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Bulletin of the Lebedev Physics Institute

Development of the quantum discriminator for compact atomic clock

A. V. Sivak, Sergei A. Zibrov

Published May 1, 2011• 0 citations
Publisher

Abstract

In the Laboratory of Frequency Standards, Quantum Radiophysics Division, Lebedev Physical Institute, a compact atomic standard of frequency and time is being developed. The results of this study make it possible to achieve a narrower metrological resonance of coherent population trapping (CPT) than in developed foreign analogues of compact atomic clocks, which will have a direct effect on the standard stability. The effect of the combination of buffer gas and antirelaxation coating on the CPT amplitude and resonance width are studied; parameters of short-cavity diode lasers (SCDLs) specially developed in collaboration with the Polyus Research and Development Institute are studied. A prototype of the compact physical discriminator for the atomic clock is developed and assembled. The dark width of the metrological resonance in the discriminator is less than 1 kHz, which theoretically implies the development of a compact atomic clock with a stability (Allan parameter) no worse than 3 · 10 −12 per hour.

DOI
10.3103/s1068335611050034

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In the Laboratory of Frequency Standards, Quantum Radiophysics Division, Lebedev Physical Institute, a compact atomic standard of frequency and time is being developed. The results of this study make it possible to achieve a narrower metrological resonance of coherent population trapping (CPT) than in developed foreign analogues of compact atomic clocks, which will have a direct effect on the standard stability. The effect of the combination of buffer gas and antirelaxation coating on the CPT amplitude and resonance width are studied; parameters of short-cavity diode lasers (SCDLs) specially developed in collaboration with the Polyus Research and Development Institute are studied. A prototype of the compact physical discriminator for the atomic clock is developed and assembled. The dark width of the metrological resonance in the discriminator is less than 1 kHz, which theoretically implies the development of a compact atomic clock with a stability (Allan parameter) no worse than 3 · 10 −12 per hour.
A brief review is made of the investigations conducted at the Laboratory of Frequency Standards at the P N Lebedev Physical Institute (FIAN) Division of Quantum Radiophysics along an important line of inquiry into the development of quantum frequency standards?the elaboration of a subminiature clock based on the coherent population trapping effect in alkali metal atoms with semiconductor laser pumping (frequency stability of 10?11?10?12 for an averaging time of ~ 104 s) and compact master oscillators/optical clocks with a frequency stability of 10?14?10?15 (for an averaging time of 1 s) based on He?Ne/CH4 and fiber femtosecond lasers.
This work reports for the first time development and experimental study of a laboratory prototype of an atomic clock with stability of 2×10–11τ –1/2 (Allan deviation) over time 1 s < τ < 1000 s (7×10–13 over 1000 s) on the basis of a coherent population trapping (CPT) resonance in 87Rb in a compact spherical cell having 1.3-cm diameter and without any buffer gas. Demonstrated are dependencies of the width of the studied CPT resonance upon the amplitude and modulation frequency of optical frequency difference (3.417 GHz) of a bichromatic circularly polarised pump field. For the first time, a considerable non-linearity was discovered in the dependence upon the reference difference frequency of the spectral position of the CPT resonance when the modulation frequency of the optical frequency difference is varied within a broad range.
The goal of this project is to optimize parameters of a prototype atomic clock to achieve the best Allan variance. We have established a prototype atomic clock using coherent population trapping (CPT) by creating a laser system to lock the laser’s frequency to an atomic resonance in rubidium and utilized radiofrequency (rf) modulation to create a sideband and carrier comb. With the system assembled, we have achieved our best Allan variance of 4 × 10−10 for a 15 torr Neon buffer gas cell, but have changed to a 5 torr Neon buffer gas cell to decrease the Allan variance.
We demonstrate a vapor-cell atomic-clock prototype based on a continuous-wave interrogation and double-modulation coherent population trapping (DM-CPT) technique. The DM-CPT technique uses a synchronous modulation of polarization and the relative phase of a bichromatic laser beam in order to increase the number of atoms trapped in a dark state, i.e., a nonabsorbing state. The narrow resonance, observed in the transmission of a Cs vapor cell, is used as a narrow frequency discriminator in an atomic clock. A detailed characterization of the CPT resonance versus numerous parameters is reported. A short-term fractional-frequency stability of 3.2×10−13τ−1/2 up to a 100-s averaging time is measured. These performances are more than one order of magnitude better than industrial Rb clocks and are comparable to those of the best laboratory-prototype vapor-cell clocks. The noise-budget analysis shows that the short- and midterm frequency stability is mainly limited by the power fluctuations of the microwave used to generate the bichromatic laser. These preliminary results demonstrate that the DM-CPT technique is well suited for the development of a high-performance atomic clock, with the potential compact and robust setup due to its linear architecture. This clock could find future applications in industry, telecommunications, instrumentation, or global navigation satellite systems.
This work presents a study of the short-term stability properties of Rb atomic clocks based on coherent population trapping (CPT) under dynamic excitation of a CPT resonance in cells without buffer gas. It is demonstrated that in an optical cell with anti-relaxation coating of its inner walls, the best stability is achieved at the scanning frequency of the frequency difference of the bichromatic pump field equal to 2 kHz at the resonance width of 450 Hz. In cells without a wall coating, the optimal scanning frequency range is found to be 1.2–2.8 kHz at the resonance width of 29 kHz. Examination of the slope of the stabilization system’s discriminant curve at zero error signal for buffer-gas-free cells uncovered an amount of correlation between the discriminant curve slope and atomic clock stability. It is demonstrated that the highest stability of atomic clocks in dynamic excitation mode is achieved at a ratio of scanning frequency and amplitude around 1.
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.