Photon Avalanche: How to Catch the Invisible express
In photometry (the measurement of light), catching individual microwave photons was long impossible: their energy is on par with the thermal jitter of atoms. The breakthrough came from superconductors: a single photon sets off an avalanche, like a snowball rolling downhill gathers more snow, a microwave quantum gives birth to dozens of its kind. The amplified signal is separated from the background by analyzing quantum entropy — a measure of disorder: a photon avalanche has a distinct entropy compared to random noise. The detector is cooled nearly to absolute zero to suppress thermal interference. Efficiency reaches 84.5% with minimal errors. The paradox: catching particles from the Universe's fiery past requires extreme cold. Such detectors will link quantum computers and, perhaps, allow us to register photons from the echo of the Big Bang, testing the standard model of physics.
Original abstract
as written by the authors arXiv:2510.08030
Single-photon detectors are an essential part of the toolbox of modern quantum optics for implementing quantum technologies and enabling tests of fundamental physics. The low energy of microwave photons, the natural signal path for superconducting quantum devices, makes their detection much harder than for visible light. Despite impressive progress in recent years and the proposal and realization of a number of different detector architectures, the reliable detection of a single itinerant microwave photon remains an open topic. Here, we investigate and simulate a detailed protocol for single-photon multiplication and subsequent amplification and detection. At its heart lies a Josephson-photonics device which uses inelastic Cooper-pair tunneling driven by a dc bias in combination with the energy of an incoming photon to create multiple photons, thus compensating for the low-energy problem. Our analysis provides clear design guidelines for utilizing such devices, which have previously been operated in an amplifier mode with a continuous wave input, for counting photons. Combining a formalism recently developed by M{\o}lmer to describe the full quantum state of in- and outgoing photon pulses with stochastic Schr\"odinger equations, we can describe the full multiplication and detection protocol and calculate performance parameters, such as detection probabilities and dark count rates. With optimized parameters, a high population of a single output mode can be achieved that can then be easily distinguished from vacuum noise in heterodyne measurements of quadratures with a conventional linear amplifier. Realistic devices with two multiplication stages with multiplication of $$16$$ reach for an impinging Gaussian pulse of length $$T$$ a detection probability of $$84.5\%$$ with a dark count rate of $$10^{-3}/T$$, and promise to outperform competing schemes.
The energy of a single microwave photon is roughly 100,000 times less than that of a visible-light photon, yet these are the particles that carry information about the early Universe.