Optical Sensor for Magnetic Fields

Kasper Jensen in the Quantop research group’s laboratories at the Niels Bohr Institute where the experiments are carried out. (Source: O. J. Joensen)

Kasper Jensen in the Quantop research group’s laboratories at the Niels Bohr Institute where the experiments are carried out. (Source: O. J. Joensen)

Small magnetic fields from the human body can usually only be picked up by very sensitive super­conducting magnetic field sensors that have to be cooled by liquid helium to near absolute zero. But now researchers from the Niels Bohr Institute at the Uni­versity of Copen­hagen have developed a much cheaper and more practical optical magnetic field sensor that even works at room tempe­rature or at body tempe­rature.

“The optical magnetic field sensor is based on a gas of caesium atoms in a small glass container. Each caesium atom is equivalent to a small bar magnet, which is affected by external mag­netic fields. The atoms and thus the magnetic field are picked up using laser light. The method is based on quantum optics and atomic physics and can be used to measure extremely small magnetic fields,” explains Kasper Jensen from the Center for Quantum Optics, Quantop, at the Niels Bohr Institute at the University of Copen­hagen.

The magnetic field sensor itself consists of a glass container, which has a channel that is approxi­mately 1cm long and 1 mm wide. At the bottom of the glass container is caesium metal. Caesium eva­porates into gas at room tempera­ture and the gas atoms rise up into the small channel in the sensor head. Each caesium atom rotates around itself and the axis is like a tiny bar magnet. Now the sensor is held close to a nerve, which emits an electrical nerve pulse. The electrical pulse has a magnetic field that causes a change in the tilt of the axes of the caesium atoms and by sending a laser beam through the gas, you can read the ultra-small magnetic fields of the nerve signals.

The magnetic field sensor is made up of a glass container embedded with caesium metal. The caesium evaporates into gas at room temperature and the gas atoms rise up into the small channel in the sensor head. Each caesium atom is like a tiny bar magnet. Now the sensor is held close to a nerve, which emits an electrical nerve pulse. The electrical pulse has a magnetic field that causes a change in the tilt of the axes of the caesium atoms and by sending a laser beam through the gas, you can read the ultra-small magnetic fields of the nerve signals. (Source: K. Jensen, NBI)

The magnetic field sensor is made up of a glass container embedded with caesium metal. The caesium evaporates into gas at room temperature and the gas atoms rise up into the small channel in the sensor head. Each caesium atom is like a tiny bar magnet. Now the sensor is held close to a nerve, which emits an electrical nerve pulse. The electrical pulse has a magnetic field that causes a change in the tilt of the axes of the caesium atoms and by sending a laser beam through the gas, you can read the ultra-small magnetic fields of the nerve signals. (Source: K. Jensen, NBI)

The labora­tory tests, which were carried out in colla­boration with researchers from the Faculty of Health and Medical Sciences, have shown that you can use the magnetic field sensor to detect the magnetic fields from the elec­trical impulses from the nervous system. The tests were done on the sciatic nerve from a frog, which in many ways resemble the nerves in the human body. For practical reasons, the nerve was removed from the frog before the tests, but it is also possible to pick up electrical impulses from live frogs or from humans.

The advantage of the optical sensor is precisely that the magnetic fields and electrical impulses can be safely and easily picked up at a distance of a few milli­metres or centi­metres – without the sensor actually coming into contact with the body. “We expect that the sensor will be used for special medical exa­minations, where it is important for the sensor not to be directly in contact with the body, for example, for diag­nosing heart problems in tiny foetuses. Here the magnetic field sensor is placed on the mother’s abdomen and you can easily and safely detect the hear­tbeat of the foetus and you will be able to diagnose any heart problems at an early stage so that the foetus can get the right treatment quickly,” explains Eugene Polzik, head of Quantop at the Niels Bohr Institute.

Polzik explains that you can calculate the speed at which the nerve impulses are moving from the measured signals. There are a large number of diseases where the nerves are damaged, for example, multiple scleroses, where the nerve impulses move more slowly than in people who are not ill. Other issues could, for example, be a number of eye diseases where you will be able to make the diagnosis without having to put electrodes on the eye or Alzheimer’s, where you will be able to measure the electrical signals in specific nerve pathways. (Source: NBI)

Reference: K. Jensen et al.: Non-invasive detection of animal nerve impulses with an atomic magnetometer operating near quantum limited sensitivity, Sci. Rep. 6, 29638; DOI: 10.1038/srep29638

Link: Center for Quantum Optics Quantop, Niels Bohr Institute, University of Copenhagen, Denmark

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