9 min•Beginner
Measurement
Measurement is where the quantum world meets our classical one. It forces a definite outcome, and destroys the superposition.
Learning objectives
- Explain what happens when we measure a qubit.
- Recognize that measurement is probabilistic.
The rule
Measuring in the standard basis returns 0 with probability |α|² and 1 with probability |β|². After measurement, the qubit collapses to that outcome.
Repeatability
Measuring twice in a row gives the same answer — the state has already collapsed.
Key terms
Collapse
The state change caused by measurement.
Basis
A set of reference states used for measurement.
Checkpoint
A qubit in state |1⟩ is measured. What outcome do we get?