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Physical Review A

Synchronous coherent population trapping and its magnetic spectral response in rubidium vapor

G. S. Pati, Renu Tripathi, Raghwinder Singh Grewal, Mauricio Pulido, Robin Depto

Published Sep 24, 2021• 13 citations
Publisher

Abstract

This paper reports experimental investigations of the spectral response of synchronous coherent population trapping (CPT) for magnetometry applications. Synchronous CPT is demonstrated using resonant two-photon excitation with a modulated bichromatic laser field in the $^{87}\mathrm{Rb} {D}_{1}$ manifold. Spectra containing multiple magnetic resonances are produced due to two-photon excitations created by various frequency components of the modulated beam. Theoretical modeling based on the atomic density-matrix equations is used to simulate the spectral response of synchronous CPT. Our results show potential advantages of synchronous CPT over synchronous optical pumping for magnetic field measurement. Magnetic resonance signals produced by synchronous CPT can be used in eliminating the dead-zone problem and also in determining the magnetic field direction.

DOI
10.1103/physreva.104.033116

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This paper reports experimental investigations of the spectral response of synchronous coherent population trapping (CPT) for magnetometry applications. Synchronous CPT is demonstrated using resonant two-photon excitation with a modulated bichromatic laser field in the 87RbD1^{87}\mathrm{Rb} {D}_{1}87RbD1​ manifold. Spectra containing multiple magnetic resonances are produced due to two-photon excitations created by various frequency components of the modulated beam. Theoretical modeling based on the atomic density-matrix equations is used to simulate the spectral response of synchronous CPT. Our results show potential advantages of synchronous CPT over synchronous optical pumping for magnetic field measurement. Magnetic resonance signals produced by synchronous CPT can be used in eliminating the dead-zone problem and also in determining the magnetic field direction.
We demonstrate a new mechanism, suitable for performing remote earth field measurement using synchronous modulation of two laser fields, which are two-photon resonant with the sodium atom’s hyperfine ground states. This mechanism relies on coherent population trapping (CPT), and thus, called the synchronous CPT scheme. We have measured the magnetic resonances in fluorescence from a sodium cell to demonstrate suitability of this scheme for remote magnetometry. A strong magnetic resonance with its dip corresponding to the Larmor frequency is produced in the presence of a transverse magnetic field. To estimate the optimal duty cycle of light modulation for attaining high performance, we measured the amplitudes and linewidths of the magnetic resonance as a function of the duty cycle, by keeping the peak intensity in CPT fields constant.
Coherent population trapping (CPT) is extensively studied for future vapor cell clocks of high frequency stability. In the constructive polarization modulation CPT scheme, a bichromatic laser field with polarization and phase synchronously modulated is applied on an atomic medium. A high contrast CPT signal is observed in this so-called double-modulation configuration, due to the fact that the atomic population does not leak to the extreme Zeeman states, and that the two CPT dark states, which are produced successively by the alternate polarizations, add constructively. Here, we experimentally investigate CPT signal dynamics first in the usual configuration, a single circular polarization. The double-modulation scheme is then addressed in both cases: one pulse Rabi interaction and two pulses Ramsey interaction. The impact and the optimization of the experimental parameters involved in the time sequence are reviewed. We show that a simple seven-level model explains the experimental observations. The double-modulation scheme yields a high contrast similar to the one of other high contrast configurations like push-pull optical pumping or crossed linear polarization scheme, with a setup allowing a higher compactness. The constructive polarization modulation is attractive for atomic clock, atomic magnetometer, and high precision spectroscopy applications.
We are studying a new spectroscopic method using laser-cooled rubidium atoms that allows us to lock the carrier frequency of modulated laser in the coherent population trapping (CPT) clock experiment. This method uses counter-propagation of the bichromatic laser field produced by the modulated laser. The counter-propagation cancels the CPT effect near the optical resonance and gives rise to enhanced Doppler-free absorptive resonances with high-contrast away from the transition. We demonstrate the CPT clock operation by locking the modulated laser using this resonance. We also describe a theoretical model that we have developed based on density-matrix formalism to simulate this spectroscopic scheme for accurately predicting the lineshape and amplitude of resonances under different excitation conditions. Our study suggests that this spectroscopic method is also quite suiatble for developing cold-atom based optical frequency standard.
A new source of two diode laser beams, spatially separated but optically phase-locked with each other, is used to study the modulation transfer spectroscopy of coherent population trapping resonance (CPT). The spectrum for the (87)Rb D2 line is obtained with narrow linewidth and high signal-to-noise ratio, and analyzed with different experimental parameters. A theoretical analysis of the CPT modulation transfer spectra is deduced from the density matrix equation of motion, and found to be in good agreement with the experimental results.