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What is quantum interference?
Quantum interference is the way amplitudes add up like waves, raising the probability of some results and cancelling others to zero.
Amplitudes are not just positive numbers; they can have different signs (phases). If two paths to the same result have amplitudes of the same sign they reinforce each other (constructive interference); if the signs are opposite they cancel (destructive interference).
Every quantum algorithm relies on interference: it cancels the amplitudes of wrong answers and amplifies the right one. For example, applying Hadamard twice to a qubit returns it with certainty to its starting state through the interference of the two paths in superposition.
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- Superposition · Superposition is the quantum state in which a qubit, before it is measured, holds the states 0 and 1 together with definite amplitudes.
- Grover’s algorithm · Grover’s algorithm finds a marked item among N unsorted items in about √N steps, a quadratic speed-up over classical search.
- Hadamard gate (H) · The Hadamard gate is a single-qubit quantum gate that turns a qubit in |0⟩ into an equal superposition of 0 and 1.