Beating Quantum Limits in Optical Spectroscopy

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In Chaps. 6 and 7 we have discussed the effect of nonclassical squeezed light on optical spectra. A variety of classic and standard problems in optical spectroscopy have been re-examined with squeezed light included in the formulations. We have seen how the introduction of squeezed light led to many unusual effects in optical spectroscopy. Examples include a reduction in the linewidth of the fluorescence spectra, population inversion and the decay to a pure state. We now turn on the subject of precision optical spectroscopy, which deals with the fundamental laws of physics imposing limits to the precision in measurements and interferometry. Consequently, this chapter begins with a discussion of the concepts of the fundamental limits in physics. The limits, called standard limits to the precision of measurements, determine how precisely a physical quantity can be measured. Three apparently distinct limits are known: The standard quantum limit and the Heisenberg limit, both imposed by quantum fluctuations of light, and the diffraction limit imposed by the wave nature of light. All detection systems are subject to these limits. After discussing the basic concepts of the standard limits, a study is made of some techniques, called quantum strategies, that have been developed to beat the diffraction and the standard quantum limits. We shall illustrate how one can beat the limits using nonclassical squeezed and entangled light. We shall see that the ability to produce squeezed light and entangled (correlated) light beams is leading us into a remarkably new domain of quantum physics in which detectors can resolve two closely spaced points or spectral lines with the minimal resolvable limit significantly reduced or even completely suppressed. This realm of physics is now known as quantum image spectroscopy or precision optical spectroscopy. Thereafter, we shall examine how one can improve the signal-to-noise ratio with a quantum squeezed field, and the spectral resolution with entangled light. Following this development, we describe several experiments that demonstrated the improvement of the spectral resolution with entangled light beams.