Parametrically activated quantum logic gates
In a general aspect, a quantum logic gate is performed in a quantum computing system. In some cases, a pair of qubits are defined in a quantum processor; the pair of qubits can include a first qubit defined by a first qubit device in the quantum processor and a second qubit defined by a tunable qubit device in the quantum processor. A quantum logic gate can be applied to the pair of qubits by communicating a control signal to a control line coupled to the tunable qubit device. The control signal can be configured to modulate a transition frequency of the tunable qubit device at a modulation frequency, and the modulation frequency can be determined based on a transition frequency of the first qubit device.
1. A method comprising:
obtaining qubit device design parameters for a plurality of qubit devices, the plurality of qubit devices comprising a fixed-frequency qubit device and a tunable qubit device;
obtaining a quantum processor circuit design criterion associated with a two-qubit quantum logic gate;
determining, based on the qubit device design parameters and by operation of one or more processors, an operating parameter for at least one of the plurality of qubit devices under an operating condition, the operating condition comprising a flux modulation applied to the tunable qubit device, wherein the flux modulation varies the amplitude of a magnetic flux applied to the tunable qubit device over time;
determining that the operating parameter meets the quantum processor circuit design criterion associated with the two-qubit quantum logic gate; and
providing the qubit device design parameters for a quantum processor circuit specification.
2. The method of claim 1 , wherein the tunable qubit device comprises a superconducting circuit loop that defines a flux quantum Φ 0 and receives a first magnetic flux that tunes a transition frequency of the tunable qubit device, and the flux modulation comprises a modulation of the first magnetic flux at a modulation frequency.
3. The method of claim 1 , wherein the fixed-frequency qubit device is a first fixed-frequency qubit device, and the plurality of qubit devices comprises:
the tunable qubit device; and
a plurality of fixed-frequency qubit devices capacitively coupled to the tunable qubit device, wherein the plurality of fixed-frequency qubit devices includes the first fixed-frequency qubit device.
4. The method of claim 3 , wherein obtaining the qubit device design parameters comprises specifying the qubit device design parameters such that each of the fixed-frequency qubit devices has a distinct qubit frequency.
5. The method of claim 4 , wherein obtaining the qubit device design parameters comprises specifying the qubit device design parameters such that a specified qubit frequency of the tunable qubit device is distinct from the qubit frequencies of the respective fixed-frequency qubit devices.
6. The method of claim 5 , wherein the specified qubit frequency of the tunable qubit device is the maximum qubit frequency of the tunable qubit device.
7. The method of claim 5 , wherein obtaining the qubit device design parameters comprises specifying the qubit device design parameters such that the qubit frequencies of the fixed-frequency qubit devices are separated from each other by at least a first minimum frequency difference.
8. The method of claim 7 , wherein obtaining the qubit device design parameters comprises specifying the qubit device design parameters such that the qubit frequencies of the fixed-frequency qubit devices are separated from a qubit frequency of the tunable qubit device by at least a second minimum frequency difference.
9. The method of claim 3 , wherein the qubit device design parameters include:
one or more qubit frequencies and an anharmonicity for the tunable qubit device;
qubit frequencies for the respective fixed-frequency qubit devices; and
anharmonicities for the respective fixed-frequency qubit devices.
10. The method of claim 1 , wherein:
determining the operating parameter comprises computing an effective coupling strength between the fixed-frequency qubit device and the tunable qubit device under the operating condition; and
determining that the operating parameter meets the quantum processor circuit design criterion includes determining that the computed effective coupling strength is greater than a minimum effective coupling strength.
11. The method of claim 1 , wherein the quantum processor circuit design criterion is a first quantum processor circuit design criterion, and the method comprises:
obtaining a second quantum processor circuit design criterion associated with the two-qubit quantum logic gate;
determining, for a range of flux modulation amplitude, a maximum gradient of tunable qubit frequency versus flux modulation amplitude for the tunable qubit device; and
determining that the maximum gradient meets the second quantum processor circuit design criterion.
12. The method of claim 11 , wherein the second quantum processor circuit design criterion includes a minimum gradient of tunable qubit frequency versus magnetic flux, and the minimum gradient is equivalent to 0.9 GigaHertz per flux quantum.
13. The method of claim 1 , wherein the quantum processor circuit design criterion is a first quantum processor circuit design criterion, and the method comprises:
obtaining a second quantum processor circuit design criterion associated with the two-qubit quantum logic gate;
determining dispersive shifts for the tunable qubit device and respective fixed-frequency qubit devices;
determining a sum of the dispersive shifts; and
determining that the sum meets the second quantum processor circuit design criterion.
14. The method of claim 1 , comprising an iterative process, wherein each iteration of the iterative process comprises:
obtaining a current set of qubit device design parameters for the plurality of qubit devices;
determining, based on the current set of qubit device design parameters and by operation of the one or more processors, a current operating parameter for at least one of the plurality of qubit devices under the operating condition; and
evaluating whether the current operating parameter meets the quantum processor circuit design criterion associated with the two-qubit quantum logic gate.
15. The method of claim 14 , wherein each iteration comprises:
if the current operating parameter does not meet the quantum processor circuit design criterion, determining an updated set of qubit device design parameters for the next iteration of the iterative process; and
if the current operating parameter does meet the quantum processor circuit design criterion and one or more other quantum processor circuit design criteria, providing the qubit device design parameters for the quantum processor circuit specification.
16. The method of claim 14 , wherein
the fixed-frequency qubit device is a first fixed-frequency qubit device, and the plurality of qubit devices comprises:
the tunable qubit device; and
a plurality of fixed-frequency qubit devices coupled to the tunable qubit device, wherein the plurality of fixed-frequency qubit devices includes the first fixed-frequency qubit device; and
each iteration of the iterative process includes:
computing detunings between a qubit frequency of the tunable qubit device and respective qubit frequencies of the fixed-frequency qubit devices; and
evaluating whether the detunings are distinct from one another.
17. The method of claim 16 , wherein each iteration of the iterative process further includes:
computing the difference between the detunings for the first fixed-frequency qubit device and a second fixed-frequency qubit device;
obtaining a first target effective coupling strength for an interaction between the first fixed-frequency qubit device and the tunable qubit device;
obtaining a second target effective coupling strength for an interaction between the second fixed-frequency qubit device and the tunable qubit device; and
evaluating whether the difference between the detunings is greater than the first and second target effective coupling strengths.
18. A computer system comprising:
one or more processors; and
memory storing instructions that are operable when executed by the one or more processors to perform operations comprising:
obtaining qubit device design parameters for a plurality of qubit devices, the plurality of qubit devices comprising a fixed-frequency qubit device and a tunable qubit device;
obtaining a quantum processor circuit design criterion associated with a two-qubit quantum logic gate;
determining, based on the qubit device design parameters and by operation of one or more processors, an operating parameter for at least one of the plurality of qubit devices under an operating condition, the operating condition comprising a flux modulation applied to the tunable qubit device, wherein the flux modulation varies the amplitude of a magnetic flux applied to the tunable qubit device over time;
determining that the operating parameter meets the quantum processor circuit design criterion associated with the two-qubit quantum logic gate; and
providing the qubit device design parameters for a quantum processor circuit specification.
19. A method comprising:
obtaining qubit device parameters for a plurality of qubit devices in a quantum processor circuit, the plurality of qubit devices comprising a fixed-frequency qubit device and a tunable qubit device; and
based on the qubit device parameters, selecting gate parameters of a two-qubit quantum logic gate for application to a pair of qubits defined by the fixed-frequency qubit device and the tunable qubit device,
wherein selecting the gate parameters comprises selecting parameters of a control signal for delivery to a control line coupled to the tunable qubit device, and the gate parameters correspond to:
a modulation amplitude of a flux modulation produced by the control signal; and
a modulation frequency of the flux modulation produced by the control signal, wherein the flux modulation varies the amplitude of a magnetic flux applied to the tunable qubit device over time.
20. The method of claim 19 , comprising determining the modulation frequency based on the sum or the difference of:
a transition frequency of the tunable qubit device, and
a transition frequency of the fixed-frequency qubit device.
21. The method of claim 20 , wherein the modulation frequency is determined based on the transition frequency of the fixed-frequency qubit device and the transition frequency of the tunable qubit device.
22. The method of claim 20 , wherein the modulation frequency is determined based on the transition frequency of the fixed-frequency qubit device and an anharmonicity of the tunable qubit device.
23. The method of claim 20 , wherein the modulation frequency is determined based on the transition frequency between the ground state and first excited state of the fixed-frequency qubit device.
24. The method of claim 20 , wherein the modulation frequency is determined based on the transition frequency between the first excited state and second excited state of the fixed-frequency qubit device.
25. The method of claim 20 , wherein the modulation frequency is determined based on the transition frequency of the tunable qubit device and an anharmonicity of the fixed-frequency qubit device.
26. The method of claim 19 , wherein the fixed-frequency qubit device is a first fixed-frequency qubit device, and the plurality of qubit devices comprises:
the tunable qubit device; and
a plurality of fixed-frequency qubit devices capacitively coupled to the tunable qubit device, wherein the plurality of fixed-frequency qubit devices includes the first fixed-frequency qubit device.
27. The method of claim 26 , wherein the qubit device parameters include:
one or more qubit frequencies and an anharmonicity for the tunable qubit device;
qubit frequencies for the respective fixed-frequency qubit devices; and
anharmonicities for the respective fixed-frequency qubit devices.