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 quantum computing method comprising:
identifying a two-qubit quantum logic gate to be applied to a pair of qubits defined in a qubit device array, the pair of qubits comprising:
a first qubit defined by a first qubit device in the qubit device array; and
a second qubit defined by a tunable qubit device in the qubit device array, wherein the tunable qubit device neighbors the first qubit device and a third qubit device in the qubit device array; and
applying the two-qubit quantum logic gate to the pair of qubits by communicating a control signal to a control line coupled to the tunable qubit device, the control signal configured to modulate a transition frequency of the tunable qubit device at a modulation frequency, the modulation frequency being determined based on a transition frequency of the first qubit device.
2 . The quantum computing method of claim 1 , wherein the first qubit device is a tunable qubit device.
3 . The quantum computing method of claim 1 , wherein the first qubit device is a fixed-frequency qubit device.
4 . The quantum computing method of claim 1 , wherein the two-qubit quantum logic gate is a first two-qubit quantum logic gate, the pair of qubits is a first pair of qubits, the control signal is a first control signal, the modulation frequency is a first modulation frequency, and the method further comprises:
identifying a second two-qubit quantum logic gate to be applied to a second pair of qubits defined in the qubit device array, the second pair of qubits comprising the second qubit and a third qubit defined by the third qubit device; and
applying the second two-qubit quantum logic gate to the second pair of qubits by communicating a second control signal to the control line coupled to the tunable qubit device, the 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 the third qubit device.
5 . The quantum computing method of claim 4 , wherein the third qubit device is a tunable qubit device.
6 . The quantum computing method of claim 4 , wherein the third qubit device is a fixed-frequency qubit device.
7 . A quantum computing system comprising:
a quantum processor comprising a qubit device array, the qubit device array comprising a tunable qubit device and a plurality of other qubit devices that neighbor the tunable qubit device in the qubit device array; and
a control system communicably coupled to the quantum processor and configured to perform operations comprising:
identifying a quantum logic gate to be applied to a pair of qubits, the pair of qubits comprising a first qubit defined by a first qubit device that neighbors the tunable qubit device in the qubit device array and a second qubit defined by the tunable qubit device; and
applying the quantum logic gate to the pair of qubits in the qubit device array by communicating a control signal to a control line coupled to the tunable qubit device, the control signal configured to modulate a transition frequency of the tunable qubit device at a modulation frequency.
8 . The quantum computing system of claim 7 , further comprising a filter between the quantum processor and the control system, wherein the filter is configured to suppress at least one range of frequencies in the control signal communicated to the control line.
9 . The quantum computing method of claim 1 , wherein the modulation frequency is determined based on the transition frequency of the first qubit device and a transition frequency of the tunable qubit device.
10 . The quantum computing method of claim 4 , wherein the second modulation frequency is determined based on the transition frequency of the third qubit device and a transition frequency of the tunable qubit device.
11 . The quantum computing method of claim 1 , wherein applying the two-qubit quantum logic gate to the pair of qubits comprises applying a controlled-Z gate to the pair of qubits.
12 . The quantum computing method of claim 1 , wherein the modulation frequency is determined based on the transition frequency of the first qubit device and the anharmonicity of the tunable qubit device.
13 . The quantum computing method of claim 1 , wherein the modulation frequency is determined based on the transition frequency between the first excited state and second excited state of the first qubit device.
14 . The quantum computing method of claim 1 , wherein applying the two-qubit quantum logic gate to the pair of qubits comprises applying an iSWAP gate to the pair of qubits.
15 . The quantum computing method of claim 1 , wherein applying the two-qubit quantum logic gate to the pair of qubits comprises applying a square-root-of-iSWAP gate to the pair of qubits.
16 . The quantum computing system of claim 7 , wherein the modulation frequency is determined based on the transition frequency of the tunable qubit device and a transition frequency of the first qubit device.
17 . The quantum computing system of claim 7 , wherein the tunable qubit device is a first tunable qubit device, and the first qubit device is a second tunable qubit device.
18 . The quantum computing system of claim 17 , wherein the modulation frequency is determined based on the transition frequency of the second tunable qubit device and the anharmonicity of the first tunable qubit device.
19 . The quantum computing system of claim 7 , wherein the first qubit device is a fixed-frequency qubit device.
20 . The quantum computing system of claim 7 , wherein the quantum logic gate is a controlled-Z gate.