A Quantum of Sensing—Atomic Scale Bolsters New Sensor Boom
Once-esoteric physics will underlie sensor revolutions in medicine, tech, and engineering
Imagine sensors that can detect the magnetic fields of thoughts, help lunar rovers detect oxygen in moon rocks, or listen to radio waves from dark matter. Just as quantum computers can theoretically find the answers to problems no classical computer could ever solve, so too can an emerging generation of quantum sensors lead to new levels of sensitivity, new kinds of applications, and new opportunities to advance a range of fields, technologies, and scientific pursuits.
Quantum technology relies on quantum effects that can arise because the universe can become a fuzzy place at its very smallest levels. For example, the quantum effect known as superposition allows atoms and other building blocks of the cosmos to essentially exist in two or more places at the same time, while another quantum effect known as entanglement can link particles so they can influence each other instantly regardless of how far apart they are.
These quantum effects are infamously fragile to outside interference. However, whereas quantum computers strive to overcome this weakness, quantum sensors capitalize on this vulnerability to achieve extraordinary sensitivity to the slightest disturbances in the environment. Below are just a small sampling of the many kinds and varieties of quantum sensors being developed and deployed today.
BRAIN SCANS: Electric currents within the brain generate magnetic fields that sensors can analyze to noninvasively scan brain activity. Now quantum sensors are enabling a wearable helmet to perform such magnetoencephalography (MEG) scans with unprecedented performance and cost.
Currently MEG scans are performed with sensors known as superconducting quantum interference devices ( SQUIDs). These require cooling with expensive liquid helium to -269 °C, making the scanners extremely large. In contrast, the new devices from startup Cerca Magnetics in Nottingham, England, are each about the size of a Lego brick.
Each device, called an optically pumped magnetometer (OPM), contains a laser that shines a beam through a cloud of rubidium atoms at a light detector. The beam can make the magnetic fields of the rubidium atoms all line up, rendering the cloud essentially transparent. Tiny magnetic fields, such as those from brain activity, can disturb these atoms, making them capable of absorbing light, which the light detector can sense, and the laser resets the cloud so it can continue responding to magnetic disturbances.
The fact these quantum sensors work at room temperature make them much less bulky than SQUIDs. This means they can get placed much closer to a person’s head, resulting in a signal at least two times better and theoretically up to five times better, for magnetic images with millimeter accuracy and millisecond resolution of surface areas of the brain, says Matthew Brookes, chairman of Cerca and a researcher at the University of Nottingham.
The small, lightweight nature of the sensors also means they can get mounted in a wearable helmet to let people move freely during scanning, instead of having them remain still for very long periods as is currently the case. In addition, it can adapt to different head shapes and sizes, making it possible to scan not just adults but also children and babies. Moreover, “MEG with OPMs is in principle quite a lot cheaper than with SQUIDs,” Brookes says. “Even now, in early days with OPMs, a full MEG imaging system is still half the price of a SQUID system for similar performance.”
The Cerca scanner can help probe neurological disorders such as epilepsy, concussions, dementia, and schizophrenia, “helping shed light on many severe and debilitating conditions,” he says.
Future research can aim to push these sensors closer to their theoretical limits of sensitivity, permit more freedom of movement to perhaps let people walk, and add virtual reality and machine learning to boost what researchers can do with the scanners on the experimental and analytical fronts, Brookes says.
GRAVITY MAPPING: A new quantum sensor that maps the strength of Earth’s gravitational field can help reveal features hidden underground.
Anything that has mass possesses a gravitational field. The strength of this field’s pull depends on a body’s mass. Since Earth’s mass is not spread out evenly, this means the planet’s gravity is stronger at some places than others.
For decades, gravity mapping has uncovered details on large-scale geological activity, but employing such gravity cartography on the scale of meters is challenging, since long measuring times are needed to account for local noise, such as vibrations from nearby traffic.
The new quantum sensor uses clouds of rubidium atoms cooled to a few millionths of a degree Celsius above absolute zero. Laser pulses drive the atoms into states of superposition, with two versions of the atoms falling down slightly different trajectories, and these atoms are then recombined. Then, due to the wave-particle duality—the quantum phenomenon where particles can act like waves, and vice versa—these atoms quantum mechanically interfere with each other, with their peaks and troughs augmenting or suppressing each other. Analyzing the nature of this interference, a technique known as atom interferometry, can reveal the extent of the slightly different gravitational pulls felt along their separate paths.
The sensor uses an hourglass design, with one cloud in each half of the device separated vertically by 1 meter. As such, the sensor can analyze the strength of Earth’s gravity at two different heights at the same location. By comparing the data from these clouds, the researchers can account for a variety of sources of noise. In experiments, the sensor could detect a 2-by-2-meter utility tunnel buried roughly 0.5 meters under a road surface between two multistory buildings in the city of Birmingham, in England.
Potential applications for the sensor include seeing hidden underground structures, detecting subterranean natural resources, discovering underground archaeological sites, and monitoring volcanic activity and groundwater flows.
The initial refrigerator-size sensor was about 300 kilograms and used about 750 watts. The scientists are now working to build a backpack-size sensor weighing about 20 kg that runs on batteries, says Michael Holynski, an experimental physicist at the University of Birmingham, in England, and director of the startup Delta-G, which is commercializing the sensor. “The current target is to reach a commercial prototype of a next-generation sensor over the next two years,” he says. “The early markets are at around the £100 million mark for the sensors themselves. However, the data they will create is more valuable, and relevant to applications that are a few percent of GDP in the U.K.”
DETECTING COVID: Another promising quantum sensor could lead to faster, cheaper, and more accurate tests for the SARS-CoV-2 virus behind the global pandemic. It relies on microscopic artificial diamonds with defects within them, in which a carbon atom is replaced with a nitrogen atom and the adjacent carbon atom is missing. This defect in the crystals behaves like a tiny magnet whose alignment is very sensitive to magnetic fields, helping such “nitrogen-vacancy centers” serve as sensors.
The new technique involves coating nitrogen-vacancy-center diamonds roughly 25 nanometers wide with magnetic compounds that detach from the gems after they bond with the specific RNA sequence of the SARS-CoV-2 virus. When these diamonds are lit with green light, they will emit a red glow. The magnetic coating dims this glow; exposing the sensors to the virus can increase this glow.
The current gold-standard test for the SARS-CoV-2 virus takes several hours to create enough copies of the virus’s genetic material to detect. Moreover, it cannot quantify the amount of virus present with high accuracy and might have false-negative rates of more than 25 percent. In contrast, computer simulations suggest that the new test can theoretically work in just a second, is sensitive enough to detect just a few hundred strands of the viral RNA, and could have false-negative rates below 1 percent.
The nano-diamonds and the other materials used in the test are cheap. In addition, this new method could be adapted to virtually any virus, including any new ones that may emerge, by adjusting the magnetic coating to match the target virus. They are currently synthesizing and testing the sensors to see how well they actually perform. “We hope to get promising results very soon,” says researcher Changhao Li, a quantum engineer at MIT.
PROBING CELLS AND MOLECULES: Quantum diamond sensors can also find use in thermometers inside cells. Nitrogen-vacancy centers in diamonds are very sensitive to small temperature fluctuations. Physicist Peter Maurer at the University of Chicago and his colleagues have injected nanometer-scale diamonds with such defects into living cells and examined how the crystals responded to laser beams in order to map temperatures within the cells to a few thousandths of a degree Celsius.
“You can imagine using such atomic-scale thermometers to investigate how temperature influences cell division, gene expression, and how molecules go in and out of cells, all major questions in medicine and biology,” says experimental physicist David Awschalom at Argonne National Laboratory and director of the Q-NEXT consortium.
In addition, Maurer and his colleagues are investigating using diamonds with nitrogen-vacancy centers to essentially perform MRI scans on molecules. “With quantum sensors, you can perform MRI to the level of single molecules to understand the relationship between their structure and function, which could radically improve our understanding of medicine,” Awschalom says.
The scientists developed a new way to tether single protein and DNA molecules onto the surface of diamonds that host nitrogen-vacancy centers. By analyzing the magnetic fields of these molecules, “you can understand the distances between atoms, the strengths of the interactions between them, where they are, and what keeps them together,” Awschalom says.
QUANTUM ACCELEROMETER: The world now relies heavily on global navigation satellite systems such as GPS, but the satellite links that help enable such positioning, navigation, and timing do not work underground or underwater and are vulnerable to jamming, spoofing, and weather. Now a quantum sensor from Imperial College London and the Glasgow-based company M Squared can help ships navigate even when GPS is denied.
The quantum sensor is an atom interferometer like the gravity-mapping device. Analyzing how the phase of its atomic wave-packets shifts can reveal any acceleration or rotation they experienced, which the device can use to calculate the change in its position with time.
This quantum accelerometer can help serve as the foundation of an inertial navigation system that does not rely on any outside signals. Whereas temperature fluctuations and other factors lead the position estimates of conventional inertial navigation systems to drift within hours without an outside reference signal, M Squared’s device experiences negligible drift even after days, says Joseph Cotter, a research fellow at Imperial College London’s Center for Cold Matter.
“The early adopters of this emerging quantum technology are likely to be those interested in long-range navigation for underwater and surface vehicles,” Cotter says. “However, as the technology develops and becomes increasingly compact and lower cost, it will have wider benefits across the transportation industry through deployment on ships, trains, and aircraft.”
The researchers have field tests planned for their latest device this summer. Currently the quantum accelerometer “is about the size of two washing machines,” Cotter notes. “We’re working to get it even more compact.”
QUANTUM SOFTWARE: Where most quantum-sensor companies focus on the hardware, Sydney-based startup Q-CTRL focuses on software to enhance quantum technology. “When you take quantum sensors out of pristine lab environments out into the field, you often see a huge degrading in performance due to noise in the platforms,” says Michael Biercuk, CEO and founder of Q-CTRL. “Our focus is recapturing this performance with our quantum control software.”
For instance, many quantum sensors use lasers to scan cold atoms to detect any changes in the environment, but any movement in the device can lead the atoms to move out of the laser beams. “With our software, we can shape the pulse of light—its frequency, amplitude, phase—to make it more resilient against motion without any changes to the hardware itself,” Biercuk says.
Q-CTRL is partnering with the Sydney-based inertial navigation company Advanced Navigation to develop a rubidium-based atom-interferometer inertial-navigation system that can fit in less than 1 cubic meter and can work in GPS-denied areas. “We aim to have the first delivery of fieldable systems in 2023,” Biercuk says.
The company also aims to place atom interferometers aboard satellites to perform gravity mapping from space at 100 times less than the current cost, with launches of demonstration payloads into low Earth orbit expected in 2025. In addition, Q-CTRL is a member of Australia’s Seven Sisters space industry consortium designing a new lunar rover in support of NASA’s Artemis program, in which Q-CTRL is working on a rubidium-based quantum atomic magnetometer to magnetically analyze lunar rocks for oxygen.
DARK MATTER, GIANT TELESCOPES: Quantum sensors may help probe matters far beyond Earth. For example, one of the greatest mysteries in the universe is the nature and identity of dark matter, the invisible substance thought to make up five-sixths of all matter in the universe. Leading theoretical candidates for dark matter include particles known as axions, which in principle have an exceedingly low mass, at most just a trillionth the mass of the proton, making them difficult to detect.
Quantum physicist Kent Irwin at Stanford University and his colleagues are developing a “ dark matter radio” to detect axions and similar dark-matter candidates. A powerful magnet in the device will convert axions into radio waves, and quantum sensors will aim to amplify and detect these extremely weak radio signals.
Since the frequencies the dark-matter radio will probe will include ones used for over-the-air broadcasting, the device will require shielding within a thin layer of superconducting niobium metal cooled in liquid helium. This should screen out artificial signals but will be easily penetrated by dark matter. “We’re planning a version of the dark-matter radio now that’s about a cubic meter in scale that we’d like to build in the next few years,” Irwin says.
Quantum physics may also help enable giant telescope arrays, Irwin says. Multiple telescopes widely separated in space can theoretically be combined to essentially form a single telescope thousands of kilometers wide.
Forming such arrays with optical telescopes imaging visible light is difficult because of random fluctuations that inevitably crop up in any fiber optics linking these telescopes. However, entanglement can in principle allow quantum teleportation of data across great distances.
Quantum optics researcher Paul Kwiat at the University of Illinois at Urbana–Champaign is currently investigating such “quantum-enhanced telescopy” with tabletop experiments. “It’s still very far off, but also a true holy grail, a moon shot that’s incredibly exciting,” Irwin says. A telescope array roughly the diameter of Earth may in principle image features the size of cities on nearby stars, he says.
UNTOLD LIMITS: Recently scientists in Austria developed the first programmable quantum sensor, a device capable of an unprecedented level of sensitivity operating near the fundamental limits imposed by the laws of quantum mechanics.
In this work, they programmed a quantum computer to find the best settings for itself with which to measure the states of its components. They found this programmable quantum sensor could optimize itself enough to approach the fundamental sensing limit up to a factor of about 1.45. (The closer a sensor approaches the ultimate sensing limit of 1, the better its performance.) They suggest that programmable quantum sensors could find use in devices such as atomic clocks and global positioning systems, as well as magnetic and inertial sensors.
All in all, “quantum sensors are emerging with exquisite precision to cover everything from single proteins all the way to questions in astronomy and cosmology,” Awschalom says.
This article appears in the June 2022 print issue as “A Guide to the Quantum-Sensor Boom.”
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