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Adiabatic quantum computation with superconducting qubitsAdiabatic quantum computation with superconducting qubits description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20080086438, Adiabatic quantum computation with superconducting qubits. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS REFERENCE TO RELATED APPLICATIONS [0001]This application claims benefit, under 35 U.S.C. .sctn. 119(e), of U.S. Provisional Patent Application No. 60/557,748, filed on Mar. 29, 2004, which is hereby incorporated by reference in its entirety. This application also claims benefit, under 35 U.S.C. .sctn. 119(e), of U.S. Provisional Patent Application No. 60/588,002, filed on Jul. 13, 2004, which is hereby incorporated by reference in its entirety. This application is further related to concurrently filed application Ser. No. ______, Attorney Docket No. 706700-999207, entitled "Adiabatic Quantum Computation with Superconducting Qubits," and application Ser. No. ______, Attorney Docket No. 706700-999208, entitled "Adiabatic Quantum Computation with Superconducting Qubits," each of which is hereby incorporated by reference in its entirety. 1. FIELD OF THE INVENTION [0002]This invention relates to superconducting circuitry. More specifically, this invention relates to devices for quantum computation. 2. BACKGROUND [0003]Research on what is now called quantum computing may have begun with a paper published by Richard Feynman. See Feynman, 1982, International Journal of Theoretical Physics 21, pp. 467-488, which is hereby incorporated by reference in its entirety. Feynman noted that a quantum system is inherently difficult to simulate with conventional computers but that observation of the evolution of an analogous quantum system could provide an exponentially faster way to solve the mathematical model of the quantum system of interest. In particular, solving a mathematical model for the behavior of a quantum system commonly involves solving a differential equation related to the Hamiltonian of the quantum system. David Deutsch noted that a quantum system could be used to yield a time savings, later shown to include exponential time savings, in certain computations. If one had a problem modeled in the form of an equation that represented the Hamiltonian of a quantum system, the behavior of the system could provide information regarding the solutions to the equation. See Deutsch, 1985, Proceedings of the Royal Society of London A 400, pp. 97-117, which is hereby incorporated by reference in its entirety. [0004]A major activity in the quantum computing art is the identification of physical systems that can support quantum computation. This activity includes finding suitable qubits as well as developing systems and methods for controlling such qubits. As detailed in the following sections, a qubit serves as the basis for performing quantum computation. 2.1 Qubits [0005]The physical systems that are used in quantum computing are quantum computers. A quantum bit or "qubit" is the building block of a quantum computer in the same way that a conventional binary bit is a building block of a classical computer. A qubit is a quantum bit, the counterpart in quantum computing to the binary digit or bit of classical computing. Just as a bit is the basic unit of information in a classical computer, a qubit is the basic unit of information in a quantum computer. A qubit is conventionally a system having two or more discrete energy states. The energy states of a qubit are generally referred to as the basis states of the qubit. The basis states of a qubit are termed the |0> and |1> basis states. In the mathematical modeling of these basis states, each state is associated with an eigenstate of the sigma-z (.sigma..sup.Z) Pauli matrix. See Nielsen and Chuang, 2000, Quantum Computation and Quantum Information, Cambridge University Press, which is hereby incorporated by reference in its entirety. [0006]The state of a qubit can be in any superposition of two basis states, making it fundamentally different from a bit in an ordinary digital computer. A superposition of basis states arises in a qubit when there is a non-zero probability that the system occupies more than one of the basis states at a given time. Qualitatively, a superposition of basis states means that the qubit can be in both basis states |0> and |1> at the same time. Mathematically, a superposition of basis states means that the wave function that characterizes the overall state of the qubit, denoted |.PSI.>, has the form |.PSI.>=a|0>+b|> where a and b are amplitudes respectively corresponding to probabilities |a|.sup.2 and |b|.sup.2. The amplitudes a and b each have real and imaginary components, which allows the phase of qubit to be modeled. The quantum nature of a qubit is largely derived from its ability to exist in a superposition of basis states, and for the state of the qubit to have a phase. [0007]To complete a quantum computation using a qubit, the state of the qubit is typically measured (e.g., read out). When the state of the qubit is measured the quantum nature of the qubit is temporarily lost and the superposition of basis states collapses to either the |0> basis state or the |1> basis state, thus regaining its similarity to a conventional bit. The actual state of the qubit after it has collapsed depends on the amplitudes a and b immediately prior to the readout operation. [0008]A survey of exemplary physical systems from which qubits can be formed is found in Braunstein and Lo (eds.), Scalable Quantum Computers, Wiley-VCH Verlag GmbH, Berlin (2001), which is hereby incorporated by reference in its entirety. Of the various physical systems surveyed, the systems that appear to be most suited for scaling (e.g., combined in such a manner such that they entangle with each other) are those physical systems that include superconducting structures such as superconducting qubits. 2.2 Superconducting Qubits in General [0009]Superconducting qubits generally fall into two categories; phase qubits and charge qubits. Phase qubits store and manipulate information in the phase states of the device. Charge qubits store and manipulate information in the elementary charge states of the device. In superconducting materials, phase is a property of the material whereas elementary charges are represented by pairs of electrons called Cooper pairs. The division of such devices into two classes is outlined in Makhlin et al., 2001, "Quantum-State Engineering with Josephson-Junction Devices," Reviews of Modern Physics 73, pp. 357-401 which is hereby incorporated by reference in its entirety. [0010]Phase and charge are related values in superconductors and, at energy scales where quantum effects dominate, the Heisenberg uncertainty principle causes certainty in phase to lead to uncertainty in charge and, conversely, causes certainty in charge to lead to uncertainty in the phase of the system. Superconducting phase qubits are devices formed out of superconducting materials having a small number of distinct phase states and many charge states, such that when the charge of the device is certain, information stored in the phase states becomes delocalized and evolves quantum mechanically. Therefore, fixing the charge of a phase qubit leads to delocalization of the phase states of the qubit and subsequent useful quantum behavior in accordance with well-known principles of quantum mechanics. [0011]Experimental realization of superconducting devices as qubits was made by Nakamura et al., 1999, Nature 398, p. 786, which is hereby incorporated by reference in its entirety. Nakamura et al. developed a charge qubit that demonstrates the basic operational requirements for a qubit. However, the Nakamura et al charge qubits have unsatisfactorily short decoherence times and stringent control parameters. Decoherence time is the duration of time that it takes for a qubit to lose some of its quantum mechanical properties, e.g., the state of the qubit no longer has a definite phase. When the qubit loses it quantum mechanical properties, the phase of the qubit is no longer characterized by a superposition of basis states and the qubit is no longer capable of supporting all types of quantum computation. [0012]Superconducting qubits have two modes of operation related to localization of the states in which information is stored. When the qubit is initialized or measured, the information is classical, 0 or 1, and the states representing that classical information are also classical in order to provide reliable state preparation. Thus, a first mode of operation of a qubit is to permit state preparation and measurement of classical information. A second mode of operation occurs during quantum computation, where the information states of the device become dominated by quantum effects such that the qubit can evolve controllably as a coherent superposition of those states and, in some instances, even become entangled with other qubits in the quantum computer. Thus, qubit devices provide a mechanism to localize the information states for initialization and readout operations, and de-localize the information states during computation. Efficient functionality of both of these modes and, in particular, the transition between them in superconducting qubits is a challenge that has not been satisfactorily resolved in the prior art. 2.2.1 Phase Qubits [0013]A proposal to build a quantum computer from superconducting qubits was published in 1997. See Bocko et al., 1997, IEEE Trans. Appl. Supercon. 7, p. 3638, which is hereby incorporated by reference in its entirety. See also Makhlin et al., 2001, Rev. Mod. Phys. 73, p. 357 which is hereby incorporated by reference in its entirety. Since then, designs based on many other types of qubits have been introduced. One such design is based on the use of superconducting phase qubits. See Mooij et al., 1999, Science 285, 1036; and Orlando et al., 1999, Phys. Rev. B 60, 15398, which are hereby incorporated by reference in their entireties. In particular, quantum computers based on persistent current qubits, which are one type of superconducting phase qubit, have been proposed. [0014]The superconducting phase qubit is well known and has demonstrated long coherence times. See, for example, Orlando et al., 1999, Phys. Rev. B 60, 15398, and Il'ichev et al., 2003, Phys. Rev. Lett. 91, 097906, which are hereby incorporated by reference in their entireties. Some other types of superconducting phase qubits comprise superconducting loops interrupted by more or less than three Josephson junctions. See, e.g., Blatter et al., 2001, Phys. Rev. B 63, 174511, and Friedman et al., 2000, Nature, 406, 43, which are hereby incorporated by reference in their entireties. [0015]FIG. 1A illustrates a persistent current qubit 101. Persistent current qubit 101 comprises a loop 103 of superconducting material interrupted by Josephson junctions 101-1, 101-2, and 101-3. Josephson junctions are typically formed using standard fabrication processes, generally involving material deposition and lithography stages. See, e.g., Madou, Fundamentals of Microfabrication, Second Edition, CRC Press, 2002, which is hereby incorporated by reference in its entirety. Methods for fabricating Josephson junctions are well known and described in Ramos et al., 2001, IEEE Trans. App. Supercond. 11, 998, for example, which is hereby incorporated by reference in its entirety. Details specific to persistent current qubits can be found in C. H. van der Wal, 2001; J. B. Majer, 2002; and J. R. Butcher, 2002, all Theses in Faculty of Applied Sciences, Delft University of Technology, Delft, The Netherlands; http://qt.tn.tudelft.nl; Kavli Institute of Nanoscience Delft, Delft University of Technology, P.O. Box 5046, 2600 GA Delft, The Netherlands, which is hereby incorporated by reference in its entirety. Common substrates include silicon, silicon oxide, or sapphire, for example. Josephson junctions can also include insulating materials such as aluminum oxide, for example. Exemplary superconducting materials useful for forming superconducting loop 103 are aluminum and niobium. The Josephson junctions have cross-sectional sizes ranging from about 10 nanometers (nm) to about 10 micrometers (.mu.m). One or more of the Josephson junctions 101 has parameters, such as the size of the junction, the junction surface area, the Josephson energy or the charging energy that differ from the other Josephson junctions in the qubit. [0016]The difference between any two Josephson junctions in the persistent current qubit is characterized by a coefficient, termed .alpha., which typically ranges from about 0.5 to about 1.3. In some instances, the term a for a pair of Josephson junctions in the persistent current qubit is the ratio of the critical current between the two Josephson junctions in the pair. The critical current of a Josephson junction is the minimum current through the junction at which the junction is no longer superconducting. That is, below the critical current, the junction is superconducting whereas above the critical current, the junction is not superconducting. Thus, for example, the term a for junctions 101-1 and 101-2 is defined as the ratio between the critical current of junction 101-1 and the critical current of junction 101-2. Continue reading about Adiabatic quantum computation with superconducting qubits... Full patent description for Adiabatic quantum computation with superconducting qubits Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Adiabatic quantum computation with superconducting qubits patent application. 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