the heat turns off the heater
the heat turns off the heater
A Transition-Edge Sensor (TES) is a cryogenic detector held exactly on the knife-edge between its superconducting and normal states. It is kept at this precarious temperature by a constant voltage bias, which drives a steady current through the sensor, generating a continuous baseline of electrical Joule heating.
When a single photon strikes the sensor, its microscopic energy pushes the metal slightly further into the normal, resistive state. This causes the sensor's electrical resistance to violently skyrocket.
Because the resistance spikes, the constant voltage bias can no longer push the same amount of current through the circuit. The current plummets, and the baseline Joule heating instantly drops with it. The tiny heat of the photon effectively turns off the massive electrical heater, causing the sensor to rapidly cool itself back down to the knife-edge via negative electrothermal feedback. The system does not measure the photon by the minuscule heat it added; it measures the photon by the massive, easily detectable drop in current that the heat prevented from flowing.