Impact of Spectators on a Two-Qubit Gate in a Tunable Coupling Superconducting Circuit

Area of Science:

• Quantum Computing
• Superconducting Circuits
• Quantum Information Science

Background:

• Cross-resonance (CR) gates are crucial for fault-tolerant quantum computation using fixed-frequency qubits.
• Tunable couplers offer enhanced control for optimizing quantum gate operations.

Purpose of the Study:

• To experimentally implement and optimize an entangling CR gate in a tunable coupling superconducting circuit.
• To investigate the impact of spectator qubits on CR gate fidelity.
• To develop a method for evaluating perturbation effects in multi-qubit systems.

Main Methods:

• Microwave-only control for entangling CR gate implementation.
• Utilizing a tunable coupler for experimental flexibility.
• Employing a three-qubit Hamiltonian tomography protocol for detailed analysis.
• Systematic investigation of gate fidelity dependence on spurious qubit interactions.

Main Results:

• CR gate fidelity is reduced by spectator qubits due to ZZ interactions and frequency detunings.
• The target spectator qubit has a more significant impact than the control spectator under standard echo pulse schemes.
• Gate fidelity degradation can reach up to 22.5% with both spectators present.
• An optimal regime for CR gate operation was identified.

Conclusions:

• Spectator qubit interactions are a key limiting factor for CR gate fidelity in superconducting quantum circuits.
• The developed Hamiltonian tomography protocol provides a method to evaluate and mitigate these effects.
• The findings are applicable to optimizing other two-qubit gates in large-scale quantum computing architectures.