A phase-control gate can leave direct probabilities balanced while changing a
later interference-basis result. In this toy model the phase-shifted equal
superposition becomes definite in the y basis with probability 1 for plus y;
real phase gates depend on pulse calibration, phase drift, decoherence, readout
basis alignment, and finite-shot uncertainty.
A physical phase shift can be modeled as an exact quantum gate. 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 physical phase shift can be a gate
The phase-control model applies a checked phase gate to an equal superposition.
S∣+x⟩=∣+y⟩
Direct balance becomes a definite y-basis row
The phase gate keeps direct probabilities balanced, then the y-basis row becomes definite.
rowdirecty basisplus211minus210total11
The controlled phase changes a later basis result
In the y basis, the modeled result has probability 1 for plus y.