Weak measurement

In quantum mechanics (and computation & information), weak measurement is a type of quantum measurement that results in an observer obtaining very little information about the system on average, but also disturbs the state very little. From Busch's theorem any quantum system is necessarily disturbed by measurement, but the amount of disturbance is described by a parameter called the measurement strength.

Weak measurement is a subset of the more general form of quantum measurement described by operators known as POVMs, where the strength of measurement is low. In the literature weak measurements are also known as unsharp, fuzzy, dull, noisy, approximate, and gentle measurements. Additionally weak measurements are often confused with the distinct but related concept of the weak value.

The most common methods of weak measurement are by coupling the quantum system to an ancilla qubit and projectively measuring the ancilla (which results in a weak measurement on the quantum system of interest), measuring a small part of large entangled systems, and for atomic physics, phase contrast imaging.