Measurement

Quantum measurement is the process of observing a qubit's state. Before measurement, a qubit can be in superposition α|0⟩ + β|1⟩. Upon measurement, the superposition collapses irreversibly to |0⟩ with probability |α|² or |1⟩ with probability |β|². This collapse is fundamental to quantum mechanics and is not a limitation of technology. Measurements are non-unitary (irreversible) and cannot be undone. Mid-circuit measurements (measuring some qubits while others continue computing) are supported on some hardware and enable quantum error correction and adaptive circuits. The eigenstate (result) of a measurement is always a classical bit. To extract a probability distribution, the circuit must be repeated many times (shots). The no-cloning theorem prevents copying a qubit to avoid measurement collapse.