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Physical Review A

Constructive and destructive features of multiphoton quantum interferences in atomic systems

Seyed M. Sadeghi, Jochen Meyer

Published Sep 1, 1998• 8 citations
Publisher

Abstract

One-photon coupling of the upper level of an atomic transition to a metastable state can generate a dark line in the atom's emission spectrum. We show in this paper that if such a coupling is replaced by a dominant two-photon coupling, the spectrum is dynamically narrowed instead. The dominant two-photon coupling is achieved here by coupling these two levels to a common broad intermediate state using two laser fields. We also show that if one exchanges the metastable state with the broad one in this system, the one-photon coupling of all coupled states becomes dominant, causing the generation of a dynamically damped dark line effect. We explain these effects in terms of constructive and destructive quantum interferences between a direct and a three-photon Ramam path in the former case, and between a direct and a two-photon Raman path in the latter case. Additionally, we show how these interfering paths provide elements of coherent control of the atomic emission spectra, in general.

DOI
10.1103/physreva.58.2534

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One-photon coupling of the upper level of an atomic transition to a metastable state can generate a dark line in the atom's emission spectrum. We show in this paper that if such a coupling is replaced by a dominant two-photon coupling, the spectrum is dynamically narrowed instead. The dominant two-photon coupling is achieved here by coupling these two levels to a common broad intermediate state using two laser fields. We also show that if one exchanges the metastable state with the broad one in this system, the one-photon coupling of all coupled states becomes dominant, causing the generation of a dynamically damped dark line effect. We explain these effects in terms of constructive and destructive quantum interferences between a direct and a three-photon Ramam path in the former case, and between a direct and a two-photon Raman path in the latter case. Additionally, we show how these interfering paths provide elements of coherent control of the atomic emission spectra, in general.
We study laser-induced transparency and dark-line effects in a two-level atomic system where the upper transition level is coupled by a laser field to a dressed system consisting of two lower-energy metastable states mixed by a second laser field (\ensuremath{\Pi} configuration). We show that the emission (absorption) spectrum of the two-level system develops two dynamic dark lines (transparency holes) with tunable central frequencies equal to the Rabi frequency of the second field with positive and negative signs, and widths proportional to that of the first field. The detailed evolution of these effects at low- and high-field intensities and the role of quantum interferences are discussed by presenting a model based on laser-induced continuum structure and adopting a dressed-state picture. We discuss various nonlinear processes in this system and show how the dynamics of emission and absorption spectra are determined by one- and two-photon coupling processes.
We study the influence of quantum interference on the spontaneous emission from an excited two-level atom when either the atomic upper or lower level is coupled by a coherent field to a third, usually higher-lying, state. In the case of the upper level coupling, destructive quantum interference between two competing decay amplitudes produces a dark line in the emission spectrum, a phenomenon that should not be confused with the well known population trapping, and a narrowing of one of the two side lobes that make up the spectral profile. Quantum interference is absent, instead, in the case of the lower level coupling, and the spectrum modified by the external driving field is just the incoherent superposition of two Lorentzian lines. We suggest a physical interpretation of these results. In addition, we compare the analytic predictions of the simplest nontrivial versions of these models with more realistic but nonanalytic descriptions and show, numerically, that the interference effects persist in the upper level case even when additional complications are taken into account.
Atomic coherence and interference manifested by electromagentically induced transparency (EIT) and coherent population trapping (CPT) plays an important role in the current studies of atom-photon interactions and has found numerous applications in optical physics. EIT is created in a three-level atomic system by a coupling field and results in destructive interference between two excitation paths of a weak probe laser interacting with the atomic medium. This leads to suppressed linear absorption and rapidly varying atomic dispersion for the probe laser near the atomic resonance, which provides the platform for a variety of applications such as nonlinear optics at low light levels, slow light manipulation, and quantum state engineering for photons and atoms. Here we extend the simple three-level EIT system to more complicated and versatile configurations in a multi-level atomic system coupled by multiple laser fields. We show that with multiple excitation paths provided by different laser fields, phase-dependent quantum interference is induced: either constructive or destructive interference can be realized by varying the relative phases among the laser fields. Two specific examples are discussed. One is a three-level system coupled by bichromatic coupling and probe fields, in which the phase dependent interference between the resonant two-photon Raman transitions can be initiated and controlled. Another is a four-level system coupled by two coupling fields and two probe fields, in which a double-EIT configuration is created by the phase-dependent interference between three-photon and one-photon excitation processes. We analyze the coherently coupled multi-level atomic system and discuss the control parameters for the onset of constructive or destructive quantum interference. We describe two experiments performed with cold Rb atoms that can be approximately treated as the coherently coupled three-level and four-level atomic systems respectively. The experimental results show the phase-dependent quantum coherence and interference in the multi-level Rb atomic system, and agree with the theoretical calculations based on the coherently coupled three-level or four-level model system.
We present a theory for two- and three-photon excitation, optical shifting, and four-wave mixing when a first laser is tuned onto, or near, a two-photon resonance and a second much more intense laser is tuned near or on resonance between the two-photon resonance and a second excited state. When the second excited state has a dipole-allowed transition back to the ground state and the concentration is sufficiently high, a destructive interference is produced between three-photon coupling of the ground state and the second excited state and one-photon coupling between the same states by the internally generated four-wave mixing field. This interference leads to several striking effects. For instance, as the onset of the interference occurs, the optical shifts in the two-photon resonance excitation line shape become smaller in copropagating geometry so that the line shapes for multiphoton ionization enhanced by the two-photon resonance eventually become unaffected by the second laser. In the same range of concentrations the four-wave mixing field evolves to a concentration-independent intensity. With counterpropagating laser beams the line shape exhibits normal optical shifts like those observed for both copropagating and counterpropagating laser beams at very low concentrations. The theoretical work presented here extends our earlier works by including the effect of laser bandwidth and by removing the restriction of having the second laser be tuned far from three-photon resonance. In this way we have now included, as a special case, the effect of both laser bandwidth and interference on laser-induced transparency. Unlike other effects related to odd-photon destructive interference, the effect of a broad bandwidth is to bring about the predicted effects at much lower concentrations. Studies in rubidium show good agreement between theory and experiment for both ionization line shapes and four-wave mixing intensity as a function of concentration.
The quantum interference in the spontaneous-emission spectrum of a driven atom, when the upper level of the atomic transition is coupled to a third level via a classical driving field, results in a dark line in the absence of the atomic decays. The existence of the dark line is independent of the Rabi frequency associated with the driving field. There is no such dark line in the spontaneous-emission spectrum when, instead of the upper level, the lower level is coupled to a third level via a driving field. In this paper, we include the level decays, incoherent pumping, and the finite bandwidth of the driving field in our analysis and study their role in the spontaneous-emission spectrum in the two configurations.
A new scheme is suggested to manipulate the probe transitions (and hence the optical properties of atomic vapors) via double-control destructive and constructive quantum interferences. The influence of phase coherence between the two control transitions on the probe transition is also studied. The most remarkable feature of the present scheme is that the optical properties (absorption, transparency and dispersion) of an atomic system can be manipulated using this double-control multi-pathway interferences (multiple routes to excitation). It is also shown that a four-level system will exhibit a two-level resonant absorption because the two control levels (driven by the two control fields) form a dark state (and hence a destructive quantum interference occurs between the two control transitions). However, the present four-level system will exhibit electromagnetically induced transparency to the probe field when the three lower levels (including the probe level and the two control levels) form a three-level dark state. The present scenario has potential applications in new devices (e.g. logic gates and sensitive optical switches) and new techniques (e.g. quantum coherent information storage).