Measurement in quantum computing is the process of extracting classical information from a qubit, fundamentally altering its quantum state in the process. When a qubit in superposition α|0⟩ + β|1⟩ is measured in the computational basis, it collapses to |0⟩ with probability |α|² or to |1⟩ with probability |β|², and the superposition is irreversibly destroyed. This is not a limitation of measurement technology — it is a fundamental property of quantum mechanics.
Measurement is typically performed at the end of a quantum circuit to read out results, but mid-circuit measurement is increasingly important for quantum error correction, where syndrome qubits are measured repeatedly during computation to detect errors without disturbing the encoded logical information. Mid-circuit measurement followed by conditional classical operations (feed-forward) enables adaptive quantum circuits and is a key capability for fault-tolerant quantum computing.
The physical implementation of measurement varies by qubit technology. In superconducting systems, a readout resonator coupled to the qubit is probed with a microwave pulse — the resonator's response frequency differs depending on whether the qubit is in |0⟩ or |1⟩. Measurement fidelity (the probability of correctly identifying the qubit's state) is a critical metric, typically 99-99.9% for modern superconducting and trapped-ion systems. Measurement errors can be mitigated through readout error correction, where a classical confusion matrix is inverted to correct systematic misidentification.