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arXiv:1710.10788

Observation of CPT for the ground hyperfine interval in $^{133}$Cs

Sumanta Khan, Vineet Bharti, Vasant Natarajan A

Published Oct 30, 2017• 0 citations
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Abstract

We use the technique of coherent population trapping (CPT) to access the ground hyperfine interval (clock transition) in $^{133}$Cs. The probe and control beams required for CPT are obtained from a single compact diode laser system. The phase coherence between the beams, whose frequency difference is the clock frequency, is obtained by frequency modulating the laser with an electro-optic modulator (EOM). The EOM is fiber coupled and hence does not require alignment, and the atoms are contained in a vapor cell. Both of these should prove advantageous for potential use as atomic clocks in satellites.

DOI
10.48550/arxiv.1710.10788

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Observation of CPT for the ground hyperfine interval in 133^{133}133Cs
We use the technique of coherent population trapping (CPT) to access the ground hyperfine interval (clock transition) in 133^{133}133Cs. The probe and control beams required for CPT are obtained from a single compact diode laser system. The phase coherence between the beams, whose frequency difference is the clock frequency, is obtained by frequency modulating the laser with an electro-optic modulator (EOM). The EOM is fiber coupled and hence does not require alignment, and the atoms are contained in a vapor cell. Both of these should prove advantageous for potential use as atomic clocks in satellites.
We demonstrate the development of an atomic clock using a diode laser and a vapor cell of Cs atoms. We use the technique of coherent population trapping (CPT). The required phase coherence between the probe and control beams, whose frequency difference is the clock frequency, is obtained by frequency modulating the laser with an electro-optic modulator (EOM). The EOM is fiber coupled and hence does not require alignment, which is advantageous for space applications. The atoms are contained in a vapor cell, which is again useful in satellite launches.
Space-borne atomic frequency standards demand a high level of performance and stability throughout the mission life in the harsh environment of space. Recently, atomic clocks employing Coherent Population Trapping (CPT), a quantum interference phenomenon in alkali atoms, have shown much promise for space application. This paper presents the development and realization of compact and reliable CPT atomic clock for ISRO's navigational satellites. In our approach, quantum interference in Rb-87 atoms are pursued for obtaining the clock resonance, owing to their larger ground state hyperfine transition energy (higher Q-factor). Quantum interference is realized by generating two resonant optical laser fields from a VCSEL diode by modulating its drive current at RF frequency which corresponds to the ground state hyperfine separation in Rb-87 atoms. The emission frequency of the laser diode is stabilized by locking it with respect to the D1 hyperfine transition with the help of a servo control loop, thereby the instabilities associated with laser frequency fluctuations are minimized. The resulting CPT resonance (<;kHz) as shown in Figure 1 is used as reference to stabilize the crystal oscillator frequency (10 MHz) that serves both as clock output and seed for generating the RF field.
Coherent population trapping (CPT) has been demonstrated as an interesting technique for miniature atomic frequency references [1,2] and quantum information. It is based on the coupling of the two hyperfine ground states of an alkali atom – namely cesium (133Cs) for atomic clocks – through excitation to a common atomic level by two phase-coherent laser fields nearly resonant with the atomic transitions. The frequency difference between the two laser fields is tuned at the atomic frequency splitting in the microwave range, equal to 9.192 GHz for 133Cs atoms. Outputs powers in the mW range and narrow-linewidth emission (<500 kHz) are required for the two laser beams.
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 article reports on the characterization of a compact coherent-population-trapping (CPT) Cs vapor cell atomic clock using the push-pull optical pumping technique. This method allows to detect high-contrast CPT resonances on the 0-0 magnetic-field insensitive clock transition. The clock short-term fractional frequency stability is measured to be 3×10−13 τ−1/2 up to 100 s averaging time, in correct agreement with the signal-to-noise ratio limit. These results are among the best performances ever reported for a CPT-based clock. The noise budget is detailed. A preliminary study is reported to evaluate limitations to the clock mid-term frequency stability. The clock performances are found to be presently mainly limited in the mid-term by laser power effects.
The authors demonstrate a simple technique to significantly improve the long-term frequency stability of atomic clocks based on coherent population trapping (CPT). In this technique, the CPT fields are created by a modulated diode laser and a slow servo is used to actively tune the laser modulation index to a value where the light shift vanishes. The observed clock frequency at this modulation index is given by the rubidium hyperfine frequency when no light fields are present, and this makes the clock frequency largely insensitive to variations in laser properties. In addition to reducing stringent requirements on the long-term stabilities of laser temperature, laser frequency, and rf modulation power, this technique may also significantly reduce frequency drifts related to laser aging. In the experiment, they demonstrate improvement by over one order of magnitude in the stability of a clock that is limited by light-shift-induced frequency drifts.