Physicists in the US have developed a tiny portable device that is sensitive enough to detect the magnetic fields from electrical signals in the brain, writes Michael Banks . Consisting of a cell filled with a vapour of rubidium atoms, the sensor can detect magnetic field changes as small as 70 fT – about one billionth of the Earth's magnetic field – and could have applications ranging from airport security to the study of epilepsy.
Physicists in the US have developed a tiny portable device that is sensitive enough to detect the magnetic fields from electrical signals in the brain, writes Michael Banks . Consisting of a cell filled with a vapour of rubidium atoms, the sensor can detect magnetic field changes as small as 70 fT – about one billionth of the Earth's magnetic field – and could have applications ranging from airport security to the study of epilepsy.
The quantum realm is often portrayed as weird and nebulous, far removed from our everyday world. Subatomic particles can be in many places at once or linked across vast distances by the intimate connection known as entanglement. But researchers are now using this quantum fuzziness to forge a new set of practical tools.
In this entanglement-assisted magnetometer, lenses, and other optical devices steer a laser beam to the small glass cell filled with cesium vapor, and a photodetector above records the effect of magnetic fields on light emitted by the cesium atoms. Image credit: Ola J. Joensen (Niels Bohr Institute, Copenhagen, Denmark).
Although quantum computers gain the most headlines, a quieter revolution in the world of sensors may be just as profound. Quantum states, such as the spin of an electron, have a peculiar delicacy that is being exploited to detect tiny variations in gravity and magnetism, opening up a range of applications that could give us a clearer view of the world beneath our feet and provide deep insights into our own biology.
This has become possible thanks to continuing advances in components such as lasers and cooling systems that allow researchers and engineers to manipulate the quantum states of atoms and measure how they are affected by their environment. “We are at this sweet spot where we have quantum systems under sufficient control,” says Kai Bongs, who leads the Quantum Sensors and Metrology Hub (1), a collaboration between academics and industry centered at the University of Birmingham in the United Kingdom.
Some of the large vacuum systems and magnetic traps now used to hold cold atoms are being replaced with chip-scale devices, while researchers are developing compact, low-power lasers that could make quantum sensors much more accessible. “The early devices are still clumsy, but now we can start …
The ability to measure tiny magnetic fields with good sensitivity can be found in many applications, from biological imaging to prospecting for buried treasure. However, the most sensitive magnetometers that operate in ambient conditions tend to be power-hungry, bulky, and heavy. Shrinking the size to just several millimeters and the power consumption to hundreds of milliwatts, Schwindt et al. have fabricated a sensitive magnetometer using microelectromechanical technology. A cloud of rubidium atoms trapped in a micromachined vapor cell is used to sense the magnetic field. The magnetic field splits the energy levels of rubidium atoms, and the extent of the splitting depends on the strength of the magnetic field. Changes in the magnetic field are then detected and tracked optically by the relative absorption changes of a laser light tuned to the split energy levels. It could be that in the not-too-distant future we could be using handheld battery-operated magnetometers. — ISO
An atomic magnetometer is a highly sensitive device designed to measure magnetic fields with exceptional precision. Unlike traditional magnetometers like flux gate magnetometers and MEMS based magnetometers, atomic magnetometers use the quantum properties of individual atoms to achieve unparalleled sensitivity. It works without any cryogenic cooling, unlike SQUIDS, making it suitable from the perspective of portability and maintenance. The magnetometer is able to work in Earth’s magnetic field, allowing the user to measure environmental magnetic field fluctuations caused by the presence of some objects (say, a submarine or unexploded ordnance) made from magnetic material or due to some structural changes in Earth.
There has been permanent interest in investigations of new magnetic sensors and their various applications (Edelstein, 2007). Last years there is a rapid progress in development of magneto-optical sensors because of their sensitivity and potential for miniaturization. Magnetometers, based on magneto-optical sensors have high sensitivity comparable to, or even surpassing this of the SQUIDs (Superconducting Quantum Interference Devices) (Kominis et al., 2003; Dang et al., 2010; Savukov, 2010; Knappe, 2010). Microfabrication of components using the techniques of Micro-Electro-Mechanical Systems (MEMS) developed for atomic clocks (Knappe, 2004) gives the opportunity for building small, low consuming, low cost and non-cryogenic (as SQUIDs) sensors (Griffith et al., 2010). Coherent optical effects can be applied for magnetic field detection and offer perspectives for development of high-precision optical magnetometers (Cox et al., 2011; Kitching et al., 2011). These magnetometers are appropriate for geomagnetic, space, nuclear and biological magnetic field measurements (cardio and brain magnetic field imaging), environmental monitoring, magnetic microscopy, investigations of fundamental physics, etc. Coherent magneto-optical resonances have many applications not only in magnetometry, but in high-resolution spectroscopy, lasing without inversion, laser cooling, ultraslow group velocity propagation of light, etc. (Gao, 2009).
Using the techniques of microelectromechanical systems, we have constructed a small low-power magnetic sensor based on alkali atoms. We use a coherent population trapping resonance to probe the interaction of the atoms’ magnetic moment with a magnetic field, and we detect changes in the magnetic flux density with a sensitivity of 50pTHz−1∕2 at 10Hz. The magnetic sensor has a size of 12mm3 and dissipates 195mW of power. Further improvements in size, power dissipation, and magnetic field sensitivity are immediately foreseeable, and such a device could provide a hand-held battery-operated magnetometer with an atom shot-noise limited sensitivity of 0.05pTHz−1∕2.
Self-oscillating atomic magnetometers, in which the precession of atomic spins in a magnetic field is driven by resonant modulation, offer high sensitivity and dynamic range. Phase-coherent feedback from the detected signal to the applied modulation creates a resonant spin maser system, highly responsive to changes in the background magnetic field. Here we show a system in which the phase condition for resonant precession is met by digital signal processing integrated into the maser feedback loop. This system uses a modest chip-scale laser and mass-produced dual-pass caesium vapour cell and operates in a 50 [Formula: see text]T field, making it a suitable technology for portable measurements of the geophysical magnetic field. We demonstrate a Cramér-Rao lower bound-limited resolution of 50 fT at 1 s sampling cadence, and a sensor bandwidth of 10 kHz. This device also represents an important class of atomic system in which low-latency digital processing forms an integral part of a coherently-driven quantum system.