Flux crosstalk as an optimization problem*

Flux crosstalk is an important control issue for scaling up quantum computers based on superconducting qubits. The number of measurements required to calibrate crosstalk usually scales at least quadratically with the number of control channels, and becomes worse when the circuit elements interact strongly. Here we propose a new method for calibrating flux crosstalk for superconducting circuits. Using the fundamental property that superconducting circuits respond periodically to external fluxes, crosstalk calibration of N flux channels can be treated as N independent optimization problems, with the optimization metric being the periodicity of a measured signal. We show experimental results on a small-scale quantum annealer, which indicates that the optimization problem is convex given reasonable initial estimates of the crosstalk, and can thus be efficiently solved.