Role of the spectral shape of quantum correlations in two-photon virtual-state spectroscopy

TL;DR

This paper clarifies that two-photon virtual-state spectroscopy relies on specific spectral shapes and frequency correlations of photon pairs, not solely on entanglement, to retrieve atomic or molecular energy level information.

Contribution

It demonstrates through a full quantum formalism that spectral shape and frequency correlations are essential for two-photon virtual-state spectroscopy, resolving debates about the role of entanglement.

Findings

Spectral shape and frequency correlations are crucial for information retrieval.

Entanglement alone is not sufficient for successful spectroscopy.

Specific two-photon sources are required for effective virtual-state spectroscopy.

Abstract

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 level structure of an atom or a molecule. The consideration of closely related techniques, such as multidimensional pump-probe spectroscopy [Roslyak et al, Phys. Rev. A 79, 063409, 2009], might suggest that spectroscopic information retrieved in the two-photon absorption process is the same regardless of the classical or quantum nature of the light source. Here, we solve this debate by making 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 the absorbed 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 the realization of two-photon virtual-state spectroscopy possible. This result helps clarifying the discussion whether entanglement is needed or not, and also, to specify the type of two-photon source that needs to be used in order to experimentally perform the two-photon virtual-state spectroscopy technique.