Tunable qubit coupler

1. A device comprising:

a first data qubit;

a second data qubit, wherein each of the first data qubit and the second data qubit comprises a corresponding elongated arm, and the elongated arm of the first data qubit is spaced apart from the elongated arm of the second data qubit by a gap;

a third qubit, wherein the third qubit is a tunable qubit coupler arranged to couple to the first data qubit and to couple to the second data qubit such that, during operation of the device, the tunable qubit coupler allows tunable coupling between the first data qubit and the second data qubit, and wherein the tunable qubit coupler comprises an elongated wire;

a first substrate, wherein the first data qubit and the second data qubit are on a principal surface of the first substrate; and

a second substrate spaced apart from and coupled to the first substrate, wherein the tunable qubit coupler is on a principal surface of the second substrate and is arranged so as to overlap a portion of the first data qubit and a portion of the second data qubit,

wherein the elongated wire overlaps the elongated arm of the first data qubit, the elongated arm of the second data qubit, and the gap.

2. The device of claim 1 , wherein the tunable qubit coupler is arranged on a first axis, wherein the first axis is:

orthogonal to a second axis that connects the first data qubit and the second data qubit; and

intersects the second axis between the first data qubit and the second data qubit.

3. The device of claim 1 , wherein a pressure in a space between the first substrate and the second substrate is less than 10 −5 Torr.

4. The device of claim 1 , wherein the tunable qubit coupler comprises a superconducting quantum interference device (SQUID) directly connected to the elongated wire.

5. The device of claim 1 , wherein the first data qubit and the second data qubit comprise a first differential qubit and a second differential qubit, respectively.

6. The device of claim 1 , wherein the tunable qubit coupler comprises a transmon qubit.

7. The device of claim 1 , wherein the principal surface of the first substrate faces the principal surface of the second substrate.

8. The device of claim 1 , wherein the first data qubit and the second data qubit are arranged on the principal surface of the first substrate such that, during operation of the device, the first data qubit and the second data qubit are directly coupled.

9. A method of controlling coupling between a first data qubit and a second data qubit using a third qubit that is a tunable qubit coupler, wherein the tunable qubit coupler is arranged to couple to the first data qubit and to couple to the second data qubit to tune the coupling in response to a control signal, the method comprising:

controlling coupling between the first data qubit and the second data qubit by varying an operating frequency of the tunable qubit coupler,

wherein the first data qubit and the second data qubit are on a principal surface of a first substrate,

wherein the tunable qubit coupler is on a principal surface of a second substrate spaced apart from and coupled to the first substrate, and

wherein the tunable qubit is arranged so as to overlap a portion of the first data qubit and a portion of the second data qubit;

tuning each of the first data qubit and the second data qubit to a same resonance frequency; and

subsequently varying the operating frequency of the tunable qubit coupler to turn coupling on between the first data qubit and the second data qubit.

10. The method of claim 9 , wherein the tunable qubit coupler comprises a superconducting quantum interference device (SQUID), and wherein varying the operating frequency of the tunable qubit coupler comprises varying a magnetic flux of the SQUID of the tunable qubit coupler.

11. The method of claim 9 , wherein varying the operating frequency of the tunable qubit coupler comprises varying the operating frequency within a range of no less than 4 GHz and no more than 6 GHz.

12. The method of claim 9 , wherein the resonance frequency of the first data qubit and the second data qubit is within a range of 4 GHz to 8 GHz.

13. The method of claim 9 , wherein the resonance frequency of the first data qubit and the second data qubit differs from the operating frequency of the tunable qubit coupler by 0.5 GHz to 7 GHz.

14. The method of claim 9 , comprising, before tuning each of the first data qubit and the second data qubit to the same resonance frequency, varying the operating frequency of the tunable qubit coupler to a first frequency to turn coupling off between the first data qubit and the second data qubit.

15. The method of claim 14 , wherein varying the operating frequency of the tunable qubit coupler to turn coupling off comprises varying the operating frequency so that a direct coupling between the first data qubit and the second data qubit is offset by an indirect coupling between the first data qubit and the second data qubit through the tunable qubit coupler.

16. The method of claim 15 , wherein a coupling between the first data qubit and the second data qubit that is a combination of the direct coupling and the indirect coupling operates over a frequency range of 45 MHz.

17. The method of claim 15 , wherein the indirect coupling between the first data qubit and the second data qubit through the tunable qubit coupler occurs across a space that is at a pressure of less than 10 −5 Torr.