Interference
Interference means amplitudes add before they are squared into probabilities, so same-sign contributions reinforce and opposite-sign contributions cancel. In H–Z–H the two contributions to outcome 0 are +0.5 and −0.5 and cancel exactly: a phase-only change becomes a deterministic 1.
The rule that makes quantum computation different from probabilistic computation is one sentence long: to get an outcome's probability, first add every amplitude contribution flowing into that outcome, then square the total.
Classical probabilities can only accumulate: mixing two routes that each reach an outcome with probability 0.25 gives 0.25 + 0.25 = 0.5. Amplitudes carry signs, so their sum can be smaller than either term. Contributions +0.5 and +0.5 add to 1.0, and 1.0² = 1 — certainty. Contributions +0.5 and −0.5 add to 0, and 0² = 0 — the outcome is eliminated. The first case is called constructive interference, the second destructive interference.
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