A detuning window test accepts a candidate mode when its frequency square falls within a fixed tolerance of the target. The candidate frequency square 1/4 is a distance 3/4 from the target 1, which is within tolerance 1, so the accepted bit is 1 exactly for the stated ideal frequency-square model; real devices have calibration drift, linewidth, noise, fabrication spread, loss, thermal photons, crosstalk, and readout uncertainty.
A detuning decision is an exact window test over checked mode squares. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
A nearby candidate stays inside the window
The target frequency square is 1 and the candidate is 1/4 per square second.
1−41=43
Inside the tolerance gives accepted bit one
The distance is 3/4, which is within tolerance 1, so the accepted bit is 1.