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Reviews of Modern Physics

Quantum interference effects in two-photon spectroscopy

M. M. Salour

Published Jul 1, 1978• 90 citations
Publisher

Abstract

Interaction of atoms with two intense coherent time-delayed short pulsed standing waves produces quantum interference effects in the atomic fluorescence. This type of quantum interference effect applied to Doppler-free two-photon spectroscopy (an extension of the Ramsey method of separated r.f. or microwave fields in atomic beam experiments) offers a number of important improvements in the presently available techniques of ultra-high-resolution spectroscopy of atoms, molecules, and crystals.

DOI
10.1103/revmodphys.50.667

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Dec 18, 2008•0 citations
Interaction of atoms with two intense coherent time-delayed short pulsed standing waves produces quantum interference effects in the atomic fluorescence. This type of quantum interference effect applied to Doppler-free two-photon spectroscopy (an extension of the Ramsey method of separated r.f. or microwave fields in atomic beam experiments) offers a number of important improvements in the presently available techniques of ultra-high-resolution spectroscopy of atoms, molecules, and crystals.
Using two coherent time-delayed short pulses, we have demonstrated that one can obtain, in the profile of Doppler-free two-photon resonances, interference fringes with a splitting 12T\frac{1}{2T}2T1​ (TTT, delay time between the two pulses) much smaller than the spectral width $\frac{1}{\ensuremath{\tau}}$ of each pulse ($\ensuremath{\tau}$ duration of each pulse). This method should lead to a number of important improvements in the presently available techniques of ultrahigh-resolution laser spectroscopy of atoms and molecules.
We have studied Doppler-free two-photon excitation of atoms with a train of phase-coherent standing-wave light pulses, originating from the same laser pulse. Quantum interference effects produce narrow spectral fringes which have a physical origin similar to that of Ramsey fringes. Linewidths much less than the Fourier-transform limit of an individual light pulse have been observed for the sodium $3S\ensuremath{-}5S$ transition; and a dramatic enhancement of the resonant signal is possible.
For the two-photon ac Stark effect in Raman-Ramsey configuration, such as the Raman clock, and pulsed CPT clock configuration, an interference phenomenon of two-photon ac Stark effect is predicted. Here we report this new physical phenomenon, the interference of two-photon ac Stark effect in two-zone Raman-Ramsey interactions, which is general in any Raman-Ramsey spectroscopy. In particular, for an atomic beam Raman clock configuration, the ac Stark shift will present an interference fringe caused by the separated oscillatory-field interaction when the shift fields interact with atoms at two zones. On the center of dispersion shape of normal ac-Stark shift, an interference pattern appeared when the scale of frequency detuning is expanded.
Two types of interference were observed using two-photon spontaneous parametric radiation from two nonlinear interaction regions. Two experimental setups analogous to the Young and Mach-Zehnder interferometers were used. An interesting feature of the two-photon Young interference is the opposite conditions for its observation by two different methods: by measuring intensity of light at a single frequency and by measuring correlation of intensities at two conjugated frequencies (method of coincidences). Two-photon Mach-Zehnder interference resembles the Ramsey method of separated fields, which is used in beam spectroscopy. A simple macroscopic quantum model agrees well with the experimental results and enables their interpretation in terms of ``biphotons'' carrying information about the pump phase.
The possibility of realization of collective resonance fluorescence of extended systems of atoms in the strong travel and standing laser waves it is demonstrated. The influence of regular arrangement of the atoms in interaction with the laser pulls on the fluorescence intensity is analyzed. Taking in to account that such interference can be realize in two distant atomic ensembles we analyzed the collective interference phenomena between the fluorescent fields in the farther field detection region. These effects depend on the exchange integrals between systems of atoms through vacuum fluorescence field. The fluorescence spectrum of these atoms is found. About similar coherence interference experiment of spontaneous emission of light from two distant solid-state ensembles of atoms that are coherently excited by a short laser pulse it is reported in the papers [1,2]. For example, in paper [1] it is observed the entanglement between two distant atomic ensembles located in distinct apparatuses on different tables. Quantum interference in the detection of a photon emitted by one of the samples projects the otherwise independent ensembles into an entangled state with one joint excitation stored remotely in 105 atoms at each site. The experiment [2] relate about the ensembles of erbium ions doped into two LiNbO3 crystals with channel wave guides, which are placed in the two arms of a Mach-Zehnder interferometer. The light that is spontaneously emitted after the excitation pulse shows first-order interference. By a strong collective enhancement of the emission, the atoms behave as ideal two-level quantum systems and no which-path information is left in the atomic ensembles after emission of a photon. Influence of geometry of the sample on the coherent phenomena between two distant systems of atoms in this report is presented.