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arXiv:0804.4627 · Physical Review A

Quantitative study of the destructive quantum-interference effect on coherent population trapping

Evelina Breschi, Georgy A. Kazakov, Roland Lammegger, G. Mileti, Boris G. Matisov, Laurentius Windholz

Published Jun 25, 2009• 26 citations
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Abstract

We investigate experimentally and theoretically the coherent population trapping (CPT) effect occurring in $^{87}\text{R}\text{b}$ ${D}_{1}$ line due to the interaction with linearly polarized laser light ($\text{lin}\ensuremath{\parallel}\text{lin}$ CPT). In this configuration, the coherence is strongly influenced by the structure of the excited state; consequently, the quantum interference between dark states is an essential feature of this interaction scheme. We study the $\text{lin}\ensuremath{\parallel}\text{lin}$ CPT resonance as a function of the laser optical detuning. The comparison between experimental and theoretical results allows us to quantify the contribution from different dark states to the total signal. Based on these results, we investigate the signal depending on both the pressure broadening of the optical transition and the laser linewidth, and we find in which conditions the laser linewidth does not degrade the $\text{lin}\ensuremath{\parallel}\text{lin}$ CPT resonance.

DOI
10.1103/physreva.79.063837

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The dependence of the width of the coherent population trapping (CPT) resonance on the atomic interaction time is studied theoretically. The optical Bloch equations are solved for the Hanle/CPT schemes corresponding to the open ${F}_{g}\ensuremath{\rightarrow}{F}_{e}$ transitions of the sodium D1{D}_{1}D1​ line interacting with linearly polarized laser light. The mechanism producing the narrowing of the Hanle/CPT resonance is examined, the role of the population losses is pointed out, and the law describing the narrowing of the width of the resonance with the interaction time is determined. Our results are compared with the Hanle/CPT linewidth dependence on the interaction time occurring for a closed optical transition.
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We present a joint theoretical and experimental characterization of the coherent population trapping (CPT) resonance excited on the D1{D}_{1}D1​ line of 87Rb^{87}\mathrm{Rb}87Rb atoms by bichromatic linearly polarized laser light. We observe high-contrast transmission resonances (up to $\ensuremath{\approx}25%$), which makes this excitation scheme promising for miniature all-optical atomic clock applications. We also demonstrate cancellation of the first-order light shift by proper choice of the frequencies and relative intensities of the two laser-field components. Our theoretical predictions are in good agreement with the experimental results.
We calculated the coherent population trapping (CPT) for the open D1D1D1 line transition ${F}_{g}=2\ensuremath{\rightarrow}{F}_{g}=1$ in 87Rb^{87}\mathrm{Rb}87Rb by solving the time-dependent optical Bloch equations, integrating the results for the total excited state populations over atomic trajectories, and averaging over velocities (components perpendicular and parallel to the laser beam) and over atom incident angles to the laser beam. We obtained a square root dependence of the CPT linewidths as a function of the laser intensity for both steplike and Gaussian transverse beam profiles. The results obtained with the Gaussian transverse laser beam profile are in good agreement with recent experimental results. Results for both steplike and Gaussian profiles of the laser beam show asymptotic 1∕d1∕\sqrt{d}1∕d​ dependence (ddd is the laser beam diameter) of the CPT linewidths, for a large range of the laser intensity.
We discuss the use of electromagnetically modified absorption to achieve selective excitation in atoms: that is, the laser excitation of one transition while avoiding simultaneously exciting another transition whose frequency is the same as or close to that of the first. The selectivity which can be achieved in the presence of coherent population trapping (CPT) is limited by the decoherence rate of the dark state. We present exact analytical expressions for this effect, and also physical models and approximate expressions which give useful insights into the phenomena. When the laser frequencies are near-resonant with the single-photon atomic transitions, CPT is essential for achieving discrimination. When the laser frequencies are far detuned, the ``bright'' two-photon Raman resonance is important for achieving selective excitation, while the ``dark'' resonance (CPT) need not be. The application to laser cooling of a trapped atom is also discussed.