Quantum computing method using magnetic flux states at a josephson junction

• US 6,563,311 B2
• Filed: 05/14/2001
• Issued: 05/13/2003
• Est. Priority Date: 12/01/1999
• Status: Expired due to Term

• Current Assignee: D-Wave Systems Inc. (D-Wave Quantum, Inc.)
• Original Assignee: D-Wave Systems Inc. (D-Wave Quantum, Inc.)
• Inventors: Zagoskin, Alexandre M.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US09/855,817
• Publication Number:
  US 20010020701A1
• Time in Patent Office: 729 Days
• Field of Search: 324/248, 257/31-34, 708/677, 708/679, 708/832, 708/400, 505/162, 505/170, 505/190, 505/829, 505/846, 505/702, 326/3-7
• US Class Current: 324/248
• CPC Class Codes: B82Y 10/00 Nanotechnology for information processing, storage or transmission, e.g. quantum computing or single electron logic
• CPC Class Codes: G06N 10/00 Quantum computing, i.e. information processing based on quantum-mechanical phenomena
• CPC Class Codes: H10N 60/124 comprising high-Tc ceramic materials
• CPC Class Codes: Y10S 505/829 Electrical computer or data processing system
• CPC Class Codes: Y10S 977/933 Spintronics or quantum computing

Abstract

A solid-state quantum computing structure includes a d-wave superconductor in sets of islands that clean Josephson junctions separate from a first superconducting bank. The d-wave superconductor causes the ground state for the supercurrent at each junction to be doubly degenerate, with two supercurrent ground states having distinct magnetic moments. These quantum states of the supercurrents at the junctions create qubits for quantum computing. The quantum states can be uniformly initialized from the bank, and the crystal orientations of the islands relative to the bank influence the initial quantum state and tunneling probabilities between the ground states. A second bank, which a Josephson junction separates from the first bank, can be coupled to the islands through single electron transistors for selectably initializing one or more of the supercurrents in a different quantum state. Single electron transistors can also be used between the islands to control entanglements while the quantum states evolve. After the quantum states have evolved to complete a calculation, grounding the islands, for example, through yet another set of single electron transistors, fixes the junctions in states having definite magnetic moments and facilitates measurement of the supercurrent when determining a result of the quantum computing.

Claims

1. A quantum computing method comprising:

cooling a structure including a superconducting bank and a superconducting mesoscopic island to a temperature that suppresses thermal excitations in said structure sufficiently to suppress thermal sources of decoherence, the structure including a clean Josephson junction between the superconducting mesoscopic island and the superconducting bank;

establishing a quantum state of a supercurrent at the junction, wherein the quantum state is an admixture of a first state having a first magnetic moment and a second state having a second magnetic moment;

allowing the quantum state of the supercurrent at the junction to evolve according to probabilities for tunneling between the first and second state; and

measuring magnetic flux at the junction to determine a result of the quantum computing.

2. The method of claim 1, wherein the supercurrent at the junction is a ground-state current arising from a d-wave superconductor in the structure.

3. The method of claim 1, wherein measuring magnetic flux comprises:

grounding the island to fix the supercurrent in the first or second state; and

measuring the magnetic flux while the island is grounded.

4. The method of claim 1, wherein establishing the quantum state comprises running a current through the bank.

9. The method of claim 1, wherein said establishing said quantum state comprises running a current through the bank.

10. The method of claim 1, wherein allowing the supercurrent at the junction to evolve includes providing a tunneling operation.

11. The method of claim 1, wherein allowing the supercurrent at the junction to evolve includes providing a bias operation.

5. A quantum computing method comprising:

cooling a structure including a superconducting bank and a plurality of superconducting mesoscopic islands to a temperature that suppresses thermal excitations in said structure sufficiently to suppress thermal sources of decoherence, the structure including a separate clean Josephson junction between each superconducting island in said plurality of superconducting mesoscopic islands and the superconducting bank;

establishing a separate quantum state of a supercurrent at each said junction in said plurality of junctions, wherein the separate quantum state is an admixture of a first state having a first magnetic moment and a second state having a second magnetic moment;

allowing each said quantum state of the supercurrent at each said junction to evolve according to probabilities for tunneling between the first and second state; and

measuring magnetic flux at each said separate junction to determine a result of the quantum computing.

6. The method of claim 5, wherein said allowing each said quantum state of the supercurrent at each said junction to evolve further comprises controlling a conductivity of a transistor that couples a first and second mesoscopic island in said plurality of mesoscopic islands together so that an entanglement of the quantum states of said first and second islands is created.

7. The method of claim 5, wherein said establishing said separate quantum state comprises running a current through the bank.

8. The method of claim 7, wherein, for each island in said plurality of mesoscopic islands, a crystal orientation is selected for said mesoscopic island in order to achieve a Josephson junction between said mesoscopic island and said bank that provides said mesoscopic island with a predetermined quantum state.

12. The method of claim 5, wherein allowing the supercurrent at the junction to evolve includes providing a tunneling operation.

13. The method of claim 5, wherein allowing the supercurrent at the junction to evolve includes providing a bias operation.

14. The method of claim 5, wherein allowing the supercurrent at the junction to evolve includes providing a entanglement operation.