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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Coherent population trapping for magnetic field measurements

Emilia A. Alipieva, C. Andreeva, Lachezar A. Avramov, G. Bevilaqua, V. Biancalana, Ekaterina G. Borisova, Evelina Breschi, S. Cartaleva, Yordanka V. Dancheva, S. Gateva, Todor S. Karaulanov, Roland Lammegger, L. Moi, Lyubomir P. Petrov, Nikolay Petrov, Dimitar G. Slavov, Elena T. Taskova, G. Todorov, Laurentius Windholz, Andrey Yanev

Published Apr 22, 2005• 7 citations
Publisher

Abstract

The purpose of this communication is to summarize and compare the results obtained under the project "New all-optical systems and methods for magnetic and electromagnetic field measurement", financed by EC (contract G6RD-CT-2001-000642). The main aim of the project is investigation of the potential of different approaches based on the effect of Coherent Population Trapping (CPT) for magnetic field measurement. The investigated approaches are: (i) CPT prepared by coupling pairs of non-degenerate Zeeman sublevels belonging to the two ground-state hyperfine components of alkali atoms by means of polychromatic laser field; (ii) CPT prepared by coupling degenerate Zeeman sublevels belonging to a single ground-state hyperfine component of alkali atoms by means of single-frequency laser field of appropriate polarization at zero magnetic field (MF); and (iii) polychromatic laser field coupls pairs of non-degenerate Zeeman sublevels belonging to a single ground-state hyperfine level of alkali atoms.

DOI
10.1117/12.617773

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The purpose of this communication is to summarize and compare the results obtained under the project "New all-optical systems and methods for magnetic and electromagnetic field measurement", financed by EC (contract G6RD-CT-2001-000642). The main aim of the project is investigation of the potential of different approaches based on the effect of Coherent Population Trapping (CPT) for magnetic field measurement. The investigated approaches are: (i) CPT prepared by coupling pairs of non-degenerate Zeeman sublevels belonging to the two ground-state hyperfine components of alkali atoms by means of polychromatic laser field; (ii) CPT prepared by coupling degenerate Zeeman sublevels belonging to a single ground-state hyperfine component of alkali atoms by means of single-frequency laser field of appropriate polarization at zero magnetic field (MF); and (iii) polychromatic laser field coupls pairs of non-degenerate Zeeman sublevels belonging to a single ground-state hyperfine level of alkali atoms.
We report our study on coherent population trapping (CPT) in a $\ensuremath{\Lambda}$ configuration coupled by magnetic dipole interactions. The $\ensuremath{\Lambda}$ configuration is formed by bichromatic radio-frequency fields coupling a pair of Zeeman sublevels in one hyperfine level of an alkali-metal atom to another sublevel in the other hyperfine level. The Zeeman sublevel at the apex is coupled to a state in the nPnPnP manifold via an optical transition on one of the spectroscopic DDD lines to induce optical pumping to a dark state. The configuration is closed without leakage to other Zeeman sublevels. We use lithium atoms in an optical trap for our experimental study. The system allows independent control of the main parameters characterizing CPT, which include the upper-state decay rate and the decoherence rate as well as the Rabi frequencies. By turning off the applied fields, the system is frozen so that its quantum state can be measured precisely. We studied the line shapes and dynamics of the CPT system, and measured the phase relation of the dark superposition state to find excellent agreement with theory. The possible application of the scheme as a method to cool optically trapped atoms below the recoil limit in a manner analogous to velocity-selective coherent population trapping is discussed.
SummaryCurrent compact atomic clock technologies rely on coherent population trapping (CPT), which employs a microwave resonance to stabilize a crystal oscillator. Standard CPT, while a simple and easy-to-implement optical preparation scheme in alkali metal atoms, suffers from low signal-to-noise ratios (SNR). The primary contributor to the low signal amplitude is the collection of atoms at the extreme ends of the Zeeman sublevels in the hyperfine levels of the alkali ground state, so-called “trap states.” Solutions to the trap-state problem have been reported previously, though the redistribution of atomic population into the CPT signal states with these methods has not yet been carefully examined. This work aims to provide an experimental basis to evaluate trap-state recovery schemes.
Coherent population trapping (CPT) resonances on degenerate two-level system of D/sub 1/ line of /sup 87/Rb were investigated with single-frequency laser excitation in Hanle configuration. Their sensitivity to stray magnetic fields was examined for linear, elliptical, and circular laser polarization. A methodology for magnetic field (MF) measurement using the CPT resonance is proposed. The sensitivity of the system to weak magnetic fields was evaluated and a magnetic field of 3 nT was unambiguously detected.
Magnetic field and its gradient measurements based on coherent population trapping resonances at the D1 line of 87 Rb are performed by means of coupling two ground-state Zeeman sublevels belonging to different hyperfine levels to a common excited state.Two coherent laser fields with frequency difference of 6.8 GHz are used.They are produced by direct current modulation of a diode laser.The resonance splitting and broadening caused by the applied magnetic field is measured by scanning the laser modulation frequency around the ground-state hyperfine frequency.
A brief introduction is presented related to three different approaches for Coherent Population Trapping (CPT) observation, namely CPT resonance prepared: (i) in degenerate two-level system, (ii) by coupling pairs of non-degenerate Zeeman sublevels of two ground levels and (iii) utilizing Zeeman sublevels within a single hyperfine transition. As an illustration of the first two approaches recent results concerning the CPT preparation on the first resonance line in Na and K are described. The first, to our best knowledge, CPT resonance observation on the second resonance line of K (violet, 404.4 nm) is reported. A transfer of the CPT resonance due to cascade transitions, without compromising its parameters, is evidenced. The presented results are promising for investigation of processes in atomic vapour and gas discharge.
Summary form only given. We have experimentally investigated the potential of narrow coherent population trapping resonances for precision applications like magnetometry or atomic frequency standards. CPT can be observed in /spl Lambda/ systems where two ground states are coupled to a common excited state by two near-resonant light fields. Effective formation of ground state coherences takes place when the frequency difference of the light fields precisely matches the ground state splitting. This process leads to reduced absorption in the medium (electromagnetically induced transparency) and thus reduced fluorescence intensity (dark resonance) at the optical resonance frequency. Extremely narrow dark resonances can be observed when the two light fields have fixed phase and frequency difference and time-of-flight broadening is suppressed by the use of a buffer gas, making this type of system a promising candidate for precision applications. The /spl Lambda/ systems under study here are the D/sub 2/ lines of Cs, /sup 87/Rb and /sup 85/Rb where the ground state hyperfine splittings are 9.2 GHz, 6.8 GHz, and 3.0 GHz, respectively. Thus in a magnetic field the CPT resonance splits into several Zeeman components whose frequency positions are determined by the Breit-Rabi formula.