Time-delayed coincidence technique for subnatural-width spectroscopy as an interference phenomenon

Abstract

© 2019 American Physical Society. A single photon, emitted in a transition between two states, has a frequency distribution of intensity that is given by a Lorentzian if the transition is only naturally broadened and the period of observation T is long compared to the lifetime T1 of the excited state. However, when the observation time T is short or comparable to T1, the frequency spectrum is appreciably broadened. If only the delayed part of the emitted radiation field is detected, then the radiation spectrum does not change. However, if the radiation field is transmitted through a resonant absorber and then detected, the transmission spectrum of the delayed radiation field is narrowed. We show that this narrowing is due to the interference of the spectral components of the incident and coherently scattered fields. Experimental spectra of absorption of Mössbauer radiation, obtained by the coincidence technique, confirm this conclusion.