Photon Avalanche: How to Catch the Invisible express
Problem: Detecting single itinerant microwave photons remains an open challenge due to their extremely low energy. Method: A photon multiplication protocol followed by amplification and detection is proposed and numerically studied. The key element is a Josephson photon multiplier that uses inelastic tunneling of Cooper pairs under constant bias and the energy of the incoming photon to generate multiple secondary photons. The full protocol is modeled using the Mølmer formalism for quantum state of pulses and stochastic Schrödinger equations. Detection probability and dark count rate are calculated. Optimization yields high occupancy of one output mode, allowing robust separation of signal from vacuum noise by heterodyne measurement of quadratures with a standard linear amplifier. For a realistic two-stage scheme with ×16 multiplication, an incoming Gaussian pulse of duration T is detected with probability 84.5% at a false-alarm rate of 10^(-3)/T, outperforming alternative approaches.
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.
🎯 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.