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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Coherent population trapping for the realization of a small, stable, atomic clock

Jaques Vanier

Published Jun 25, 2003• 12 citations
Publisher

Abstract

The coherent population trapping phenomenon (CPT) has created interesting avenues in the practical realization of atomic frequency standards using laser optical pumping. In practice two different types of standards can be implemented: a passive clock based on the dark state created in the ensemble by the CPT phenomenon and a maser based on the ground state oscillating magnetization induced by the same phenomenon. The paper describes the basic physics involved in the approach and summarizes the progress made thus far in the practical implementation of such frequency standards.

DOI
10.1109/freq.2002.1075920

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The coherent population trapping phenomenon (CPT) has created interesting avenues in the practical realization of atomic frequency standards using laser optical pumping. In practice two different types of standards can be implemented: a passive clock based on the dark state created in the ensemble by the CPT phenomenon and a maser based on the ground state oscillating magnetization induced by the same phenomenon. The paper describes the basic physics involved in the approach and summarizes the progress made thus far in the practical implementation of such frequency standards.
The paper describes the practical realization and characteristics of a small, totally autonomous, passive frequency standard based on the coherent population trapping (CPT) phenomenon. A description of the frequency standard, including physical characteristics, block diagrams of the optical control CPT resonance system and of the digital control electronics is given. The characteristics of the frequency standard relative to general functionality and frequency stability are given, as measured on several units.
The paper summarizes the relative advantages and disadvantages of coherent population trapping (CPT) or intensity optical pumping (IOP) for the implementation of a passive atomic frequency standard using the isotope /sup 87/Rb. This paper outlines the basic principles common to both CPT and IOP when using laser optical pumping, and makes explicit their similarities and their differences. This paper describes experimental results obtained in the same cell on the characteristics of the CPT and IOP /sup 87/Rb-hyperfine-resonance line. The measurements showed that the signal contrast is larger in CPT than in IOP for the same resulting line width; the light shift is smaller in CPT than in IOP, and is easier to control; in principle, a passive frequency standard based on CPT has a smaller size than that based on IOP, due to the absence of a microwave cavity. Conclusions on overall expectations for the future of such frequency standards are drawn.
We proposed a scheme to implement coherent population trapping (CPT) atomic clock based on the transient CPT phenomenon. We proved that the transient transmitted laser power in a typical Λ system near CPT resonance features as a damping oscillation. Also, the oscillating frequency is exactly equal to the frequency detuning from the atomic hyperfine splitting. Therefore, we can directly measure the frequency detuning and then compensated to the output frequency of microwave oscillator to get the standard frequency. By this method, we can further simplify the structure of CPT atomic clock, and make it easier to be digitized and miniaturized.
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.
The paper gives an overview of our present theoretical understanding of coherent population trapping in alkali metal atoms as used in the implementation of an atomic frequency standard. Two approaches are examined: The passive approach in which the microwave hyperfine resonance is detected directly on the transmitted radiation and the maser approach in which the same resonance is observed by means of stimulated emission in a microwave cavity. The theoretical analysis reviewed is supported by experimental results.