THE OBSTACLE IS THE SIGNAL
A superconducting nanowire single-photon detector (SNSPD) operates on the edge of a cliff. A massive bias current flows through a microscopic meander of niobium nitride, pushed right to the absolute limit of what the superconducting state can carry without breaking. It is a silent, frictionless river.
When a single photon strikes the wire, it does not add energy to the stream. Instead, it breaks a few Cooper pairs, nucleating a microscopic hotspot of normal, resistive metal.
The current, encountering this sudden wall of resistance, violently shunts sideways out of the wire and into a parallel 50-ohm readout amplifier. This diversion creates a massive, sub-nanosecond voltage spike. The photon did not create the current; it merely blocked the path. The signal we read is not the presence of the photon, but the temporary displacement of the river. The obstacle is the signal.
