An RF SQUID readout for a flux qubit-based microwave single photon counter

Abstract

An analysis of the measurement of the magnetic flux in superconducting qubits based on RF SQUIDs was carried out with a 800 MHz bandwidth low power consumption cryogenic high-electron-mobility transistor (HEMT) amplifier. The preliminary experimental results obtained at temperatures 2 K and 4 K for RF SQUIDs in hysteretic and in two non-hysteretic regimes with a pump frequency of about $30$~MHz are discussed. Parameters of RF SQUIDs in the hysteretic and non-hysteretic modes are analyzed within the framework of the Resistively and Capacitively Shunted Junction (RCSJ) model for Josephson junctions. Its sensitivity at the temperature of 30 mK and frequency band (speed) are calculated and optimized to read the states of a flux qubit used as single microwave photon counter. It is shown that an RF SQUID, operated in an adiabatic non-hysteretic mode for qubit readout, allows to minimize its back-action effect and the dark count rate. This is owing to the absence of Josephson generation, the small amplitude of the resonator electromagnetic field, and the selection of the pump frequency that does not coincide with the characteristic frequencies of the flux qubit.

Control of the effective value of the critical current of the RF SQUID by the high-frequency electromagnetic field

“…Results from numerical simulations 6 indicate that weak HF fields will increase the slope of the "steps" in the hysteretic regime and lead to a decrease in sensitivity. Essentially, this is associated with an expansion of the probability density of phase jumps in the interferometer, 3 resulting in additional losses. The effect of strong HF electromagnetic fields can be relatively easily revealed experimentally.…”