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Izmeritel`naya Tekhnika

Thermal compensation in quantum frequency standards based on the effect of coherent population trapping

Danil A. Paryohin

Published Jan 1, 2021• 0 citations
Publisher

Abstract

The problem of developing a thermal compensation system in quantum frequency standards based on the effect of coherent population trapping is considered. The development of such a system significantly reduces the temperature coefficient of frequency, the value of which is an order of magnitude higher than in rubidium frequency standards. One of the ways to reduce the temperature coefficient of frequency is temperature compensation. A method of thermal compensation based on the Zeeman effect for the shift of the actual frequency is proposed. A method for determining the minimum value of the magnetic field at which there is no influence of magnetosensitive resonances on the reference resonance is considered. The results of the operation of the quantum frequency standard before and after switching on the thermal compensation system are presented.

DOI
10.32446/0368-1025it.2021-6-8-12

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•May 4, 2015•0 citations
The problem of developing a thermal compensation system in quantum frequency standards based on the effect of coherent population trapping is considered. The development of such a system significantly reduces the temperature coefficient of frequency, the value of which is an order of magnitude higher than in rubidium frequency standards. One of the ways to reduce the temperature coefficient of frequency is temperature compensation. A method of thermal compensation based on the Zeeman effect for the shift of the actual frequency is proposed. A method for determining the minimum value of the magnetic field at which there is no influence of magnetosensitive resonances on the reference resonance is considered. The results of the operation of the quantum frequency standard before and after switching on the thermal compensation system are presented.
The problem of developing control algorithms in quantum frequency standards based on the effect of coherent population trapping is considered. The development of such algorithms will make it possible to create frequency standards with metrological characteristics that are not inferior to the characteristics of rubidium frequency standards, with reduced power consumption and overall dimensions. A method for studying the dependence of the actual value and frequency instability of quantum frequency standards based on the effect of coherent population trapping on the operating modes of individual parts of the standards is presented. As a criterion for optimizing the laser injection current, the output power of the microwave generator, and the cell temperature, we chose to minimize the effect on the shift of the actual frequency of the standard. A comparative analysis of methods for changing the radiation intensity of a surface-emitting laser with a vertical resonator has been carried out and control algorithms have been developed that take into account the features of these methods. Based on these algorithms, software and tuning methods have been developed in quantum frequency standards based on the effect of coherent population trapping. For the prototype standard, the results of measuring the frequency instability are shown. It is shown that control algorithms and tuning methods can qualitatively change the metrological characteristics of quantum frequency standards based on the effect of coherent population trapping.
We studied the relationship between the frequency shift of coherent population trapping resonance and the cell temperature of (85)Rb. Results show that the temperature coefficient of the frequency shift can be reduced by buffer gas pressure adjustment and light shift optimization. When the contribution of buffer gas collision to temperature coefficient of frequency shift is less than 0.3 Hz/K, the contribution of light shift to the temperature coefficient of frequency shift becomes obvious. Under this cancelling effect, we can reduce the rate of total frequency shift to near zero.
We investigate the magnetic-field-induced frequency shift (MFS) of the clock “0–0” transition in the microwave quantum frequency standard (atomic clock) based on coherent population trapping (CPT) in 87Rb vapor. To scan the CPT resonance and to form the error signal, a method analogous to the Pound–Drever–Hall (PDH) technique in the optical frequency range is employed, where the modulating frequency (fm) significantly exceeds the resonance linewidth (FWHM). The experiments demonstrate that this technique offers brilliant capabilities for controlling the sensitivity of the clock transition frequency to magnetic field variations in the vapor cell compared to the conventional method with low-frequency modulation (fm≪FWHM). Specifically, the PDH technique provides several optimal values of the bias magnetic field generated by the solenoid, at which the “0–0” transition frequency exhibits extremely low sensitivity to small variations in the external magnetic field. Furthermore, these magnetic field values can be easily adjusted by changing fm, which is relevant for the optimization of the atomic clock’s operating regime. The experimental results show that by using the PDH technique, the influence of MFS on the clock transition can be suppressed down to ≈3.2×10−13δB2mG−2. These findings can be leveraged both to relax stringent requirements for magnetic field shielding in state-of-the-art CPT-based miniature atomic clocks and to build a new generation of such clocks with long-term frequency stability better than 10−12.
Summary form only given. We have experimentally investigated the potential of narrow coherent population trapping resonances for precision applications like magnetometry or atomic frequency standards. CPT can be observed in /spl Lambda/ systems where two ground states are coupled to a common excited state by two near-resonant light fields. Effective formation of ground state coherences takes place when the frequency difference of the light fields precisely matches the ground state splitting. This process leads to reduced absorption in the medium (electromagnetically induced transparency) and thus reduced fluorescence intensity (dark resonance) at the optical resonance frequency. Extremely narrow dark resonances can be observed when the two light fields have fixed phase and frequency difference and time-of-flight broadening is suppressed by the use of a buffer gas, making this type of system a promising candidate for precision applications. The /spl Lambda/ systems under study here are the D/sub 2/ lines of Cs, /sup 87/Rb and /sup 85/Rb where the ground state hyperfine splittings are 9.2 GHz, 6.8 GHz, and 3.0 GHz, respectively. Thus in a magnetic field the CPT resonance splits into several Zeeman components whose frequency positions are determined by the Breit-Rabi formula.
An infrared frequency standard based on the NH $\ensuremath{\mid}{n}_{v}=0,{N}_{0}=0,J=1,{F}_{1}=3∕2,F=5∕2,{M}_{F}=5∕2⟩\ensuremath{\rightarrow}\ensuremath{\mid}{n}_{v}=1,{N}_{0}=0,J=1,{F}_{1}=3∕2,F=5∕2,{M}_{F}=5∕2⟩$ transition $(3\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m})$ is proposed. There is no Zeeman shift with this transition frequency, because the Zeeman coefficient is not dependent on the vibrational state. NH molecules, precooled by collisions with liquid 3He^{3}\mathrm{He}3He vapor, are trapped by an inhomogeneous magnetic field. The temperature of the trapped NH molecules holds the potential to be reduced down to several $\ensuremath{\mu}\mathrm{K}$ by evaporative cooling. The uncertainty of the clock transition can potentially be reduced to lower than ${10}^{\ensuremath{-}17}$.
In this work, a portable atomic frequency standard based on coherent population trapping is designed and demonstrated. To achieve a portable prototype, in the system, a single transverse mode 795nm VCSEL modulated by a 3.4GHz RF source is used as a pump laser which generates coherent light fields. The pump beams pass through a vapor cell containing atom gas and buffer gas. This vapor cell is surrounded by a magnetic shield and placed inside a solenoid which applies a longitudinal magnetic field to lift the Zeeman energy levels’ degeneracy and to separate the resonance signal, which has no first-order magnetic field dependence, from the field-dependent resonances. The electrical control system comprises two control loops. The first one locks the laser wavelength to the minimum of the absorption spectrum; the second one locks the modulation frequency and output standard frequency. Furthermore, we designed the micro physical package and realized the locking of a coherent population trapping atomic frequency standard portable prototype successfully. The short-term frequency stability of the whole system is measured to be 6×10−11 for averaging times of 1s, and reaches 5×10−12 at an averaging time of 1000s.