Abstract

The principles of quantum mechanics allow one to measure many times a single observable of a quantum system. However, the first precise measurement, which can be regarded as preparing the system, in a well-defined eigenstate of the measured observable, introduces uncertainties in some observable of the system. This "measurement noise" may usually come back to the measured quantity, preventing one to retrieve the result initially found. In a back-action evading (BAE) measurement, the fluctuations resulting from the coupling between the system and the measurement apparatus are kept into an observable that is conjugated with the measured quantity.¹

© 1996 Optical Society of America