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.
A consequence of the deferred measurement principle is that measuring commutes with conditioning.
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.
Thanks to the deferred measurement principle, measurements in a quantum circuit can often be shifted around so they happen at better times.
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.
