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

TL;DR

This paper proposes a broadband microwave-photon detector based on a non-equilibrium Josephson threshold detector that is insensitive to thermal noise, enabling detection at the quantum limit with high dynamic range.

Contribution

It introduces a non-equilibrium Josephson threshold detector (NEJTD) that operates effectively in high thermal noise environments, improving sensitivity and broadband capabilities.

Findings

NEJTD can detect weak microwave signals at the energy quantum limit.

The detector's phase dynamics are insensitive to thermal noise at high sweep rates.

The proposed detector offers a broad dynamic range and photon-number resolution.

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.