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Detection of 5 GHz photons using Al Josephson junctions at 0.7 K

Dmitry A. Ladeynov, A. L. Pankratov, L. S. Revin, Anna V. Gordeeva, Alexander V. Chiginev, Sergey A. Razov, Evgeni V. Il’ichev

Published Jun 27, 2025• 2 citations
Publisher

Abstract

We predict that threshold detectors based on Al Josephson junctions with critical currents below 100 nA exhibiting a phase diffusion regime can be exploited for microwave photon detection at both 17 mK and 700 mK. We demonstrate the detection of two- and one-photon energies at 5 GHz with 90% and 15% efficiency and dark count times of about 0.1 s and 0.01 s, respectively. The weak temperature dependence of the detector’s performance observed in the sub-kelvin range fully confirms its phase diffusion mode of operation. On the other hand, these results show that inevitable thermal fluctuations are not the main source of detector noise. Consequently, there is still room to optimize the detector’s performance. These results are important for axion search experiments in the range of 5–25 GHz (20–100 μeV).

DOI
10.20935/acadquant7780

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We predict that threshold detectors based on Al Josephson junctions with critical currents below 100 nA exhibiting a phase diffusion regime can be exploited for microwave photon detection at both 17 mK and 700 mK. We demonstrate the detection of two- and one-photon energies at 5 GHz with 90% and 15% efficiency and dark count times of about 0.1 s and 0.01 s, respectively. The weak temperature dependence of the detector’s performance observed in the sub-kelvin range fully confirms its phase diffusion mode of operation. On the other hand, these results show that inevitable thermal fluctuations are not the main source of detector noise. Consequently, there is still room to optimize the detector’s performance. These results are important for axion search experiments in the range of 5–25 GHz (20–100 μeV).
We predict that the threshold detectors based on Al Josephson junctions, with critical currents below 100 nA, exhibiting a phase diffusion regime, can be exploited for the microwave photon detection both at 17 mK and 700 mK. We demonstrate a detection of two- and one-photon energies at 5 GHz with 90% and 15% efficiency and dark count time of about 0.1 s and 0.01 s, respectively. The observed weak temperature dependence of the detector's performance in the sub-kelvin range fully confirms its phase-diffusion mode of operation. On the other hand, these results show that inevitable thermal fluctuations are not the main source of the detector noise. Consequently, there is still a room to optimize the detector's performance. These results are important for axion search experiments in the range of 5-25 GHz (20-100 μμμeV).
Abstract The major task of detecting axions or axion-like particles has two challenges. On the one hand, the ultimate sensitivity is required, down to the energy of a single microwave photon of the yoctojoule range. On the other hand, since the detected events are supposed to be rare, the dark count rate of the detector must be extremely low. We show that this trade-off can be approached due to the peculiar switching dynamics of an underdamped Josephson junction in the phase diffusion regime. The detection of a few photons’ energy at 10 GHz with dark count time above 10 s and the efficiency close to unity was demonstrated. Further enhancements require a detailed investigation of the junction switching dynamics.
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.
Abstract Josephson junctions, in appropriate configurations, can be excellent candidates for detection of single photons in the microwave frequency band. Such possibility has been recently addressed in the framework of galactic axion detection. Here are reported recent developments in the modelling and simulation of dynamic behaviour of a Josephson junction single microwave photon detector. For a Josephson junction to be enough sensitive, small critical currents and operating temperatures of the order of ten of mK are necessary. Thermal and quantum tunnelling out of the zero-voltage state can also mask the detection process. Axion detection would require dark count rates in the order of 0.001 Hz. It is, therefore, is of paramount importance to identify proper device fabrication parameters and junction operation point.
When measuring electromagnetic radiation of frequency f, the most sensitive detector counts single quanta of energy hf. Single photon detectors have been demonstrated from γ-rays to infrared wavelengths, with ongoing efforts to extend their range to microwaves. Here we show that an underdamped Josephson junction can detect 14 GHz thermal photons, with energy 10 yJ or 50 μeV, stochastically emitted by a microwave copper cavity at millikelvin temperatures. After characterizing the source and the detector, we vary the cavity temperature and measure the photon rate. The device achieves 45% efficiency and a dark count rate of 0.1 Hz over several GHz. Demonstrated super-Poissonian photon statistics is a signature of thermal light and a hallmark of quantum chaos. We discuss applications in dark matter axion searches and note its relevance to quantum information and fundamental physics. It is shown that 14 GHz photons, thermally emitted from a copper cavity, can be detected by a Josephson junction with 45% efficiency and 0.1 Hz dark count rate. Demonstrated super-Poissonian photon statistics is a signature of thermal light and a hallmark of quantum chaos.
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.