Quantum coherence and interference in optics and laser spectroscopy

Theodor W. Hänsch, HERBERT WALTHER

Fabio Di Teodoro, Elizabeth F. McCormack

Diagrammatic perturbation theory and the spherical tensor formalism are used to model the signal observed in time-resolved, two-colour resonant four-wave mixing (TC-RFWM). An analysis using the Liouville formalism illustrates how TC-RFWM can be used to perform state-selective quantum beat spectroscopy in three-level systems by suitably designing three experimental features: the excitation scheme for the matter-field interaction, the time ordering of the laser pulses and the polarization of the incident laser beams. In contrast to traditional fluorescence quantum beat spectroscopy, time-resolved TC-RFWM results in a background-free, coherent signal that allows for high-resolution detection of the spectrally unresolved energy structure within a selected material state of the three-level system. The beats are shown to result from the quantum interference between different Liouville-space pathways in the laser-induced evolution of the material density operator and appear as a temporal modulation in the amplitude of the light beam emerging as the output of the mixing process. This type of control of a coherent optical field, based on the selective preparation of superposition states, is reminiscent of other recently developed optical techniques such as coherent population trapping and lasing without inversion.

R.W. Smith

Alan Corney

Abstract The paper surveys the techniques of Doppler-free spectroscopy based on the creation and detection of Hertzian coherence in atomic and molecular systems. The preparation of individual atoms in a coherent superposition of states with the use of broadband optical excitation is discussed, with particular reference to excitation by pulsed tunable dye lasers. Time-resolved observations of the fluorescent light emitted by these atoms reveal the interference phenomenon known as quantum beats and enable Landé g-factors, fine and hyperfine splittings of excited levels to be determined. An account of progress in recent experiments with probe-beam monitoring and pulse trains from mode-locked dye lasers completes this review.

Richard M. Sillitto

Xiang Zhang, Qi Lu, Yalei Zhu +14 more

Quantum interference occurs frequently in the interaction of laser radiation with materials, leading to a series of fascinating effects such as lasing without inversion, electromagnetically induced transparency, Fano resonance, etc. Such quantum interference effects are mostly enabled by single-photon resonance with transitions in the matter, regardless of how many optical frequencies are involved. Here, we report on quantum interference driven by multiple photons in the emission spectroscopy of nitrogen ions that are resonantly pumped by ultrafast infrared laser pulses. In the spectral domain, Fano resonance is observed in the emission spectrum, where a laser-assisted dynamic Stark effect creates the continuum. In the time domain, the fast-evolving emission is measured, revealing the nature of free-induction decay arising from quantum radiation and molecular cooperativity. These findings clarify the mechanism of coherent emission of nitrogen ions pumped with mid-infrared pump laser and are found to be universal. The present work opens a route to explore the important role of quantum interference during the interaction of intense laser pulses with materials near multiple photon resonance.