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Quantum vector magnetometer based on the CPT effect in 87Rb vapour

Valeriy Andryushkov, Daba A. Radnatarov, Sergey M Kobtsev, Aldar Dambuev

Published Aug 22, 2020• 2 citations
Publisher

Abstract

Developed and characterised for the first time is a quantum vector magnetometer using coherent population trapping (CPT) in 87Rb vapour and external magnetic field compensation by 3-axis Helmholtz coils to stabilize the amplitude and spectral position of the CPT resonance excited on a magneto-sensitive (mf ≠ 0) energy level transition. To measure the magnetic field components orthogonal to the magnetometer optical axis, resonance amplitude dependence upon the transverse field strength is used, which has a maximum at zero transverse field. The proposed approach allowed measurement of the external magnetic field vector with a sensitivity exceeding 500 pT/Hz1/2.

DOI
10.1117/12.2568340

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jun 21, 2004•6 citations
Developed and characterised for the first time is a quantum vector magnetometer using coherent population trapping (CPT) in 87Rb vapour and external magnetic field compensation by 3-axis Helmholtz coils to stabilize the amplitude and spectral position of the CPT resonance excited on a magneto-sensitive (mf ≠ 0) energy level transition. To measure the magnetic field components orthogonal to the magnetometer optical axis, resonance amplitude dependence upon the transverse field strength is used, which has a maximum at zero transverse field. The proposed approach allowed measurement of the external magnetic field vector with a sensitivity exceeding 500 pT/Hz1/2.
The method is presented for converting a coherent population trapping (CPT) atomic clock into a CPT vector (compass-) magnetometer without mechanically moving parts through a relatively simple add-on. The system of 3D Helmholtz coils was used for compensation of the external magnetic field, allowing measurement of both strength and direction of a magnetic field at the sensitivity level of s u b − n T / H z 1 / 2 within a 10-Hz bandwidth. Angular resolution of the developed vector magnetometer amounts to about 1 0 − 2 degrees.
Experimentally compared for the first time is the sensitivity of two magnetometer types implemented on a common base of rubidium atomic clock with coherent population trapping (CPT). The first magnetometer used a magneto-sensitive optically excited CPT resonance, the other measured field strength through the electron paramagnetic resonance (EPR) excited with alternating magnetic field. We show advantages and limitations of these promising quantum sensing technologies. Sensitivity of both types was measured under resonance excitation on the D1 line of 87Rb. We demonstrate that the СРТ magnetometer sensitivity may exceed that of the EPR device by two orders of magnitude.
We demonstrate a real-time, single-shot vector magnetic measurement technique using synchronous coherent population trapping (SCPT) in an apparatus consisting of a small rubidium vapor cell. Vector modality of our magnetometer is enabled by designing a feedback system based on performing a peak-lock on a particularly strong 2ΩL magnetic resonance produced in the SCPT spectrum and compensating the external magnetic field via a three-axis field coil. With its current design, this magnetometer exhibits high sensitivities of approximately 155, 129, and 57pT/Hz in measuring the magnetic field vector components along the (x,y,z) axes. Sensitivities closer to the shot-noise limit can be achieved in the future by reducing the technical noise of our equipment and by employing a differential detection and polarization rotation measurement scheme in our system.
We have set up a coherent population trapping (CPT)-based magnetometer prototype with the D1 line of ⁸⁷Rb atoms. The dichromatic light field is derived from a fiber electro-optic modulator (FEOM) connected to an external cavity laser diode. A CPT resonance signal with a 516 Hz linewidth is observed. By feeding back the derivative of the resonance curve to the FEOM with a proportional integral controller, of which the voltage output is directly converted to the measured magnetic field intensity, the resonance peak is locked to the environmental magnetic field. The measurement data we have achieved are well matched with the data measured by a commercial fluxgate magnetometer within 2 nT, and the sensitivity is better than 8 pT/√Hz in a parallel B field.
Coherent population trapping (CPT) resonances on the degenerate two-level system of the (Fg=2→Fe=1) transition of the 87Rb D1 line were investigated in an uncoated vacuum cell by means of level-crossing type experiments-sweeping a magnetic field around zero value and using a single laser beam. The two-photon resonance manifests itself as fluorescence canceling when the system of levels is degenerate (at zero point of magnetic field). The influence of weak transverse magnetic fields on the amplitude and width of the resonance was investigated. The CPT resonances were observed with linear, circular and elliptical polarized laser light. Transverse magnetic field widens the resonances obtained by linear and circular polarization but splits the resonance created by slightly elliptically polarized laser light. The dependence of this splitting on the elliptical ratio and experimental geometry were investigated. Due to its narrow width and sensitivity to low magnetic fields (< 1 G) this resonance is very promising for small magnetic field measurement.