The Loop Answers for the Body
The Loop Answers for the Body
Seed: SQUID (Superconducting Quantum Interference Device).
Central fact: A SQUID does not measure magnetic flux directly; it converts flux into a periodic voltage by exploiting the fact that Cooper-pair phase around a superconducting loop can only accumulate in quantized units, making interference across one or two Josephson junctions the actual sensing mechanism.
A magnetometer could try to measure a magnetic field directly, the way a ruler measures a length. A SQUID refuses the direct approach. It builds a superconducting loop, interrupted by one or two thin insulating breaks, and lets the phase of the Cooper pairs circling that loop answer instead.
The loop cannot hold just any flux — only whole multiples of a single quantum, the smallest unit the universe permits. A magnetic field trying to thread the loop is rounded, forced, quantized, and the interference between the two paths around the break turns that quantization into a periodic voltage the loop can be made to speak in. The sensor never touches the field. It only listens to what the field forces its own geometry to become.