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arXiv:1702.04891

Exponential decrease of Ramsey linewidth via coherent atomic phase tracking

Gongxun Dong, Jinda Lin, Jianliao Deng, Yuzhu Wang

Published Feb 16, 2017• 0 citations
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Abstract

We propose an effective method to decrease the Ramsey linewidth of microwave atomic clock by tracking the coherent atomic phase through nondestructive measurement. The free evolution time T between two Ramsey pulses is divided into N subsections. In each subsection, the coherent phase of the atomic ensemble is probed by an additional large-detuning laser. The probed phase in the previous subsection is used to change the phase of the microwave pulse in the following subsection. After tracking the coherent atomic phase for N-1 times, the final linewidth of Ramsey fringe decreases quickly as N/(2N-1). We then demonstrate this proposal in a vapor cell atomic clock. The linewidth of the Ramsey fringe is decreased down to 27 Hz after 5 successive sub-interrogations, which is much narrower than the linewidth of 142 Hz obtained by normal Ramsey spectroscopy.

DOI
10.48550/arxiv.1702.04891

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We propose an effective method to decrease the Ramsey linewidth of microwave atomic clock by tracking the coherent atomic phase through nondestructive measurement. The free evolution time T between two Ramsey pulses is divided into N subsections. In each subsection, the coherent phase of the atomic ensemble is probed by an additional large-detuning laser. The probed phase in the previous subsection is used to change the phase of the microwave pulse in the following subsection. After tracking the coherent atomic phase for N-1 times, the final linewidth of Ramsey fringe decreases quickly as N/(2N-1). We then demonstrate this proposal in a vapor cell atomic clock. The linewidth of the Ramsey fringe is decreased down to 27 Hz after 5 successive sub-interrogations, which is much narrower than the linewidth of 142 Hz obtained by normal Ramsey spectroscopy.
The linewidth of CPT atomic resonances observed in a vapor cell are usually limited by optical saturation and collisions due to buffer gas. We have applied here the Ramsey method with optical CPT pulses generated by a phase-locked device. Fringes are then produced giving narrow atomic signals only dependent on interrogation time between the two pulses. The possibility to obtain narrow fringe widths below 100 Hz with short or long pulses is shown experimentally and analysed using the classical Schrodinger state representation.
High contrast transparent Ramsey fringes are observed using double microwave pulses interaction with the prepared atomic coherent state in a warm 87 Rb vapor with mixture buffer gases in a closed cell.The Ramsey fringes are generated by the pulsed technique, a strong coupling light pulse and a weak signal light pulse are applied to prepare the atomic coherent state, followed by the application of double microwave pulses to interact with the atomic coherent state.Afterwards, the light pulses are applied again with weaker intensity and detecting the signal transmission is delected.The central line of the transparent Ramsey fringes has narrow linewidth of 125 Hz and high contrast of 21%.The light shift is dramatically reduced since the interrogating process is not involved the light field, and the cavity pulling effect is negligible due to the low Q requirement, which is promising for building small, compact, and stable atomic clocks.
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.
The optical Ramsey spectrum is experimentally realized in an 87Sr optical lattice clock, and the measured linewidth agrees well with theoretical expectation. The coherence time between the clock laser and the atoms, which indicates the maximum free evolution period of using Ramsey detection to measure the atom-laser phase information, is determined as 340(23) ms by measuring the fringe contrasts of the Ramsey spectrum as a function of the free evolution period. Furthermore, with the same clock duty cycle of about 0.1, the clock stability is measured by using the Ramsey and Rabi spectra, respectively. The experimental and theoretical results show approximately the same stability as the two detection methods, which indicates that Ramsey detection cannot obviously improve the clock stability until the clock duty cycle is large enough. Thus, it is of great significance to choose the detection method of a specific clock.
In general, the linewidth of the Ramsey central fringe (RCF) is equal to 1/(2T), where T is the Ramsey free-evolution time. We demonstrate that the RCF linewidth of a pulsed optically pumped (POP) atomic clock with orthogonal polarization detection based on the magneto-optical rotation effect can be narrowed down to 1/(4T). The Allan deviation of the POP atomic clock decreases from 2.4×10−13τ−1/2 to 1.4×10−13τ−1/2. This corresponds to an improvement in the frequency stability by about 60%. We also estimate the main noise sources that limit the short-term frequency stability of the POP atomic clock.
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.