Dispersive readout of a flux qubit at the single photon level

TL;DR

This paper demonstrates dispersive readout of a flux qubit at the single photon level, achieving high visibility and minimal qubit relaxation, enabling continuous quantum state monitoring.

Contribution

It introduces a dispersive readout method using a superconducting resonator and amplifier that improves visibility at low photon numbers without degrading qubit coherence.

Findings

Readout visibility increased by 4.5 times at  photons.

No reduction in qubit lifetime observed from 0.008 to 0.1 photons.

Potential for continuous qubit state monitoring.

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 $\bar{n}$ = 1.5 photons, the readout visibility is increased by a factor of 4.5 over that using a cryogenic semiconductor amplifier. As $\bar{n}$ is increased from 0.008 to 0.1, no reduction in $T_1$ is observed, potentially enabling continuous monitoring of the qubit state.