Credit: Sean Kelley/NIST A voltage is applied across the semiconductor. When a photon hits the detector, its absorbed energy kicks an electron off an atom in the semiconductor – the same photoelectric effect that generates electricity in solar panels. That loose electron is accelerated by the applied voltage and causes a sort of chain reaction in which large numbers of adjacent atoms release an “avalanche” of electrons just as a small added stress can prompt an entire mountainside of snow to collapse. That avalanche current is the output signal. Finally, the device is reset by quenching the current with a counter-voltage and restoring the initial applied voltage. Because the avalanche involves such a large number of electrons, getting the entire system back to a quiet state where it is ready to detect another photon is challenging.