Four-terminal system for reading the state of a phase qubit

Details Four-terminal system for reading the state of a phase qubit Four-terminal system for reading the state of a phase qubit

2001-06-05

2003-06-17

Nelms, David (Department: 2818)

Active solid-state devices (e.g., transistors, solid-state diode

Heterojunction device

Heterojunction formed between semiconductor materials which...

C257S031000, C257S033000, C257S034000, C257S035000, C257S036000

Reexamination Certificate

active

06580102

ABSTRACT:

BACKGROUND
1. Field of the Invention
This invention relates to quantum computing and to solid-state devices that use superconductive materials to implement the coherent quantum states used in quantum computing.
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. Milestones were the discoveries of the Shor and Grover algorithms. See, e.g., P. Shor, SIAM J. of Comput. 26, 1484(1997); L. Grover, Proc. 28th STOC, 212(ACM Press, New York, 1996); and A. Kitaev, LANL preprint quant-ph/9511026. 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/9803056and references therein.
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).
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 qubits the states evolve. 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 entangled 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. 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, 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. (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. A technology that is excellently suited for large-scale integration involves superconducting phase qubits.
One implementation of a phase qubit involves a micrometer-sized loop with three (or four) Josephson junctions. See J. E. Mooij, T. P. Orlando, L. Levitov, L. Tian, C. H. van der Wal, and S. Lloyd, “Josephson Persistent-Current Qubit,” Science 285, 1036(1999) and references therein. The energy levels of this system correspond to differing amounts of magnetic flux threading the loop. Application of a static magnetic field normal to the loop may bring two of these energy levels (or basis states) into degeneracy. Typically, external AC electromagnetic fields are also applied, to enable tunneling between non-degenerate states.
A radio-frequency superconducting quantum-interference device (rf-SQUID) qubit is another type of phase qubit having a state that can be read by inductively coupling the rf-SQUID to rapid single-flux-quantum (RSFQ) circuitry. See R. C. Rey-deCastro, M. F. Bocko, A. M. Herr, C. A. Mancini, and M. J. Feldman, “Design of an RSFQ Control Circuit to Observe MQC on an rf-SQUID,” IEEE Trans. Appl. Supercond. 11, 1014 (2001) and references therein, which is hereby incorporated by reference in its entirety. A timer controls the readout circuitry and triggers the entire process with a single input pulse, producing an output pulse only for one of the two possible final qubit states. The risk of this readout method lies in the inductive coupling with the environment causing decoherence or disturbance of the qubit state during quantum evolution. The circuitry attempts to reduce decoherence by isolating the qubit with intermediate inductive loops. Although this may be effective, the overhead is large, and the method becomes clumsy for large numbers of qubits.
In both above systems, an additional problem is the use of basis states that are not naturally degenerate. Accordingly, the strength of the biasing field for each qubit has to be precisely controlled to achieve the desired tunneling between its basis states. This is possible for one qubit, but becomes extremely difficult with several qubits.
U.S. patent application Ser. No. 09/452,749, “Permanent Readout Superconducting Qubit,” filed Dec. 1, 1999, and Ser. No. 09/479,336, “Qubit using a Josephson Junction between s-Wave and d-Wave Superconductors,” filed Jan. 7, 2000, which are hereby incorporated by reference in their entirety, describe Permanent Readout Superconducting Qubits (PRSQs). An exemplary PRSQ consists of a bulk superconductor, a grain boundary, a superconductive mesoscopic island [i.e., a superconductive region having a size such that a single excess Cooper pair (pair of electrons) is noticeable], and a means for grounding the island. The material used in the bulk or the island has a superconducting order containing a dominant component whose pairing symmetry has non-zero angular momentum, and a sub-dominant component with any pairing symmetry. As a result, the qubit has the states ±&PHgr;
0
, where &PHgr;
0
is the minimum-energy phase of the island with respect to the bulk superconductor.
The area in which the phase is maintained is much more localized in a PRSQ than in prior qubits such as an rf-SQUID qubit. Thus, the rate of decoherence is minimized, making the PRSQ a strong candidate for future solid-state quantum-computing implementations.
The state of a PRSQ can be characterized by the direction of the magnetic field, H↑ or H↓, inside the junction between the bulk and the island. This difference in field direction can be used to read out the state of a PRSQ, for instance using a SQUID. However, the proposed readout methods introduce an interaction with the environment that potentially disturbs the state of the qubit, which necessitates complicated and time-consuming error-correction and/or re-initialization procedures. Attempts to “switch off”the readout circuit during quantum evolution face severe practical constraints. For example, physicall

Affiliated with

Also associated with

Rating

Four-terminal system for reading the state of a phase qubit does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Four-terminal system for reading the state of a phase qubit, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Four-terminal system for reading the state of a phase qubit will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3090298