Introduction.
Quantum noise reduction in optical systems and the determination of quasi-distribution functions, in particular Wigner functions, to describe quantum optical states, have been central issues in quantum optics for a number of years. The generation of sources of squeezed light has opened very exciting possibilities of ultrahigh precision measurements beyond the standard quantum limit, by either interferometric or spectroscopic means. In the present issue, the paper by Mundarain and Orszag discusses the interferometric detection of ultrasmall signals, in particular gravitational waves, when one injects a squeezed state in the unused port of the interferometer. The authors find that the sensitivity of the system is very strongly affected by the quantum efficiency of the non-ideal photodetectors, when the squeezed signal is injected, as opposed to the ordinary vacuum case where the effects are rather small. This imposes strong conditions on the quality of those photodetectors. The standard quantum limit and the corresponding minimum detectable gravitational amplitude is also discussed in general, for short time measurements.
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