Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2001-03-16
2003-01-07
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S017000, C257S021000, C257S432000
Reexamination Certificate
active
06504172
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to quantum computing and to solid state devices that use superconductive materials to create and maintain coherent quantum states such as required for quantum computing.
2. Description of Related Art
Research on what is now called quantum computing traces back to Richard Feynman. See R. Feynman, Int. J. Theor. Phys., 21, 467-488 (1982). Feynman noted that quantum systems are inherently difficult to simulate with conventional computers but that observing the evolution of a quantum system could provide a much faster way to solve some computational problems. In particular, solving a theory for the behavior of a quantum system commonly involves solving a differential equation related to the Hamiltonian of the quantum system. Observing the behavior of the quantum system provides information regarding the solutions to the equation.
Further efforts in quantum computing were initially concentrated on “software development” or building of the formal theory of quantum computing. Milestones in these efforts were the discoveries of the Shor and Grover algorithms. See P. Shor, SIAM J. of Comput., 26:5, 1484_
14
1509
(1997); L. Grover, Proc.
28
th STOC, 212-219 (1996); and A. Kitaev, LANL preprint quant-pb/9511026 (1995). In particular, the Shor algorithm permits a quantum computer to factorize natural numbers. Showing that fault-tolerant quantum computation is theoretically possible opened the way for attempts at practical realizations of quantum computers. See E. Knill, R. Laflamme, and W. Zurek, Science, 279, p. 342 (1998).
In such an application, a quantum computer could render obsolete all existing encryption schemes that use the “public key” method. In another application, quantum computers (or even a smaller scale device, a quantum repeater) could enable absolutely safe communication channels, where a message, in principle, cannot be intercepted without being destroyed in the process. See H. J. Briegel et al., LANL preprint quantph/9803056 (1998) and the references therein.
Quantum computing generally involves initializing the states of N qubits (quantum bits), creating controlled entanglements among the N qubits, allowing the quantum states of the qubits to evolve under the influence of the entanglements, and reading the qubits after they have evolved. A qubit is conventionally a system having two degenerate quantum states, and the state of the qubit can have non-zero probability of being found in either degenerate state. Thus, N qubits can define an initial state that is a combination of 2
N
states. The entanglements control the evolution of the distinguishable quantum states and define calculations that the evolution of the quantum states performs. This evolution, in effect, can perform 2
N
simultaneous calculations. Reading the qubits after evolution is complete determines the states of the qubits and the results of the calculations.
Several physical systems have been proposed for the qubits in a quantum computer. One system uses chemicals having degenerate nuclear spin states. Nuclear magnetic resonance (NMR) techniques can read the spin states. These systems have successfully implemented a search algorithm and a number-ordering algorithm. See M. Mosca, R. H. Hansen, and J. A. Jones, “Implementation of a quantum search algorithm on a quantum computer,”
Nature,
393:344-346, 1998 and Lieven M. K. Vandersypen, Matthias Steffen, Gregory Breyta, Costantino S. Yannoni, Richard Cleve and Isaac L. Chuang, “Experimental Realization Of Order-Finding With A Quantum Computer,” LANL preprint quant-pb/0007017 (2000) and the references therein. These search processes are related to the quantum Fourier transform, an essential element of both Shor's algorithm for factoring of a natural number and Grover's Search Algorithm for searching unsorted databases. See T. F. Havel, S. S. Somaroo, C. -H. Tseng, and D. G. Cory, “Principles And Demonstrations Of Quantum Information Processing By NMR Spectroscopy,” 2000 and the references therein, which are hereby incorporated by reference in their entirety. However, efforts to expand such systems up to a commercially useful number of qubits face difficult challenges.
Another physical system for implementing a qubit includes a superconducting reservoir, a superconducting island, and a dirty Josephson junction that can transmit a Cooper pair (of electrons) from the reservoir into the island. The island has two degenerate states. One state is electrically neutral, but the other state has an extra Cooper pair on the island. A problem with this system is that the charge of the island in the state having the extra Cooper pair causes long range electric interactions that interfere with the coherence of the state of the qubit. The electric interactions can force the island into a state that definitely has or lacks an extra Cooper pair. Accordingly, the electric interactions can end the evolution of the state before calculations are complete or qubits are read. This phenomenon is commonly referred to as collapsing the wavefunction, loss of coherence, or decoherence. See “Coherent Control Of Macroscopic Quantum States In A Single-Cooper-Pair Box,” Y. Nakamura; Yu, A. Pashkin and J. S. Tsai,
Nature
Volume 398 Number 6730 Page 786-788 (1999) and the references therein.
Another physical system for implementing a qubit includes a radio frequency superconducting quantum interference device (RF-SQUID). See J. E. Mooij, T. P. Orlando, L. Levitov, Lin Tian, Caspar H. van der Wal, and Seth Lloyd, “Josephson Persistent-Current Qubit,”
Science
Aug. 13, 1999; 285: 1036-1039, and the references therein, which are hereby incorporated by reference in their entirety. The RF-SQUID's energy levels correspond to differing amounts of magnetic flux threading the SQUID ring. Application of a static magnetic field normal to the SQUID ring may bring two of these energy levels, corresponding to different magnetic fluxes threading the ring, into resonance. Typically, external AC magnetic fields can also be applied to pump the system into excited states so as to maximize the tunneling frequency between qubit basis states. A problem with this system is that the basis states used are not naturally degenerate and the biasing field required has to be extremely precise. This biasing is possible for one qubit, but with several qubits, fine tuning this bias field becomes extremely difficult. Another problem is that the basis states used are typically not the ground states of the system, but higher energy states populated by external pumping. This requires the addition of an AC field-generating device, whose frequency will differ for each qubit as the individual qubit parameters vary.
Research is continuing and seeking a structure that implements a quantum computer having a sufficient number of qubits to perform useful calculations.
SUMMARY
In accordance with one embodiment of the invention, a qubit includes a dot formed of a superconductor having a pairing symmetry that contains a dominant component with non-zero angular momentum, and a sub-dominant component that can have any pairing symmetry. The high temperature superconductors 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
. are examples of superconductors that have non-zero angular momentum (dominant d-wave pairing symmetry), whereas the low temperature superconductor Sr
2
RuO
4
, or the heavy fermion material CeIrIn
5
, are examples of p-wave superconductors that also have non-zero angular momentum.
In such qubits, persistent equilibrium currents arise near the outer boundary of the superconducting dot. These equilibrium currents have two degenerate ground states that are related by time-reversal symmetry. One of the ground states corresponds to persistent currents circulating in a clockwise fashion around the superconducting dot, while the other ground state corresponds to persistent currents circulating counter-clockwise around the dot. The circulating currents induce magnetic fluxes and therefore
Amin Mohammad H. S.
Franz Marcel
Hilton Jeremy P.
Rose Geordie
Zagoskin Alexandre M.
D-Wave Systems Inc.
Nelms David
Pennie & Edmonds LLP
Tran Mai-Huong
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