Readout in Superconducting Qubits

Readout in superconducting qubits must determine the presence or absence of a single quantum of energy on the order of 20 micro electron volts, in a time on the order of 0.1 microsecond. Measuring such a small signal in a way that can be switched on and off presents a challenge that has persisted in the field for almost thirty years. The history, principles, and current challenges of qubit readout are presented in a language bridging physics and electrical engineering. Points of focus will be the requirements on readout error and speed, fundamental physics and engineering of dispersive readout, and the challenge of scaling up the off-chip components of dispersive readout receivers.