ARE SQUEEZED STATES NECESSARY - A CASE-STUDY OF PHOTON DETECTION BASED ON QUANTUM INTERFERENCE

Squeezed states are the particular quantum states of light with the very attractive property of having quantum fluctuations of one of the quadratures reduced below the vacuum limit. The present paper suggests an alternative view on the phenomenon of reduction of quantum noise by showing that an entirely equivalent effect can be achieved by using the appropriate photon detection. For this purpose the specific scheme of a new photon counter that employs the mechanism of quantum interference is introduced. The detection of signal photons in such a device takes place by the coherent combination of the processes of photon absorption and stimulated emission. We start our analysis with a brief review of the properties of squeezed states, putting the emphasis on the notion of reduction of quantum noise. Then. the standard theory of photon counting is reconsidered and revised from the new perspective, and the model of the new detecting device is formulated and thoroughly analyzed. It is proven, using the theory of laser-induced continuum structures, that the photon counting statistics obtained by using this device for detection of ordinary coherent states is strictly equivalent to that of standard detection of the appropriate squeezed states of light. The implicit degree of effective squeezing, moreover, can be manipulated by the appropriate tuning of the detector. The performance characteristics of the new detector in various circumstances (different detector parameters and various measured signals) are investigated in detail. Also, the various sources of incoherence that may degrade the operation of the detector (spontaneous emission, thermal noise) are carefully examined. Throughout, the question of feasibility of practical realization of this thought experiment is constantly addressed, and it is argued that, albeit not easy, it is in principle possible.