Tunable interactions for implementing two-qubit gates, and extensible circuits built therefrom
A system and method provide two-qubit gates and quantum computing circuits built therefrom. Pairs of qubits are inductively or capacitively coupled using a coupler that is driven using an off-resonant microwave drive. By controlling the drive, the ZZ interaction between the qubits can be precisely controlled. In particular, the interaction may be selectively reduced or suppressed, thereby isolating the qubits from each other, or the interaction may be increased to provide fast, controlled-Z, two-qubit gates with high fidelity. Moreover, qubits and couplers may be arranged according to their resonant frequencies into unit cells and replicated to arbitrary size, thereby forming a quantum computer.
1. A system comprising:
a first qubit associated with a first resonant frequency;
a second qubit associated with a second resonant frequency;
a qubit coupler inductively or capacitively coupling the first qubit to the second qubit according to a ZZ interaction strength; and
a microwave driver for driving the qubit coupler with microwaves according to a driver power and a third driver frequency that is different than the first resonant frequency and the second resonant frequency,
wherein the microwave driver is configurable to selectively increase or decrease the ZZ interaction strength, and
wherein the microwave driver is configurable to reduce spurious interactions between the first qubit and the second qubit by selectively decreasing the ZZ interaction strength, and
wherein the microwave driver is configurable to form a two-qubit gate by selectively increasing the ZZ interaction strength, and
wherein the microwave driver is configurable for selectively tuning to obtain a given leakage rate, or a given gate speed, or a balance between a leakage rate and a gate speed.
2. The system of claim 1 , wherein either the first qubit or the second qubit comprises a fixed-frequency transmon, or a tunable-frequency transmon, or a fluxonium.
3. The system of claim 1 , wherein the qubit coupler comprises a split-transmon, an inductively-shunted transmon, or a fluxonium.
4. The system of claim 1 , wherein the microwave driver is configurable by selectively tuning either the driver power, or the driver frequency, or both.
5. The system of claim 1 , further comprising one or more additional qubits, wherein the microwave driver is further configurable to selectively decrease a ZZZ interaction strength or a ZZZZ interaction strength between the first qubit, the second qubit, and the one or more additional qubits.
6. A method of tuning ZZ interactions between a first qubit and a second qubit that are inductively or capacitively coupled using a qubit coupler, the method comprising:
driving the qubit coupler using microwaves having a driver power, and having a driver frequency that is different than a resonant frequency of the first qubit and a resonant frequency of the second qubit;
tuning a resonant frequency of the qubit coupler to reduce spurious interactions between the first qubit and the second qubit by selectively decreasing the ZZ interaction strength; and
driving the qubit coupler to selectively increase the ZZ interaction strength, thereby forming a two-qubit gate,
wherein driving the qubit coupler selectively increases or decreases a ZZ interaction strength between the first qubit and the second qubit through the qubit coupler, and wherein driving the qubit coupler comprises selectively tuning to obtain a given leakage rate, or a given gate speed, or a balance between a leakage rate and a gate speed.
7. The method of claim 6 , wherein driving the qubit coupler comprises selectively tuning either the driver power, or the driver frequency, or both.
8. The method of claim 6 , further comprising driving the qubit coupler to selectively decrease a ZZZ interaction strength or ZZZZ interaction strength between the first qubit, the second qubit, and one or more additional qubits.
9. A multi-qubit processor comprising:
a plurality of qubits, each qubit in the plurality of qubits being associated with a respective resonant frequency;
a plurality of qubit couplers, each qubit coupler in the plurality of qubit couplers being associated with a respective resonant frequency and inductively or capacitively coupling a respective pair of qubits in the plurality of qubits; and
a plurality of microwave drivers, each microwave driver configurable for driving a corresponding qubit coupler with microwaves according to a driver power and a driver frequency, the driver power and driver frequency being selected to reduce crosstalk between the qubits inductively coupled by the corresponding qubit coupler.
10. The multi-qubit processor of claim 9 , further comprising at least one qubit coupler that inductively or capacitively couples a first pair of qubits in the plurality of qubits to a second pair of qubits in the plurality of qubits.
11. The multi-qubit processor of claim 9 , wherein the plurality of qubits and the plurality of qubit couplers comprise a two-dimensional repeating pattern of unit cells, each unit cell having at least two qubits and at least one qubit coupler.
12. The multi-qubit processor of claim 11 , wherein each unit cell is characterized by a minimum Manhattan distance between qubits in the plurality of qubits, or couplers in the plurality of couplers, that have the same resonant frequency.
13. The multi-qubit processor of claim 9 , wherein at least one microwave driver is configured according to a driver power and driver frequency selected to minimize or cancel crosstalk between the qubits inductively coupled by the corresponding qubit coupler.