Dispersive readout of a flux qubit at the single-photon level

Abstract

A superconducting flux qubit is inductively coupled to a superconducting quantum interference device (SQUID) magnetometer, capacitively shunted to form a 1.294-GHz resonator. The qubit-state-dependent resonator frequency is weakly probed with a microwave signal and detected with a microstrip SQUID amplifier. At a mean resonator occupation $\overline{n}=1.5$ photons, the readout visibility is increased by a factor of 4.5 over that using a cryogenic semiconductor amplifier. As $\overline{n}$ is increased from 0.008 to 0.1, no reduction in ${T}_{1}$ is observed, potentially enabling continuous monitoring of the qubit state.