The rate of absorption of entangled photon pairs is linear in the photon-flux density. We demonstrate that the two-photon absorption cross section is a nonmonotonic function of the entanglement time; it vanishes for certain energy-level configurations and values of the entanglement time. This entanglement-induced two-photon transparency arises from the coherent summation of transition-amplitude contributions over the finite entanglement time. As an example, the entangled two-photon cross section for the 1S-2S electronic transition in atomic hydrogen is obtained.
A new form of absorption spectroscopy is introduced in which the energy levels and matrix elements of virtual-state transitions in a medium are determined by the use of continuous-wave degenerate entangled photons without varying their wavelengths. Extractable spectroscopic information is embedded in the entangled-photon absorption cross section when measured over a range of entanglement and interbeam-delay times. This technique exploits the fundamental quantum interference that arises during the interaction of an entangled quantum state with a quantum system. [S0031-9007(98)05928-6]
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