Core Concept: Quantum sensors probe uncharted territories, from Earth’s crust to the human brain
Stephen Battersby
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.
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 iterating down to make them smaller, cheaper, easier to use, and more robust,” says Bongs.
Ground States
Universities and companies around the world are harnessing the intricacies of quantum mechanics for many tasks, such as navigation using quantum gyroscopes, but the two biggest targets at the moment are magnetic and gravitational fields.
The big payoff for quantum gravity sensors is picking up the subtle signals of buried infrastructure. “Whenever you break ground, the first thing you come across is buried hazards — foundations, pipes, and so on,” says Nicole Metje, a professor of civil engineering at the University of Birmingham. Digging holes to find these obstacles can be enormously time consuming, so it would be much more efficient to peer through the ground and identify what lies beneath. Ground-penetrating radar and probes that measure the electrical conductivity of the ground already offer a limited subterranean view, but they struggle to spot small, deeply buried objects: a water-filled pipe with a 1-meter diameter would be undetectable at more than 5 meters down, for example. Electrically conducting soil, such as wet clay, also blocks many of these methods.
Gravity is immune to such shielding effects. Mass generates gravity, so any cavity or object with a lower density than its surroundings will result in a minuscule decrease in the local force of gravity.
Existing gravimeters are based on a small mass hanging from a spring, and ground vibrations can muddle their readings. Surveyors rarely use this technique because it can take several minutes to cancel out these vibrations for each measurement.
The quantum gravimeters being developed at the University of Birmingham should be able to do much better. They use lasers and magnetic fields to trap and move a cloud of cold rubidium atoms inside a vacuum chamber. According to quantum mechanics, each atom is described by a wavefunction — an oscillating mathematical function that determines the probability of finding the atom in a given position and state. To make a gravity measurement, a laser first puts each atom into a hybrid quantum state known as a superposition, which allows it to simultaneously have slightly lower and higher energies. Then the laser gives the cloud a kick so that the higher-energy half of the wavefunction moves up from the low-energy half by a few centimeters. Another kick then moves the two halves back together again so that the wavefunctions overlap and interfere with one another, like two converging sets of ocean waves.
The gravitational potential energy of each cloud affects its wavefunction, so the cloud that spent more time higher up has oscillated less than the one that lingered below. When they recombine and interfere, this skews the ratio of the two energy states. Measuring that ratio with a laser beam reveals the strength of gravity (2).
“The beauty of quantum technology is that we are far from its physical limits.”
— Kai Bongs
Instruments known as gradiometers perform these measurements with two separate clouds at different heights to find the gradient in the gravitational field. This allows the instrument to cut out the effects of vibrations, so it can make measurements in seconds instead of minutes.
The big challenge has been getting these quantum systems to work outside the lab in uncontrolled environments, says Bongs. But the University of Birmingham team successfully field-tested a prototype gradiometer called Gravity Imager in 2018, and a more sensitive version is on the way. This should be able to spot a water-filled pipe buried twice as deep as any existing technology could manage. The team also hopes to mount a smaller version of the device on a drone.
Quantum gravity sensors could also beat existing methods to scan archaeological sites, explore for mineral resources, monitor volcanic activity, search for underground rock formations where CO2 can be safely stored, and survey aquifers to help manage water resources. “We aim to do surveys faster and potentially see things we can’t see at the moment,” says Metje.
Brainwaves
While gravity sensors promise to probe the planet, quantum magnetic sensors have a wide variety of different targets. Defects in the crystal structure of diamond form microscopic sensors, which can be placed so close to a sample researchers could even potentially monitor the electromagnetic activity of individual cells. Collective states of ultracold atoms known as Bose–Einstein condensates might probe advanced materials and microelectronics (see Core Concept: How Bose–Einstein condensates keep revealing weird physics, https://www.pnas.org/content/114/23/5766). Perhaps most intriguing is a quantum device that already probes the human brain better in some ways than other methods.
This scanner is based on optically pumped magnetometers (OPMs), which contain a vapor of cesium or rubidium atoms. The single outer electron of each atom has a spin that gives it a magnetic field like a small bar magnet, and a laser beam can make all these spins point in the same direction. Any magnetic field from outside alters this settled state, making the spins wobble around in a motion called precession. This affects the laser light, providing a way to measure the magnetic field.
OPMs are now sensitive enough to probe the brain. All electrical currents generate a magnetic field — even the weak currents flowing between neurons — and the technique known as magnetoencephalography (MEG) aims to detect these feeble fields to monitor brain activity. In the past, MEG has only been possible by using superconducting detectors, which require cooling with liquid helium. That makes the scanners expensive and bulky, which means that the detectors can’t get very close to a patient’s skull. This is critical because the brain’s already tiny magnetic fields fade rapidly with distance, which makes it especially difficult to scan the smaller heads of children.
In 2018, a team at the University of Nottingham, in the United Kingdom, demonstrated a prototype MEG scanner using OPMs made by QuSpin, a company based in Louisville, CO. The researchers put 13 of these OPMs in a 3D-printed helmet tailored to the head of the wearer so that the sensors sat within a few millimeters of the skull. The scanner showed clear activity in the motor cortex when the subject spread out his or her fingers, revealing how the activity of brain regions just a few millimeters across changed on a timescale of a few milliseconds (3). This combination can’t be matched by either electroencephalograms (EEGs), which have lower spatial resolution, or functional MRI, which can only see changes on a timescale of a few seconds or more.
Researchers at University College London in the United Kingdom plan to use the system to study epilepsy in children, replacing the EEG, which can involve the highly invasive procedure of attaching electrodes to the brain. In the future, the high sensitivity and high space and time resolution of OPM magnetometers could be used to investigate a wide range of disorders including dementia and Parkinson disease, says Matt Brookes, who leads the University of Nottingham project. He also thinks that the technology could enable researchers to trace brain development through childhood. “What fundamentally changes before and after we can walk, talk, read?” says Brookes. “This is a huge scientific question that you can’t address with existing technology.” Children are difficult to scan because their heads are small and because they move around a lot (see Inner Workings: Sitting still, for science, https://www.pnas.org/content/113/8/1960), but both of those problems are solved by a wearable scanner.
How Quantum?
In Copenhagen, a group led by Eugene Polzik has moved magnetometers to the next quantum level. A laser entangles groups of atoms, putting them into a shared quantum state in which their spins are strongly correlated (4). This correlation can be used to reduce the random jitter that is normally present in quantum states. The practical upshot is to cut noise, making the magnetometer even more sensitive and able to monitor activity deeper within the body, for example, in the brain’s hippocampus. The electrical signals that regulate heartbeats also generate magnetic fields, which can be picked up by an entangled magnetometer (5). “I hope to measure fetal heartbeat, which is rarely done by doctors because of the difficulty of access,” says, Kasper Jensen, who recently moved from Polzik’s group to the University of Nottingham.
Quantum sensors are just on the verge of commercial reality, and with such a young technology, there is still much room for improvement. Quantum gravimeters, for example, could eventually offer a million-fold increase in sensitivity, says Bongs: “The beauty of quantum technology is that we are far from its physical limits.”
Published under the PNAS license.
