Quantum explainers: Optically pumped magnetometers
Why do we need to measure magnetic fields in medicine?
When neurons fire in the brain or the heart contracts, tiny magnetic fields are produced. These signals can reveal valuable information about conditions such as epilepsy, Parkinson's disease and cardiac arrhythmias. However, measuring them is extremely difficult because they are very weak - millions of times weaker than the Earth's magnetic field.
For decades, scientists have relied on large, expensive systems to detect these signals. While highly effective, these technologies can be restrictive as they often require people to remain completely still during scans, making measurements more challenging for children and some patient groups.
Optically pumped magnetometers (OPMs) are changing that. Small enough to be worn and sensitive enough to detect the body's faint magnetic signals, they are opening up new possibilities for studying the brain and heart in more natural and comfortable settings.
What are optically pumped magnetometers?
Optically pumped magnetometers (OPMs) are ultra-sensitive instruments that measure magnetic fields using light and the quantum properties of atoms rather than traditional coils or electronics. Because they rely on quantum physics, they can detect extremely small magnetic signals, even those produced by the human brain or heart.
How do OPMS work?
Inside an OPM is a small glass cell containing a vapour of atoms, typically alkali metals such as rubidium or caesium, although some designs use helium. These atoms behave like tiny magnetic compasses, with their spins responding to the magnetic field around them.
To prepare the atoms for measurement, a process called optical pumping is used. A laser beam at a precisely defined frequency shines through the vapour, aligning the spins of the atoms so they all point in the same direction.
When a magnetic field is present, these aligned spins start to wobble, in the same way that a spinning top tilts under gravity. This wobbling, called precession, happens at a rate that depends on the strength of the magnetic field.
A second laser, called a probe laser, measures this precession (although some systems use one laser to perform both the optical pumping and probing). As it passes through the vapour, the properties of the light change slightly depending on how the atoms are spinning. By measuring these changes with a photodetector, it is possible to calculate the strength and, in some cases, the direction of the magnetic field.
What are the benefits over existing technology?
Measuring magnetic signals from the brain can be done with a technique called magnetoencephalography (MEG).
Traditional MEG systems use superconducting quantum interference devices (SQUIDs). These use loops of superconducting material and while they can detect extremely weak magnetic signals from the brain, they need to be cooled to extreme cryogenic temperatures using liquid helium to work. Because of this, the sensors are in a fixed position, which means that patients need to be completely still when being scanned.
Using OPMs for MEG (OP-MEG) helps overcome some of these issues. OPMs operate at room temperature so they don’t need cryogenic cooling, which makes them simpler to use and more practical than SQUID-MEG systems.
OPMs are also very compact – each one is about the size of a Lego brick. This allows them to be placed close to the scalp, improving sensitivity for brain measurements.
Their small size also means that they can be incorporated into a wearable helmet that moves with the patient. Researchers can therefore measure brain activity while a person moves naturally, something that is not possible with conventional systems. As a result, children and patients with conditions such as Parkinson's disease can be scanned more comfortably and in more natural settings.
What can OPMs be used for?
Brain imaging
Magnetoencephalography (MEG) measures magnetic signals from the brain to study conditions like epilepsy or Parkinson’s. Current SQUID-MEG systems are large and require patients to stay still. OPMs make MEG wearable and more comfortable, enabling longer scans and even measurements while patients move.
Heart monitoring
OPMs can also be used to detect magnetic signals from the heart (a technique called magnetocardiography or MCG), and could help doctors to diagnose conditions like arrhythmias without invasive procedures.
Magnetic relaxometry thermometry
Some cancer therapies use magnetic nanoparticles heated by an external magnetic field to destroy cancer cells, but it is crucial to measure the temperature of the particles to prevent damage to healthy tissue. As magnetic nanoparticles relax (i.e. lose their magnetisation) at a rate that depends strongly on temperature, Q-BIOMED researchers are investigating how we could use OPMs to develop systems that measure the temperature of these particles remotely.
By making magnetic sensing more precise, portable and patient-friendly, OPMs are transforming how we study the brain and heart. Researchers are also exploring entirely new applications, from advanced diagnostics to monitoring emerging cancer therapies, opening the door to a new generation of quantum-enabled healthcare technologies. While OPM-based systems are not yet widely used in hospitals, ongoing research is helping to bring these technologies closer to clinical practice.
What can OPMs be used for?
Brain imaging
Magnetoencephalography (MEG) measures magnetic signals from the brain to study conditions like epilepsy or Parkinson’s. Current SQUID-MEG systems are large and require patients to stay still. OPMs make MEG wearable and more comfortable, enabling longer scans and even measurements while patients move.
Heart monitoring
OPMs can also be used to detect magnetic signals from the heart (a technique called magnetocardiography or MCG), and could help doctors to diagnose conditions like arrhythmias without invasive procedures.
Magnetic relaxometry thermometry
Some cancer therapies use magnetic nanoparticles heated by an external magnetic field to destroy cancer cells, but it is crucial to measure the temperature of the particles to prevent damage to healthy tissue. As magnetic nanoparticles relax (i.e. lose their magnetisation) at a rate that depends strongly on temperature, Q-BIOMED researchers are investigating how we could use OPMs to develop systems that measure the temperature of these particles remotely.
By making magnetic sensing more precise, portable and patient-friendly, OPMs are transforming how we study the brain and heart. Researchers are also exploring entirely new applications, from advanced diagnostics to monitoring emerging cancer therapies, opening the door to a new generation of quantum-enabled healthcare technologies. While OPM-based systems are not yet widely used in hospitals, ongoing research is helping to bring these technologies closer to clinical practice.
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