Optically pumped magnetometers

Optically pumped magnetometers (OPMs) are high-sensitivity quantum sensors that take advantage of the unique alkali vapor properties and their interaction with external magnetic and laser fields to detect magnetic fields with unpresentable sensitivity. 

OPMs use optical methods to spin-polarise an atomic medium and detect its coherent response to a magnetic field. OPMs surpass the sensitivity of superconducting sensors, while also working without cryogenics, making them potentially disruptive in the bio magnetic applications of magnetoencephalography (MEG) and magneto-cardiography (MCG). Deploying OPMs in these applications requires them to be simultaneously sensitive, fast, with a large dynamic range, close packable, manufacturable, and reliable in large numbers. 

Objective: in macQsimal, partners work closely together for the design, development, and validation of Compact Optically Pumped Magnetometers (OPM) for low-frequency and radio-frequency magnetic fields for biomagnetic scientific and medical applications.

Related macQsimal publication

Sub-pT optical magnetometry with squeezed light
The Institute of Photonic Sciences (ICFO), 2020, micro-WOPM 2020, online.

Magnetic field modeling with surface currents. Part II. Implementation and usage of bfieldtools
Zettera et all (2020) Journal of Applied Physics, 128(6), 10897550. A preprint available on arXiv.

Magnetic-field modeling with surface currents. Part I. Physical and computational principles of bfieldtools
Mäkinena et all (2020) Journal of Applied Physics, 128(6), 063906. A preprint  available on arXiv.

Scale-invariant spin dynamics and the quantum limits of field sensing
Mitchell, M.W. (2020) New Journal of Physics, 22(5), 053041. A preprint available on arXiv.


Measurement-induced, spatially-extended entanglement in a hot, strongly-interacting atomic system
Kong, J., Jiménez-Martínez, R., Troullinou, C., Lucivero, V., Tóth, G., Mitchell, M.W. (2020) Nature Communications, 11(1), 2415.

Quantum limits to the energy resolution of magnetic field sensors
Mitchell, M.W., Palacios Álvarez, S. (2020) Reviews of Modern Physics, 92(2), 021001.
A preprint available on arXiv.

Detection of low-conductivity objects using eddy current measurements with an optical magnetometer
Jensen et all (2019) Physical Review Research, 1(3), 033087.

Laboratory setup for OPMs

In macQsimal we characterise, test and optimise the different components of the sensor with the ultimate goal to construct an OPM device with a superior balance between magnetic sensitivity, size and other operating constraints, e.g. proximity to the cerebral cortex, in MEG.  

One of the novel characteristics of the instrumentation employed by macQsimal partner, ICFO,  is the use of integrated three-axis biplanar pcb coils, specially designed for maximum magnetic field homogeneity at the cell region in all three directions and minimal stray field, enabling many sensors to be packed close together for arrayed operation. ICFO’s test bench (photo on the right) for OPM development occupies a full optical table. However,  each final prototype sensor will be the size of a fingertip.

Researchers working in the experimental setup in the lab at ICFO.

Quantum-enhanced OPMs

The magnetic sensitivity of OPMs is fundamentally limited by quantum noise. In these sensors, two quantum systems – atoms and light – interact to produce the signal. Understanding and controlling the quantum noise in this interacting system is an outstanding challenge. In macQsimal we are applying quantum resources like optical and spin squeezing techniques, and quantum entanglement to improve the performance of state-of-the-art atomic sensors as OPMs.  

Researchers from ICFO have recently demonstrated the first case in which optical squeezing improves a high-performance optically pumped magnetometer.

Squeezed light magnetometer setup, combining the squeezer setup and the Bell Bloom magnetometer.
Figure showing generated squeezed light probe interacting with processing polarized atoms from the recently published paper.

OPMs package

The optimisation of the design and the selection of the materials are the key issues in order to have a non-magnetic package. macQsimal partner VTT manufactures a ceramic sensor package with a low footprint using low temperature co-fired ceramic technology.  This technology consists of several process steps, including screen-printing of conductors, stacking of tape layers, lamination of the stack, and co-firing to manufacture a multi-layer substrate. The OPM package contains the atomic vapor cell. The cell is assembled on a ceramic isostatic holder which is then integrated into the LTCC package.

Vapor cell oven OPM test bench.

Towards market aplicability

Since OPM sensors can be placed directly on the scalp, the measured MEG signals are larger than those from the superconducting sensors conventionally used in MEG. We have shown this improvement by simulations and also by experiments with our custom MEG system we have built using commercially available OPM sensors.
OPM sensors have strengths and weaknesses when compared to superconducting sensors: They are compact and don’t require cryogenics, but they are expensive, the outer surface of the sensor can be quite hot, the amplitude response is nonlinear, and there can be significant crosstalk when the sensors are placed in dense arrays.
macQsimal partners, AALTO and MEGIN focus on these weaknesses.

Magnetoencephalography (MEG) traditional system – MEGIN. With aim of quantum enhanced sensors for higher accuracy in brain screening.