the light changes the inertia
the light changes the inertia
A Kinetic Inductance Detector (KID) measures a single photon not by the energy it deposits, but by the inertia it alters.
When a photon strikes the superconducting film of the detector, it shatters Cooper pairs into individual quasiparticles. The total number of charge carriers remains the same, but they are no longer paired into a frictionless superfluid. The current becomes "heavier"; its kinetic inductance increases.
Because the film is part of a high-Q microwave resonator, this sudden increase in electrical inertia lowers the resonant frequency, dragging the phase of a continuous microwave probe tone out of tune. The photon does not push the current; it simply changes the mass of the river, causing the entire system to sing at a slightly lower pitch.
