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Novel Optical Systems, Methods, and Applications

Comparison of quantum CPT and EPR magnetometers

Daba A. Radnatarov, Valeriy Andryushkov, Sergey M Kobtsev

Published Aug 21, 2020• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.2568335

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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.
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.
We present an experimental and theoretical investigation of the coherent population trapping (CPT) resonance excited on the D1 line of 87 Rb atoms by bichromatic linearly polarized laser light. The experimental results show that a lin ‖ lin transition scheme is a promising alternative to the conventional circular—circular transition scheme for an atomic magnetometer. Compared with the circular light transition scheme, linear light accounts for high-contrast transmission resonances, which makes this excitation scheme promising for high-sensitivity magnetometers. We also use linear light and circular light to detect changes of a standard magnetic field, separately.
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.
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.
An atomic magnetometer based on coherent population trapping (CPT) resonances in microfabricated vapor cells is demonstrated. Fabricated by the micro-electro-mechanical-system (MEMS) technology, the cells are filled with Rb and Ne at a controlled pressure. An experimental apparatus is built for characterizing properties of microfabricated vapor cells via the CPT effects. The typical CPT linewidth is measured to be about 3 kHz (1.46 kHz with approximately zero laser intensity) for the rubidium D1 line at about 90 °C. The effects of pressure, temperature and laser intensity on CPT linewidth are studied experimentally. A closed-loop atomic magnetometer is finally finished with a sensitivity of 210.5 pT/Hz1/2 at 1 Hz bandwidth. This work paves the way for developing an integrated chip-scale atomic magnetometer in the future.