention. The phase interference switch 130 functions as a switch that controls the interaction between the phase shift element 110 and the charge element 120. In operation, the external flux Φ _x is used to control the state of 130 and, consequently, the interaction of phase shift element 110 and charge element 120. The external flux Φ ₀ can be applied by providing a second loop 135 that is inductively coupled to the switch 130. The control loop 135 includes a current source I _x and an inductor 135-1. Control of the current through the loop 135 can adjust the switch 130 such that the phase element 110 is interference coupled to the charge element 120. Switch 130 may be any interference switch that controllably couples phase element 110 to charge element 120. For example, another switch useful for such applications is an interference SSET, e.g., `` The Radio-Frequency Single-Electron Transistor (RF-SET): A Fast and See Ultrasensitive Electrometer (RF-SET), Science, 280, 1238 (May 1998), which is incorporated herein by reference in its entirety.

  FIG. 4 further includes circuitry that controls charge element 120 and interacts with charge element 120. The circuit of FIG. 4 includes a shunt resistor 371, a current source 370, a ground 190-3, and a voltmeter 373. Further, FIG. 4 illustrates grounds 190-C and 190-Q that connect the gate voltages of charge element 120 and phase element 110 to circuit ground, respectively, and phase current source 370-Q that controls through phase element 110. To do. The shunt resistor 371 serves to change the behavior of the charging element 120 to the non-hysteresis overdamped mode. The shunt resistor 371 may be a normal metal or, for example, a Josephson junction with a large standard conductance and a small resistance. Methods for providing current sources 370 and 370-Q are well known in the art. The current source 370 can be controlled from a room temperature device such as a suitable cryogenic filter. The grounds 190-3, 190-C and 190-Q may be, for example, macroscopic superconductors. Methods for providing voltmeters 373 and 373-Q are well known in the art. In some embodiments of the invention, the lead connecting to the voltmeter 373 passes through a cold amplifier that is sampled at room temperature.

  A readout circuit similar to that illustrated in FIG. 4 that interacts with the charging element has been described in the art. For example, “Quantum-Limited Electrometer Based on Single Cooper Pair Tunneling” by A. Zorin, Phys. Rev. Lett., 76, 4408 (June 1996) and References cited therein are referenced and incorporated herein by reference in their entirety.

In the embodiment of the present invention, the capacitance Cp of the phase element is considerably larger than the capacitance Cj of the charge element. Furthermore, Cj is much larger than the capacitance Cg of the charge element gate and the capacitance Cjs of the switch separating the phase element and the charge element, leading to the following relationship:
Cq >> Cj >> Cg, Cjs (Formula 2)
The first inequality (Cq >> Cj) takes into account that the phase element is in phase regime and the charge element is in charge regime. The last inequality (Cj >> Cg, Cjs) assumes that the interaction between the charge of the charge element and the charge of the phase element is small.

In one embodiment of the invention according to FIG. 4, the Josephson junction 120-1 has a Josephson energy E _{j1 on} the order of 2.38 K (Kelvin) and a critical current I _c0 on the order of 100 nA (nanoamperes). Furthermore, the Josephson junction 120-2 has a charge energy E _J2 on the order of 1.015 * E _j1 , the mesoscopic island region 120-4 has a charge energy E _cg on the order of E _j1 , and the switch 130 is 3 * _Has Josephson energy E _js on the order of E _j1 . For embodiments of the present invention that meet the above ratio, the E _j2 / E _j1 ratio changes, making the invention robust against manufacturing errors. Further, in some embodiments of the present invention, the shunt resistor 371 can have a resistance value on the order of 100Ω.

  Embodiments of the present invention operate at a temperature such that thermal excitation is sufficiently suppressed to perform with quantum interference operation. In some embodiments of the invention, such temperature may be on the order of 1K or less. In other embodiments of the invention, such temperature may be on the order of 50 mK or less.

  Any superconducting Josephson device has a switch current or critical current that changes from one having a DC Josephson behavior to one having an AC Josephson behavior. This charge is related to the mechanical effect of the Josephson device. Thus, if the critical current of such a superconducting device exceeds a certain bias current, the voltage can be measured across the device. However, when the bias current is smaller than the critical current of the device, the voltage cannot be measured.

Referring to FIG. 4, some embodiments of the present invention provide a method for reading the state of the mesoscopic phase element 110. The method includes interferometric coupling of the mesoscopic phase element 110 to the mesoscopic charge element 120 using the phase shift element 115 and measuring the state of the mesoscopic charge element 120. In some embodiments, interfering the mesoscopic phase element 110 to the mesoscopic charge element 120 further comprises providing an interference connection between the mesoscopic phase element 110 and the mesoscopic charge element 120. In some embodiments, the interference connection includes a phase element 15. The interference connection maintains the quantum state of the mesoscopic phase element and the quantum state of the mesoscopic charge element. The step of interference coupling the mesoscopic phase element 110 to the mesoscopic charge element 120 further comprises providing a phase interference switch 130 that controls the interference connection between the mesoscopic phase element 110 and the mesoscopic charge element 120. Further, the step of interference coupling the mesoscopic phase element 110 to the mesoscopic charge element 120 includes adjusting the state of the phase shift interference switch 130 for a duration t. In some embodiments, the phase shift element is a π / 2 phase shift element. In some embodiments, the phase interference switch 130 is a superconducting loop that includes a first Josephson junction in the first branch and a second Josephson junction in the first branch. In one embodiment, the state of the phase interference switch 130 is adjusted by applying an external flux Φ _x (applied magnetic flux) for a time t. In one embodiment, the phase interference switch 130 connects to an interference connection where the first lead of the SSET leads to the mesoscopic phase element 110 and connects the interference connection where the second lead of the SSET leads to the mesoscopic charge element 120. A superconducting single electron transistor (SSET) connected to an interference connection. In some embodiments of the invention, the state of the phase interference switch 130 is adjusted by applying a gate voltage to SSET for a time t. In some embodiments, the time t (eg, the time during which the external flux Φ _x or the gate voltage is applied) when the phase interference switch 130 is adjusted is about 2 microseconds or less. In some embodiments, the duration t correlates with the rotational speed of the quantum state of the mesoscopic phase element 110. The rotational speed of the quantum state of the mesoscopic phase element 110 correlates with its tunneling amplitude, and therefore the duration t correlates with the tunneling amplitude of the mesoscopic phase element 110.

In some embodiments, measuring the state of the mesoscopic device includes driving a bias current across the leads of the mesoscopic charge device 120 for the duration of time t _b and then measuring the voltage drop across the device 120. In some embodiments, this measurement is made by placing element 120 in parallel with shunt resistor 371 (FIG. 4), and measuring the voltage drop across the mesoscopic charge element includes the voltage drop across shunt resistor 371. In some embodiments, the shunt resistor 371 is a regular metal or a Josephson junction with a large normal conductance and a small resistance. In certain embodiments, the duration t _b is about 1 nanosecond or less. In other embodiments, duration t _b is 1 microsecond or less. In some embodiments, the voltage drop measured across the mesoscopic charge element 120 correlates with the state of the mesoscopic phase element 110.

In some embodiments of the present invention, quantum phase charge coupled devices are used to read (measure) quantum states. In order to perform a read out operation with the phase element 110, the phase of the mesoscopic island region 110-1 (FIG. 1) can be measured. Preferably, an embodiment of the present invention provides a method for reading the phase of a phase element. The method of the present invention includes coupling a phase element to a charge element, driving a bias current across the lead of the charge element, and measuring the resulting voltage. In some embodiments of the invention, the voltage resulting from applying the current bias depends on the phase state of the phase element. For example, if the phase element has a first phase state and a second phase state, the first phase state is correlated with the first critical current I _C1 of the charge element, and the second phase state is Correlates with the second critical current I _C2 . The current through the charge element 120 is
I = I _c sinφ / 2 × cosθ (Formula 3)
Where φ is the sum of the phase differences across each Josephson junction of the charge element 120. Furthermore, θ is the phase of the mesoscopic island of the charge element 120. Therefore, the current flowing through the charge element 120 is affected by the phase of the mesoscopic island of the charge element 120. This relationship is part of the Josephson effect. The charge state of the mesoscopic island affects this phase. The value of cos θ takes a negative or positive value depending on whether the extra Cooper pair is on the mesoscopic island of the charge element 120 that conducts the negative and positive current 120. When one applies current to the charge element 120, there are two critical current values (I _C1 and I _C2 ) because the current already exists so that the Josephson effect can add or reduce the applied current. .

Another embodiment of a method for reading the state of the charge device 120 includes applying a bias current. The bias current exceeds both critical current values I _C1 and I _C2 that correlate with the first and second states of the charge device. In such a method, the resulting voltage is measured across the charge element. Since the bias current is greater than both the critical current values I _C1 and I _C2 , the charge element enters a voltage state and the resulting voltage depends on the state of the charge element.

  The circuit illustrated in FIG. 4 can be controlled separately over both charge and phase elements. If the switch is open, the phase element 110 and the charge element 120 are disconnected from each other, and the circuits attached to each other can be used separately to control and measure the state of each element. For example, when switch 130 is open, the state of phase element 110 can be initialized using current source 370 -Q and the state of charge element 120 uses current source 370 and gate voltage 125. Can be controlled. Furthermore, once disconnected, the state of each element can be read separately. Methods for reading the state of each element separately are well known in the art.

In an embodiment of the present invention, the method for reading the phase state of the phase element includes biasing the charge element with a bias current that is greater than the first critical current I _{C1 and} less than the second critical current I _C2 . Referring to FIG. 4, since I _C1 and I _C2 correlate with the phase state of phase element 110, if charge element 120 has a first critical current I _C1 , a voltage is developed across shunt resistor 371 and charge element 120. Does not develop across the shunt resistor 371 if has a second critical current I _C2 .

In an embodiment of the present invention, the phase qubit is interferometrically and controllably coupled to the mesoscopic charge element, and the bias state of the qubit is correlated to the critical current I _C1 of the mesoscopic charge element. The critical current of a mesoscopic charge device represents the maximum current that can be handled before the device becomes non-superconducting and is substantially determined by the characteristics of the main components of the mesoscopic charge device (see above). For example, the first bias state of the phase qubit may be | 0>, correlating with the first critical current I _C1 of the mesoscopic charge element, and the second bias state of the phase qubit is | 1>. Correlating with the second critical current I _C2 of the mesoscopic charge device. If the applied current exceeds the critical current of the mesoscopic charge element, a voltage will develop across the lead, and if the applied voltage does not exceed the critical current of the mesoscopic charge element, it will remain superconducting and substantially between the leads Has a zero voltage drop.

According to an embodiment of the present invention, a method for performing a read operation of a qubit includes applying a bias current across the SSET lead, the SSET mesoscopic island being interferometrically coupled to the phase qubit, It further includes measuring the voltage generated across the SSET leads. The bias current I _b correlates with the bias state of the phase qubit and is selected to be between the critical currents of the mesoscopic islands of the SSET element, and if the phase qubit occupies the first bias state, I _b is If the critical current is exceeded, driving the SSET into dynamic regime, causing a voltage drop across the SSET lead and the phase qubit occupying the second bias state, _Ib will not exceed the SSET critical current and the lead There is no current measured across.

  According to embodiments of the present invention, a phase-charge element can include multiple elements of an array, and the element can be either a phase element or a charge element. Each of the phase or charge elements is coupled to each other and the two elements of the array are interferometrically coupled. In an embodiment of the invention, the array consists of phase elements, each coupled to a single charge element via a separate interference switch, and the phase elements of the array can be connected to a charge element if desired. .

  FIG. 5 illustrates a phase charge element 505 that includes a charge element connected to a plurality of phase elements 500-1 through 500-N. Each of the phase elements 500-1 to 500-N is arranged in parallel, and each branch includes a switch 130-1 to 130-N, respectively, and when the switch 130-1 to 130-N is closed, 500 is coupled to charge element 120 and each phase element 500 is disconnected from charge element 120 when open. FIG. 5 is further similar to reading the circuit illustrated in FIG. 4 and includes reading a circuit including a shunt resistor or Josephson junction 371, a current source 370, an electrometer 373, and ground 190-3. Illustrated. In embodiments of the present invention, the readout circuit used to determine the state of the charge element 120 varies.

  Referring again to FIG. 5, the charge element 120, the phase shift element 115, the phase elements 500-1 to 500-N, and the switch 130-1 form a circuit that entangles the state of the phase elements in the array. In addition, element 505 includes a bus 501 that each qubit branch can be coupled or disconnected by controlling switches 130-1 through 130-N. Previous proposals for implementing quantum register entanglement operations include only coupling with the nearest neighbor. For example, Phys. Rev A 022312 (2001) by A. Blais and “Quantum state engineering with Josephson-junction devices” by Y. Makhlin, G. Schon, A. Shnirman See LANL Preprint, cond-mat / 0011269 (November 2000), which is incorporated herein by reference in its entirety. However, embodiments of the present invention provide a mechanism for coupling with neighbors that are not the closest. According to the present invention, an array of phase elements is formed and a mechanism for entanglement of the state of the phase elements of the array can be achieved beyond the nearest neighbor coupling.

An embodiment of a method for providing an entanglement operation includes selecting a first phase element 500 (FIG. 5), the first phase element interacting with the charge element 120 for a duration t ₁ and sustaining. Selecting a second phase element 500 for time t ₂ , the state of the second phase element interacts with the charge element 120, and thus the state of the first phase element and the second phase element Including entanglement. The durations t ₁ and t ₂ may be 10 microseconds or less and depend on the embodiment of the present invention. In some embodiments of the invention, durations t ₁ and t ₂ may be 100 nanoseconds or less. Such a method can be repeated for a desired number of phase elements 500.

  To finish the entanglement operation, the state of charge element 120 can be untangled by reading the state of charge element 120 and phase element 500 is deselected. The entanglement operation ensures that information from previous entanglements is removed and is independent of the new coupling formed between the phase element 500 and the charge element 120 of the array.

  The entanglement method can further be used to collect information about the state of the array. For example, to perform large-scale quantum computation operations, the quantum states in the quantum register can be encoded to reduce the disappearance of coherence. For example, US Pat. No. 5,768,297 by PW Shor “Method for reducing decoherence in quantum computer memory” and US Pat. No. 6,128,764 by D. Gottesman “Quantum error- See correcting codes and devices. Such encoding methods typically involve applying an indeterminate set of operations in measuring the state of one or more qubits in the quantum register. Thus, during the entanglement process, the state of the charge element helps to collect the information required to maintain the encoded state and therefore increases the coherency of the quantum register.

  The method for reading the state of the phase element 500 arranged in series with the charge element 120 in parallel with the other phase elements 500 of the array selects each phase element 500 of the array and sets the state of the charge element 120 to Including reading. Selection of a particular phase element 500 requires a phase element 500 coupled to charge element 120, the state of phase element 500 correlates with the state of charge element 120, and as detailed above, other phase elements 500 of the array Is disconnected from the charge element 120.

  A method for reading the state of charge elements 120 arranged in series with a parallel array of phase elements 500 selects each phase element 500 to be read, biases charge element 120 with a bias current, and voltage across charge element 120. Includes measuring descent. In an embodiment of the present invention, selecting the phase element 500 includes closing the switch 130 to couple each phase element 500 to the charge element 120. The readout method can then be repeated for each of the phase elements 500 in the array. For example, referring to FIG. 5, reading the state of phase elements 500-1 through 500-N closes switch 130-1, uses current source 370 to drive the bias current, and uses electrometer 373. Measuring the voltage drop, deselecting or opening switch 130-1, and repeating the process for elements 500-2 through 500-N.

  According to the present invention, the phase charge coupled device is a two-qubit quantum register. Each of the qubits is controlled via initialization and readout, and furthermore, the quantum state of each charge and phase qubit can be intertwined. Such a quantum register can be fully scaled with respect to a quantum register containing multiple qubits. Each qubit of the plurality of qubits may be either a charge qubit or a phase qubit as described above. Such a quantum register is defined as a non-uniform quantum register. In an embodiment of the present invention, a method for performing an entanglement operation between a non-uniform pair of qubits including a phase qubit and a charge qubit uses a coupling mechanism to combine the pair for a certain duration t. including. Such a coupling mechanism is described in detail above. The duration t of the entanglement operation can be on the order of microseconds or less, depending on the embodiment of the present invention. In some embodiments of the invention, the duration t is on the order of nanoseconds or less. In one embodiment, the duration t of the entanglement operation correlates with the tunneling amplitude of one of the non-uniform pairs of qubits.

  Referring again to FIG. 5, the charge element 120 may be a charge qubit and the phase elements 500-1 to 500-N may be phase qubits. When one of the phase qubits of the array or quantum register is selected, the state of each phase qubit is entangled with the state of the charge qubit 120. Furthermore, each phase qubit can be entangled with the charge qubit by simultaneously selecting the array's phase qubits from each other. In the regime where the charge element 120 is not a charge qubit, the array shown in FIG. 5 is used to entangle the phase qubits 500-1 through 500-N and read out the state of the phase qubits 500-1 through 500-N. Used to.

  FIG. 6 illustrates an embodiment of the present invention. In circuit 600, a parallel array of charge and phase elements 500-1 to 500-N is connected to the readout circuit. The readout circuit is the same as that described in conjunction with the discussion of FIGS. 4 and 5 above. The circuit 600 of FIG. 6 further illustrates a coupling mechanism between the branches adjacent to the array that includes each of the switches 130-1 to 130-N-1 and the Josephson junctions 115-1 to 115-N-1. Each branch of the circuit includes phase or charge elements 500-1 to 500-N and switches 685-1 to 685-N for coupling each phase or charge element to the readout circuit.

Certain embodiments of the present invention provide a method for reading the state of a non-uniform quantum register qubit. The non-uniform quantum register includes a first plurality of phase qubits and a second plurality of charge qubits. The method includes selecting a first qubit of the quantum register. The first qubit is a first charge qubit or a first phase qubit. The first qubit is interferometrically connected to the mesoscopic charge element for a certain duration t _c . The state of the mesoscopic charge element is then read after the duration t _c has elapsed. In some embodiments, the first qubit is coupled to the mesoscopic charge element of the quantum register by adjusting the state of a phase interference switch that adjusts the connection between the qubit and the mesoscopic charge element. In some embodiments, the phase interference switch is adjusted by applying a flux. In some embodiments, the phase interference switch is adjusted by applying a gate voltage. In some embodiments, the duration t _c correlates with the tunneling amplitude of the first qubit. In other embodiments, the duration t _c is about 1 microsecond or less. In yet other embodiments, the duration t _c is about 1 nanosecond or less. In one embodiment, the state of the mesoscopic charge element is read by driving a bias current across the leads of the mesoscopic charge element and measuring the voltage drop across the mesoscopic charge element.

  In certain embodiments, adjacent elements of the array are of several regimes. Such an arrangement includes charge-charge pairs as well as phase-phase pairs. In such a situation, the phase shift element 115 is not required between adjacent elements. An embodiment of a method for reading the state of a phase or charge element includes selecting each of the phase or charge elements, driving a bias current across the element, and measuring a voltage drop across the element.

An embodiment for coupling a charge element and a phase element includes closing a coupling switch between the respective elements. Referring again to FIG. 6, charge element 500-1 is connected to phase element 500-2 by closing switch 130-1 for a certain duration t _c , and then elements 500-1 and 500-2 are connected. The switch 130-1 is opened so as not to be coupled. Any two adjacent elements of the circuit illustrated in FIG. 6 can be similarly coupled. In embodiments of the present invention, the phase and coupling elements may be phase qubits and charge qubits, respectively, 600 for read, initialization, and entanglement operations, and for each of qubits 500-1 through 500-N. It operates as a quantum register that provides other important quantum computation operations such as sigma z or bias operations. The method for performing each operation of the 600 qubits is described in US patent application 09 / 872,495 “Quantum Processing,” filed June 1, 2001 by M. Amin, G. Rose, A. Zagoskin and J. Hilton. System for Superconducting Phase Qubit ”and references cited therein, the entire contents of which are hereby incorporated by reference.

  FIG. 7 illustrates the phase element 110 coupled to the charge element 120, which is coupled to the readout circuit 790. Read circuit 790 includes a tank circuit 790-1 connected in series to charge element 120 and ground 190-3. Connection 791 is an input and output line through which wireless signals can enter and exit the circuit. The readout circuit 790 illustrated in FIG. 7 is a wireless SET (rf-SET) and is described for use as an electrometer in the art. For example, “Radio-Frequency Single-Electron Transistor as Readout Device for Qubits: Charge Sensitivity and Backaction” by A. Assime, G. Johansson, G. Wendin, R. Schoelkopf and P. Delsing. Electronic Transistors: Charge Sensitivity and Back Action) ”Phys. Rev. Lett., 86, 3376 (April 2001), which is incorporated herein by reference in its entirety.

  In this reference, the rf-SET element is used as an electrometer or a charge readout device for charge qubits. However, phase qubit readout is not considered and charge qubit readout is accomplished by measuring the charge state of the qubit.

  FIG. 8 illustrates another readout circuit 800 that provides inductive coupling between the tank circuit 895 and the superconducting loop 896, which is formed by connecting the leads of the charge element 120 together. 894 represents the inductive coupling between the two elements. The readout circuit is described in A. Zorin's “Cooper-pair qubit and Cooper-pair electrometer in one device” and its references, which are hereby incorporated by reference in their entirety. Incorporate.

Certain aspects of the present invention provide a method for performing quantum computational entanglement operations between a phase qubit and a charge qubit. In that method, an interference connection is provided between the phase qubit and the charge qubit. The interference connection allows the quantum state of the phase qubit and the quantum state of the charge qubit to interact with each other. Furthermore, the interference connection is adjusted for a duration t _e . Phase qubit, for at least a duration t _e to intertwining controllably quantum states of the quantum state and charge qubit phase qubit can be connected to a charge qubit. In some embodiments, the duration t _e is 1 microsecond or less. In some embodiments, the duration t _e is 1 nanosecond or less.

  In certain embodiments, the interference connection includes the use of a phase shifting element. In some embodiments, the interference connection is adjusted by adjusting a phase interference switch. In some embodiments, the phase interference switch is a superconducting loop that includes a first Josephson junction of a first branch and a second Josephson junction of a second branch. In yet another embodiment, adjusting the phase interference switch includes controlling the applied magnetic flux or gate voltage. In one embodiment of the invention, the phase interference switch is a SSET connected to the interference connection, the first lead of the SSET is connected to the interference connection leading to the phase qubit, and the second lead of the SSET is to the charge qubit. Connect to the interfering connection to be connected.

Another aspect of the invention provides a method for intertwining non-uniform quantum register qubits. A non-uniform quantum register includes a plurality of phase qubits and a plurality of charge qubits. In this method, a quantum register having a bus is used. Any qubit of a non-uniform quantum register can be connected to or disconnected from the bus in an interfering manner. The first qubit is selected from a plurality of qubits in the non-uniform quantum register. Furthermore, the first qubit is coupled to the mesoscopic charge element for a duration t ₁ . The second qubit is selected from a plurality of qubits in the non-uniform quantum register. Second qubit for a duration t _2, is connected to the mesoscopic charge device. In some embodiments, selecting the first qubit includes adjusting a first correlated interference switch. The first phase interference switch correlates with the first qubit, and when the first phase interference switch is closed, the first qubit is connected to the bus, and when the first phase interference switch is opened, One qubit is isolated from the bus. In some embodiments, selecting the second qubit includes adjusting a second phase interference switch. The second phase interference switch is correlated with the second qubit, and when the second phase interference switch is closed, the second qubit is connected to the bus, and when the second phase interference switch is opened, the second qubit is The qubit is isolated from the bus. In some embodiments, coupling the first qubit to the mesoscopic charge element includes selecting the mesoscopic charge element, the mesoscopic charge element being connected to the bus for a duration t _s . In some embodiments, the mesoscopic charging element includes adjusting a phase interference switch, and when the phase interference switch is open, the mesoscopic charge element is disconnected from the selected first qubit and the phase interference switch is closed. The mesoscopic charge element is connected to the selected first qubit. In some embodiments, the duration t ₁ is about 1 microsecond or less. In other embodiments, duration t ₁ is about 1 nanosecond or less. In certain embodiments, duration t ₂ is about 1 microsecond or less. In yet another embodiment, the duration t ₂ is about 1 nanosecond or less.

  In certain embodiments, the method further includes cleaning information from the mesoscopic charge element by reading the state of the mesoscopic charge element. In some embodiments, reading the state of the mesoscopic charge device provides information that is used as feedback to perform an error correction algorithm on the non-uniform quantum register.

  In some embodiments, the method includes cleaning the mesoscopic charge device such that the non-uniform quantum resistor is ready to begin a new entanglement operation. In certain embodiments, cleaning the mesoscopic charge device includes reading the quantum state of the mesoscopic charge device. In some embodiments, the clean operation provides information about the quantum state of the quantum register. In some embodiments, the clean operation provides information useful for performing error correction operations on the quantum state of the quantum register. In some embodiments, the quantum operation is performed on the qubits of the quantum register subject to information resulting from performing the clean operation. Although the invention has been described with reference to a few specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications may be made by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.

  All references cited herein are for the same scope as each individual publication or patent or patent application is clearly and individually indicated to be incorporated by reference in its entirety for all purposes, and All of which are incorporated herein by reference.

1 illustrates a superconducting structure, also referred to as a quantum phase charge coupled device, according to an embodiment of the invention. 1 illustrates a method for fabricating a quantum phase charge coupled device according to an embodiment of the invention. 1 illustrates a method for fabricating a quantum phase charge coupled device according to an embodiment of the invention. 1 illustrates a method for fabricating a quantum phase charge coupled device according to an embodiment of the invention. 1 illustrates a quantum phase charge coupled device including a flux qubit according to an embodiment of the present invention. 4 illustrates a quantum phase charge coupled device including circuitry that interacts with and controls a charge device, according to another embodiment of the invention. 4 illustrates a quantum phase charge-coupled device including an array of phase elements and circuitry that interlaces the array of phase elements, according to another embodiment of the invention. FIG. 6 illustrates a superconducting structure including a parallel array of charge and phase elements connected to a readout circuit, according to another embodiment of the invention. FIG. 4 illustrates a quantum phase charge coupled device in which a charge device is connected to a readout circuit and a phase device, according to an embodiment of the present invention. Figure 4 illustrates a readout circuit used in some embodiments of the present invention.

Claims (115)

A mesoscopic phase element;
A mesoscopic charge element;
A mechanism for coupling the mesoscopic phase element and the mesoscopic charge element so that the quantum state of the mesoscopic phase element interacts with the quantum state of the mesoscopic charge element;
A superconducting structure characterized by comprising: The mesoscopic phase element includes a superconducting mesoscopic island characterized by a charge energy E _C and a Josephson coupling energy E _J , wherein the charge energy E _{C of the} mesoscopic island is at least 10 times greater than the Josephson coupling energy E _J. The superconducting structure according to claim 1, wherein the superconducting structure is large. 2. The super of claim 1, wherein the mesoscopic phase element includes a superconducting mesoscopic island, and the charge energy E _C of the mesoscopic island is in the same order as the Josephson coupling energy E _{J of the} mesoscopic island. Conductive structure.   The superconducting structure according to claim 1, wherein the mesoscopic phase element is made of a superconducting material that disturbs time reversal symmetry.   The superconducting structure according to claim 4, wherein the superconducting material disturbing time reversal symmetry is a d-wave superconductor. The d-wave superconducting material selected from the group consisting of _{Yba 2 Cu 3 O 7-x} , Bi 2 Sr 2 Ca n-1 Cu n O 2n + 4, Tl 2 Ba 2 CuO 6 + x and HgBa ₂ CuO ₄ 6. The superconducting structure according to claim 5, wherein x is smaller than 0.6 and larger than 0.   The superconducting structure according to claim 5, wherein the superconducting material is a p-wave superconductor. The superconducting structure according to claim 7, wherein the superconducting material is Sr ₂ RuO ₄ . The superconducting structure according to claim 7, wherein the superconducting material is CeIrIn ₅ . Mesoscopic phase element
Mesoscopic island,
A bulk region;
A clear Josephson junction separating the mesoscopic island from the bulk region;
The superconducting structure according to claim 1, wherein Mesoscopic phase element
An inner superconducting loop including one or more Josephson junctions;
An outer superconducting loop comprising two or more Josephson junctions;
Including
The superconducting structure of claim 1, wherein the inner superconducting loop is inductively coupled to the outer superconducting loop.   The superconducting structure according to claim 11, wherein the inner superconducting loop of the mesoscopic phase element is made of a normal superconducting material.   The superconducting structure according to claim 12, wherein the normal superconducting structure is selected from the group consisting of aluminum (Al), niobium (Nb), and lead (Pb).   The superconducting structure according to claim 11, wherein the inner superconducting loop is made of a superconducting material that disturbs time reversal symmetry.   The superconducting structure according to claim 14, wherein the superconducting material is a d-wave superconductor. The d-wave superconducting material selected from the group consisting of _{Yba 2 Cu 3 O 7-x} , Bi 2 Sr 2 Ca n-1 Cu n O 2n + 4, Tl 2 Ba 2 CuO 6 + x and HgBa ₂ CuO ₄ The superconducting structure according to claim 15, wherein x is smaller than 0.6 and larger than 0.   The superconducting structure according to claim 11, wherein the outer superconducting loop is made of a normal superconducting material.   The superconducting structure according to claim 17, wherein the normal superconducting material is selected from the group consisting of aluminum (Al), niobium (Nb), and lead (Pb).   The superconducting structure of claim 11, wherein the outer superconducting loop includes a phase shift element. Comparing the mesoscopic charge device comprises a superconducting mesoscopic island characterized by charge energy E _C and Josephson coupling energy E _J, the charge energy E _C of the mesoscopic superconducting region, and the Josephson coupling energy E _J The superconducting structure according to claim 1, wherein the superconducting structure is small. 2. The superconducting structure according to claim 1, wherein the mesoscopic charge element includes a superconducting mesoscopic island, and a charge energy E _C of the mesoscopic island is equal to or lower than a Josephson coupling energy E _{J of the} mesoscopic island. body. The mesoscopic charge element is
An element lead;
Mesoscopic island,
Two Josephson junctions connected to the element leads and the two Josephson junctions being coupled to the mesoscopic islands to insulate the mesoscopic islands from the element leads;
An electrode capacitively coupled to the mesoscopic island;
Controlling the charge of the capacitively coupled electrodes.
The superconducting structure according to claim 1.   The superconducting structure according to claim 1, wherein the mesoscopic charge element includes a normal superconducting material.   The superconducting structure of claim 1, wherein the mechanism for coupling includes a mechanism for introducing a phase shift.   The superconducting structure of claim 24, wherein the mechanism for coupling includes a mechanism for controlling a phase shift between the mesoscopic phase element and the mesoscopic charge element.   The superconducting structure according to claim 25, wherein the mechanism for controlling phase shift is a π / 2 phase shift element.   27. The superconducting structure according to claim 26, wherein the π / 2 phase shift element is a phase shift Josephson junction.   28. The superconducting structure of claim 27, wherein the phase-shifted Josephson junction includes a ferromagnetic material, and the ferromagnetic material is disposed between leads of the phase-shifted Josephson junction.   A phase-shifting Josephson junction includes an unusual superconducting material, and the lead of the phase-shifting Josephson junction is connected across the Josephson junction grain boundary, so that the phase shift accumulates at the transition across the grain boundary. The superconducting structure according to claim 27, wherein:   30. The superconducting structure according to claim 29, wherein an anisotropy angle of the crystal across the grain boundary is 0 to 45 degrees with respect to the grain boundary angle.   The superconducting structure according to claim 1, wherein the mesoscopic phase element is a qubit.   The superconducting structure according to claim 1, wherein the mesoscopic charge device is a qubit.   The superconducting structure according to claim 1, wherein the mechanism for coupling the mesoscopic phase element and the mesoscopic charge element coherently couples the mesoscopic phase element and the mesoscopic charge element.   The superconducting device according to claim 1, wherein the mechanism for coupling the mesoscopic phase element and the mesoscopic charge element is capable of intertwining the mesoscopic phase element and the mesoscopic charge element. Structure.   26. The superconducting structure of claim 25, wherein the mechanism for controlling phase shift produces a [pi] / 2 phase shift.   26. The superconducting structure of claim 25, wherein the mechanism for controlling phase shift produces a [pi] /2±0.20 [pi] phase shift.   26. The superconducting structure of claim 25, wherein the mechanism for controlling phase shift produces a π / 2 ± 0.10π phase shift.   26. The superconducting structure of claim 25, wherein the mechanism for controlling phase shift produces a π / 2 ± 0.05π phase shift. A mesoscopic phase element;
A mesoscopic charge element;
A mechanism for coupling the mesoscopic phase element to the mesoscopic charge element such that a quantum state of the mesoscopic phase element interacts with a quantum state of the mesoscopic charge element;
A mechanism for reading the quantum state of the mesoscopic charge device;
A superconducting structure characterized by comprising: The mesoscopic charge element includes an element lead, and the mechanism for reading a quantum state of the mesoscopic charge element includes:
Shunt resistance,
A current source;
A mechanism for measuring the voltage drop at the lead of the mesoscopic charge device;
A mechanism for grounding the lead of the mesoscopic charge element;
Including
40. The shunt resistance, the current source, the mechanism for measuring a voltage drop, and the mechanism for grounding are in parallel with the element lead of the mesoscopic charge element. Superconducting structure. The mesoscopic charge element includes an element lead, and the mechanism for reading a quantum state of the mesoscopic charge element includes:
Josephson junction,
A current source;
A mechanism configured to measure a voltage drop at a lead of the mesoscopic charge device;
A mechanism for grounding the lead of the mesoscopic charge element;
Including
The Josephson junction, the current source, the mechanism configured to measure a voltage drop, and the mechanism for grounding are arranged in parallel with the element lead of the mesoscopic charge element. 40. A superconducting structure according to claim 39, characterized in that: The mechanism for reading the quantum state of the mesoscopic charge device comprises:
A first lead of the mesoscopic charge element grounded;
A second lead of the mesoscopic charge element connected to a tank circuit;
The tank circuit includes an inductor and a capacitor;
40. The superconducting structure of claim 39, wherein the characteristics of the tank circuit depend on the quantum state of the mesoscopic charge device.   The superconducting structure according to claim 42, wherein the characteristic of the tank circuit is an impedance of the tank circuit. The mechanism for reading the quantum state of the mesoscopic charge device comprises:
Connecting a first lead of the mesoscopic charge element to a second lead of the mesoscopic charge element such that the lead of the mesoscopic charge element forms a loop;
43. The superconducting structure of claim 42, wherein a tank circuit and the tank circuit are inductively coupled to the loop, and characteristics of the tank circuit depend on a quantum state of the mesoscopic charge device. body.   The superconducting structure according to claim 44, wherein the characteristic of the tank circuit is an impedance of the tank circuit. A plurality of additional mesoscopic phase elements, each arranged in parallel;
Coupling the first additional mesoscopic phase element of the plurality of additional mesoscopic phase elements to the mesoscopic charge element such that the state of the first additional mesoscopic phase element interacts with the state of the mesoscopic charge element. And a mechanism for
40. The superconducting structure according to claim 39.   47. The mesoscopic phase element of the group consisting of the mesoscopic phase element and the plurality of additional mesoscopic phase elements is a phase qubit, and the superconducting structure forms a quantum resistor. Superconducting structure.   The superconducting structure according to claim 39, further comprising a mechanism for reading a state of a mesoscopic phase element. A plurality of mesoscopic phase elements, each mesoscopic phase element of the plurality of mesoscopic phase elements is arranged in parallel,
A mesoscopic charge element;
A mechanism for coupling the first mesoscopic phase element of the plurality of mesoscopic phase elements with the mesoscopic charge element such that the state of the first mesoscopic phase element and the state of the mesoscopic charge element interact;
A mechanism for reading the state of the mesoscopic charge element;
A superconducting structure characterized by comprising: The mesoscopic charge element includes a lead, and the mechanism for reading the state of the mesoscopic charge element includes:
Shunt resistance,
A current source;
A mechanism for measuring the voltage drop at the lead of the mesoscopic charge device;
A mechanism for grounding the lead of the mesoscopic charge element;
Including
50. The shunt resistor, the current source, the mechanism for measuring a voltage drop, and the mechanism for grounding are in parallel with the element lead of the mesoscopic charge element. Superconducting structure. The mesoscopic charge element includes an element lead, and the mechanism for reading the state of the mesoscopic charge element includes:
Josephson junction,
A current source;
A mechanism for measuring a voltage drop across the leads of the mesoscopic charge device;
A mechanism for grounding the lead of the mesoscopic charge element;
Including
50. The method of claim 49, wherein the Josephson junction, the current source, the mechanism for measuring a voltage drop, and the mechanism for grounding are disposed in parallel with a lead of the mesoscopic charge element. The superconducting structure described. The mechanism for reading the state of the mesoscopic charge element is:
Grounding the first lead of the mesoscopic charge element;
Connecting a second lead of the mesoscopic charge element to a tank circuit;
The tank circuit includes an inductor and a capacitor;
The characteristics of the tank circuit depend on the state of the mesoscopic charge element,
The superconducting structure according to claim 49, wherein   53. The superconducting structure according to claim 52, wherein the characteristic of the tank circuit is an impedance of the tank circuit.   50. The superconducting structure of claim 49, wherein each mesoscopic phase element of the plurality of mesoscopic phase elements is a phase qubit, and the superconducting structure forms a quantum resistor. A plurality of mesoscopic phase elements, each arranged in parallel;
A plurality of mesoscopic charge devices;
The first mesoscopic phase element and the first mesoscopic charge element first so that the state of the first phase element of the plurality of mesoscopic phase elements interacts with the state of the first mesoscopic charge element. A mechanism for coupling a mesoscopic charge element;
A mechanism for reading the state of the first mesoscopic charge element or the mesoscopic phase element;
A superconducting structure characterized by comprising: The mechanism for reading the state of the mesoscopic charge element is:
Shunt resistance,
A current source;
A mechanism for measuring the voltage drop at the lead of the mesoscopic charge device;
A mechanism for grounding the lead of the mesoscopic charge element;
Including
56. The shunt resistor, the current source, the mechanism for measuring a voltage drop, and the mechanism for grounding are in parallel with the element lead of the mesoscopic charge element. Superconducting structure. Each of the mesoscopic charge elements includes a lead, and the mechanism for reading the state of the first mesoscopic charge element comprises:
Josephson junction,
A current source;
A mechanism for measuring a voltage drop at a lead of the first mesoscopic charge device;
A mechanism for grounding a lead of the first mesoscopic charge device;
Including
The Josephson junction, the current source, the mechanism for measuring a voltage drop, and the mechanism for grounding are arranged in parallel with a lead of the first mesoscopic charge element. Item 56. The superconducting structure according to Item 55. The mechanism for reading the state of the first mesoscopic charge element;
Grounding the first lead of the first mesoscopic charge element;
Connecting a second lead of the mesoscopic charge element to a tank circuit;
The tank circuit includes an inductor and a capacitor;
56. The superconducting structure of claim 55, wherein characteristics of the tank circuit depend on a state of the first mesoscopic charge device.   The superconducting structure according to claim 55, wherein the characteristic of the tank circuit is an impedance of the tank circuit.   The mesoscopic phase element of the plurality of mesoscopic phase elements and the mesoscopic charge element of the plurality of mesoscopic charge elements are qubits, and the superconducting structure constitutes a non-uniform quantum register. Item 56. The superconducting structure according to Item 55. A method for reading a quantum state of a mesoscopic phase element,
The method is
Coherently coupling the mesoscopic phase element to a mesoscopic charge element using a phase shift element;
Measuring the quantum state of the mesoscopic charge device,
A method comprising steps. Coherently coupling the mesoscopic phase shift element to the mesoscopic charge element further comprises:
Providing a coherent connection between the mesoscopic phase element and the mesoscopic charge element, wherein the coherent connection includes the phase shift element, and the coherent connection includes a quantum state of the mesoscopic phase element. Maintaining the quantum state of the mesoscopic charge device,
Providing a phase interference switch for controlling the coherent connection between the mesoscopic phase element and the charge element;
Adjusting the quantum state of the phase interference switch for a duration t;
62. The method of claim 61, comprising steps.   62. The method of claim 61, wherein the mesoscopic phase element and the mesoscopic charge element are connected or disconnected for the duration t.   62. The method of claim 61, wherein the phase shift element is a [pi] / 2 phase shift element.   63. The method of claim 62, wherein the phase interference switch is a superconducting loop that includes a first Josephson junction in a first branch and a second Josephson junction in the second branch.   63. The method of claim 62, wherein adjusting the phase interference switch comprises controlling an applied external flux Φx.   Connecting the first lead of the SSET to the coherent connection leading to the mesoscopic phase element, and connecting the second lead of the SSET to the coherent connection leading to the second mesoscopic charge element. 64. The method of claim 62, wherein a phase interference switch is connected to the coherent connection.   64. The method of claim 62, wherein adjusting the quantum state of the phase interference switch comprises controlling a gate voltage.   63. The method of claim 62, wherein the duration t is on the order of the tunneling amplitude of the mesoscopic phase element.   64. The method of claim 62, wherein the duration is about 2 microseconds or less. Stimulating the quantum state of the mesoscopic charge device comprises:
Driving a bias current across the lead of the mesoscopic charge device, with a quick hit of a certain duration t _b ,
Measuring a voltage drop across the mesoscopic charge element;
63. The method of claim 62, comprising steps.   72. The method of claim 71, wherein the mesoscopic charge element is disposed in parallel with a shunt resistor, and the measurement of the voltage drop across the mesoscopic charge element includes a voltage drop across the shunt resistor.   The method of claim 72, wherein the shunt resistor is a Josephson junction. The method of claim 71, wherein the duration t _b is less than about 1 microsecond. The method of claim 71, wherein the duration t _b is less than about 1 nanosecond.   72. The method of claim 71, wherein the measured voltage drop is correlated with a quantum state of the mesoscopic phase element.   62. The method of claim 61, wherein the mesoscopic phase element is a phase qubit.   78. The method of claim 77, wherein the method of reading the quantum state of the phase-shifting qubit is a quantum read operation used in quantum computation. A method for reading a quantum state of a qubit of a non-uniform quantum register comprising a plurality of first phase qubits and a plurality of second charge qubits, the method comprising:
Selecting a first charge qubit or a first phase qubit in the non-uniform quantum register;
Coherently coupling the first phase qubit or the first charge qubit to a mesoscopic charge element for a duration t _c ,
Reading the quantum state of the mesoscopic charge device after the duration t _c has elapsed,
A method comprising steps.   The first phase qubit or the first charge qubit adjusts a quantum state of a phase interference switch that adjusts a connection between the first phase qubit or the first charge qubit and the mesoscopic charge element. 80. The method of claim 79, wherein:   81. The method of claim 80, wherein the phase interference switch is adjusted by applying a flux.   81. The method of claim 80, wherein the phase interference switch is adjusted by applying a gate voltage. The method of claim 79, wherein the duration t _c, characterized in that close to the tunneling amplitude of the first phase qubit or the first charge qubit. 80. The method of claim 79, wherein the duration _tc is about 1 microsecond or less. 80. The method of claim 79, wherein the duration _tc is about 1 nanosecond or less. Reading the quantum state of the mesoscopic charge device,
Driving a bias current in the lead of the mesoscopic charge device;
Measuring a voltage drop across the mesoscopic charge element;
80. The method of claim 79, comprising steps. A method for performing a quantum computation entanglement operation between a phase qubit and a charge qubit, comprising:
The method is
Providing a coherent connection between the phase-shifting qubit and the charge qubit that allows the quantum state of the phase-shifting qubit and the quantum state of the charge qubit to interact with each other;
Adjusting the coherent connection for a duration t _e , wherein the phase qubit has a duration t in order to controllably entangle the quantum state of the phase-shifting qubit and the quantum state of the charge qubit. _The method comprising: being connected to the charge qubit for at least a portion of _e .   88. The method of claim 87, wherein the coherent connection further includes a phase shift element.   88. The method of claim 87, wherein adjusting the coherent connection comprises adjusting a phase interference switch.   90. The method of claim 89, wherein the phase interference switch is a superconducting loop that includes a first Josephson junction in a first branch and a second Josephson junction in a second branch.   90. The method of claim 89, wherein adjusting the phase interference switch comprises controlling an applied magnetic flux.   The phase interference switch connects the first lead of the SSET to the coherent connection leading to the phase qubit and connects the second lead of the SSET to the coherent connection leading to the charge qubit. 90. The method of claim 89, wherein the method is a superconducting single electron transistor (SSET) connected to the connection.   90. The method of claim 89, wherein adjusting the state of the phase interference switch includes controlling a gate voltage. 90. The method of claim 89, wherein the duration _te is about 1 microsecond or less. 95. The method of claim 94, wherein the duration _te is about 1 microsecond or less.   90. The method of claim 88, wherein the phase shift element produces a [pi] / 2 phase shift.   90. The method of claim 88, wherein the phase shift element produces a π / 2 ± 0.20π phase shift.   90. The method of claim 88, wherein the phase shift element produces a [pi] /2±0.10 [pi] phase shift.   90. The method of claim 88, wherein the phase shift element produces a π / 2 ± 0.05π phase shift. A method of intertwining a non-uniform quantum register qubit comprising a plurality of phase-shifting qubits and a plurality of charge qubits,
The method is
Providing a quantum register having a bus configured such that each qubit of the non-uniform quantum register is connected and disconnected coherently;
Selecting a first qubit from the plurality of qubits of the non-uniform quantum register;
Coupling the first qubit to a mesoscopic charge element for a duration t ₁ ;
Selecting a second qubit from the plurality of qubits of the non-uniform quantum register;
Coupling the second qubit to the mesoscopic charge element for a duration t ₂ ;
A method comprising steps.   The step of selecting a first qubit includes adjusting a first phase interference switch, wherein the first phase interference switch is correlated with the first qubit and the first phase interference switch is closed. 101. The device of claim 100, wherein when the first qubit is connected to the bus and the first phase interference switch is open, the first qubit is isolated from the bus. The method described.   The step of selecting a second qubit includes adjusting a second phase interference switch, wherein the second phase interference switch is correlated with the second qubit and the second phase interference switch is closed. 101. The 100 of claim 100, wherein when the second qubit is connected to the bus and the second phase interference switch is open, the second qubit is isolated from the bus. the method of. Coupling the first qubit to the mesoscopic charge element comprises selecting the mesoscopic charge element, the mesoscopic charge element being coupled to the bus for a duration t _s. 102. The method of claim 101, wherein: Selecting the mesoscopic charge element comprises:
When the phase interference switch is open, the mesoscopic charge element is disconnected from the selected first qubit, and when the phase interference switch is closed, the mesoscopic charge element is the selected first qubit. Adjusting the phase interference switch to be coupled to
104. The method of claim 103. The method of claim 100, wherein the duration t ₁ is less than about 1 microsecond. The method of claim 100, wherein the duration t ₁ is less than about 1 nanosecond. The method of claim 100, wherein the duration t ₂ is less than about 1 microsecond. The method of claim 100, wherein the duration t ₂ is less than about 1 nanosecond.   101. The method of claim 100, further comprising clearing information from the mesoscopic charge element by reading a state of the mesoscopic charge element.   110. The method of claim 109, wherein reading the state of the mesoscopic charge device provides information used as feedback to perform an error correction algorithm on the non-uniform quantum register. .   101. The method of claim 100, further comprising clearing the mesoscopic charge device such that the non-uniform quantum register is in a proper state to initiate a new entanglement operation.   112. The method of claim 111, wherein the clearing of the mesoscopic charge device comprises reading a quantum state of the mesoscopic charge device.   113. The method of claim 112, wherein the clear operation is providing information about a quantum state of the quantum register.   113. The method of claim 112, wherein the clear operation is providing information useful for performing an error correction operation of a quantum state of the quantum register.   113. The method of claim 112, wherein a quantum operation is performed on a qubit of the quantum register subject to information resulting from performing the clean operation.

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