Quantum Physics

Quantum tech is going green.

A new take on highly sensitive magnetic field sensors ditches the power-hungry lasers that previous devices have relied on to make their measurements and replaces them with sunlight. Lasers can gobble 100 watts or so of power — like keeping a bright lightbulb burning. The innovation potentially untethers quantum sensors from that energy need. The result is an environmentally friendly prototype on the forefront of technology, researchers report in an upcoming issue of Physical Review X Energy.

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The big twist is in how the device uses sunlight. It doesn’t use solar cells to convert light into electricity. Instead, the sunlight does the job of the laser’s light, says Jiangfeng Du, a physicist at the University of Science and Technology of China in Hefei.   

Quantum magnetometers often include a powerful green laser to measure magnetic fields. The laser shines on a diamond that contains atomic defects (SN: 2/26/08). The defects result when nitrogen atoms replace some of the carbon atoms that pure diamonds are made of. The green laser causes the nitrogen defects to fluoresce, emitting red light with an intensity that depends on the strength of the surrounding magnetic fields.

The new quantum sensor needs green light too. There’s plenty of that in sunlight, as seen in the green wavelengths reflected from tree leaves and grass. To collect enough of it to run their magnetometer, Du and colleagues replaced the laser with a lens 15 centimeters across to gather sunlight. They then filtered the light to remove all colors but green and focused it on a diamond with nitrogen atom defects. The result is red fluorescence that reveals magnetic field strengths just as laser-equipped magnetometers do.

Green-colored light shining on the diamond-based sensor in a quantum device can be used to measure magnetic fields. In this prototype, a lens (top) collects sunlight, which is filtered to leave only green wavelengths of light. That green light provides an environmentally friendly alternative to the light created by power-hungry lasers that conventional quantum devices rely on.Yunbin Zhu/University of Science and Technology of China

Changing energy from one type to another, as happens when solar cells collect light and produce electricity, is an inherently inefficient process (SN: 7/26/17). The researchers claim that avoiding the conversion of sunlight to electricity to run lasers makes their approach three times more efficient than would be possible with solar cells powering lasers.

“I’ve never seen any other reports that connect solar research to quantum technologies,” says Yen-Hung Lin, a physicist at the University of Oxford who was not involved with the study. “It might well ignite a spark of interest in this unexplored direction, and we could see more interdisciplinary research in the field of energy.”

Quantum devices sensitive to other things, like electric fields or pressure, could also benefit from the sunlight-driven approach, the researchers say. In particular, space-based quantum technology might use the intense sunlight available outside Earth’s atmosphere to provide light tailored for quantum sensors. The remaining light, in wavelengths that the quantum sensors don’t use, could be relegated to solar cells that power electronics to process the quantum signals.

The sunlight-driven magnetometer is just a first step in the melding of quantum and environmentally sustainable technology. “In the current state, this device is primarily for developmental purposes,” Du says. “We expect that the devices will be used for practical purposes. But there [is] lots of work to be done.” More

Stealthy communication just got more secure, thanks to quantum entanglement.

Quantum physics provides a way to share secret information that’s mathematically proven to be safe from the prying eyes of spies. But until now, demonstrations of the technique, called quantum key distribution, rested on an assumption: The devices used to create and measure quantum particles have to be known to be flawless. Hidden defects could allow a stealthy snoop to penetrate the security unnoticed.

Now, three teams of researchers have demonstrated the ability to perform secure quantum communication without prior confirmation that the devices are foolproof. Called device-independent quantum key distribution, the method is based on quantum entanglement, a mysterious relationship between particles that links their properties even when separated over long distances.

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In everyday communication, such as the transmission of credit card numbers over the internet, a secret code, or key, is used to garble the information, so that it can be read only by someone else with the key. But there’s a quandary: How can a distant sender and receiver share that key with one another while ensuring that no one else has intercepted it along the way?

Quantum physics provides a way to share keys by transmitting a series of quantum particles, such as particles of light called photons, and performing measurements on them. By comparing notes, the users can be sure that no one else has intercepted the key. Those secret keys, once established, can then be used to encrypt the sensitive intel (SN: 12/13/17). By comparison, standard internet security rests on a relatively shaky foundation of math problems that are difficult for today’s computers to solve, which could be vulnerable to new technology, namely quantum computers (SN: 6/29/17).

But quantum communication typically has a catch. “There cannot be any glitch that is unforeseen,” says quantum physicist Valerio Scarani of the National University of Singapore. For example, he says, imagine that your device is supposed to emit one photon but unknown to you, it emits two photons. Any such flaws would mean that the mathematical proof of security no longer holds up. A hacker could sniff out your secret key, even though the transmission seems secure.

Device-independent quantum key distribution can rule out such flaws. The method builds off of a quantum technique known as a Bell test, which involves measurements of entangled particles. Such tests can prove that quantum mechanics really does have “spooky” properties, namely nonlocality, the idea that measurements of one particle can be correlated with those of a distant particle. In 2015, researchers performed the first “loophole-free” Bell tests, which certified beyond a doubt that quantum physics’ counterintuitive nature is real (SN: 12/15/15).

“The Bell test basically acts as a guarantee,” says Jean-Daniel Bancal of CEA Saclay in France. A faulty device would fail the test, so “we can infer that the device is working properly.”

In their study, Bancal and colleagues used entangled, electrically charged strontium atoms separated by about two meters. Measurements of those ions certified that their devices were behaving properly, and the researchers generated a secret key, the team reports in the July 28 Nature.

Typically, quantum communication is meant for long-distance dispatches. (To share a secret with someone two meters away, it would be easier to simply walk across the room.) So Scarani and colleagues studied entangled rubidium atoms 400 meters apart. The setup had what it took to produce a secret key, the researchers report in the same issue of Nature. But the team didn’t follow the process all the way through: The extra distance meant that producing a key would have taken months.

In the third study, published in the July 29 Physical Review Letters, researchers wrangled entangled photons rather than atoms or ions. Physicist Wen-Zhao Liu of the University of Science and Technology of China in Hefei and colleagues also demonstrated the capability to generate keys, at distances up to 220 meters. This is particularly challenging to do with photons, Liu says, because photons are often lost in the process of transmission and detection.

Loophole-free Bell tests are already no easy feat, and these techniques are even more challenging, says physicist Krister Shalm of the National Institute of Standards and Technology in Boulder, Colo. “The requirements for this experiment are so absurdly high that it’s just an impressive achievement to be able to demonstrate some of these capabilities,” says Shalm, who wrote a perspective in the same issue of Nature.

That means that the technique won’t see practical use anytime soon, says physicist Nicolas Gisin of the University of Geneva, who was not involved with the research.

Still, device-independent quantum key distribution is “a totally fascinating idea,” Gisin says. Bell tests were designed to answer a philosophical question about the nature of reality — whether quantum physics really is as weird as it seems. “To see that this now becomes a tool that enables something else,” he says, “this is the beauty.” More

Machine learning can get a boost from quantum physics.

On certain types of machine learning tasks, quantum computers have an exponential advantage over standard computation, scientists report in the June 10 Science. The researchers proved that, according to quantum math, the advantage applies when using machine learning to understand quantum systems. And the team showed that the advantage holds up in real-world tests.

“People are very excited about the potential of using quantum technology to improve our learning ability,” says theoretical physicist and computer scientist Hsin-Yuan Huang of Caltech. But it wasn’t entirely clear if machine learning could benefit from quantum physics in practice.

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In certain machine learning tasks, scientists attempt to glean information about a quantum system — say a molecule or a group of particles — by performing repeated experiments, and analyzing data from those experiments to learn about the system.

Huang and colleagues studied several such tasks. In one, scientists aim to discern properties of the quantum system, such as the position and momentum of particles within. Quantum data from multiple experiments could be input into a quantum computer’s memory, and the computer would process the data jointly to learn the quantum system’s characteristics.

The researchers proved theoretically that doing the same characterization with standard, or classical, techniques would require exponentially more experiments in order to learn the same information. Unlike a classical computer, a quantum computer can exploit entanglement — ethereal quantum linkages — to better analyze the results of multiple experiments.

But the new work goes beyond just the theoretical. “It’s crucial to understand if this is realistic, if this is something we could see in the lab or if this is just theoretical,” says Dorit Aharonov of Hebrew University in Jerusalem, who was not involved with the research.

So the researchers tested machine learning tasks with Google’s quantum computer, Sycamore (SN: 10/23/19). Rather than measuring a real quantum system, the team used simulated quantum data, and analyzed it using either quantum or classical techniques.

Quantum machine learning won out there, too, even though Google’s quantum computer is noisy, meaning errors can slip into calculations. Eventually, scientists plan to build quantum computers that can correct their own errors (SN: 6/22/20). But for now, even without that error correction, quantum machine learning prevailed. More

The best way to find buried treasure may be with a quantum gravity sensor.

In these devices, free-falling atoms reveal subtle variations in Earth’s gravitational pull at different places. Those variations reflect differences in the density of material beneath the sensor — effectively letting the instrument peer underground. In a new experiment, one of these machines teased out the tiny gravitational signature of an underground tunnel, researchers report in the Feb. 24 Nature.

“Instruments like this would find many, many applications,” says Nicola Poli, an experimental physicist at the University of Florence, who coauthored a commentary on the study in the same issue of Nature.

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Poli imagines using quantum gravity sensors to monitor groundwater or magma beneath volcanoes, or to help archaeologists uncover hidden tombs or other artifacts without having to dig them up (SN: 11/2/17). These devices could also help farmers check soil quality or help engineers inspect potential construction sites for unstable ground.

“There are many tools to measure gravity,” says Xuejian Wu, an atomic physicist at Rutgers University in Newark, N.J., who wasn’t involved in the study. Some devices measure how far gravity pulls down a mass hanging from a spring. Other tools use lasers to clock how fast an object tumbles down a vacuum chamber. But free-falling atoms, like those in quantum gravity sensors, are the most pristine, reliable test masses out there, Wu says. As a result, quantum sensors promise to be more accurate and stable in the long run than other gravity probes.

Inside a quantum gravity sensor, a cloud of supercooled atoms is dropped down a chute. A pulse of light then splits each of the falling atoms into a superposition state — a quantum limbo where each atom exists in two places at once (SN: 11/7/19). Due to their slightly different positions in Earth’s gravitational field, the two versions of each atom feel a different downward tug as they fall. Another light pulse then recombines the split atoms.

Thanks to the atoms’ wave-particle duality — a strange rule of quantum physics that says atoms can act like waves — the reunited atoms interfere with each other (SN: 1/13/22). That is, as the atom waves overlap, their crests and troughs can reinforce or cancel each other out, creating an interference pattern. That pattern reflects the slightly different downward pulls that the split versions of each atom felt as they fell — revealing the gravity field at the atom cloud’s location.

Extremely precise measurements made by such atom-based devices have helped test Einstein’s theory of gravity (SN: 10/28/20) and measure fundamental constants, such as Newton’s gravitational constant (SN: 4/12/18). But atom-based gravity sensors are highly sensitive to vibrations from seismic activity, traffic and other sources.

“Even very, very small vibrations create enough noise that you have to measure for a long time” at any location to weed out background tremors, says Michael Holynski, a physicist at the University of Birmingham in England. That has made quantum gravity sensing impractical for many uses outside the lab.  

Holynski’s team solved that problem by building a gravity sensor with not one but two falling clouds of rubidium atoms. With one cloud suspended a meter above the other, the instrument could gauge the strength of gravity at two different heights in a single location. Comparing those measurements allowed the researchers to cancel out the effects of background noise.

Holynski and colleagues tested whether their sensor — a 2-meter-tall chute on wheels tethered to a rolling cart of equipment — could detect an underground passageway on the University of Birmingham campus. The 2-by-2-meter concrete tunnel lay beneath a road between two multistory buildings. The quantum sensor measured the local gravitational field every 0.5 meters along an 8.5-meter line that crossed over the tunnel. Those readouts matched the predictions of a computer simulation, which had estimated the gravitational signal of the tunnel based on its structure and other factors that could influence the local gravitational field, such as nearby buildings.

Based on the machine’s sensitivity in this experiment, it could probably provide a reliable gravity measurement at each location in less than two minutes, the researchers estimate. That’s about one-tenth the time needed for other types of gravity sensors.

The team has since built a downsized version of the gravity sensor used in the tunnel-detecting experiment. The new machine weighs about 15 kilograms, compared with the 300-kilogram beast used for the tunnel test. Other upgrades could also boost the gravity sensor’s speed.

In the future, engineer Nicole Metje envisions building a quantum gravity sensor that could be pushed from place to place like a lawn mower. But portability isn’t the only challenge for making these tools more user-friendly, says Metje, a coauthor on the study who is also at the University of Birmingham. “At the moment, we still need someone with a physics degree to operate the sensor.”

So hopeful beachcombers may be waiting a long time to trade in their metal detectors for quantum gravity sensors. More

From Data to QuantaSlobodan PerovićUniv. of Chicago, $45

Ever since Max Planck introduced the idea of the quantum to the world, physicists have argued about whether reality is more like sand or water.

Planck’s famous 1900 discovery that energy is grainy — at least when absorbed or emitted — moved him to label those smallest bits of energy grains “quanta.” But he believed that once emitted, as in light from a fire, those grains merged into smooth, continuous waves, just as water seems a smooth liquid to human perception. Einstein, on the other hand, insisted that light quanta traveled through space on their own, behaving like particles later called photons.

By the mid-1920s, both the wave and particle views of light had gained experimental support, with the additional paradox that electrons — supposedly particles — could sometimes disguise themselves as waves.

Into this arena of controversy stepped the famed Danish physicist Niels Bohr, the pioneer of exploring the architecture of the atom. Bohr announced that resolving the wave-particle paradox required a new view of reality, in which both notions shared a role in explaining experimental phenomena. In experiments designed to observe waves, waves you would find, whether electrons or light. In experiments designed to detect particles, you’d see particles. But in no experiment could you demonstrate both at once. Bohr called this viewpoint the principle of complementarity, and it successfully guided the pursuit of quantum mechanics during the following decades.

More recently, as philosopher Slobodan Perović recounts in From Data to Quanta, Bohr’s success has been questioned by some physicists and philosophers and even popular science writers (SN: 1/19/19, p. 26). Complementarity has been derided as an incoherent application of vague philosophy expressed in incomprehensible language. But as Perović’s investigations reveal, such criticisms are rarely rooted in any deep understanding of Bohr’s methods. Rather than Bohr’s philosophy contaminating his science, Perović argues, it is his opponents’ philosophical prejudices that have led to misstatements, misunderstandings and misrepresentations of Bohr’s physics. And Bohr can’t be understood by attempting to understand his philosophy, Perović asserts, because philosophy did not guide him — experiments did.

In fact, Bohr’s drive to understand the wave-particle paradox was fueled by a deep devotion to comprehending the experimental evidence in its totality. It was the same approach the younger Bohr took when developing his model of the atom in 1913 (SN: 7/13/13, p. 20). Various experiments suggested properties of the atom that seemed irreconcilable. But Bohr forged those experimental clues into a “master hypothesis” that produced a thoroughly novel understanding of the atom and its structure.

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Perović describes how Bohr’s process began with lower-level hypotheses stemming from features directly given by experiment. Spectral lines — different specific colors of light emitted by atoms — led to basic hypotheses that some vibratory process, of an atom itself or its constituents, produced electromagnetic radiation exhibiting precise patterns. Intermediate hypotheses about the structure of the atom did not explain such lines, though. And then Ernest Rutherford, on the basis of experiments in his lab, inferred that an atom was mostly empty space. It contained a dense, tiny central nucleus encompassing most of the mass, while lightweight electrons orbited at a distance. But that hypothesis didn’t mesh with the precise patterns of spectral lines. And such an atom would be unstable, persisting for less than a millisecond. From all these disparate experiment-based hypotheses, Bohr applied Planck’s quantum idea to construct a master hypothesis. He reconciled the spectral lines and Rutherford’s nuclear atom with a new atomic model, in which electrons maintained stability of the atom but jumped from one orbit to another, emitting specific patterns of spectral lines in the process.

As Perović demonstrates, Bohr followed a similar course in arriving at complementarity. While numerous experiments showed that light was a wave, by the early 1920s other experiments established that X-rays, highly energetic light, collided with electrons just as though both were particles (momentum and energy were conserved in the collisions just as the particle view required). Bohr’s master hypothesis, complementarity, seemed the only way forward.

Throughout the book, Perović relates how Bohr has been misinterpreted, his views misleadingly conflated with those of others (like John von Neumann and Werner Heisenberg), and his philosophy incorrectly portrayed as antirealist — suggesting that only observations brought reality into existence. Bohr never said any such thing, and in fact cautioned against using language so loosely.

Perović’s account offers a thorough survey of other historical investigations into Bohr’s work and draws liberally from Bohr’s own writings. It’s a nuanced and insightful presentation of the interplay of experiment and theory in the scientific process. This book is not easy reading, though. It’s not the place to seek clear explanations of quantum physics and Bohr’s interpretation of it. Perović opts for scholarly thoroughness and careful reasoning with a propensity for long sentences. But then again, Bohr’s writings were no breeze, either. In fact, a major complaint against Bohr has been expressed by authors who say his writings are very difficult to understand. It’s unfortunate that so many seem to think that because they can’t understand Bohr, he must have been wrong. Perović’s book provides a useful antidote to that attitude.

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Some quantum particles gotta get right back to where they started from.

Physicists have confirmed a theoretically predicted phenomenon called the quantum boomerang effect. An experiment reveals that, after being given a nudge, particles in certain materials return to their starting points, on average, researchers report in a paper accepted in Physical Review X.

Particles can boomerang if they’re in a material that has lots of disorder. Instead of a pristine material made up of orderly arranged atoms, the material must have many defects, such as atoms that are missing or misaligned, or other types of atoms sprinkled throughout.

In 1958, physicist Philip Anderson realized that with enough disorder, electrons in a material become localized: They get stuck in place, unable to travel very far from where they started. The pinned-down electrons prevent the material from conducting electricity, thereby turning what might otherwise be a metal into an insulator. That localization is also necessary for the boomerang effect.

To picture the boomerang in action, physicist David Weld of the University of California, Santa Barbara imagines shrinking himself down and slipping inside a disordered material. If he tries to fling away an electron, he says, “it will not only turn around and come straight back to me, it’ll come right back to me and stop.” (Actually, he says, in this sense the electron is “more like a dog than a boomerang.” The boomerang will keep going past you if you don’t catch it, but a well-trained dog will sit by your side.)

Weld and colleagues demonstrated this effect using ultracold lithium atoms as stand-ins for the electrons. Instead of looking for atoms returning to their original position, the team studied the analogous situation for momentum, because that was relatively straightforward to create in the lab. The atoms were initially stationary, but after being given kicks from lasers to give them momenta, the atoms returned, on average, to their original standstill states, making a momentum boomerang.

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The team also determined what’s needed to break the boomerang. To work, the boomerang effect requires time-reversal symmetry, meaning that the particles should behave the same when time runs forward as they would on rewind. By changing the timing of the first kick from the lasers so that the kicking pattern was off-kilter, the researchers broke time-reversal symmetry, and the boomerang effect disappeared, as predicted.

“I was so happy,” says Patrizia Vignolo, a coauthor of the study. “It was perfect agreement” with their theoretical calculations, says Vignolo, a theoretical physicist at Université Côte d’Azur based in Valbonne, France.

Even though Anderson made his discovery about localized particles more than 60 years ago, the quantum boomerang effect is a recent newcomer to physics. “Nobody thought about it, apparently, probably because it’s very counterintuitive,” says physicist Dominique Delande of CNRS and Kastler Brossel Laboratory in Paris, who predicted the effect with colleagues in 2019.

The weird effect is the result of quantum physics. Quantum particles act like waves, with ripples that can add and subtract in complicated ways (SN: 5/3/19). Those waves combine to enhance the trajectory that returns a particle to its origin and cancel out paths that go off in other directions. “This is a pure quantum effect,” Delande says, “so it has no equivalent in classical physics.” More

If you’re superstitious, a black cat in your path is bad luck, even if you keep your distance. Likewise, in quantum physics, particles can feel the influence of magnetic fields that they never come into direct contact with. Now scientists have shown that this eerie quantum effect holds not just for magnetic fields, but for gravity too — and it’s no superstition.

Usually, to feel the influence of a magnetic field, a particle would have to pass through it. But in 1959, physicists Yakir Aharonov and David Bohm predicted that, in a specific scenario, the conventional wisdom would fail. A magnetic field contained within a cylindrical region can affect particles — electrons, in their example — that never enter the cylinder. In this scenario, the electrons don’t have well-defined locations, but are in “superpositions,” quantum states described by the odds of a particle materializing in two different places. Each fractured particle simultaneously takes two different paths around the magnetic cylinder. Despite never touching the electrons, and hence exerting no force on them, the magnetic field shifts the pattern of where particles are found at the end of this journey, as various experiments have confirmed (SN: 3/1/86).

In the new experiment, the same uncanny physics is at play for gravitational fields, physicists report in the Jan. 14 Science. “Every time I look at this experiment, I’m like, ‘It’s amazing that nature is that way,’” says physicist Mark Kasevich of Stanford University.

Kasevich and colleagues launched rubidium atoms inside a 10-meter-tall vacuum chamber, hit them with lasers to put them in quantum superpositions tracing two different paths, and watched how the atoms fell. Notably, the particles weren’t in a gravitational field–free zone. Instead, the experiment was designed so that the researchers could filter out the effects of gravitational forces, laying bare the eerie Aharonov-Bohm influence.

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The study not only reveals a famed physics effect in a new context, but also showcases the potential to study subtle effects in gravitational systems. For example, researchers aim to use this type of technique to better measure Newton’s gravitational constant, G, which reveals the strength of gravity, and is currently known less precisely than other fundamental constants of nature (SN: 8/29/18).

A phenomenon called interference is key to this experiment. In quantum physics, atoms and other particles behave like waves that can add and subtract, just as two swells merging in the ocean make a larger wave. At the end of the atoms’ flight, the scientists recombined the atoms’ two paths so their waves would interfere, then measured where the atoms arrived. The arrival locations are highly sensitive to tweaks that alter where the peaks and troughs of the waves land, known as phase shifts.

At the top of the vacuum chamber, the researchers placed a hunk of tungsten with a mass of 1.25 kilograms. To isolate the Aharonov-Bohm effect, the scientists performed the same experiment with and without this mass, and for two different sets of launched atoms, one which flew close to the mass, and the other lower. Each of those two sets of atoms were split into superpositions, with one path traveling closer to the mass than the other, separated by about 25 centimeters. Other sets of atoms, with superpositions split across smaller distances, rounded out the crew. Comparing how the various sets of atoms interfered, both with and without the tungsten mass, teased out a phase shift that was not due to the gravitational force. Instead, that tweak was from time dilation, a feature of Einstein’s theory of gravity, general relativity, which causes time to pass more slowly close to a massive object.

The two theories that underlie this experiment, general relativity and quantum mechanics, don’t work well together. Scientists don’t know how to combine them to describe reality. So, for physicists, says Guglielmo Tino of the University of Florence, who was not involved with the new study, “probing gravity with a quantum sensor, I think it’s really one of … the most important challenges at the moment.” More