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Chinese Journal of Lasers

Research on Phase Modulation of Ramsey Fringes in Integrating Sphere Cold Atom Clocks

刘 鹏 Liu Peng, 成华东 Cheng Huadong, 孟艳玲 Meng Yanling, 万金银 Wan Jinyin, 王秀梅 Wang Xiumei, 王亚宁 Wang Yaning, 肖玲 Xiao Ling, 刘亮 Liu Liang

Published Jan 1, 2016• 2 citations
Publisher

Abstract

In the Ramsey scheme, if one modulates two microwave pulses at the same frequency but with pi/2 phase differences, the Ramsey fringes with zero amplitude at the central frequency can be obtained. By monitoring the variance of probe light absorption by atoms at the central frequency, one can get the error signals which feedbacks to the local oscillator. Theoretically, this method doubles the absorption signal intensity, compared with the case of Ramsey fringes with the same phase. Experimentally, the phase modulated Ramsey pattern in the integrating sphere cold atom clock system is the same with that from theoretical calculation. Moreover, the Ramsey fringe oscillation amplitude here is 1.6 times of that of Ramsey fringes with the same phase.

DOI
10.3788/cjl201643.1112001

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In the Ramsey scheme, if one modulates two microwave pulses at the same frequency but with pi/2 phase differences, the Ramsey fringes with zero amplitude at the central frequency can be obtained. By monitoring the variance of probe light absorption by atoms at the central frequency, one can get the error signals which feedbacks to the local oscillator. Theoretically, this method doubles the absorption signal intensity, compared with the case of Ramsey fringes with the same phase. Experimentally, the phase modulated Ramsey pattern in the integrating sphere cold atom clock system is the same with that from theoretical calculation. Moreover, the Ramsey fringe oscillation amplitude here is 1.6 times of that of Ramsey fringes with the same phase.
Sophisticated Ramsey-based interrogation protocols using composite laser pulse sequences have been recently proposed to provide next-generation high-precision atomic clocks with a near perfect elimination of frequency shifts induced during the atom-probing field interaction. We propose here a simple alternative approach to the autobalanced Ramsey interrogation protocol and demonstrate its application to a cold-atom microwave clock based on coherent population trapping (CPT). The main originality of the method, based on two consecutive Ramsey sequences with different dark periods, is to sample the central Ramsey fringes with frequency jumps finely adjusted by an additional frequency-displacement concomitant parameter, scaling as the inverse of the dark period. The advantage of this displaced frequency-jump Ramsey method is that the local oscillator (LO) frequency is used as a single physical variable to control both servo loops of the sequence, simplifying its implementation and avoiding noise associated with controlling the LO phase. When tested using a CPT cold-atom clock, the DFJR scheme reduces the sensitivity of the clock frequency to variations of the light shifts by more than an order of magnitude compared with the standard Ramsey interrogation. This simple method can be applied in a wide variety of Ramsey-spectroscopy based applications including frequency metrology with CPT-based and optical atomic clocks, mass spectrometry, and precision spectroscopy.
We describe a method to stabilize the amplitude of the interrogating microwave field in compact atomic clocks working in a Ramsey approach. In this technique, we take advantage of the pulsed regime to use the atoms themselves as microwave amplitude discriminators. Specifically, in addition to the dependence on the microwave detuning, the atomic signal after the Ramsey interrogation acquires a dependence on the microwave pulse area (amplitude times duration) that can be exploited to implement an active stabilization of the microwave field amplitude, in a similar way in which the Ramsey clock signal is used to lock the local oscillator frequency to the atomic reference. This stabilization results in a reduced sensitivity of the clock frequency to microwave amplitude fluctuations that are transferred to the atoms through the cavity-pulling effect. The proposed technique is then effective to improve the clock stability and drift on medium and long term. We demonstrate the method for a vapor-cell clock working with a hot sample of atoms but it can be extended to cold-atom compact clocks.
Abstract The peculiarities of Ramsey resonance and its sensitivity to the light shift from an optically dense medium of cold atoms are investigated. Different composite pulse protocols for clock spectroscopy, including hyper-Ramsey, modified and generalised hyper-Ramsey schemes, are compared. Error signals change significantly due to the processes of absorption and dispersion in the atomic medium. The dependence of the position of the central fringe resonance with a residual uncompensated light shift of the atomic transition is studied with the attenuation of the radiation intensity in the medium taken into account. It is shown that using a combination of generalised hyper-Ramsey error signals allows one to suppress the sensitivity to the light shift for any length of the medium.
A compact frequency standard based on an expanding cold Cs133 cloud is under development in our laboratory. In a first experiment, Cs cold atoms were prepared by a magneto-optical trap in a vapor cell, and a microwave antenna was used to transmit the radiation for the clock transition. The signal obtained from fluorescence of the expanding cold atoms cloud is used to lock a microwave chain. In this way the overall system stability is evaluated. A theoretical model based on a two-level system interacting with the two microwave pulses enables interpretation for the observed features, especially the poor Ramsey fringes contrast.