Parametrically activated quantum logic gates

Patent metadata

Field	Value
Publication number	US-10483980-B2
Application number	US-201816012551-A
Country	US
Kind code	B2
Filing date	Jun 19, 2018
Priority date	Jun 19, 2017
Publication date	Nov 19, 2019
Grant date	Nov 19, 2019

Abstract

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.

First claim

What is claimed is:

1. A method comprising:
obtaining operating parameters for one or more of 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, the operating parameters based on measurements of the quantum processor circuit under an operating condition, the operating condition comprising a flux modulation applied to the tunable qubit device; and
based on the operating 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;
generating a control signal configured to modulate, at a modulation frequency, a transition frequency of the tunable qubit device, the modulation frequency being determined based on a transition frequency of the fixed-frequency qubit device; and
applying the two-qubit quantum logic gate to the pair of qubits by communicating the control signal to a control line coupled to the tunable qubit device;
wherein the control signal is a first control signal, the modulation frequency is a first modulation frequency, the fixed-frequency qubit device is a first fixed-frequency qubit device that defines a first qubit, the two-qubit quantum logic gate is a first two-qubit quantum logic gate, the pair of qubits is a first pair of qubits, and the method further comprises:
generating a second control signal configured to modulate the transition frequency of the tunable qubit device at a second modulation frequency, the second modulation frequency being determined based on a transition frequency of a second fixed-frequency qubit device in the quantum processor circuit; and
applying a second two-qubit quantum logic gate to a second pair of qubits in the quantum processor circuit by communicating the second control signal to the control line coupled to the tunable qubit device, the second pair of qubits comprising a second qubit defined by the tunable qubit device and a third qubit defined by the second fixed-frequency qubit device.

2. The method of claim 1 , wherein the operating parameters include an effective coupling strength between the qubit devices during the flux modulation.

3. The method of claim 1 , wherein the operating parameters are obtained for a range of modulation amplitude of the flux modulation.

4. The method of claim 1 , wherein selecting gate parameters comprises selecting initial gate parameters, and the method further comprises generating refined gate parameters from the initial gate parameters.

5. The method of claim 4 , wherein the refined gate parameters are generated based on spectroscopic measurements of the two-qubit quantum logic gate applied to the quantum processor circuit.

6. The method of claim 1 , wherein the gate parameters include a flux modulation amplitude and a flux modulation frequency.

7. The method of claim 6 , further comprising selecting a filter for a flux modulation control line based on the flux modulation frequency.

8. The method of claim 1 , wherein selecting gate parameters of a two-qubit quantum logic gate comprises selecting parameters of the control signal for delivery to the control line coupled to the tunable qubit device, and the gate parameters correspond to:
a modulation amplitude of the flux modulation produced by the control signal;
the modulation frequency of the flux modulation produced by the control signal; and
a duration of the control signal.

9. The method of claim 8 , further comprising identifying single-qubit phase corrections for the two-qubit quantum logic gate.

10. The method of claim 1 , wherein selecting gate parameters of a two-qubit quantum logic gate comprises selecting gate parameters of a Bell-Rabi gate, a controlled-Z gate, an iSWAP gate or a square-root-of-iSWAP gate.

11. The method of claim 1 , further comprising benchmarking the two-qubit quantum logic gate based on the selected gate parameters.

12. The method of claim 1 , wherein:
the tunable qubit device comprises a superconducting circuit loop that receives a first magnetic flux that tunes the transition frequency of the tunable qubit device, and the control line comprises a flux-bias device that is inductively coupled to the superconducting circuit loop to control the first magnetic flux; and
generating the control signal to modulate the transition frequency of the tunable qubit device at the modulation frequency comprises generating the control signal to modulate the first magnetic flux at half the modulation frequency.

13. The method of claim 1 , wherein:
the tunable qubit device comprises a superconducting circuit loop that receives a first magnetic flux that tunes the transition frequency of the tunable qubit device, and the control line comprises a flux-bias device that is inductively coupled to the superconducting circuit loop to control the first magnetic flux; and
generating the control signal to modulate the transition frequency of the tunable qubit device at the modulation frequency comprises generating the control signal to modulate the first magnetic flux at the modulation frequency.

14. The method of claim 1 , wherein the operating condition comprises the flux modulation applied to the tunable qubit device at a flux modulation frequency between 0.02 GigaHertz and 0.40 GigaHertz.

15. The method of claim 1 , wherein the operating condition comprises the flux modulation applied to the tunable qubit device at a flux modulation frequency between 40 MegaHertz and 160 MegaHertz.

16. A method comprising:
obtaining measurements of a coherence time of a qubit defined by a tunable qubit device in a quantum processor circuit comprising a plurality of qubit devices, the plurality of qubit devices comprising a fixed-frequency qubit device coupled to the tunable qubit device, the measurements representing values of the coherence time during a flux modulation applied to the tunable qubit device, the measurements corresponding to a range of modulation amplitudes of the flux modulation;
based on the measurements, identifying a modulation amplitude control value associated with a maximum coherence time of the qubit; and
based on the identified modulation amplitude control value, selecting a gate parameter of a two-qubit quantum logic gate for the fixed-frequency qubit device and the tunable qubit device;
wherein identifying the modulation amplitude control value comprises identifying a first modulation amplitude control value corresponding to the maximum measurement of the coherence time.

17. The method of claim 16 , 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.

18. The method of claim 17 , wherein the modulation amplitude control value represents a voltage amplitude of a control signal that produces the modulation of the first magnetic flux.

19. The method of claim 17 , wherein the modulation amplitude control value corresponds to a flux modulation amplitude between 0.45Φ 0 and 0.65Φ 0 .

20. The method of claim 16 , 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 each capacitively coupled to the tunable qubit device, wherein the

Assignees

• Rigetti & Co Inc

Inventors

• Sete Eyob A
• Didier Nicolas
• Da Silva Marcus Palmer
• Rigetti Chad Tyler
• Reagor Matthew J
• Caldwell Shane Arthur
• Tezak Nikolas Anton
• Ryan Colm Andrew
• Hong Sabrina Sae Byul
• Sivarajah Prasahnt
• Papageorge Alexander
• Abrams Deanna Margo

Classifications

• H03K19/195 Primary

  using superconductive devices · CPC title

• G06N10/00

  Quantum computing, i.e. information processing based on quantum-mechanical phenomena · CPC title

• G06N10/20

  Models of quantum computing, e.g. quantum circuits or universal quantum computers · CPC title

• G06N10/70

  Quantum error correction, detection or prevention, e.g. surface codes or magic state distillation · CPC title

• G06N10/40

  Physical realisations or architectures of quantum processors or components for manipulating qubits, e.g. qubit coupling or qubit control · CPC title