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arXiv:2110.02466 · Physical Review A

Magnetic-field-insensitive coherent-population-trapping resonances excited by bichromatic linearly polarized fields on the D1 line of Cs133

Kenta Matsumoto, Sota Kagami, Akihiro Kirihara, Shinya Yanagimachi, Takeshi Ikegami, Atsuo Morinaga

Published Feb 22, 2022• 6 citations
arXivHTMLPDF

Abstract

We have experimentally demonstrated that magnetic-field-insensitive coherent-population-trapping (CPT) resonances are generated between the ground hyperfine levels on the ${D}_{1}$ line of $^{133}\mathrm{Cs}$ using a two-photon $\mathrm{\ensuremath{\Lambda}}$ scheme excited by lin||lin polarizations. The frequency shift of the CPT resonance is 0.04 Hz for the deviation of $1\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{T}$ at a ``magic'' magnetic field of $139\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{T}$ and is 50 times smaller than that of the conventional clock transition at a typical bias magnetic field for the clock operation. The amplitude of the CPT spectrum excited by lin||lin polarizations is enhanced as the excitation intensity increases and is well explained by the three-level model without trap levels. Thus, the CPT resonance on the ${D}_{1}$ line of $^{133}\mathrm{Cs}$ excited by a simple lin||lin scheme will be one of the best candidates for frequency reference of miniature atomic clocks.

DOI
10.1103/physreva.105.023110

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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.
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We present an experimental and theoretical investigation of the coherent population trapping (CPT) resonance excited on the D1 line of 87 Rb atoms by bichromatic linearly polarized laser light. The experimental results show that a lin ‖ lin transition scheme is a promising alternative to the conventional circular—circular transition scheme for an atomic magnetometer. Compared with the circular light transition scheme, linear light accounts for high-contrast transmission resonances, which makes this excitation scheme promising for high-sensitivity magnetometers. We also use linear light and circular light to detect changes of a standard magnetic field, separately.
State-of-the-art miniature atomic clocks (MACs) are based on the phenomenon of coherent population trapping (CPT) in alkali-metal atomic vapors (Rb\mathrm{Rb}Rb or Cs\mathrm{Cs}Cs). Improving the stability of the clock frequency is a crucial issue; resolving it would result in significant progress in numerous applications. Here, we examine the configuration of the light field, which is composed of two bichromatic beams with opposing handedness of circular polarization. The beams are in resonance with optical transitions in the Cs\mathrm{Cs}Cs D1{D}_{1}D1​ line ($\ensuremath{\lambda}\ensuremath{\approx}895$ nm). This configuration has previously been shown to produce observed CPT resonances with higher contrast than a standard single-beam scheme; however, our scheme implements pump and probe beams that operate independently, with the transmission of the probe beam being monitored separately. This approach differs from previous studies. The experiments are carried out using a cubic $5\ifmmode\times\else\texttimes\fi{}5\ifmmode\times\else\texttimes\fi{}5\phantom{\rule{0.2em}{0ex}}{\mathrm{mm}}^{3}$ vapor cell filled with buffer gas. It is shown that the line shape of the resonance acquires an asymmetry that can be efficiently controlled by a microwave (Raman) phase between the beams. As a proof of concept, we study the way in which this asymmetry can help to mitigate the influence of the ac Stark (light) shift on the long-term frequency stability of CPT clocks based on vapor microcells ($V\ensuremath{\ll}1\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{3}$). Experimental verification is performed with both a distributed-Bragg-reflector laser and a vertical-cavity surface-emitting laser. The latter is of particular importance for the development of MACs. The experimental results are in qualitative agreement with the analytical theory based on a double-$\mathrm{\ensuremath{\Lambda}}$ scheme of atomic energy levels.
We study the peculiarities of the coherent population trapping (CPT) dark resonance in atomic vapor confined in a finite-size cell. Taking into account the scattering of atoms on anti-relaxation coating of the walls, we analyze the line shape of the CPT resonance in 133 C s depending on various parameters, such as the width of laser radiation spectrum, the type of pumping, and the magnitude of static magnetic field for both lin || lin and l i n ⊥ l i n polarization schemes. We show that for different parameters, the dependence of the line shape on the size of a cell is different, i.e., different factors affect the CPT resonance nonadditively. Thus, in the case when CPT resonance is excited by a narrowband laser, the shape of the resonance weakly depends on the size of a cell due to the laser-induced narrowing effect. We also show that the distortion of the line shape of the CPT resonance caused by nonresonant hyperfine sublevels of atomic excited states is modified with the size of a cell. Our calculation reveals the important role of accounting for the hyperfine and Zeeman structure of atomic levels. We have shown that the contrast of the CPT resonance differs about 5 times from the prediction of the simple three-level Λ model. The obtained results can be used for the optimization of the quantum frequency standards based on the CPT effect. In the final part of the paper, on the basis of the shot noise approximation we estimate the short-term stability of such standards.
The authors presented investigations on the application of CPT effect as alternative to the traditional optical microwave double resonance in small atomic clocks. An important factor which may limit the performances of CPT-based atomic clocks is the reduced signal contrast as compared to optical pumping. To overcome this limit, several excitation schemes have been proposed in the last years. Particularly, a significant enhancement of the CPT contrast can be obtained when co-propagating laser waves with parallel linear polarization (lin | | lin CPT) are resonant with the excited state Fe=l of alkali atoms.