Quantum enhanced spectroscopy with entangled multi-photon states
  Traditionally, spectroscopy is performed by examining the position of absorption lines. However, at frequencies near the transition frequency, additional information can be obtained from the phase shift. In this work we consider the information about the transition frequency obtained from both the absorption and the phase shift, as quantified by the Fisher information in an interferometric measurement. We examine the use of multiple single-photon states, NOON states, and numerically optimized states that are entangled and have multiple photons. We find the optimized states that improve over the standard quantum limit set by independent single photons for some atom number densities. 
Hossein T. Dinani, Manish K. Gupta, Jonathan P. Dowling, Dominic W.   Berry
quant-ph