Plasma and quantum information sciences: Entanglement Enhanced Laser absorption and stimulated emission

Conventional single-frequency laser absorption on ions or neutrals in a plasma involves sweeping the wavelength of a tunable laser through an absorption transition. The result is a line-integral of the velocity-resolved quantum-state population. The technique is difficult when absorption is weak and laser-intensity fluctuations are larger than the absorption signal. Here we propose an enhanced absorption spectroscopy method that employs the simultaneous generation of two photons to eliminate the effect of laser power fluctuations. The laser probe beam is created from the parametric down conversion of short wavelength laser emission. Two entangled photons are created from the down conversion, one at the target absorption wavelength, and one is sent through the plasma while the other is sent directly to a single photon detector. Coincidence is used to detect transmitted photon pairs. To measure the upper and lower velocity-resolved state densities in the same scan, 50% beam splitters and another detector are added to each path. A 3^rd identical photon due to stimulated emission from the upper quantum state detected in the plasma leg provides the upper state density and the 4^th detector in the reference leg rejects accidental double-pair generation events from the down-conversion.