Stabilizing Lasers for Applications in Quantum Optics

In our field of quantum optics, it is now quite usual for us to discuss squeezed radiation states with sub-shot-noise levels of fluctuation. The pioneering experiments [1] of Slusher, Hollberg, Yurke, Mertz and Valley were the first to demonstrate an unambiguous level of squeezing,and it was natural for such a demonstration to prefer to squeeze the vacuum or zero-photon state. Indeed, one of the most exciting applications of squeezing —- that of enhancing the sensitivity of gravity-wave interferometers —- will be able to make powerful use of such a well-squeezed vacuum state input into the reference port, along the lines discussed by Caves [2 J, for example. Still, in some ways it is an even more exciting prospect to have a strongly-squeezed radiation field with substantial intensity. Such a field could be applied to a single cooled, trapped, two-level ion for some amusing and perhaps fundamental contact between subdisciplines of atomic physics, radiation physics, and quantum measurement theory. It may also be the field of choice for illuminating various types of sensitive interferometers, reducing the fluctuating radiation-pressure forces which lead to perturbation of the mirrors. Such a highly-squeezed strong field would also facilitate systematic investigation of the several apparently distinct physics models which discuss the origins of noise in heterodyne detection. In the contribution by H. J. Kimble and the present author in these proceedings, we give a progress report on our experimental approach to generation of such a squeezed, high-intensity field by doubly-resonant intracavity second harmonic generation. With this background of the quantum optics ideas then, let me explain the purpose of the present article: Here we try to give an indication of the performance level of absorption spectroscopy and laser stabilization, assuming that the laser fields employed are utilized merely with shot-noise limited performance. To make the discussion here compelling and self- contained, it is useful to begin with a few tutorial remarks about the nature and behavior of servo control systems, including for example those which stabilize a laser s intensity or frequency.