Capacitively-shunted asymmetric DC-SQUID for qubit readout and reset

1. A superconducting device comprising:

a capacitive device shunting a set of asymmetric DC-SQUIDs to form a tunable resonator, an asymmetric DC-SQUID in the set of asymmetric DC-SQUIDs comprising a first Josephson junction and a second Josephson junction, wherein a first critical current of the first Josephson junction is different from a second critical current of the second Josephson junction;

a coupling between the tunable resonator and a qubit such that the capacitively-shunted asymmetric DC-SQUIDs can read a quantum state of the qubit; and

an external magnetic flux, wherein the external magnetic flux is set to a first value and applied to the tunable resonator, wherein a first value of the external magnetic flux causes the tunable resonator to tune to a first frequency, and wherein the tunable resonator tuning to the first frequency causes active reset of the qubit.

2. The superconducting device of claim 1 , further comprising:

the external magnetic flux changed to a second value, wherein the second value of the external magnetic flux causes the tunable SQUID to tune to a second frequency, wherein the first frequency is within a first frequency difference from a resonance frequency of the qubit, wherein the second frequency is detuned from the resonance frequency of the qubit by at least a second frequency difference, and wherein the tunable resonator tuning to the second frequency enables a dispersive readout operation of the quantum state of the qubit to be performed.

3. The superconducting device of claim 2 , wherein the second frequency is a maximum frequency in a frequency resonance range of the tunable resonator.

4. The superconducting device of claim 2 , wherein the second frequency difference is a function of a degree of asymmetry between the first Josephson junction and the second Josephson junction.

5. The superconducting device of claim 1 , wherein the first frequency difference is zero and the first frequency is the resonance frequency of the qubit.

6. The superconducting device of claim 1 , wherein the first frequency being within the first frequency difference from the resonance frequency of the qubit causes the qubit to release a photon, the releasing causing the qubit to relax to a ground energy state.

7. The superconducting device of claim 6 , wherein forcing the qubit to the ground energy state is faster than an energy decay time constant of the qubit.

8. The superconducting device of claim 1 , wherein the set of asymmetric DC-SQUIDs includes only the asymmetric DC-SQUID.

9. The superconducting device of claim 1 , further comprising:

a series connection connecting a plurality of asymmetric DC-SQUIDs from the set of asymmetric DC-SQUIDs.

10. The superconducting device of claim 9 , wherein the series connection comprises a superconducting wire.

11. The superconducting device of claim 1 , further comprising:

a first pad formed on a first side of the set of asymmetric DC-SQUIDs; and

a second pad forming on a second side of the set of asymmetric DC-SQUIDs, wherein the first pad and the second pad are separated by a distance, and wherein the first pad and the second pad together form the capacitive device.

12. A method comprising:

forming a tunable resonator by shunting a set of asymmetric DC-SQUIDs with a capacitive device, an asymmetric DC-SQUID in the set of asymmetric DC-SQUIDs comprising a first Josephson junction and a second Josephson junction, wherein a first critical current of the first Josephson junction is different from a second critical current of the second Josephson junction;

coupling the tunable resonator to a qubit such that the tunable resonator can dispersively read a quantum state of the qubit; and

actively resetting the qubit by applying an external magnetic flux of a first value to the tunable resonator, wherein a first value of the external magnetic flux causes the tunable resonator to tune to a first frequency, wherein the first frequency is within a first frequency difference from a resonance frequency of the qubit.

13. The method of claim 12 , further comprising:

changing the external magnetic flux to a second value, wherein the second value of the external magnetic flux causes the tunable resonator to tune to a second frequency, wherein the second frequency is detuned from the resonance frequency of the qubit by at least a second frequency difference; and

performing, using the tunable resonator tuned to the second frequency, a dispersive readout operation of the quantum state of the qubit.

14. The method of claim 13 , wherein the second frequency is a maximum frequency in a frequency resonance range of the tunable resonator.

15. The method of claim 13 , wherein the second frequency difference is a function of a degree of asymmetry between the first Josephson junction and the second Josephson junction.

16. The method of claim 12 , wherein the first frequency difference is zero and the first frequency is the resonance frequency of the qubit.

17. The method of claim 12 , wherein the first frequency being within the first frequency difference from the resonance frequency of the qubit causes the qubit to release a photon, the releasing causing the qubit to relax to a ground state.

18. The method of claim 17 , wherein forcing the qubit to the ground energy state is faster than an energy decay time constant T 1 of the qubit.

19. The method of claim 12 , wherein the set of asymmetric DC-SQUIDs includes only the asymmetric DC-SQUID.

20. The method of claim 12 , further comprising:

connecting a plurality of asymmetric DC-SQUIDs from the set of asymmetric DC-SQUIDs in a series.

21. The method of claim 20 , wherein the plurality of asymmetric DC-SQUIDs is connected in series using a superconductor.

22. The method of claim 12 , further comprising:

forming a first pad on a first side of the set of asymmetric DC-SQUIDs; and

forming a second pad on a second side of the set of asymmetric DC-SQUIDs, wherein the first pad and the second pad are separated by a distance, and wherein the first pad and the second pad together form the capacitive device.

23. A superconducting fabrication system which when operated to fabricate a tunable resonator device performing operations comprising:

forming the tunable resonator by shunting a set of asymmetric DC-SQUIDs with a capacitive device, an asymmetric DC-SQUID in the set of asymmetric DC-SQUIDs comprising a first Josephson junction and a second Josephson junction, wherein a first critical current of the first Josephson junction is different from a second critical current of the second Josephson junction;

coupling the tunable resonator to a qubit such that the tunable resonator can dispersively read a quantum state of the qubit; and

actively resetting the qubit by applying an external magnetic flux of a first value to the tunable resonator, wherein a first value of the external magnetic flux causes the tunable resonator to tune to a first frequency, wherein the first frequency is within a first frequency difference from a resonance frequency of the qubit.

24. The superconducting fabrication system of claim 23 , further comprising:

changing the external magnetic flux to a second value, wherein the second value of the external magnetic flux causes the tunable resonator to tune to a second frequency, wherein the second frequency is detuned from the resonance frequency of the qubit by at least a second frequency difference; and

performing, using the tunable resonator tuned to the second frequency, a dispersive readout operation of the quantum state of the qubit.

25. The superconducting fabrication system of claim 24 , wherein the second frequency is a maximum frequency in a frequency resonance range of the tunable resonator.