R. de J. León-Montiel, J. Svozilík, L. J. Salazar-Serrano, J. P. Torres
It is controversial what is the true role of entanglement in two-photon virtual-state spectroscopy [Saleh et al., Phys. Rev. Lett. 80, 3483, (1998)], a two-photon absorption spectroscopic technique that can retrieve information about the energy levels of an atom or a molecule. The consideration of closely related techniques, such as multidimensional pump-probe spectroscopy [Rolyak et al., Phys. Rev. A 79, 063409, (2009)], might suggest that the spectroscopic information obtained in the two-photon absorption process is the same regardless of the classical or quantum nature of the light source. Here, we make use of a full quantum formalism to show that the ability to obtain information about the energy level structure of a medium requires the existence of temporal (frequency) correlations between photons. Moreover, we show that these correlations are not the only requisite for retrieving such information. In fact, it is a combination of both, the presence of frequency correlations and its specific spectral shape, which makes possible the realization of two-photon virtual-state spectroscopy.
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http://arxiv.org/abs/1209.5620
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