Abstract
We find that stimulated Raman excitation of an atom by a two-photon pulse can be enhanced by orders of magnitude if the photons are simultaneously frequency correlated and spatially anticorrelated. That is, a correlated photon pair must have an inherent time delay between its constituent photons. This counterintuitive feature is a manifestation of the uncertainty principle, which yields that frequency-correlated photons cannot be time (spatially) correlated. This is opposite to two-photon absorption by a three-level atom, for which the enhancement occurs if photons in the pulse are frequency anticorrelated and spatially correlated, that is, photons in the pair simultaneously interact with the atom. Our findings can be useful for imaging and spectroscopy of biological samples which demand low illumination intensity.
Highlights
Interaction of nonclassical radiation fields with atoms is a subject of long-standing interest
The use of nonclassical states of light can open the way to realize a low-intensity microscopy at intensity levels not achievable with classical sources and to achieve the same fluorescence signal at much lower excitation light intensity [10]. This reduction of the probing light intensity is a critically important advantage for sensing [11] and biological applications since high light intensity could result in damage of sensitive chemical and biological samples
It has been shown that efficiencies of the two-photon absorption [12,13,14,15,16,17,18] and upconversion of light [19,20] can be enhanced by using quantum correlated photon pairs
Summary
Interaction of nonclassical radiation fields with atoms is a subject of long-standing interest. We assume that the atom is being excited by a two-photon pulse described by the field state vector Equation (17) shows that in the limit γ− γ+ and | 2 − 1|γ− 1, the atom’s excitation probability for the ladder scheme is by a factor of 2γ−/γ+ 1 greater than that for a separable two-photon state [cf Eq (14)].
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