In this paper, we present a quantum information-theoretic formulation of electric dipole estimation in quantum spectroscopy of two-level systems. In an explicit quantum optical model, fundamental quantum limits of dipole strength estimation stem from absorption and/or spontaneous emission pathways. For entangled probes where only one of the photons interacts with the sample, we also establish that there is no advantage due to time-energy entanglement for resonant estimation.

The PRA doi is : https://doi.org/10.1103/PhysRevA.107.062601.