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Glossary

Quantum glossary

Measurement

Also: collapse, readout

Reading out a qubit, which returns a classical bit sampled by the Born rule and leaves the qubit in the state matching it.

A measurement in the computational basis only asks “0 or 1?”, and in answering it discards the phase information. Measuring in a different basis means rotating first — which is why the same state can look random in one basis and certain in another.

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Related terms

  • Born rule — The rule that the probability of a measurement outcome is the squared magnitude of its amplitude.
  • Superposition — A quantum state that is a weighted combination of basis states, with complex amplitudes.