Tiny sensor detects a mouse heart's magnetic pulse

 作者:姜觑     |      日期:2019-03-01 03:05:02
By Mason Inman A tiny sensor that is not much bigger than a grain of rice can detect magnetic fields as weak as those produced by a mouse’s heart. The sensor, known as an atomic magnetometer, could be used for a wide range of applications, from portable helmets for sensing brain activity to detectors of explosives, researchers say. It can detect a field as weak as 70 femtoteslas – about a billionth of the Earth’s magnetic field. The sensor’s sensitivity approaches that of much more complex devices called superconducting quantum interference devices (SQUIDs). These are the most sensitive magnetometers available, but they need to be cooled to near absolute zero to function, so they are relatively large and consume a lot of energy. The new sensor uses an alternative approach, known as optical magnetometry. A minute container traps a group of atoms – in this case about 100 billion atoms of rubidium gas. When hit with a laser, the spin of each atom aligns with in the direction of the laser beam. When the sensor is close to say, a brain, the magnetic fields produced by firing neurons perturbs these atoms, which blocks some of the laser light. The stronger the magnetic field, the less light gets through to a detector. John Kitching of the US National Institute of Standards and Technology in Boulder, Colorado, US, and colleagues made the new device, which is about 1000 times more sensitive than previous devices of a similar size. It is not just much smaller than a SQUID, but also operates at much higher temperatures, at around 150 °C. Currently the complete device is a few millimetres on each side. “The small size and high performance of this sensor will open doors to applications that we could previously only dream of,” Kitching says. Kitching and colleagues made the new magnetometers through photolithography, the same process used to make computer chips. “You can make very large numbers of the devices in parallel on a single wafer [of silicon],” Kitching says. “That will reduce the cost.” In October, Kitching and colleagues reported the first use of such a device for recording the magnetic field of a mouse heart. It could be used to study people too, and could generate magnetocardiograms that provide similar information to an electrocardiogram (ECG), without requiring electrodes on the patient’s body. “You could do it from outside clothing,” Kitching says. The detectors are even sensitive enough to detect alpha waves from the human brain, which produce magnetic fields of about 1000 femtoteslas just outside the skull. To pick up the full range of magnetic fields emanating from the human head, though, they would need to be more sensitive – down to 10 femtoteslas. Kitching says this should be possible in the future. The sensors could then be used to make portable magnetoencephalography (MEG) helmets. “If you could have a very cheap magnetometer, it becomes a very compelling device,” Kitching says. Because the devices run on relatively little power, even with the laser and heating components, they could also be used for distributed sensors, Kitching says. Using a technique called nuclear quadrupole resonance that uses excited atoms to identify different molecules, they could perhaps detect explosives. The new sensor has “a great combination of size and sensitivity for some applications,” says Michael Romalis of Princeton University. The small size allows the sensor to get closer to the heart or brain it is measuring, and lets it run on much less power than larger atomic magnetometers or SQUIDs, Romalis says. “I think there is a lot of potential for this approach.” Journal reference: Nature Photonics (DOI: