Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Tunneling through region of reduced conductivity
Reexamination Certificate
2000-08-11
2003-09-30
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
C257S030000, C257S033000, C257S035000, C438S002000
Reexamination Certificate
active
06627915
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to quantum computing and to solid state devices that use superconducting materials to create and maintain coherent quantum states such as are required for quantum computing.
2. Description of Related Art
The field of quantum computing has generated interest in physical systems capable of performing quantum calculations. Such systems should allow the formation of well-controlled and confined quantum states. The superposition of these quantum states are then suitable for performing quantum computation. The difficulty is that microscopic quantum objects are typically very difficult to control and manipulate. Therefore, their integration into the complex circuits required for quantum computations is a difficult task.
Research on what is now called quantum computing traces back to Richard Feynman, [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.
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, 1484-1509 (1997); L. Grover, Proc. 28th STOC, 212-219 (1996); and A. Kitaev, LANL preprint quant-ph/9511026 (1995)]. In particular, the Shor algorithm permits a quantum computer to factorize natural numbers. The 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).]
One proposed application of a quantum computer is factoring of large numbers. In such an application, a quantum computer could render obsolete all existing encryption schemes that use the “public key” method. In other applications, quantum computers (or even a smaller scale device, a quantum repeater) could allow 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 quant-ph/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 initial state of the qubit typically has non-zero probabilities of being found in either degenerate state. Thus, N qubits define an initial state that is a combination of 2
N
degenerate 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, performs 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 spin states. Nuclear magnetic resonance (NMR) techniques can read the spin states. These systems have successfully implemented the Shor algorithm for factoring of a natural number (15). 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 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.
Another macroscopic alternative to microscopic quantum objects relies on superconducting line structures containing Josephson junctions. Superconductivity is a macroscopically coherent quantum phenomenon and therefore superconducting systems are attractive candidates for utilization in quantum computing circuits.
FIG. 1
a
shows an example of a Josephson junction
110
in a SQUID qubit
100
. A Josephson junction refers to two superconducting electrodes separated by a thin tunnel barrier formed by a dielectric.
FIG. 1
a
shows a SQUID (superconduction quantum interference device) qubit
100
. Qubit
100
has a continous superconducting loop
101
with endpoints separated by a gap to form junction
110
. Junction
110
is filled with a thin dielectric forming a potential barrier between the endpoints of line
101
. A quantum superposition of magnetic flux states in the superconducting loop containing Josephson junction
101
, qubit
100
, is called macroscopic quantum coherence (MQC).
If an external magnetic field applied to qubit
100
provides a magnetic flux equal to one half the magnetic flux quantum, &PHgr;
0
/2, then the potential energy presented by the magnetic flux (&PHgr;
int
) states of qubit
100
has two symmetric minima as is shown in
FIG. 1
b
. A magnetic flux trapped in qubit
100
can then tunnel between the two symmetric minima of the magnetic flux potential energy function. The degenerate ground states of a magnetic flux in qubit
100
are linear combinations of the states corresponding to the minima of the potential energy function (i.e. |0>+|1> and |0>−|1>). These degenerate states are split by the energy difference related to the tunneling matrix element. Therefore, if the coherence can be maintained long enough, the magnetic flux will quantum-mechanically oscillate back and forth between the two degenerate states.
The drawback of qubit
100
is that it is an open system and transition from one potential well to another is accompanied by the inversion of the magnetic field and superconducting screening currents surrounding qubit
100
. The energy of this redistribution and the unknown external influences can be relatively large and can therefore cause decoherence (i.e., collapsing of the quantum mechanical wave functions). Moreover, the potential energy barrier, and therefore the energy split between the two degenerated states, cannot be controlled in situ, unless the Josephson junction is substituted by a small SQUID with independently-tunable critical current.
There is a continuing need for a structure for implementing a quantum computer. Further, there is a need for a structure having a sufficient number of qubits to perform useful calculations.
SUMMARY
In accordance with the present invention, a qubit having a shaped long Josephson junction which can trap fluxons is presented. A qubit according to the present invention can offer a well-isolated system that allows independent control over the potential energy profile of a magnetic fluxon trapped on the qubit and dissipation of fluxons. Additionally, shaped long Josephson junctions can have shapes resulting in nearly any arbitrary desired potential energy function for a magnetic fluxon trapped on the junction.
Therefore, a superconducting qubit according to the present invention includes a long Josephson junction having a shape such as to produce a selected potential energy function ind
Koval Yu
Ustinov Alexey V.
Walraff Andreas
D-Wave Systems Inc.
Flynn Nathan J.
Pennie & Edmonds LLP
Wilson Scott R.
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