= Initial-state dependence of Ramsey contrast
The contrast of a phase-sensitive pulse sequence depends on the state \i[during] phase accumulation. Coherent preparation can create two populated paths whose <relative quantum phase> is converted to a population by readout; preparation into one path gives no such signal. For the <Hadamard-phase-Hadamard qubit rotation>, a pure $z$-polarized input yields full contrast, a pure $x$-polarized input yields none, and an unpolarized input also yields none. The last two cases have different <purity> despite identical measured populations. <Purity> alone is therefore not a phase-sensitivity criterion.
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