Abstract

At the quantum level, a measurement disturbs the system being measured. The probing laser wave in an interferometer carries quantum fluctuations proportional to the square root of the laser power P, causing by the fluctuating radiation pressure a corresponding displacement noise. On the other hand, the readout uncertainty due to quantum noise scales with $1/\sqrt{p}$ and dominates at low laser powers. An optimum laser power exists that minimizes the sum of the readout noise and that due to back action. The measurement accuracy corresponding to this optimum readout precision is the standard quantum limit (SQL),¹ which limits the sensitivity with which an external force acting on the macroscopic body may be detected using coherent light. The SQL has not been observed so far.

© 1998 Optical Society of America