Electrons spin detection using a superconducting flux qubit with a circuit-QED architecture

A superconducting flux qubit utilizes a magnetic flux as a tuning knob to adjust its resonance frequency. This implies that a flux qubit can be a sensitive detector of a magnetic field. The technique has been applied to the detection of magnetization from electrons in solid state material and electron spin resonance (ESR) spectroscopy. In our previous studies, the readout of the flux qubit is performed using a superconducting quantum interference device (SQUID). In this case, the sensitivity is limited by 1/f type magnetic noise. To reduce the immunity to the magnetic noise, we implement the flux qubit with a circuit quantum electrodynamics (QED) architecture and evaluate the sensitivity. Since the readout method in a circuit QED architecture only uses a linear resonator, the system instability from the magnetic noise can be reduced, which enables signal integration for longer time. Device parameters of the current device and measurement system show the electron spin sensitivity of 65 spins/√Hz without affected by 1/f type magnetic noise. The sensitively can be improved further by introducing Josephson parametric amplifier (JPA) to the measurement system and by optimizing device parameters. Detailed analysis of the parameters of the current device shows that the sensitivity can be improved to the level of single spin detection.