Operating a coupler device to perform quantum logic gates
In a general aspect, a quantum logic gate can be performed by tuning a coupler device. One or more coupler control signals can be received at a coupler device in a quantum processor cell. In some instances, in response to the coupler control signals, a coupler operating frequency of the coupler device changes toward a qubit operating frequency of a qubit device, and a phase shift arises in a quantum state of the qubit device due to an interaction between the qubit device and the coupler device. In some instances, in response to the control signals, the coupler operating frequency changes toward a first qubit operating frequency of a first qubit device, then changes toward a second qubit operating frequency of a second qubit device, and a controlled-phase shift arises in a quantum state of the qubit devices due to interactions between the coupler device and the respective qubit devices.
1. A quantum information control method for performing a quantum logic gate, the method comprising:
receiving a coupler control signal at a coupler device in a quantum processor cell; and
in response to the coupler control signal:
changing a coupler operating frequency of the coupler device toward a qubit operating frequency of a qubit device in the quantum processor cell;
producing a phase shift in a quantum state of the qubit device by an interaction between the qubit device and the coupler device; and
varying the coupler operating frequency over a time period of the interaction,
wherein the interaction produces a phase shift
φ
=
∫
0
t
g
2
Δ
(
t
′
)
dt
′
,
where t represents the time period, g represents a coupling strength between the coupler device and the qubit device, and Δ(t′) represents a difference between the coupler operating frequency and the qubit operating frequency.
2. The quantum information control method of claim 1 , comprising maintaining the coupler device in a ground state during the interaction.
3. The quantum information control method of claim 1 , comprising, in response to the control signal, maintaining the coupler operating frequency at a constant level during at least a portion of the time period.
4. The quantum information control method of claim 1 , wherein the phase shift is controlled, at least in part, by a duration of the coupler control signal and an amplitude of the coupler control signal.
5. The quantum information control method of claim 1 , wherein the coupler control signal comprises a bias signal that varies an offset electromagnetic field experienced by the coupler device.
6. The quantum information control method of claim 1 , wherein the coupler device comprises a fluxonium qubit device and the qubit device comprises a transmon qubit device.
7. The quantum information control method of claim 1 , wherein the interaction induces an energy level repulsion that gives rise to the phase shift.
8. The quantum information control method of claim 1 , wherein the control signal comprises a direct current (DC) electrical signal.
9. A quantum computing system comprising:
a coupler device in a quantum processor cell;
a qubit device in the quantum processor cell; and
a control system communicably coupled to the quantum processor cell and operable to:
generate a coupler control signal configured to tune a coupler operating frequency of the coupler device toward a qubit operating frequency of the qubit device; and
communicate the coupler control signal to the coupler device to:
generate, between the qubit device and the coupler device, an interaction that produces a phase shift in a quantum state of the qubit device; and
vary the coupler operating frequency over a time period of the interaction,
wherein the interaction produces a phase shift
φ
=
∫
0
t
g
2
Δ
(
t
′
)
dt
′
,
where t represents the time period, g represents a coupling strength between the coupler device and the qubit device, and Δ(t′) represents a difference between the coupler operating frequency and the qubit operating frequency.
10. The quantum computing system of claim 9 , wherein the control system is operable to maintain the coupler device in a ground state during the interaction.
11. The quantum computing system of claim 9 , wherein the control system is operable to control the phase shift based, at least in part, on controlling a duration of the coupler control signal and an amplitude of the coupler control signal.
12. The quantum computing system of claim 9 , wherein the coupler device comprises a fluxonium qubit device and the qubit device comprises a transmon qubit device.
13. The quantum computing system of claim 9 , wherein the control signal is configured to produce, between the qubit device and the coupler device, an energy level repulsion that gives rise to the phase shift.
14. The quantum computing system of claim 9 , wherein the control signal comprises a direct current (DC) electrical signal.