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Optical and Quantum Sensing and Precision Metrology

Synchronous coherent population trapping for remote magnetic field measurement

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

Published Mar 4, 2021• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.2582433

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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.
We have demonstrated a technique for remote magnetometry using fluorescence measurements in a sodium cell. This technique utilizes two-photon resonance instead of single-photon resonance to generate magnetic resonances with synchronous modulation of two laser fields. A strong magnetic resonance with its dip matching the Larmor frequency, ΩL, is produced when a magnetic field perpendicular to the laser propagation direction is applied. A magnetic field sensitivity of 45 pT/√Hz is measured at a 35% modulation duty cycle where ΩL resonance has a maximum amplitude. We have discussed ways to further improve the sensitivity closer to the shot-noise limit by making changes to our existing setup. An additional magnetic resonance at 3ΩL is also formed due to two-photon resonance, which can be utilized in determining the magnetic field orientation. We have developed a theoretical model based on density matrix equations to study the magnetic resonances under different conditions and also for validating our experimental observations.
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 have developed a field-deployable vector magnetometer using a small rubidium vapor cell, which is capable of measuring the magnitude and direction of the earth's magnetic field with high accuracy and sensitivity. The magnetometer measures Larmor precession of rubidium atoms using the phenomenon, called synchronous coherent population trapping (SCPT). This gives the advantage of producing a high contrast and narrow linewidth resonance needed for measuring the magnetic field with high accuracy and sensitivity. The magnetometer exhibits sub-nT/√Hz sensitivities in measuring the field components along the three directions.
Measurement and monitoring of geomagnetic field in the mesosphere has many potential applications such as detection of oceanic currents, mapping of large-scale magnetic structures, and study of electric-current fluctuations in the ionosphere. Remote measurement of the geomagnetic field can be performed by optical pumping of sodium atoms in the mesosphere with an amplitude modulated (AM) light at the Larmor precession frequency, and observing the magnetic resonance in fluorescence at a ground station. In this context, we have conducted an experiment in the laboratory to demonstrate remote magnetic field measurement using fluorescence signal from a sodium cell. Sodium atoms in the cell are interrogated with AM light produced by an acousto-optic modulator (AOM), introduced in the beam path of a frequency-doubled amplified sodium laser made in our laboratory. Magnetic resonance observed in sodium cell fluorescence is studied as a function of varying magnetic field intensity. Characteristics of the magnetic resonance such as linewidth and contrast (or SNR), and their dependence on optical power density, AM duty cycle, B-field orientation and intensity are studied. Theoretical modeling and experimental measurements are also carried out to determine the sensitivity of the magnetometer for realizing remote magnetometry with mesospheric sodium atoms.
An automated magnetometer suitable for long lasting measurement under stable and controllable experimental conditions has been implemented. The device is based on coherent population trapping (CPT) produced by a multifrequency excitation. CPT resonance is observed when a frequency comb, generated by diode laser current modulation, excites Cs atoms confined in a π/4×(2.5)2×1 cm3, 2 TorrN2 buffered cell. A fully optical sensor is connected through an optical fiber to the laser head allowing for truly remote sensing and minimization of the field perturbation. A detailed analysis of the CPT resonance parameters as a function of the optical detuning has been made in order to get high sensitivity measurements. The magnetic field monitoring performances and the best sensitivity obtained in a balanced differential configuration of the sensor are presented.