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arXiv:2402.04234 · Physical Review A

Role of spontaneously transferred coherence in laser cooling

Rajnandan Choudhury Das, Thilagaraj Ravi, Samir Khan, Kanhaiya Pandey

Published Sep 3, 2024• 5 citations
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Abstract

The well-known sub-Doppler polarization gradient cooling in a type-I transition (${F}_{\mathrm{e}}={F}_{\mathrm{g}}+1$) is caused by red-detuned lasers. On the other hand, in a type-II transition (${F}_{\mathrm{e}}\ensuremath{\le}{F}_{\mathrm{g}}$), sub-Doppler cooling takes place through blue-detuned lasers. This opposite behavior for the two types of transitions is due to spontaneously transferred coherence (STC). In the absence of STC, both types of transitions show blue-detuned cooling. In this paper, we experimentally and theoretically demonstrate blue-detuned cooling for both types of transitions in $^{87}\mathrm{Rb}$. For completeness, we compare the temperatures in various configurations.

DOI
10.1103/physreva.110.033101

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Sisyphus polarization gradient laser cooling has been performed in a one-dimensional lin lin laser beam configuration on each of the three fine structure transitions of . Hence for the first time Sisyphus polarization gradient cooling on a transition with has been demonstrated. We have investigated the qualitative and quantitative differences between the cooling process for three transitions with . A major qualitative difference of the laser cooling on the and transitions has been observed as expected from simple theoretical arguments. In the case of sub-Doppler cooling is achieved when the laser detuning is blue. Even though the situation for the and case is qualitatively similar, we find quantitative differences which are in good agreement with Monte Carlo wavefunction simulations. For experiments where narrow velocity distributions are essential, such as atom lithography, these results can be applied.
We have observed a new type of sub-Doppler laser cooling with neither polarization gradients nor magnetic fields. All known sub-Doppler laser cooling methods require one of these two to resdistribute the atomic population among the energy levels. This allows the irreversible processes of optical pumping vels. This allows the irreversible processes of optical pumping (OP) to manipulate the conservative force of light shifts (e.g. resonant exchange between the two beams comprising a standing wave) into a damping force. Our scheme employs stimulated Raman transitions between ground state hyperfine levels of Rb with a bichromatic standing wave, accompanied by a momentum change of {Delta}p = 2{h_bar}k. OP, with {Delta}p = 1{h_bar}k, creates a population imbalance allowing repetition of the Raman process. When the detuning of the light from Raman resonance {delta} is properly chosen, the net momentum exchange damps the atomic motion. Changing {delta} by only {gamma}/6(1 MHz) replaces cooling by heating. This process separates the velocity selective Raman resonance and OP processes differently from others, and helps clarify the role of these different mechanisms.
We study several magneto-optical trapping configurations in 87Rb^{87}\mathrm{Rb}87Rb. These unconventional MOTs all use type-II transitions, where the angular momentum of the ground state is greater than or equal to that of the excited state. Some use red-detuned light, and others blue-detuned light. The properties of these MOTs are strongly influenced by the balance between opposing Doppler and Sisyphus forces, and vary widely from one configuration to another. In the blue-detuned MOT, Sisyphus cooling dominates over Doppler heating for all relevant speeds and magnetic fields. We measure the capture velocity of this MOT as a function of intensity and detuning, finding a maximum of $3.8\ifmmode\pm\else\textpm\fi{}0.1$ m/s. Atomic densities are particularly high in the blue-detuned MOT, and its lifetime is limited by collisions between the trapped atoms. We present measurements of the loss rate due to these ultracold collisions as a function of laser intensity and detuning. In the red-detuned MOTs, Sisyphus heating dominates at low speeds and Doppler cooling at higher speeds. Consequently, temperatures in the red-detuned MOTs are up to a thousand times higher than in the blue-detuned MOTs. One MOT forms large ring structures, with no density at the center, showing how atoms driven towards a non-zero equilibrium speed remain trapped by orbiting around the center. Another MOT demonstrates that magnetic mixing of the excited-state hyperfine levels can be an important mechanism in type-II MOTs.
Sub-Doppler laser cooling requires optical pumping among differently light-shifted ground-state sublevels. We describe a study of the simplest possible angular-momentum configuration that permits all sub-Doppler cooling phenomena. The Jg = 1 ⇒ Je = 0 angular-momentum configuration shows recoil-limited cooling in the two most well-known types of polarization gradient, magnetically induced laser cooling, velocity-selective resonances, transient cooling, and velocity-selective population trapping.
We observed a new type of sub-Doppler cooling that employs neither polarization gradients nor magnetic fields, and involves neither a damping force nor significant diffusive heating. Instead, light shifts combined with optical pumping (OP) to levels not coupled by the laser field gives transient cooling, and steady state is not achieved. The time scale is set by the OP rate. We observe both cooling and heating, but often with the opposite detuning from that of steady state. A semiclassical and a fully quantum mechanical calculation of transient cooling agree very well with one another and with our data.
We exploit red- and blue-detuned magneto-optical trapping (MOT) of 87Rb benefitting from a simplified setup and a novel approach based on liquid crystal variable retarders (LCVR). To maintain the trapping forces when switching from a red- to a blue-detuned MOT, the handedness of the circular polarization of the cooling beams needs to be reversed. LCVRs allow fast polarization control and represent compact, simple, and cost-efficient components, which can easily be implemented in existing laser systems. This way, we achieve a blue-detuned type-II MOT for 8.7 × 108 atoms of 87Rb with sub-Doppler temperatures of 44 μK well below the temperatures reached in a conventional 87Rb type-I MOT. The phase space density is increased by more than two orders of magnitude compared to the standard red-detuned type-I MOT. The setup can readily be transferred to any other systems working with 87Rb.