dispersive readout

Dispersive readout is the standard trick for asking a superconducting qubit whether it is a 0 or a 1 without poking it too hard. You don't look at the qubit directly; you attach it to a small microwave resonator, ping that resonator with a brief pulse, and listen to how the echo comes back. The qubit never absorbs the probe tone, so the measurement is gentle and the qubit usually survives the question.

It works because the qubit and resonator are tuned far apart in frequency (the dispersive regime). In this regime the qubit can't trade energy with the resonator, but it still nudges the resonator's resonant frequency by a tiny amount, +chi if the qubit is in 0 and -chi if it is in 1. So the resonator's tone comes back with a slightly different phase and amplitude depending on the qubit state. By averaging that returning signal, the control electronics sort each shot into a 0 cloud or a 1 cloud and read off the answer.

The honest catch: it is fast and repeatable, but not free. The probe pulse must be strong and long enough to tell the two clouds apart, yet a too-strong pulse can shake the qubit out of the dispersive regime and corrupt the state. Stray photons left in the resonator also dephase the qubit, so designers add filters and isolators (which are still bulky and hard to fit on-chip) and carefully budget how many photons each measurement uses.