Deferred measurement principle

The deferred measurement principle is a result in quantum computing which states that delaying measurements until the end of a quantum computation doesn't affect the probability distribution of outcomes.

The choice of whether to measure a qubit before, after, or during an operation conditioned on that qubit will have no observable effect on a circuit's final expected results.

For example, measuring qubits as early as possible can reduce the maximum number of simultaneously stored qubits; potentially enabling an algorithm to be run on a smaller quantum computer or to be simulated more efficiently.

Alternatively, deferring all measurements until the end of circuits allows them to be analyzed using only pure states.

This quantum mechanics-related article is a stub.

Two equivalent quantum logic circuits . One where measurement happens first, and one where an operation conditioned on the to-be-measured qubit happens first.