Four-Photon Mixing in Ghost Fiber Spectroscopy

We report an experimental implementation of linear absorption spectroscopy using a fiber source of frequency-correlated photon pairs. The object under study is illuminated by a signal beam of a photon pair entangled in frequency. Due to the tight correlation of entangled photons, the absorption spectrum of the object can be measured from the idler beam, counting coincidences in both channels. The main advantages of ghost optics are analyzed, including significant noise reduction due to detecting only paired coincidences and cutting off all single background photons, as well as the potential sensitivity of studying samples, which is especially important when working with biological objects.