More stories

  • in

    The quantum ‘boomerang’ effect has been seen for the first time

    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.

    Sign Up For the Latest from Science News

    Headlines and summaries of the latest Science News articles, delivered to your inbox

    Thank you for signing up!

    There was a problem signing you up.

    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

  • in

    Quantum particles can feel the influence of gravitational fields they never touch

    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.

    Sign Up For the Latest from Science News

    Headlines and summaries of the latest Science News articles, delivered to your inbox

    Thank you for signing up!

    There was a problem signing you up.

    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

  • in

    Quantum physics requires imaginary numbers to explain reality

    Imaginary numbers might seem like unicorns and goblins — interesting but irrelevant to reality. 

    But for describing matter at its roots, imaginary numbers turn out to be essential. They seem to be woven into the fabric of quantum mechanics, the math describing the realm of molecules, atoms and subatomic particles. A theory obeying the rules of quantum physics needs imaginary numbers to describe the real world, two new experiments suggest.

    Imaginary numbers result from taking the square root of a negative number. They often pop up in equations as a mathematical tool to make calculations easier. But everything we can actually measure about the world is described by real numbers, the normal, nonimaginary figures we’re used to (SN: 5/8/18). That’s true in quantum physics too. Although imaginary numbers appear in the inner workings of the theory, all possible measurements generate real numbers.

    Quantum theory’s prominent use of complex numbers — sums of imaginary and real numbers — was disconcerting to its founders, including physicist Erwin Schrödinger. “From the early days of quantum theory, complex numbers were treated more as a mathematical convenience than a fundamental building block,” says physicist Jingyun Fan of the Southern University of Science and Technology in Shenzhen, China.

    Sign Up For the Latest from Science News

    Headlines and summaries of the latest Science News articles, delivered to your inbox

    Thank you for signing up!

    There was a problem signing you up.

    Some physicists have attempted to build quantum theory using real numbers only, avoiding the imaginary realm with versions called “real quantum mechanics.” But without an experimental test of such theories, the question remained whether imaginary numbers were truly necessary in quantum physics, or just a useful computational tool.

    A type of experiment known as a Bell test resolved a different quantum quandary, proving that quantum mechanics really requires strange quantum linkages between particles called entanglement (SN: 8/28/15). “We started thinking about whether an experiment of this sort could also refute real quantum mechanics,” says theoretical physicist Miguel Navascués of the Institute for Quantum Optics and Quantum Information Vienna. He and colleagues laid out a plan for an experiment in a paper posted online at arXiv.org in January 2021 and published December 15 in Nature.

    In this plan, researchers would send pairs of entangled particles from two different sources to three different people, named according to conventional physics lingo as Alice, Bob and Charlie. Alice receives one particle, and can measure it using various settings that she chooses. Charlie does the same. Bob receives two particles and performs a special type of measurement to entangle the particles that Alice and Charlie receive. A real quantum theory, with no imaginary numbers, would predict different results than standard quantum physics, allowing the experiment to distinguish which one is correct.

    Fan and colleagues performed such an experiment using photons, or particles of light, they report in a paper to be published in Physical Review Letters. By studying how Alice, Charlie and Bob’s results compare across many measurements, Fan, Navascués and colleagues show that the data could be described only by a quantum theory with complex numbers.

    Another team of physicists conducted an experiment based on the same concept using a quantum computer made with superconductors, materials which conduct electricity without resistance. Those researchers, too, found that quantum physics requires complex numbers, they report in another paper to be published in Physical Review Letters. “We are curious about why complex numbers are necessary and play a fundamental role in quantum mechanics,” says quantum physicist Chao-Yang Lu of the University of Science and Technology of China in Hefei, a coauthor of the study.

    But the results don’t rule out all theories that eschew imaginary numbers, notes theoretical physicist Jerry Finkelstein of Lawrence Berkeley National Laboratory in California, who was not involved with the new studies. The study eliminated certain theories based on real numbers, namely those that still follow the conventions of quantum mechanics. It’s still possible to explain the results without imaginary numbers by using a theory that breaks standard quantum rules. But those theories run into other conceptual issues, making them “ugly,” he says. But “if you’re willing to put up with the ugliness, then you can have a real quantum theory.”

    Despite the caveat, other physicists agree that the quandaries raised by the new findings are compelling. “I find it intriguing when you ask questions about why is quantum mechanics the way it is,” says physicist Krister Shalm of the National Institute of Standards and Technology in Boulder, Colo. Asking whether quantum theory could be simpler or if it contains anything unnecessary, “these are very interesting and thought-provoking questions.” More

  • in

    Physicists have coaxed ultracold atoms into an elusive form of quantum matter

    An elusive form of matter called a quantum spin liquid isn’t a liquid, and it doesn’t spin — but it sure is quantum.

    Predicted nearly 50 years ago, quantum spin liquids have long evaded definitive detection in the laboratory. But now, a lattice of ultracold atoms held in place with lasers has shown hallmarks of the long-sought form of matter, researchers report in the Dec. 3 Science.

    Quantum entanglement goes into overdrive in the newly fashioned material. Even atoms on opposite sides of the lattice share entanglement, or quantum links, meaning that the properties of distant atoms are correlated with one another. “It’s very, very entangled,” says physicist Giulia Semeghini of Harvard University, a coauthor of the new study. “If you pick any two points of your system, they are connected to each other through this huge entanglement.” This strong, long-range entanglement could prove useful for building quantum computers, the researchers say.

    The new material matches predictions for a quantum spin liquid, although its makeup strays a bit from conventional expectations. While the traditional idea of a quantum spin liquid relies on the quantum property of spin, which gives atoms magnetic fields, the new material is based on different atomic quirks.

    Sign Up For the Latest from Science News

    Headlines and summaries of the latest Science News articles, delivered to your inbox

    Thank you for signing up!

    There was a problem signing you up.

    A standard quantum spin liquid should arise among atoms whose spins are in conflict. Spin causes atoms to act as tiny magnets. Normally, at low temperatures, those atoms would align their magnetic poles in a regular pattern. For example, if one atom points up, its neighbors point down. But if atoms are arranged in a triangle, for example, each atom has two neighbors that themselves point in opposite directions. That arrangement leaves the third one with nowhere to turn — it can’t oppose both of its neighbors at once.

    So atoms in quantum spin liquids refuse to choose (SN: 9/21/21). Instead, the atoms wind up in a superposition, a quantum combination of spin up and down, and each atom’s state is linked with those of its compatriots. The atoms are constantly fluctuating and never settle down into an orderly arrangement of spins, similarly to how atoms in a normal liquid are scattered about rather than arranged in a regularly repeating pattern, hence the name.

    Conclusive evidence of quantum spin liquids has been hard to come by in solid materials. In the new study, the researchers took a different tack: They created an artificial material composed of 219 trapped rubidium atoms cooled to a temperature of around 10 microkelvins (about –273.15° Celsius). The array of atoms, known as a programmable quantum simulator, allows scientists to fine-tune how atoms interact to investigate exotic forms of quantum matter.

    In the new experiment, rather than the atoms’ spins being in opposition, a different property created disagreement. The researchers used lasers to put the atoms into Rydberg states, meaning one of an atom’s electrons is bumped to a very high energy level (SN: 8/29/16). If one atom is in a Rydberg state, its neighbors prefer not to be. That setup begets a Rydberg-or-not discord, analogous to the spin-up and -down battle in a traditional quantum spin liquid.

    The scientists confirmed the quantum spin liquid effect by studying the properties of atoms that fell along loops traced through the material. According to quantum math, those atoms should have exhibited certain properties unique to quantum spin liquids. The results matched expectations for a quantum spin liquid and revealed that long-range entanglement was present.

    Notably, the material’s entanglement is topological. That means it is described by a branch of mathematics called topology, in which an object is defined by certain geometrical properties, for example, its number of holes (SN: 10/4/16). Topology can protect information from being destroyed: A bagel that falls off the counter will still have exactly one hole, for example. This information-preserving feature could be a boon to quantum computers, which must grapple with fragile, easily destroyed quantum information that makes calculations subject to mistakes (SN: 6/22/20).

    Whether the material truly qualifies as a quantum spin liquid, despite not being based on spin, depends on your choice of language, says theoretical physicist Christopher Laumann of Boston University, who was not involved with the study. Some physicists use the term “spin” to describe other systems with two possible options, because it has the same mathematics as atomic spins that can point either up or down. “Words have meaning, until they don’t,” he quips. It all depends how you spin them. More

  • in

    Scientists finally detected a quantum effect that blocks atoms from scattering light

    A cloud of ultracold atoms is like a motel with a neon “no vacancy” sign.

    If a guest at the motel wants to switch rooms, they’re out of luck. No vacant rooms means there’s no choice but to stay put. Likewise, in new experiments, atoms boxed in by crowded conditions have no way to switch up their quantum states. That constraint means the atoms don’t scatter light as they normally would, three teams of researchers report in the Nov. 19 Science. Predicted more than three decades ago, this effect has now been seen for the first time.

    Under normal circumstances, atoms interact readily with light. Shine a beam of light on a cloud of atoms, and they’ll scatter some of that light in all directions. This type of light scattering is a common phenomenon: It happens in Earth’s atmosphere. “We see the sky as blue because of scattered radiation from the sun,” says Yair Margalit, who was part of the team at MIT that performed one of the experiments.

    But quantum physics comes to the fore in ultracold, dense atom clouds. “The way they interact with light or scatter light is different,” says physicist Amita Deb of the University of Otago in Dunedin, New Zealand, a coauthor of another of the studies.

    Sign Up For the Latest from Science News

    Headlines and summaries of the latest Science News articles, delivered to your inbox

    Thank you for signing up!

    There was a problem signing you up.

    According to a rule called the Pauli exclusion principle, atoms in the experiments can’t take on the same quantum state — namely, they can’t have the same momentum as another atom in the experiment (SN: 5/19/20). If atoms are packed together in a dense cloud and cooled to near absolute zero, they’ll settle into the lowest-energy quantum states. Those low-energy states will be entirely filled, like a motel with no open rooms.

    When an atom scatters light, it gets a kick of momentum, changing its quantum state, as it sends light off in another direction. But if the atom can’t change its state due to the crowded conditions, it won’t scatter the light. The atom cloud becomes more transparent, letting light through instead of scattering it.  

    To observe the effect, Margalit and colleagues beamed light through a cloud of lithium atoms, measuring the amount of light it scattered. Then, the team decreased the temperature to make the atoms fill up the lowest energy states, suppressing the scattering of light. As the temperature dropped, the atoms scattered 37 percent less light, indicating that many atoms were prevented from scattering light. (Some atoms can still scatter light, for example if they get kicked into higher-energy quantum states that are unoccupied.)

    In another experiment, physicist Christian Sanner of the research institute JILA in Boulder, Colo., and colleagues studied a cloud of ultracold strontium atoms. The researchers measured how much light was scattered at small angles, for which the atoms are jostled less by the light and therefore are even less likely to be able to find an unoccupied quantum state. At lower temperatures, the atoms scattered half as much light as at higher temperatures.

    The third experiment, performed by Deb and physicist Niels Kjærgaard, also of the University of Otago, measured a similar scattering drop in an ultracold potassium atom cloud and a corresponding increase in how much light was transmitted through the cloud.

    Because the Pauli exclusion principle also governs how electrons, protons and neutrons behave, it is responsible for the structure of atoms and matter as we know it. These new results reveal the wide-ranging principle in a new context, says Sanner. “It’s fascinating because it shows a very fundamental principle in nature at work.”

    The work also suggests new ways to control light and atoms. “One could imagine a lot of interesting applications,” says theoretical physicist Peter Zoller of the University of Innsbruck in Austria, who was not involved with the research. In particular, light scattering is closely related to a process called spontaneous emission, in which an atom in a high-energy state decays to a lower energy by emitting light. The results suggest that decay could be blocked, increasing the lifetime of the energetic state. Such a technique might be useful for storing quantum information for a lengthier period of time than is normally possible, for example in a quantum computer.

    So far, these applications are still theoretical, Zoller says. “How realistic they are is something to be explored in the future.” More

  • in

    Scientists are one step closer to error-correcting quantum computers

    Mistakes happen — especially in quantum computers. The fragile quantum bits, or qubits, that make up the machines are notoriously error-prone, but now scientists have shown that they can fix the flubs.

    Computers that harness the rules of quantum mechanics show promise for making calculations far out of reach for standard computers (SN: 6/29/17). But without a mechanism for fixing the computers’ mistakes, the answers that a quantum computer spits out could be gobbledygook (SN: 6/22/20).

    Combining the power of multiple qubits into one can solve the error woes, researchers report October 4 in Nature. Scientists used nine qubits to make a single, improved qubit called a logical qubit, which, unlike the individual qubits from which it was made, can be probed to check for mistakes.

    “This is a key demonstration on the path to build a large-scale quantum computer,” says quantum physicist Winfried Hensinger of the University of Sussex in Brighton, England, who was not involved in the new study.

    Sign Up For the Latest from Science News

    Headlines and summaries of the latest Science News articles, delivered to your inbox

    Thank you for signing up!

    There was a problem signing you up.

    Still, that path remains a long one, Hensinger says. To do complex calculations, scientists will have to dramatically scale up the number of qubits in the machines. But now that scientists have shown that they can keep errors under control, he says, “there’s nothing fundamentally stopping us to build a useful quantum computer.”

    In a logical qubit, information is stored redundantly. That allows researchers to check and fix mistakes in the data. “If a piece of it goes missing, you can reconstruct it from the other pieces, like Voldemort,” says quantum physicist David Schuster of the University of Chicago, who was not involved with the new research. (The Harry Potter villain kept his soul safe by concealing it in multiple objects called Horcruxes.)

    In the new study, four additional, auxiliary qubits interfaced with the logical qubit, in order to identify errors in its data. Future quantum computers could make calculations using logical qubits in place of the original, faulty qubits, repeatedly checking and fixing any errors that crop up.

    To make their logical qubit, the researchers used a technique called a Bacon-Shor code, applying it to qubits made of ytterbium ions hovering above an ion-trapping chip inside a vacuum, which are manipulated with lasers. The researchers also designed sequences of operations so that errors don’t multiply uncontrollably, what’s known as “fault tolerance.”

    Thanks to those efforts, the new logical qubit had a lower error rate than that of the most flawed components that made it up, says quantum physicist Christopher Monroe of the University of Maryland in College Park and Duke University.

    However, the team didn’t quite complete the full process envisioned for error correction. While the computer detected the errors that arose, the researchers didn’t correct the mistakes and continue on with computation. Instead, they fixed errors after the computer was finished. In a full-fledged example, scientists would detect and correct errors multiple times on the fly.

    Demonstrating quantum error correction is a necessity for building useful quantum computers. “It’s like achieving criticality with [nuclear] fission,” Schuster says. Once that nuclear science barrier was passed in 1942, it led to technologies like nuclear power and atomic bombs (SN: 11/29/17).

    As quantum computers gradually draw closer to practical usefulness, companies are investing in the devices. Technology companies such as IBM, Google and Intel host major quantum computing endeavors. On October 1, a quantum computing company cofounded by Monroe, called IonQ, went public; Monroe spoke to Science News while on a road trip to ring the opening bell at the New York Stock Exchange.

    The new result suggests that full-fledged quantum error correction is almost here, says coauthor Kenneth Brown, a quantum engineer also at Duke University. “It really shows that we can get all the pieces together and do all the steps.” More

  • in

    One of nature’s key constants is much larger in a quantum material

    A crucial number that rules the universe goes big in a strange quantum material.

    The fine-structure constant is about 10 times its normal value in a type of material called quantum spin ice, physicists calculate in the Sept. 10 Physical Review Letters. The new calculation hints that quantum spin ice could give a glimpse at physics within an alternate universe where the constant is much larger.

    With an influence that permeates physics and chemistry, the fine-structure constant sets the strength of interactions between electrically charged particles. Its value, about 1/137, consternates physicists because they can’t explain why it has that value, even though it is necessary for the complex chemistry that is the basis of life (SN: 11/2/16).

    If the fine-structure constant throughout the cosmos were as large as the one in quantum spin ices, “the periodic table would only have 10 elements,” says theoretical physicist Christopher Laumann of Boston University. “And it probably would be hard to make people; there wouldn’t be enough richness to chemistry.”

    Sign Up For the Latest from Science News

    Headlines and summaries of the latest Science News articles, delivered to your inbox

    Thank you for signing up!

    There was a problem signing you up.

    Quantum spin ices are a class of substances in which particles can’t agree. The materials are made up of particles with spin, a quantum version of angular momentum, which makes them magnetic. In a normal material, particles would come to a consensus below a certain temperature, with the magnetic poles lining up in either the same direction or in alternating directions. But in quantum spin ices, the particles are arranged in such a way that the magnetic poles, or equivalently the spins, can’t agree even at a temperature of absolute zero (SN: 2/13/11).

    The impasse occurs because of the materials’ geometry: The particles are located at the corners of an array of pyramids that are connected at the corners. Conflicts between multiple sets of neighbors mean that the closest these particles can get to harmony is arranging themselves so that two spins face out from each pyramid, and two face in.

    In quantum spin ices, particles (black dots) are located at the corners of an array of pyramids (red). Normally, the spins of the particles (green arrows) arrange so that two are pointing into the pyramid and two out. If that rule is broken, as illustrated, quasiparticles called spinons (orange and blue) form.S.D. Pace et al/PRL 2021

    This uneasy truce can give rise to disturbances that behave like particles within the material, or quasiparticles (SN: 10/3/14). Flip particles’ spins around and you can get what are called spinons, quasiparticles that can move through the material and interact with other spinons in a manner akin to electrons and other charged particles found in the world outside the material. The material re-creates the theory of quantum electrodynamics, the piece of particles physics’ standard model that hashes out how electrically charged particles do their thing. But the specifics, including the fine-structure constant, don’t necessarily match those in the wider universe.

    So Laumann and colleagues set out to calculate the fine-structure constant in quantum spin ices for the first time. The team pegged the number at about 1/10, instead of 1/137. What’s more, the researchers found that they could change the value of the fine-structure constant by tweaking the properties of the theoretical material. That could help scientists study the effects of altering the fine-structure constant — a test that’s well out of reach in our own universe, where the fine-structure constant is fixed.

    Unfortunately, scientists haven’t yet found a material that definitively qualifies as quantum spin ice. But one much-studied prospect is a group of minerals called pyrochlores, which have magnetic ions, or electrically charged atoms, arranged in the appropriate pyramid configuration. Scientists might also be able to study the materials using a quantum computer or another quantum device designed to simulate quantum spin ices (SN: 6/29/17).

    If scientists succeed in creating quantum spin ice, the materials could reveal how quantum electrodynamics and the standard model would work in a universe with a much larger fine-structure constant. “That would be the hope,” says condensed matter theorist Shivaji Sondhi of the University of Oxford, who was not involved with the research. “It’s interesting to be able to make a fake standard model … and ask what would happen.” More

  • in

    Physicists used LIGO’s mirrors to approach a quantum limit

    Quantum mechanics usually applies to very small objects: atoms, electrons and the like. But physicists have now brought the equivalent of a 10-kilogram object to the edge of the quantum realm.

    Scientists with the Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, reduced vibrations in a combination of the facility’s mirrors to nearly the lowest level allowed by quantum mechanics, they report in the June 18 Science.

    The researchers quelled differences between the jiggling of LIGO’s four 40-kilogram mirrors, putting them in near-perfect sync. When the mirrors are combined in this way, they behave effectively like a single, 10-kilogram object.

    LIGO is designed to measure gravitational waves, using laser light that bounces between sets of mirrors in the detector’s two long arms (SN: 2/11/16). But physicist Vivishek Sudhir of MIT and colleagues instead used the laser light to monitor the mirrors’ movements to extreme precision and apply electric fields to resist the motion. “It’s almost like a noise-canceling headphone,” says Sudhir. But instead of measuring nearby sounds and canceling out that noise, the technique cancels out motion.

    The researchers reduced the mirrors’ relative motions to about 10.8 phonons, or quantum units of vibration, close to the zero-phonon quantum limit.

    The study’s purpose is not to better understand gravitational waves, but to get closer to revealing secrets of quantum mechanics. Scientists are still trying to understand why large objects don’t typically follow the laws of quantum mechanics. Such objects lose their quantum properties, or decohere. Studying quantum states of more massive objects could help scientists pin down how decoherence happens.

    Previous studies have observed much smaller objects in quantum states. In 2020, physicist Markus Aspelmeyer of the University of Vienna and colleagues brought vibrations of a nanoparticle to the quantum limit (SN: 1/30/20). LIGO’s mirrors are “a fantastic system to study decoherence effects on super-massive objects in the quantum regime,” says Aspelmeyer. More