Measuring weak microwave signals via a current-biased Josephson junction: Suppressing its thermal noise via fast bias current sweeps

Abstract

It is well known that the current-biased Josephson junction can serve as a Josephson threshold detector (JTD) for the sensitive detection of weak microwave signals. In a recent work [Phys. Rev. B 111, 024501 (2025)], we showed that the achievable detection sensitivity of the usual JTD can approach only the energy limit of the detected microwave signal, due to the always-on thermal noise. Here, we numerically demonstrate that the thermal noise can be effectively suppressed by increasing the sweep rate of the biased current, driving the JTD to be operated in a nonequilibrium state (It only refers to insensitivity to the equilibrium thermal noise, rather than the realistic thermodynamic nonequilibrium state.) with the measurable switching current distribution (SCD) being insensitive to the thermal noise. Therefore, by identifying the measured SCDs of the nonequilibrium JTD with and without the microwave signal input, the weak microwave signal could be sensitively detected at its energy quantum limit. Some of the achievable performance indexes of the nonequilibrium JTD, such as the dynamic range, detection bandwidth, and the photon-number resolvability, etc., have also been numerically estimated when it serves as a wideband microwave single-photon detector.

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