Calibration of a Cross-Resonance Two-Qubit Gate between Directly Coupled Transmons

Abstract

Quantum computation requires the precise control of the evolution of a quantum system, typically through application of discrete quantum-logic gates on a set of qubits. Here, we use the cross-resonance interaction to implement a gate between two superconducting transmon qubits with a direct static dispersive coupling. We demonstrate a practical calibration procedure for the optimization of the gate, combining continuous and repeated-gate Hamiltonian tomography with stepwise reduction of dominant two-qubit coherent errors through mapping to microwave control parameters. We show experimentally that this procedure can enable a ZX^-π/2 gate with a fidelity F=97.0(7)%, measured with interleaved randomized benchmarking. We show this in an architecture with out-of-plane control and readout that is readily extensible to larger-scale quantum circuits.