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Characterization of a quantum sensing system with an optically pumped magnetometer for micro-target detection

Barbecho Barrado, Adrià

Published Jul 10, 2023• 0 citations
OpenAlex

Abstract

Quantum sensing has appeared as a promising technique to push the sensitivities of sensors to inherent classical physical limitations. In particular, quantum magnetometers opens the possibilities of new range of applications where extreme sensitivities are needed. A zero field Optically Pumped Magnetometer (OPM), coupled to a conical magnetic guide (MG) is known as a promising technique to improve the spatial resolution of the sensor. This coupling, though, can produce disturbances in the optimal sensing point. In this work a set of compensating coils is designed to reduce the impact of the magnetic guide in the initial sub-15 $\fT/\sqrt{Hz}$ sensitivity of the commercial QuSpin QZFM. First, the system and different strategies are studied theoretically and later implemented experimentally, improving the sensitivity in the OPM-MG by an order of magnitude, from 100 $fT/\sqrt{Hz}$ originally, to 10 $fT/\sqrt{Hz}$.

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Quantum sensing has appeared as a promising technique to push the sensitivities of sensors to inherent classical physical limitations. In particular, quantum magnetometers opens the possibilities of new range of applications where extreme sensitivities are needed. A zero field Optically Pumped Magnetometer (OPM), coupled to a conical magnetic guide (MG) is known as a promising technique to improve the spatial resolution of the sensor. This coupling, though, can produce disturbances in the optimal sensing point. In this work a set of compensating coils is designed to reduce the impact of the magnetic guide in the initial sub-15 \fT/Hz\fT/\sqrt{Hz}\fT/Hz​ sensitivity of the commercial QuSpin QZFM. First, the system and different strategies are studied theoretically and later implemented experimentally, improving the sensitivity in the OPM-MG by an order of magnitude, from 100 fT/HzfT/\sqrt{Hz}fT/Hz​ originally, to 10 fT/HzfT/\sqrt{Hz}fT/Hz​.
Abstract To address the demands in healthcare and industrial settings for spatially resolved magnetic imaging, we present a modular optically pumped magnetometer (OPM) system comprising a multi-sensor array of highly sensitive quantum magnetometers. This system is designed and built to facilitate fast prototyping and testing of new measurement schemes by enabling quick reconfiguration of the self-contained laser and sensor modules as well as allowing for the construction of various array layouts with a shared light source. The modularity of this system facilitates the development of methods for managing high-density arrays for magnetic imaging. The magnetometer sensitivity and bandwidth are first characterised in both individual channel and differential gradiometer configurations before testing in a real-world magnetoencephalography environment by measuring alpha rhythms from the brain of a human participant. We demonstrate the OPM system in a first-order axial gradiometer configuration with a magnetic field gradient sensitivity of 10 fT / cm / Hz at a baseline of 4.5 cm. Single-channel operation achieved a sensitivity of 65 fT / Hz . Bandwidths exceeding 200 Hz were achieved for two independent modules. The system’s increased temporal resolution allows for the measurement of spinal cord signals, which we demonstrate by using phantom signal trials and comparing with an existing commercial sensor.
Experimentally achieving optimal performance of quantum sensors, such as optically pumped magnetometers, is usually challenging and time consuming due to the complexity of the physical system. For a given experiment scheme, there are usually many experiment parameters that need to be fine tuned for achieving a satisfying performance of the quantum sensor. By employing global optimization algorithms, we propose an automatic and efficient way to optimize the sensor performance. We take the most sensitive magnetic field sensor, i.e., the optically pumped magnetometer, as an example to demonstrate how these algorithms facilitate the automatic optimization of quantum sensors. Both the optimization of zero-field magnetometers in a magnetic shield and the finite-field magnetometers in open Earth's field are demonstrated. A noise floor of approximately 10fT/Hz1/210\phantom{\rule{0.2em}{0ex}}\mathrm{fT}/{\mathrm{Hz}}^{1/2}10fT/Hz1/2 is achieved within a short time. This method provides an efficient way for quantum sensor optimization and will be helpful for the quantum sensor design.
Abstract Quantum sensing provides advanced technologies which significantly improve sensitivity and accuracy for sensing changes of motion, gravity, electric and magnetic field. Therein, quantum sensors for the detection of magnetic fields, so-called quantum magnetometers, are one of the most promising technological realizations. We firstly will provide a brief overview on methods in geophysical exploration benefitting from quantum magnetometers with resolution at the physical and technical limit. We will introduce recent developments on SQUID and OPM based sensors as specific implementations of a quantum magnetometer systems and application examples.
The most efficient way to improve the sensitivity of an optically pumped magnetometer (OPM) is to maximize the spin-polarization lifetime of the atoms. The spin exchange relaxation-free (SERF) OPM that operates in zero field environment is one of the most sensitive magnetic sensors. This paper proposes a new three-axis magnetic field compensation method for the single-beam OPM, which zeroes the magnetic field around the sensor to make it work in SERF regime. The method simultaneously employs two modulation magnetic fields and two low-frequency sweeping magnetic fields along two directions perpendicular to the laser beam during the compensation process. The output signal of the photodiode is demodulated at different frequencies, resulting in two outputs that reflect the three components of the magnetic field. This method does not require iterate to tune the three-axis field, which increases the speed of the compensation process significantly. The magnetic field resolutions of the method for the x-axis, y-axis and z-axis are 0.6 pT, 0.6 pT and 5 pt, respectively. A compact OPM with noise density about 20 fT / Hz1/2@10 Hz is developed and tested, demonstrating the feasibility of the method.
The optically pumped magnetometer (OPM) is a class of atomic devices that rely on the measurement of Larmor precession of an atomic spin ensemble in a magnetic field. They need the specific frequency light to complete the pumping and detection process. OPMs typically work under the Earth's magnetic field or a limited field. When operating in near-zero field, they can realize the spin-exchange relaxation-free (SERF) regime, thereby promoting a substantial increase in sensitivity. At present, OPMs have been widely used in magnetoencephalography, magnetocardiography, geomagnetic detection, and inertia measurement (comagnetometer). There are a large number of researchers engaged in the study of relevant mechanisms, devices, technologies, and applications. More importantly, supported by MEMS technology and micro/nano optics, OPMs have the potential to move toward chip-scale sensors. This special issue contains some latest developments in OPMs and related areas. We hope the reprint can provide valuable references for researchers in the related field.
We demonstrate a compact single-beam optically pumped magnetometer (OPM) employing a pure 87Rb microfabricated vapor cell with 500 Torr buffer gas. The residual magnetic field is actively minimized to below 400 pT through a combination of mu-metal magnetic shielding and internal compensation coils. We systematically characterize the OPM response as a function of laser detuning and optical intensity, and derive the magnetic-field sensitivity from the signal slope and noise amplitude spectral density. A best sensitivity of 11 fT/Hz^1/2 is achieved near the resonance transition at an optimal optical intensity, using a small cell volume of only 0.027 cm^3. Furthermore, a miniaturized OPM prototype was developed using a folded flexible printed circuit board (FPCB) to integrate the vapor cell, a compact heater, and magnetic-field coils into a cubic sensing head. Two coil configurations patterned directly on the FPCB were investigated. Under optimized operating conditions, the OPM prototype achieved a sensitivity of 66 fT/Hz^1/2. These results demonstrate that high-buffer-pressure single-beam OPMs can achieve excellent sensitivity, establishing a pathway toward compact and scalable arrays.