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arXiv:2502.14300 · Applied Physics Letters

Experimental demonstrations of Josephson threshold detectors for broadband microwave photons detection

J. X. He, P. H. Ouyang, Y. Q. Chai, Hong Chang, Qing He, L. F. Wei

Published May 19, 2025• 5 citations

Abstract

In this Letter, we experimentally demonstrated that Josephson threshold detectors (JTDs) could be developed to implement the sensitive detection of broadband microwave photons. Beginning with the numerical simulations of the damped dynamics for the macroscopic phase particle generated by a Josephson junction (JJ), we showed that the time-domain threshold voltage state switched measurements of the JJ driven by a low-frequency triangular wave current, which could be utilized to implement the sensitive microwave photons detection. The detectable signal power can be determined by using the Kumar–Caroll index to quantify the distinguishability between two statistical distributions on the measured threshold switched durations of the JJ with and without the signal input. Experimentally, we fabricated the Al/AlOx/Al JJ and measured its threshold responses, at 50 mK temperature, to the input microwave photons at the frequency of ∼5 GHz. The experimental results indicated that the weak microwave signals, which are phenomenologically threatened by the additional thermal noise that modifies the damped phase dynamics of the JJ, can be detected by the fabricated JTD. The detectable power of the microwave signals demonstrated here was calibrated as about −92 dBm (corresponding to the photons rate of ∼2×108 photons/ms). Based on the numerical simulations for optimizing the relevant performance indicators of the detectors, we further argue that the JTDs could be utilized to achieve the sensitive detection of broadband microwave photons at the single-photon level.

DOI: 10.1063/5.0259463

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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.

It is well known that the current-biased Josephson junction (CBJJ) can serve as a Josephson threshold detector (JTD) for the sensitive detection of weak microwave signals. Based on the recent work (PRB {\bf 111}, 024501 (2025)) on the detection sensitive limit of the usual equilibrium JTD, here we numerically demonstrate that a non-equilibrium JTD can be alternatively utilized to implement the higher sensitive detection of a weak microwave signal, arriving at its energy quantum limit. In the presence of thermal noise, we numerically simulate the phase dynamics for the CBJJ in the JTD with the different sweep rates of the biased currents, and find that the SCDs of the JTD with and without the microwave signal input show different behaviors. It is demonstrated that, depending on how high the sweep rate of the biased current being applied, the JTD can be operated in either the equilibrium- or the non-equilibrium state. Specifically, under the rapidly non-adiabatic driving, the SCDs of the JTD are obviously insensitive to the thermal noises, which means that the non-equilibrium JTD can possess a higher achievable detection sensitivity, compared with its equilibrium state counterpart. Consequently, the non-equilibrium JTD can be utilized to implement the desired single microwave-photon detection. Also, some of the achievable performance indexes, such as the dynamic range, detection bandwidth, and the photon-number resolvability, etc., of the non-equilibrium JTD have been estimated, when it serves as a wideband microwave single-photon detector.

Single-photon detection is an energy quantum limit detection (EQLD) of extremely weak electromagnetic waves. Given that the sensitivity of the conventional electromagnetic induction microwave receiver working at room temperature is very limited, as a result of the significantly strong thermal noise, here we analyze the possibility of approaching the EQLD of the weak microwave signal by using a current-biased Josephson junction (CBJJ) detector. By numerically simulating the dynamics for the phase particle of the CBJJ, we propose an approach to describe the discriminability between the probabilistically escaped events of the phase particle with and without the microwave current driving, by measuring the minimum

{d}_{\mathrm{KC}}

index. Our predictions indicate that the experimentally demonstrated CBJJ detector possesses the ability to resolve about a dozen photons. Furthermore, with the minimum detectable signal strength and the saturation threshold, we discuss how to determine the dynamic range of the proposed CBJJ detector. The feasibility of using the proposed CBJJ detector to achieve the EQLD in a microwave band is also discussed.

An aluminium Josephson junction (JJ), with a critical current suppressed by a factor of three compared with the maximal value calculated from the gap, is experimentally investigated for application as a threshold detector for microwave photons. We present the preliminary results of measurements of the lifetime of the superconducting state and the probability of switching by a 9 GHz external signal. We found an anomalously large lifetime, not described by the Kramers’ theory for the escape time over a barrier under the influence of fluctuations. We explain it by the phase diffusion regime, which is evident from the temperature dependence of the switching current histograms. Therefore, phase diffusion allows for a significant improvement of the noise immunity of a device, radically decreasing the dark count rate, but it will also decrease the single-photon sensitivity of the considered threshold detector. Quantization of the switching probability tilt as a function of the signal attenuation for various bias currents through the JJ is observed, which resembles the differentiation between N and N + 1 photon absorption.

The possibility of the implementation of a single-photon detector in the microwave range has been discussed. It has been shown that, for these purposes, it is possible to use an unshunted Josephson junction, which is switched from the superconducting state into a finite-voltage state at the presence of an external signal. The sensitivity of this detector is determined by the distribution of switchings in the absence of an external signal. It has been demonstrated that there is a Josephson junction with a noise temperature below 60 mK at a nominal external temperature of 10 mK. A specific detector intended for the measurement of external microwave signals has been proposed, designed, and fabricated. The first experimental results have been presented.

We propose a scheme for the detection of microwave-induced photons through a current-biased Josephson junction, from the point of view of the statistical decision theory. Our analysis is based on the numerical study of the zero-voltage lifetime distribution in response to a periodic train of pulses, that mimics the absorption of photons. The statistical properties of the detection are retrieved comparing the thermally induced transitions with the distribution of the switchings to the finite voltage state due to the joint action of thermal noise and of the incident pulses. The capability to discriminate the photon arrival can be quantified through the Kumar-Caroll index, which is a good indicator of the signal-to-noise ratio. The index can be exploited to identify the system parameters best suited for the detection of weak microwave photons.