Measurement

In quantum systems, a system’s quantum state is distinct from the information obtained when the system is measured. This differs fundamentally from the classical model, in which a system’s state can generally be described by the value obtained when it is measured.

For example, consider a qubit in an equal superposition of |0⟩ and |1⟩. Its quantum state is:

(1/√2)|0⟩ + (1/√2)|1⟩

When this qubit is measured in the computational basis, the measurement produces either 0 or 1, with a 50% probability of each outcome. Thus, the quantum state describes the probabilities of possible measurement outcomes, while the measurement produces a specific observed result.