Quantum information processing with superconducting circuits: Purcell effect and the measurement problem

With rapid advances in fabrication, measurement, and control, superconducting circuits have become one of the leading architectures for quantum information processing. Yet, scaling such systems up towards practical applications poses remarkable challenges. At the hardware level, a strict requirement is to have fast, high-fidelity gate operations and state measurement. For theorists, this requires developing accurate models describing the complex dynamics of superconducting circuits under drives. In this talk we pedagogically introduce this experimental platform, and discuss a ubiquitously used protocol for qubit measurement, which is closely related to the well-known radiative Purcell decay of an atom. We then focus on the so-called “measurement problem”: experiments show that qubit relaxation rates can be strongly dependent on the power of the measurement drives, which hinders efforts to improve readout speed and fidelity.  We explain this by finding the leading nonlinear corrections to the Purcell decay, and identify stimulated emission as the principal mechanism at play.  Time permitting, we will present some applications of the same formalism to the study of two-qubit gates, correlated noise and crosstalk in multi-qubit systems.