Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Tunneling through region of reduced conductivity
Reexamination Certificate
2003-01-23
2004-11-23
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Tunneling through region of reduced conductivity
C257S031000, C257S036000, C257S039000
Reexamination Certificate
active
06822255
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to quantum computing and, in particular, to superconducting quantum computing systems.
2. Description of Related Art
Research on what is now called quantum computing traces back to Richard Feynman. See, e.g., R. P. Feynman, Int. J. Theor. Phys. 21, 467 (1982). He noted that quantum systems are inherently difficult to simulate with classical (i.e., conventional, non-quantum) computers, but that this task could be accomplished by observing the evolution of another quantum system. In particular, solving a theory for the behavior of a quantum system commonly involves solving a differential equation related to the system's Hamiltonian. Observing the behavior of the system provides information regarding the solutions to the equation.
Further efforts in quantum computing were initially concentrated on building the formal theory or on “software development” or extension to other computational problems. Discovery of the Shor and Grover algorithms were important milestones in quantum computing. See, e.g., P. Shor, SIAM J. of Comput. 26, 1484 (1997); L. Grover, Proc. 28th STOC, 212 (ACM Press, New York, 1996), which is hereby incorporated by reference in its entirety; and A. Kitaev, LANL preprint quant-ph/9511026, which is hereby incorporated by reference in its entirety. In particular, the Shor algorithm permits a quantum computer to factorize large natural numbers efficiently. In this application, a quantum computer could render obsolete all existing “public-key” encryption schemes. In another application, quantum computers (or even a smaller-scale device such as a quantum repeater) could enable absolutely safe communication channels where a message, in principle, cannot be intercepted without being destroyed in the process. See, e.g., H. J. Briegel et al., preprint quant-ph/9803056 and references therein, which is hereby incorporated by reference in its entirety. Showing that fault-tolerant quantum computation is theoretically possible opened the way for attempts at practical realizations. See, e.g., E. Knill, R. Laflamme, and W. Zurek, Science 279, 342 (1998), which is hereby incorporated by reference in its entirety.
Quantum computing generally involves initializing the states of N qubits (quantum bits), creating controlled entanglements among them, allowing these states to evolve, and reading out the states of the qubits after the evolution. A qubit is conventionally a system having two degenerate (i.e., of equal energy) quantum states, with a non-zero probability of being found in either state. Thus, N qubits can define an initial state that is a combination of 2
N
classical states. This initial state undergoes an evolution, governed by the interactions that the qubits have among themselves and with external influences. This evolution of the states of N qubits defines a calculation or, in effect, 2
N
simultaneous classical calculations. Reading out the states of the qubits after evolution is complete determines the results of the calculations.
Several physical systems have been proposed for the qubits in a quantum computer. One system uses molecules having degenerate nuclear-spin states. See N. Gershenfeld and I. Chuang, “Method and Apparatus for Quantum Information Processing,” U.S. Pat. No. 5,917,322, which is hereby incorporated by reference in its entirety. Nuclear magnetic resonance (NMR) techniques can read the spin states. These systems have successfully implemented a search algorithm, see, e.g., M. Mosca, R. H. Hansen, and J. A. Jones, “Implementation of a quantum search algorithm on a quantum computer,” Nature 393, 344 (1998) and references therein, which is hereby incorporated by reference in its entirety, and a number-ordering algorithm, see, e.g., L. M. K. Vandersypen, M. Steffen, G. Breyta, C. S. Yannoni, R. Cleve, and I. L. Chuang, “Experimental realization of order-finding with a quantum computer,” preprint quant-ph/0007017 and references therein, which is hereby incorporated by reference in its entirety. (The number-ordering algorithm is related to the quantum Fourier transform, an essential element of both Shor's factoring algorithm and Grover's algorithm for searching unsorted databases.) However, expanding such systems to a commercially useful number of qubits is difficult. More generally, many of the current proposals will not scale up from a few qubits to the 10
2
~10
3
qubits needed for most practical calculations.
Further, current methods for entangling qubits are susceptible to loss of coherence. Entanglement of quantum states of qubits can be 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 hereby incorporated by reference in its entirety. Current methods for entangling phase qubits require the interaction of the flux in each of the qubits, see Yuriy Makhlin, Gerd Schon, Alexandre Shnirman, “Quantum state engineering with Josephson-junction devices,” LANL preprint, cond-mat/0011269 (November 2000), which is hereby incorporated by reference in its entirety. This form of entanglement is sensitive to the qubit coupling with surrounding fields, which cause decoherence and loss of information.
As discussed above, currently proposed methods for readout, initialization, and entanglement of a qubit involve detection or manipulation of magnetic fields at the location of the qubit, which make these methods susceptible to decoherence and limits the overall scalability of the resulting quantum computing device. Thus, there is a need for an efficient quantum register where decoherence and other sources of noise is minimized but where scalability is maximized.
SUMMARY OF THE INVENTION
In accordance with the present invention, a quantum register is presented. A quantum register according to the present invention includes one or more finger SQUID qubit devices.
A finger SQUID qubit device according to an embodiment of the present invention can include a superconducting loop and a superconducting finger, wherein the superconducting finger extends from the superconducting loop towards the interior of the superconducting loop. The superconducting loop may have multiple branches. Each branch may have a Josephson junction. The Josephson junction may be a grain boundary junction. The finger SQUID qubit device may have leads capable of conducting current to and from the superconducting loop. The leads may be capable of conducting supercurrent.
When structures are referred to as “superconducting” herein, they are fabricated from a material capable of superconducting and so may superconduct under the correct conditions. For example, the superconducting loop and superconducting finger may be fabricated from a d-wave superconductor and so will superconduct under appropriate physical conditions. For example, the superconducting loop and finger will superconduct at an appropriate temperature, magnetic field, and current. However, the “superconducting loop” will not superconduct under other physical conditions. For example, when the temperature is too high, the superconducting loop will not be in a superconducting state. Additionally, structures such as superconducting SETs and other superconducting switches mentioned herein are capable of superconducting under appropriate physical conditions.
A device in accordance with an embodiment of the invention generally operates at a temperature such that thermal excitations in the superconducting crystal lattice are sufficiently suppressed to perform quantum computation. In some embodiments of the invention, such a temperature can be on the order of 1K or less. In some other embodiments of the invention, such a temperature can be on the order of 50 mK or less. Furthermore, other dissipative sources, such as magnetic fields for example, should be minimized to an extent such that quantum computing can be performed with a minimum of dissipation and decoherence.
The material capable of superconducting used in embodiments of the invention may be a material that violates time-reversa
Hilton Jeremy P.
Ivanov Zdravko
Tzalenchuk Alexander
D-Wave Systems Inc.
Flynn Nathan J.
Jones Day
Lovejoy Brett
Wilson Scott R.
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