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The emerging technology of atomic sensors exploits the quantum properties of atoms to enable a new generation of highly sensitive and versatile detectors with varied applications in science, industry and medicine.
One type of atomic sensor measures very weak magnetic fields. Until now, the speed of these optical magnetometers has been limited by fundamental physics: the more precise the measurement, the longer it takes. For some research purposes, that can be as long as several days. The EU-funded QUTEMAG project was set up to improve the performance of atomic sensors.
‘If the sensor is still responding to what happened yesterday, how do you interpret what it is telling you today?’ asks lead researcher Ricardo Jiménez. ‘This is a very important question to ask in real-time sensing applications, such as outdoor and indoor navigation, autonomous robotic systems, non-destructive testing and so on.’
He says that optical magnetometers have so far been used mainly in research labs for precision measurements of very weak but unchanging fields. ‘When you are interested in measuring things that change with time – dynamic variables – then the speed of these sensors becomes an issue and you have to pay the price of poor sensitivity.’
Kalman filtering
The principal innovation in the QUTEMAG project was to use a mathematical technique known as Kalman filtering which improves the precision of a measured quantity by taking account of its recent history and the physical laws that govern it.
It came to prominence in the 1960s when it was used in the design of the navigational computers for the Apollo moon-landing programme and has been widely used in many subsequent applications, especially in navigation and control.
‘While Kalman filtering techniques are well known in the engineering community, their use in atomic sensors has not been fully explored,’ says Jiménez. ‘The work of this project represents the first experimental demonstration of these techniques to track both the state of the atoms and external signals coupled to them.’
Jiménez worked with colleagues at the Institute of Photonic Sciences in Barcelona to show that the precision and sensitivity of magnetometry measurements could be doubled by Kalman filtering. They also demonstrated how the technique could be used to track a rapidly varying signal in real time.
‘The method evades the trade-off between sensitivity and time resolution and may enable fast and precise measurements with atomic sensors,’ he says.
Quantum technology
Miniature but highly sensitive magnetometers could be used in medicine to monitor the very weak magnetic signals coming from the brain or the heart. ‘The European Space Agency, ESA, also has a very interesting programme where they have developed these magnetometers to map the Earth’s magnetic field from space,’ Jiménez says.
While the bulk of the work in QUTEMAG has focused on magnetometry, the tools developed could be used with other atomic sensors to measure quantities such as electric field, temperature, gravity and accelerations. The last of these could have great importance for inertial navigation systems – devices used on vehicles such as ships, aircraft, submarines, guided missiles and spacecraft.
Atomic sensors are examples of quantum technology, an area ripe for rapid growth, as shown by the EU’s recent launch of the Quantum Flagship, a EUR 1-billion, 10-year programme to consolidate and expand European scientific leadership and excellence in this research area and kick-start a new and competitive European industrial sector.
The QUTEMAG project was supported through the EU’s Marie Skłodowska-Curie fellowship programme.