System and method for controlling superconducting qubits

• US 20040016918A1
• Filed: 12/17/2002
• Published: 01/29/2004
• Est. Priority Date: 12/18/2001
• Status: Abandoned Application

• Current Assignee: D-Wave Systems Inc. (D-Wave Quantum, Inc.)
• Original Assignee: D-Wave Systems Inc. (D-Wave Quantum, Inc.)
• Inventors: Amin, Mohammad H. S., Rose, Geordie, Hilton, Jeremy P., Zagoskin, Alexandre
• Application Number:
  US10/321,867
• Publication Number:
  US 20040016918A1
• Time in Patent Office: Days
• US Class Current: 257/14
• CPC Class Codes: B82Y 10/00 Nanotechnology for information processing, storage or transmission, e.g. quantum computing or single electron logic
• CPC Class Codes: G01R 33/0358 coupling the flux to the SQUID gradiometer coils G01R33/022; coils with superconductive winding H01F6/06
• CPC Class Codes: G06N 10/00 Quantum computing, i.e. information processing based on quantum-mechanical phenomena
• CPC Class Codes: H10N 69/00 Integrated devices, or assemblies of multiple devices, comprising at least one superconducting element covered by group H10N60/00
• CPC Class Codes: Y10S 977/933 Spintronics or quantum computing

Abstract

A system and method for controlling an information state of a superconducting qubit having a superconducting loop that includes a bulk loop portion, a mesoscopic island portion, and two Josephson junctions separating the bulk loop portion from the mesoscopic island portion is described. The method includes applying a bias across the mesoscopic island portion. In one embodiment, the method includes driving a bias current in the superconducting loop. In one embodiment, the method includes driving a bias current in the superconducting loop by coupling a magnetic flux into the superconducting loop. In one embodiment, the control system includes a tank circuit inductively coupled to the superconducting loop. In one embodiment, entanglement between qubits is provided by connections between qubits to be entangled.

Claims

1. A method for controlling the state of a qubit, comprising:

controlling a current in a loop of superconducting material, said loop having at least two Josephson junctions that isolate a mesoscopic island of the loop.

2. A method for initializing a two-junction superconducting loop qubit, comprising:

providing a bias current in the loop I₀=±

I_i for a time duration t such that the qubit enters one of its basis states.

3. A method for reading out a state of a qubit having two Josephson junctions that connect a mesoscopic portion of the qubit to a bulk portion, and a control Josephson junction in said bulk portion, comprising:

providing a bias current in the loop I=I_DC+IAC for some duration t; and

measuring a potential drop V_R across said control Josephson junction.

4. A method for reading a state of a qubit having two Josephson junctions that connect a mesoscopic portion of the qubit to a bulk portion of the qubit, comprising;

applying a bias field across said mesoscopic island portion; and

measuring a resulting potential related to said bias field.

5. The method of claim 4, wherein the qubit is coupled to a dc-SQUID and said measuring a resulting potential includes measuring the potential across said dc-SQUID.

6. The method of claim 4, wherein the qubit is coupled to a tank circuit and said measuring a resulting potential includes measuring an impedance of said tank circuit.

7. A quantum register comprising;

a first qubit, wherein said first qubit includes a superconducting loop, said superconducting loop comprising a bulk loop portion and a mesoscopic island portion, said superconducting loop further comprising at least two Josephson junctions separating said bulk loop portion from said mesoscopic island portion, and

an apparatus for coupling said first qubit to a second qubit.

8. The quantum register of claim 7, wherein said apparatus for coupling comprises a bus, wherein said bus is capacitively coupled to at least said first qubit and said second qubit.

9. The quantum register of claim 8, wherein said bus comprises a switch.

10. The quantum register of claim 9, wherein said switch comprises a single electron transistor.

11. The quantum register of claim 8, further comprising a capacitive connection between said bus and said first qubit.

12. The quantum register of claim 11, wherein said capacitive connection comprises a switch, wherein said switch has a first state and a second state, and wherein said first state capacitively connects said first qubit to said bus and said second state isolates said first qubit from said bus.

13. The quantum register of claim 12, wherein said switch comprises a single electron transistor.

14. The quantum register of claim 12, wherein said bus comprises a switch for connecting said bus to said first qubit.

15. A method for performing an entanglement operation between a first qubit and a second qubit comprising;

biasing a first qubit, wherein said first qubit comprises a first superconducting loop, said first superconducting loop comprising a first bulk loop portion and a first mesoscopic island portion, said first superconducting loop further comprising at least two Josephson junctions separating said first bulk loop portion from said first mesoscopic island portion; and

biasing a second qubit, wherein said second qubit comprises a second superconducting loop, said second superconducting loop comprising a second bulk loop portion and a second mesoscopic island portion, said second superconducting loop further comprising at least two Josephson junctions separating said second bulk loop portion from said second mesoscopic island portion, wherein said first and second qubits share mutual inductance with each other and wherein said first and second qubits share flux when mutually biased.

16. The method of claim 15, wherein said shared flux between said first and second qubits does not disrupt the qubit states when said first and second qubits are not mutually biased.