We present a proof-of-concept experiment aimed at increasing the sensitivity of temperature sensors implemented with Fiber Bragg gratings by making use of a weak value amplification scheme. The technique requires only linear optics elements for its i
mplementation, and appears as a promising method for extending the range of temperatures changes detectable to increasingly lower values than state-of the-art sensors can currently provide. The device implemented here is able to generate a shift of the centroid of the spectrum of a pulse of $mathrm{sim 0.035,nm/^{circ}C}$, a nearly fourfold increase in sensitivity over the same Fiber Bragg Grating system interrogated using standard methods.
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 st
ructure 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.
