A checked pair of stages can return a state to a basis outcome. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The second stage receives a balanced state
A balanced input to a recombination gate can be checked directly by the gate box.
H∣+⟩=∣0⟩
The paired-stage rows return to a basis state
The first row is balanced, the middle row is the checked gate action, and the final row is a basis outcome.
rowinputgatecheckedpzero2111pone2100
Paired stages can undo each other
The checked output is zero with probability 1. This is a stage self-check, not a device calibration curve.