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Conference on Lasers and Electro-Optics

Quantum limit sensitivity of coherent dark-state magnetometers

Hwang Lee, Igor K. Kulikov, A.P. Dowling

Published Jun 25, 2003• 0 citations
Publisher

Abstract

Summary form only given. The measurement of magnetic fields is done by magnetometers. In an atomic cell experiment the medium is Doppler broadened and the decay rate of the coherence between the lower lying levels are determined by the time of flight. For a homogeneously broadened medium the trapped neutral atoms can be used.
DOI
10.1109/cleo.2002.1033398

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An optical magnetometer based on atomic coherence effects is analyzed using a quantum Langevin approach. The large dispersion of a phase-coherent atomic medium (``phaseonium'') at a point of vanishing absorption is used to detect magnetic level shifts via optical phase measurements in a Mach-Zehnder interferometer with sensitivities potentially superior to state-of-the-art devices. Effects of Doppler broadening and fluctuations of the driving field are discussed and a comparison to standard optical-pumping magnetometers is made.
Summary form only given. By now there have been proposed and partly realized in experiments several optical schemes for the precise measurement of a magnetic field. Among them the most promising methods are based on atomic coherence (first proposed by Scully and Fleischhauer in 1992). In one of optical schemes the interaction with spatially separated fields was used, following to the Ramsey method. We propose a new variant of magnetometer, where the advantages of the Ramsey method are combined with the high-contrast detection provided by the coherent population trapping effect. In our scheme for the measurement of arbitrary directed magnetic field B a cell with atomic vapor is used.
24 In recent years interest has been growing into the fascinating properties of dark resonances. Most of the experiments have been carried out in alkali atoms due to their favourable level structure and the commercially available diode lasers, resulting in numerous applications. Teams from Siena and Sofia developed an all-optical magnetometer based on such dark resonances. The dark-state effect was discovered in 1976 by G. Alzetta, A. Gozzini, L. Moi and G. Orriols [1], while they were working on optical-pumping experiments with a multimode dye laser. They wanted to make the observation of multi-photon radio-frequency resonances in sodium atoms easy, i.e., observable by the naked eye. To this end, they introduced a magnetic field (MF) that was constant in time but spatially inhomogeneous along the laser beam, so as to produce Zeeman splitting of groundstate hyperfine levels (Fig.1a, left). In this way, an applied radio-frequency field was resonant only if it matched the Zeeman splitting. As a result, a bright radio-frequency resonance appeared (Fig.1a, right). During these experiments, besides the bright resonances, a new “dark” resonance was observed, which does not require any radio-frequency field. In the introductory illustration only the dark resonance is shown. The new dark state (DS) effect appears in a position along the laser beam where the frequency difference (ω10-ω20) between two optical frequencies is equal to the frequency splitting ω12 between two Zeeman sublevels (Fig.1b, left). This is the resonant condition for the stimulated Raman transitions between the two longliving ground levels. Theoretical studies described the dark resonance as a coherent superposition of two long-living ground levels prepared by the bi-chromatic laser field, and ‘atomic population trapping’ was introduced, leading to the term Coherent Population Trapping (CPT) [2]. l Luigi Moi1, Stefka Cartaleva2 DOI: 10.1051/epn/2012603
Summary form only given, as follows.The 30 kHz linewidth and extremely steep dispersion of a coherent dark resonance in a caesium vapor cell allows measurements of magnetic fields to about 10 nT in 0.1 s integration time, limited by stray fields. The narrow resonance should make possible a compact frequency standard. In a preliminary experiment the authors used a coherent dark resonance in a 2 cm long caesium vapor cell to measure magnetic fields with about 10-5 uncertainty. Two diode lasers (852 nm wavelength) were phase-locked onto each other with a 9.2 GHz frequency offset, corresponding to the frequency ground state separation of the two caesium levels. The difference frequency is derived from a stable tunable rf source. Absorption spectrum of the caesium coherent population trapping resonance for linear laser polarizations is illustrated; it is split into seven components by a magnetic field parallel to the laser polarization direction. The width of the individual lines is only 130 kHz due to power broadening. The steep slope of the resonance could be used to lock the frequency of the 9.2 GHz rf source. This would provide a compact frequency reference.
We investigate the experimental sensitivity limit of a scalar optical magnetometer based on coherent population trapping on the D 2 line of a thermal cesium vapor. We find the expected strong dependence on averaging time, with a detection limit below 4 pT for integration times longer than 1 s, limited by slow drifts of the cell temperature and of the applied test magnetic-field itself. A detailed noise analysis shows that for shorter averaging times the demonstrated limit of about 12 pT/√Hz is dominated by the frequency noise of the laser source. The magnetometer can be operated for real-time detection in flux densities up to at least milliteslas and is robust enough for application outside the laser laboratory.
The combination of unmanned aerial vehicles and atomic magnetometers can be used for detection applications such as mineral resource exploration, environmental protection, and earthquake monitoring, as well as the detection of sunken ships and unexploded ordnance. A dark-resonance atomic magnetometer offers the significant advantages of a fully optical probe and omnidirectional measurement with no dead zones, making it an ideal choice for airborne applications on unmanned aerial vehicles. Enhancing the sensitivity of such atomic magnetometers is an essential task. In this study, we sought to enhance the sensitivity of a dark-state resonance atomic magnetometer. Initially, through theoretical analysis, we compared the excitation effects of coherent population trapping (CPT) resonance on the D1 and D2 transitions of 133Cs thermal vapor. The results indicate that excitation via the D1 line yields an increase in resonance contrast and a reduction in linewidth when compared with excitation through the D2 line, aligning with theoretical predictions. Subsequently, considering the impact of various quantum system parameters on sensitivity, as well as their interdependent characteristics, two experimental setups were developed for empirical investigation. One setup focused on parameter optimization experiments, where we compared the linewidth and contrast of CPT resonances excited by both D1 and D2 transitions; this led to an optimization of atomic cell size, buffer gas pressure, and operating temperature, resulting in an ideal parameter range. The second setup was employed to validate these optimized parameters using a coupled dark-state atom magnetometer experiment, achieving approximately a 10-fold improvement in sensitivity.