9.11 Quantum non- demolition measurements
A quantum non-demolition (QND) measurement is a measurement in which an apparatus extracts information only on the observable to be measured and transfers the whole back action onto the canonical conjugate observable. In other words, the observable to be measured remains unperturbed, while the canonically conjugate one is perturbed precisely to the minimal extent allowed by the uncertainty relations. In order to examine the properties of a QND measurement, we need to introduce first the concept of indirect measurement.
9.11.1 Indirect measurement
We can use the distinction between the two aspects of measurement ...