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Measurement

Measurement gate tile from the Operations catalog

Measurement reads out a qubit's state as a classical bit - the one operation in a quantum circuit that isn't reversible.
Qubits 1 quantum register + 1 classical register
Parameters Which classical bit to store the result in
Reversible? No
Also called Measure

What it does

Measurement collapses a qubit's superposition into a definite outcome - \( 0 \) or \( 1 \) - with probability given by the squared magnitude of that basis state's amplitude, and writes the result into a classical bit. Any superposition or entanglement the qubit had is destroyed by this collapse.

Example

Measuring a qubit prepared with H gives 0 or 1 with 50/50 probability each time you run the circuit - matching the bars shown in Probabilities. Every quantum wire also has a plain end-of-wire circle showing the default readout point, whether or not you've placed an explicit measurement - see Quantum and classical registers.

Other notations

Naming conventions across ecosystems; exact syntax can vary by library version, and this does not claim to be Qompile's generated output unless otherwise noted.

Language Typical form
OpenQASM 2.0 measure q[0] -> c[0];
Qiskit circuit.measure[q[0], c[0]]
Cirq cirq.measure(q[0], key='c0')
Q# let result = M(q[0]);