A broadband single microwave-photon detector insensitive to the thermal noise

Abstract

Thermal noise is one of the physical obstacles that constrain the achievable detection sensitivities of various detectors. Indeed, as we showed in a recent paper (PRB 111, 024501 (2025)), the usual Josephson threshold detector (JTD) operated in an equilibrium state can be utilized to implement a weak microwave signal, just approaching (but not arriving at) its energy quantum limit, even though its physical parameters have been optimized. In this letter, we further demonstrate numerically that the phase dynamics of a current-biased Josephson junction (CBJJ) can be insensitive to the always-on thermal noise if the sweep rate of the biased current is significantly high. As a consequence, the JTD can be operated alternatively in a non-equilibrium state. Based on the statistical binary detection criterion, we demonstrate how such a non-equilibrium JTD (NEJTD) can be utilized to implement the weak microwave signal, arriving at its energy quantum limit level. The dynamic range and photon-number resolvability of the proposed NEJTD are also discussed when it serves as a broadband single microwave-photon detector.

Figures (8)

• Figure 1: Numerical simulations of the SCDs for a JTD biased by the currents with the typical sweep rates. Here, the relevant parameters are set as: $\dot{\varphi}_{0}=0$, $\beta=10^{-4}$, $v=1\times10^{-5}$, and $2\beta k_{B}T/E_{J0}=1\times10^{-7}$ which corresponds to the typical thermal noise level of a microampere-scale JJ around 50 mK, with a plasma frequency of several GHz.
• Figure 2: Numerical verification of the thermal-noise sensitivity in the SCDs with $\varphi_{0}=0.1$; (a, b) for the EJTD with $\kappa=0.2$, and (c, d) for the NEJTD with $\kappa=5$. The parameters are the same as in Fig. \ref{['F1']}.
• Figure 3: Schematic of the weak microwave signal detection scheme by using a NEJTD. From top to bottom: incident microwave signal, triangular bias current, external magnetic field for phase initialization, and real-time voltage response of the JTD.
• Figure 4: Simulated distinguishability of the SCDs with and without the continuous-wave signal input, for different initial phases. (a, b) for an EJTD, while (d,e) for a NEJTD. (c, f) show the corresponding distinguishabilities. Here, the relevant parameters are set as: $\omega_{MW}=1$, $\varphi_0=0.1$ in (a, d) and $\varphi_0=0.2$ in (b, e). The other parameters as in Fig. \ref{['F1']}.
• Figure 5: Numerical optimization of the operational parameters of a NEJTD for single-photon signal detection. (a) $\kappa$ parameter, (b) $\varphi_0$ parameter, and (c) signal intensity. Here, $\omega_{MW}=1$ and the other parameters as in Fig. \ref{['F1']}.