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International Conference Laser Optics (ICLO)

Multifactor optimization of the CPT miniature quantum frequency standards

К. А. Баранцев, A. N. Litvinov, Е. Н. Попов, V. M. Petrov

Published Jun 1, 2016• 0 citations
Publisher

Abstract

In this work the mathematical model of coherent population trapping effect which takes into account the distortion of the light spectrum and changing in light correlations inside the gas cell has been constructed. It was shown that decreasing in spectral width of the sidebands increases contrast of the CPT signal. We demonstrated the method of light shift suppression in optically dense medium. Optimization of the short-term stability of CPT based quantum frequency standard was done.

DOI
10.1109/lo.2016.7549810

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In this work the mathematical model of coherent population trapping effect which takes into account the distortion of the light spectrum and changing in light correlations inside the gas cell has been constructed. It was shown that decreasing in spectral width of the sidebands increases contrast of the CPT signal. We demonstrated the method of light shift suppression in optically dense medium. Optimization of the short-term stability of CPT based quantum frequency standard was done.
The present work is dedicated to optimization of the laser radiation parameters and optically thickness of the gas cell for improvement of short-term stability of quantum frequency standard based on coherent population trapping effect in the cell with 87Rb working gas and buffer gas. The effects associated with the optical density of the medium are investigated, such as laser spectrum distortion and appearance of the light shifts of the coherent population trapping resonance due to different absorption of frequency components.
We investigate the influence of optical thickness on the coherent population trapping (CPT) resonance interrogated by the autobalanced Ramsey method as well as by the method of combined error signals (CESs). We analyze sensitivity of the CPT resonance to the light shift of the etalon transition for the microwave quantum frequency standards based on cold or hot atomic ensembles. When the CPT resonance is excited, two-frequency laser radiation can be significantly absorbed at the atomic transitions of the optical range. The effects of absorption and phase incursion lead to additional shift of the reference resonance. This is an additional factor that degrades the accuracy of the frequency standards because this additional shift can vary with intensity and temperature fluctuations. It was shown that the CES method is more robust in an optically dense medium than is the autobalanced Ramsey method, but it also undergoes the light shift. We found that by choosing the normalization factor of the CES method it is possible to suppress the light shift for a certain density of the medium or suppress its sensitivity to temperature fluctuations.
In recent years, the coherent population trapping (CPT) phenomenon using a very small cell has attracted considerable attention for its potential application to ultra-compact atomic clock. For the miniaturization of CPT clock, it is important to clarify influence by the collision with atoms and cell walls. We study the effect of cell geometry on the CPT spectrum using a thin Cs vapor cell without buffer gas. When the cell length is below 1 mm and the laser intensity is sufficiently weak, the transit-time broadening is largely suppressed because only slow atoms have enough time to settle down on the dark state and contribute to the CPT signals. Furthermore, we show that introduction of buffer gas may broaden the CPT resonance in extremely thin cells. We also show that sub-Doppler signal which can be used for stabilization of laser frequency is obtained by using the CPT signal.
The light (ac Stark) shift of coherent population trapping (CPT) resonances in cesium vapor is studied under excitation by radiation from a vertical-cavity surface-emitting laser, whose current is modulated at a microwave frequency (≈ 4.6 GHz). This approach is used in some state-of-the-art microwave quantum frequency standards (QFS). One of the main factors leading to degradation of long-term frequency stability of QFS is associated with the light shift of the CPT resonance due to variations in optical power P in the vapor cell. In this work, it is shown that using an additional electro- optic modulator assembled as a Mach-Zehnder interferometer makes it possible to effectively control the amplitudes of sidebands in the radiation spectrum. This allows finding such an optimal optical power value near which the resonance shift is insensitive to its small changes. The results are of interest for the development of CPT-based QFS.
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.
Abstract In this study, the light shift of coherent population trapping (CPT) resonances is studied in a miniature glass cell (~0.1 cm3) filled with 133Cs vapor. The atoms exposed to the radiation from a vertical-cavity surface-emitting laser (VCSEL). The laser current is modulated at a microwave frequency (≈4.6 GHz), which leads to the frequency modulation (FM) of output radiation. In addition, the light beam is transmitted through an electro-optic modulator (EOM), which is assembled as a Mach–Zehnder interferometer, acquiring amplitude modulation (AM). This dual (FM–AM) modulation leads to essentially nonlinear behavior of the function describing the light shift of the CPT resonance versus the total optical power in the cell. In particular, this function has an extremum that can be used to suppress the influence of small optical power variations in the cell on the stability of the CPT-based quantum frequency standard (QFS). The proposed scheme is characterized by a number of parameters such as the phase difference between the FM and AM signals as well as the bias voltage at EOM, which are additional degrees of freedom for controlling the behavior of the light shift of the CPT resonance. This can be used for the optimization of the QFS operation regime.