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Journal of Experimental and Theoretical Physics Letters

Contrast of Ramsey-CPT Fringes in Quenching and Depolarizing Gases

Dmitry S. Chuchelov, Maria I. Vaskovskaya, Eugene A. Tsygankov, Sergei A. Zibrov, Kirill M. Sabakar, Vitali V. Vassiliev, Vladimir L Velichansky

Published Dec 25, 2023• 3 citations
Publisher

Abstract

Molecular nitrogen is often used as a buffer gas in cells with alkali metals due to its known ability to quench resonant fluorescence. It is widely believed that the suppression of spontaneous emission decreases the width of coherent population trapping resonance. However, our recent results have not confirmed this positive action of molecular nitrogen within the typical range of 87Rb concentrations and buffer gas pressures. On the opposite, we have observed the negative influence of quenching, the decrease in contrast of the coherent population trapping resonance in $${{\sigma }^{ + }}$$ – $${{\sigma }^{ + }}$$ configuration. In this work, we further confirm these results implementing the Ramsey spectroscopy, and compare the characteristics of the central fringe in nitrogen and neon, and show that the latter provides a significantly better contrast-to-width ratio.

DOI
10.1134/s0021364023603615

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Molecular nitrogen is often used as a buffer gas in cells with alkali metals due to its known ability to quench resonant fluorescence. It is widely believed that the suppression of spontaneous emission decreases the width of coherent population trapping resonance. However, our recent results have not confirmed this positive action of molecular nitrogen within the typical range of 87Rb concentrations and buffer gas pressures. On the opposite, we have observed the negative influence of quenching, the decrease in contrast of the coherent population trapping resonance in σ+{{\sigma }^{ + }}σ+ – σ+{{\sigma }^{ + }}σ+ configuration. In this work, we further confirm these results implementing the Ramsey spectroscopy, and compare the characteristics of the central fringe in nitrogen and neon, and show that the latter provides a significantly better contrast-to-width ratio.
We investigate the effect of buffer gases on the coherent population trapping resonance induced by a $\ensuremath{\sigma}$-polarized optical field in 87Rb{}^{87}\mathrm{Rb}87Rb atoms. Our experimental results show that inert gases, which depolarize the excited state of alkali-metal atoms, provide higher contrast than nitrogen that effectively quenches their fluorescence. We also demonstrate that elimination of spontaneous radiation does not significantly decrease the width at moderate temperatures of an atomic medium. Therefore, a mixture of inert gases can be preferable over a mixture with nitrogen for atomic clocks.
We investigate the effect of buffer gases on the coherent population trapping resonance induced by a σσσ-polarized optical field in 87^{87}87Rb atoms. Our experimental results show that inert gases, which depolarize the excited state of the alkali-metal atoms, provide higher contrast than nitrogen that effectively quenches their fluorescence. We also demonstrate that elimination of the spontaneous radiation does not significantly decrease the width at moderate temperatures of an atomic medium. Therefore, a mixture of inert gases can be preferable over a mixture with nitrogen for atomic clocks.
We report the results of experimental investigation of the Raman – Ramsey method of pulsed excitation of coherent population trapping (CPT) resonances in a rubidium vapour cell filled with a mixture of Ar and Ne buffer gases. For the development of compact quantum microwave frequency standards, this method is an alternative to the pulsed optical pumping method recently implemented using the same rubidium cell. Both methods have advantages over the traditional double radio-optical resonance technique, the main of which is the substantial suppression of the light shift of the clock transition frequency, since the evolution of the coherence of atomic states forming the clock transition occurs in the absence of laser radiation in the cell. The narrow (110 – 240 Hz) Raman – Ramsey resonances are obtained using the scheme of pulsed excitation of CPT resonances on the D 1 lines of the 87 Rb atom with the same linear (lin||lin) polarisations of the bichromatic laser radiation components. The process of optimising the central fringe linewidth, its contrast, and the linewidth-to-contrast ratio is described. The magnetic dependence of the Raman – Ramsey line central fringe linewidth is experimentally investigated.
The Ramsey method has been applied by means of optical coherent population trapping (CPT) pulses through a cesium vapor cell with N/sub 2/ buffer gas at room temperature, using two phase-locked lasers. With this method, CPT resonance spectral widths are no longer limited by optical saturation and collision effects, but only depend on free evolution time between the two pulses. A fringe width below 100 Hz is reported. Experimental Raman-Ramsey fringes are analyzed using the classical wavefunction formalism.
Pulsed coherent population trapping (CPT) spectroscopy is applied in buffer-gas filled Cs–Ne vapor microfabricated cells. The properties of the Ramsey-CPT clock transition central fringe (linewidth, signal amplitude, and contrast) are studied versus several experimental parameters, including the Ramsey sequence and the input laser intensity, and are compared to those obtained in the conventional continuous (CW) interrogation mode. In the pulsed case and for short Ramsey-free evolution times TR, the central fringe linewidth is found to exhibit a small but visible power broadening and is measured to be narrower than the value of 1/(2TR) that is reached for long TR. The microwave hyperfine coherence lifetime T2 in the Cs–Ne microcells is measured to be in the 50–500 μs range. Its dependence to the buffer gas pressure or cell temperature is briefly studied.
Coherent population trapping (CPT) on 87 Rb atoms in neon atmosphere has been studied in small-size glass absorption cells under conditions of pumping with narrow-band laser radiation at the D 2 line of the main doublet. Parameters of the absorption signal have been measured in 3-mm-diameter cells at buffer gas (Ne) pressures varied within 200–400 Torr, cell temperatures within 65–120°C, and pumping radiation power densities within 30–400 μW/cm 2 . Optimum values of the buffer gas pressures, cell temperature, and pumping power are determined at which the short-term instability of the resonance line is at minimum. Orientational shifts of the CPT resonance signal in gas-filled cells and small-size cells with antirelaxation coating have been compared.