If there’s one law of physics that seems easy to grasp, it’s the second law of thermodynamics: Heat flows spontaneously from hotter bodies to colder ones. But now, gently and almost casually, Alexssandre de Oliveira Jr. has just shown me I didn’t truly understand it at all.
Take this hot cup of coffee and this cold jug of milk, the Brazilian physicist said as we sat in a café in Copenhagen. Bring them into contact and, sure enough, heat will flow from the hot object to the cold one, just as the German scientist Rudolf Clausius first stated formally in 1850. However, in some cases, de Oliveira explained, physicists have learned that the laws of quantum mechanics can drive heat flow the opposite way: from cold to hot.
This doesn’t really mean that the second law fails, he added as his coffee reassuringly cooled. It’s just that Clausius’ expression is the “classical limit” of a more complete formulation demanded by quantum physics.
Physicists began to appreciate the subtlety of this situation more than two decades ago and have been exploring the quantum mechanical version of the second law ever since. Now, de Oliveira, a postdoctoral researcher at the Technical University of Denmark, and colleagues have shown that the kind of “anomalous heat flow” that’s enabled at the quantum scale could have a convenient and ingenious use.
It can serve, they say, as an easy method for detecting “quantumness”—sensing, for instance, that an object is in a quantum “superposition” of multiple possible observable states, or that two such objects are entangled, with states that are interdependent—without destroying those delicate quantum phenomena. Such a diagnostic tool could be used to ensure that a quantum computer is truly using quantum resources to perform calculations. It might even help to sense quantum aspects of the force of gravity, one of the stretch goals of modern physics. All that’s needed, the researchers say, is to connect a quantum system to a second system that can store information about it, and to a heat sink: a body that’s able to absorb a lot of energy. With this setup, you can boost the transfer of heat to the heat sink, exceeding what would be permitted classically. Simply by measuring how hot the sink is, you could then detect the presence of superposition or entanglement in the quantum system.
Quantum computers are already here, even though it’s not readily apparent. Now, researchers say quantum advantage—the field’s long-promised milestone of outperforming classical computers—appears to have finally arrived. But the story comes with an important caveat.
Research by scientists at the University of Texas at Austin and Colorado computing firm Quantinuum devised and carried out an experiment that demonstrates “unconditional” quantum advantage, sometimes referred to as quantum supremacy. As the researchers phrased it, their “result is provable and permanent: no future development in classical algorithms can close this gap.” The preprint, which has yet to be peer reviewed, was made available on arXiv earlier this month.
Gizmodo reached out to several experts in the field, who affirmed the new results. They added that the experiment, while commendable, isn’t the most practical use of a quantum computer—which already gets flak for its uselessness to everyday users.
Then again, “quantum advantage” is a weird, surprisingly malleable concept with many possible applications. Overall, the results are definitely worth a closer look.
Alice and Bob make a cameo
Quantum enthusiasts may be familiar with Alice and Bob, two fictional characters often summoned for quantum thought experiments. In the context of the new experiment, Alice and Bob are two researchers collaborating on a computation using a single device. They receive different inputs at different points in time, but only Alice can send Bob a message, and not the other way around. Based on Alice’s message, Bob must decide how to measure and interpret to produce a final output.
According to the paper, “the use of a quantum message can provably reduce the amount of communication required by an exponential factor compared to any protocol that uses classical communication alone.” In other words, a small quantum message can replace a much larger classical one. To prove their point, the team repeated the experiment 10,000 times on Quantinuum’s H1-1 trapped-ion quantum computers, coupled with a careful mathematical validation of their protocol.
Surprisingly, they found that a quantum computer only needed 12 qubits (qubits are the smallest unit of information for quantum computers) to solve this problem. By contrast, even the most efficient classical computers needed 330 bits.
A different way to play the game
“This is a very different type of quantum advantage than we have seen before—not better or worse, but it’s just proving something completely different from past experiments,” Bill Fefferman, a computer scientist at the University of Chicago, told Gizmodo in an email. Fefferman previously collaborated with senior author Scott Aaronson but wasn’t involved in the new study.
Fefferman explained that scientists typically equate quantum advantage to “striving to perform a computation on a quantum computer that can be solved dramatically faster than any classical computer.” By contrast, the new experiment achieves “quantum information supremacy,” in which the focus isn’t so much on speed as it is on using fewer qubits to solve a problem that classical computers need many more bits to crack.
“It is true that their result is unconditional, in the sense that it doesn’t rely on unproven assumptions,” Fefferman said. “This is, of course, a great feature of this new experiment, but it’s also inherited by this ‘moving of the goalposts.’”
Gizmodo contacted the study’s authors, who said they couldn’t comment until the paper is formally published.
Pressing the advantage
The results raise questions about the broader goals of proving quantum advantage. As IBM Quantum’s director told Gizmodo in a previous interview, a potential answer is to ask how quantum computers can enhance computing problems we’re already familiar with.
But as Fefferman noted, there isn’t necessarily a better or worse approach for arriving at quantum advantage—although this “goalpost” appears to be the holy grail for the field’s struggle to prove its worth.
That may be a product of quantum computing’s history, Giuseppe Carleo, a computational physicist at EPFL in Switzerland who wasn’t involved in the new work, explained to Gizmodo in a video call. The rapid growth of quantum computing makes it easy to forget how recently the right hardware became available to test theory.
“So the field has developed historically in the past 20, 30 years much closer to mathematics, rather than an applied field where, if you want, you can use a machine to run things,” said Carleo, who spoke with Gizmodo about the history of quantum computing. As a result, most of the analysis in the field remained at theoretical levels for a longer time than scientists would’ve hoped.
But with hardware advances and a fast-growing industry, this trend is gradually shifting—as it should, Carleo said. More projects are moving away from designing quantum advantage experiments “specifically tailored to show advantage,” he said, turning instead to places where quantum computers can help, not necessarily upend.
That’s actually closer to the field’s “origins,” he added. Richard Feynman, the physicist instrumental to quantum computing’s foundations, suggested that quantum computers should predict quantum phenomena. Sure, there might not be so much “money attached to it,” but they are “of tremendous interest for theoretical physics,” particularly with regard to fundamental questions about our universe, Carleo explained.
Quantum-anything never makes it easy
The new experiment might struggle to prove its immediate connection to practicality. But in a way, the preprint does adhere to Feynman’s advice. It’s certainly a theoretically robust demonstration of using quantum hardware to investigate quantum concepts.
At this very moment, that makes it seem detached from reality. Then again, when has anything quantum ever given easy answers? Yet, if science history is any guide, the best discoveries come from the most unexpected, seemingly impractical pursuits. We’ll just have to keep watch.
Of all the eccentricities of the quantum realm, time crystals—atomic arrangements that repeat certain motions over time—might be some of the weirdest. But they certainly exist, and to provide more solid proof, physicists have finally created a time crystal we can actually see.
In a recent Nature Materials paper, physicists at the University of Colorado Boulder presented a new time crystal design: a glass cell filled with liquid crystals—rod-shaped molecules stuck in strange limbo between solid and liquid. It’s the same stuff found in smartphone LCD screens. When hit with light, the crystals jiggle and dance in repeating patterns that the researchers say resemble “psychedelic tiger stripes.”
“They can be observed directly under a microscope and even, under special conditions, by the naked eye,” said Hanqing Zhao, study lead author and a graduate student at the University of Colorado Boulder, in a release. Technically, these crystalline dances can last for hours, like an “eternally spinning clock,” the researchers added.
An asymmetrical curiosity
Time crystals first appeared in a 2012 paper by Nobel laureate Frank Wilczek, who pitched an idea for an impossible crystal that breaks several rules of symmetry in physics. Specifically, a time crystal breaks symmetry because its atoms do not lock into a continuous lattice, and their positions change over time.
Physicists have since demonstrated versions of Wilczek’s proposal, but these crystals lasted for a terribly short time and were microscopic. Zhao and Ivan Smalyukh, the study’s senior author and a physicist at the University of Colorado Boulder, wanted to see if they could overcome these limitations.
Finding the molecular ‘kink’
For the new time crystal, the duo exploited the molecules’ “kinks”—their tendency to cluster together when squeezed in a certain way. Once together, these kinks behave like whole atoms, the researchers explained.
“You have these twists, and you can’t easily remove them,” Smalyukh said. “They behave like particles and start interacting with each other.”
The team coated two glass cells with dye molecules, sandwiching a liquid crystal solution between the layers. When they flashed the setup with polarized light, the dye molecules churned inside the glass, squeezing the liquid crystal. This formed thousands of new kinks inside the crystal, the researchers explained.
“That’s the beauty of this time crystal,” said Smalyukh. “You just create some conditions that aren’t that special. You shine a light, and the whole thing happens.”
The team believes its iteration of the time crystal could have practical uses. For instance, a “time watermark” printed on bills could be used to identify counterfeits. Also, stacked layers could serve as a tiny data center.
It’s rare for quantum systems to be visible to the naked eye. Only time will tell if this time crystal amounts to anything—the researchers “don’t want to put a limit on the applications right now”—but even if it doesn’t, it’s still a neat demonstration of how physical theories exist in strange, unexpected corners of reality.
“It provides a natural framework, or a bookkeeping mechanism, to assemble very large numbers of Feynman diagrams,” said Marcus Spradlin, a physicist at Brown University who has been picking up the new tools of surfaceology. “There’s an exponential compactification in information.”
Carolina Figueiredo, a graduate student at Princeton University, noticed a striking coincidence where three species of seemingly unrelated quantum particles act identically.
Photograph: Andrea Kane/Institute for Advanced Study
Unlike the amplituhedron, which required exotic particles to provide a balance known as supersymmetry, surfaceology applies to more realistic, nonsupersymmetric particles. “It’s completely agnostic. It couldn’t care less about supersymmetry,” Spradlin said. “For some people, me included, I think that’s really been quite a surprise.”
The question now is whether this new, more primitive geometric approach to particle physics will allow theoretical physicists to slip the confines of space and time altogether.
“We needed to find some magic, and maybe this is it,” said Jacob Bourjaily, a physicist at Pennsylvania State University. “Whether it’s going to get rid of space-time, I don’t know. But it’s the first time I’ve seen a door.”
The Trouble with Feynman
Figueiredo sensed the need for some new magic firsthand during the waning months of the pandemic. She was struggling with a task that has challenged physicists for more than 50 years: predicting what will happen when quantum particles collide. In the late 1940s, it took a yearslong effort by three of the brightest minds of the postwar era—Julian Schwinger, Sin-Itiro Tomonaga, and Richard Feynman—to solve the problem for electrically charged particles. Their eventual success would win them a Nobel Prize. Feynman’s scheme was the most visual, so it came to dominate the way physicists think about the quantum world.
When two quantum particles come together, anything can happen. They might merge into one, split into many, disappear, or any sequence of the above. And what will actually happen is, in some sense, a combination of all these and many other possibilities. Feynman diagrams keep track of what might happen by stringing together lines representing particles’ trajectories through space-time. Each diagram captures one possible sequence of subatomic events and gives an equation for a number, called an “amplitude,” that represents the odds of that sequence taking place. Add up enough amplitudes, physicists believe, and you get stones, buildings, trees, and people. “Almost everything in the world is a concatenation of that stuff happening over and over again,” Arkani-Hamed said. “Just good old-fashioned things bouncing off each other.”
There’s a puzzling tension inherent in these amplitudes—one that has vexed generations of quantum physicists going back to Feynman and Schwinger themselves. One might spend hours at a chalkboard sketching byzantine particle trajectories and evaluating fearsome formulas only to find that terms cancel out and complicated expressions melt away to leave behind extremely simple answers—in a classic example, literally the number 1.
“The degree of effort required is tremendous,” Bourjaily said. “And every single time, the prediction you make mocks you with its simplicity.”
NEW YORK, September 25, 2024 (Newswire.com)
– For over a decade, Quanta Magazine has challenged and delighted fans with stories about the most fundamental questions in science. Today, in its most ambitious project to date, the magazine brings its audience into what might be the deepest mystery of all: the nature of reality itself.
Many physicists now believe that space and time are not fundamental features of the universe, but rather properties that emerge as the result of something else going on underneath. Why do physicists believe this? How can we know if it is true? And if space-time isn’t the fabric of reality, what is?
The new series, “The Unraveling of Space-Time,” tangles with those questions. It includes nine new pieces of writing and media, brought together in a rich, interactive experience designed by Quanta and HLabs, an award-winning digital design agency. Senior Editor Natalie Wolchover, who, along with Quanta, was awarded the 2022 Pulitzer Prize in Explanatory Reporting, oversaw the project’s development.
“Physicists have made genuine progress recently in studying the underpinnings of space-time,” Wolchover said. “We wanted to be as ambitious in our coverage as the subject deserves, by laying out the history, motivations and context behind the developments with the help of alluring art, animation and infographics, and a beautifully designed hub that brings it all together.”
The series includes:
Two deep-dive features — one about new progress in understanding space-time as a hologram and another about the geometric underpinnings of quantum physics and space-time
Two explainers, on the phenomenon of duality and the thermodynamics of black holes
A dynamic exploration of thought experiments that expose problems with space-time
A historical essay about physicist John Wheeler by science writer Amanda Gefter
A video documentary by Senior Producer Emily Buder
Interviews with physicist Latham Boyle and philosopher of science Karen Crowther
30 original visuals from five artists, under the guidance of Art Director Samuel Velasco
“These articles resulted from more than 60 hours of interviews with more than 30 quantum gravity researchers,” said Staff Writer Charlie Wood, author of five of the pieces in the series. “It’s next to impossible to talk about anything without leaning on the concepts of space and time. And yet many physicists suspect that our current picture is holding us back.”
Conceived by Wood, Wolchover, Executive Editor Michael Moyer, and Samir Patel, Quanta’s new editor-in-chief, “The Unraveling of Space-Time” is the first of several planned editorial projects that will engage some of the biggest questions in basic science and math today.
“Quanta has never shied away from exploring the frontiers of knowledge — however challenging, abstract, or esoteric — with ambitious storytelling and visual panache,” Patel said. “‘The Unraveling of Space-Time’ is an evolutionary step for us, and we can’t wait to do it again.”
“I hope that our audience will enjoy exploring the many facets of this series,” Wolchover added, “and will come away with a far deeper understanding of physicists’ ultimate quest.”
Those who enjoy the series will have an opportunity to put their questions to members of Quanta’s staff. From 1:30–4:30 p.m. ET on Friday, September 27, Wolchover and Wood will answer questions about the series in a Reddit “Ask Me Anything” discussion on r/IAmA, a forum for community-driven Q&A discussions with subject experts.
Quanta Magazine is an award-winning, editorially independent online publication of the Simons Foundation.
Patel, Wolchover and Wood are available for media interviews about the series and its contents.
But not all questions about quantum systems are easier to answer using quantum algorithms. Some are equally easy for classical algorithms, which run on ordinary computers, while others are hard for both classical and quantum ones.
To understand where quantum algorithms and the computers that can run them might offer an advantage, researchers often analyze mathematical models called spin systems, which capture the basic behavior of arrays of interacting atoms. They then might ask: What will a spin system do when you leave it alone at a given temperature? The state it settles into, called its thermal equilibrium state, determines many of its other properties, so researchers have long sought to develop algorithms for finding equilibrium states.
Whether those algorithms really benefit from being quantum in nature depends on the temperature of the spin system in question. At very high temperatures, known classical algorithms can do the job easily. The problem gets harder as temperature decreases and quantum phenomena grow stronger; in some systems it gets too hard for even quantum computers to solve in any reasonable amount of time. But the details of all this remain murky.
“When do you go to the space where you need quantum, and when do you go to the space where quantum doesn’t even help you?” said Ewin Tang, a researcher at the University of California, Berkeley, and one of the authors of the new result. “Not that much is known.”
In February, Tang and Moitra began thinking about the thermal equilibrium problem together with two other MIT computer scientists: a postdoctoral researcher named Ainesh Bakshi and Moitra’s graduate student Allen Liu. In 2023, they’d all collaborated on a groundbreaking quantum algorithm for a different task involving spin systems, and they were looking for a new challenge.
“When we work together, things just flow,” Bakshi said. “It’s been awesome.”
Before that 2023 breakthrough, the three MIT researchers had never worked on quantum algorithms. Their background was in learning theory, a subfield of computer science that focuses on algorithms for statistical analysis. But like ambitious upstarts everywhere, they viewed their relative naïveté as an advantage, a way to see a problem with fresh eyes. “One of our strengths is that we don’t know much quantum,” Moitra said. “The only quantum we know is the quantum that Ewin taught us.”
The team decided to focus on relatively high temperatures, where researchers suspected that fast quantum algorithms would exist, even though nobody had been able to prove it. Soon enough, they found a way to adapt an old technique from learning theory into a new fast algorithm. But as they were writing up their paper, another team came out with a similar result: a proof that a promising algorithm developed the previous year would work well at high temperatures. They’d been scooped.
Sudden Death Reborn
A bit bummed that they’d come in second, Tang and her collaborators began corresponding with Álvaro Alhambra, a physicist at the Institute for Theoretical Physics in Madrid and one of the authors of the rival paper. They wanted to work out the differences between the results they’d achieved independently. But when Alhambra read through a preliminary draft of the four researchers’ proof, he was surprised to discover that they’d proved something else in an intermediate step: In any spin system in thermal equilibrium, entanglement vanishes completely above a certain temperature. “I told them, ‘Oh, this is very, very important,’” Alhambra said.
From left: Allen Liu, Ainesh Bakshi, and Ankur Moitra collaborated with Tang, drawing on their background in a different branch of computer science. “One of our strengths is that we don’t know much quantum,” Moitra said.
Photographs: From left: Courtesy of Allen Liu; Amartya Shankha Biswas; Gretchen Ertl
A key question was the origin of the logarithmic scaling of the greenhouse effect—the 2-to-5-degree temperature rise that models predict will happen for every doubling of CO2. One theory held that the scaling comes from how quickly the temperature drops with altitude. But in 2022, a team of researchers used a simple model to prove that the logarithmic scaling comes from the shape of carbon dioxide’s absorption “spectrum”—how its ability to absorb light varies with the light’s wavelength.
This goes back to those wavelengths that are slightly longer or shorter than 15 microns. A critical detail is that carbon dioxide is worse—but not too much worse—at absorbing light with those wavelengths. The absorption falls off on either side of the peak at just the right rate to give rise to the logarithmic scaling.
“The shape of that spectrum is essential,” said David Romps, a climate physicist at the University of California, Berkeley, who coauthored the 2022 paper. “If you change it, you don’t get the logarithmic scaling.”
The carbon spectrum’s shape is unusual—most gases absorb a much narrower range of wavelengths. “The question I had at the back of my mind was: Why does it have this shape?” Romps said. “But I couldn’t put my finger on it.”
Consequential Wiggles
Wordsworth and his coauthors Jacob Seeley and Keith Shine turned to quantum mechanics to find the answer.
Light is made of packets of energy called photons. Molecules like CO2 can absorb them only when the packets have exactly the right amount of energy to bump the molecule up to a different quantum mechanical state.
Carbon dioxide usually sits in its “ground state,” where its three atoms form a line with the carbon atom in the center, equidistant from the others. The molecule has “excited” states as well, in which its atoms undulate or swing about.
A photon of 15-micron light contains the exact energy required to set the carbon atom swirling about the center point in a sort of hula-hoop motion. Climate scientists have long blamed this hula-hoop state for the greenhouse effect, but—as Ångström anticipated—the effect requires too precise an amount of energy, Wordsworth and his team found. The hula-hoop state can’t explain the relatively slow decline in the absorption rate for photons further from 15 microns, so it can’t explain climate change by itself.
The key, they found, is another type of motion, where the two oxygen atoms repeatedly bob toward and away from the carbon center, as if stretching and compressing a spring connecting them. This motion takes too much energy to be induced by Earth’s infrared photons on their own.
But the authors found that the energy of the stretching motion is so close to double that of the hula-hoop motion that the two states of motion mix with one another. Special combinations of the two motions exist, requiring slightly more or less than the exact energy of the hula-hoop motion.
This unique phenomenon is called Fermi resonance after the famous physicist Enrico Fermi, who derived it in a 1931 paper. But its connection to Earth’s climate was only made for the first time in a paper last year by Shine and his student, and the paper this spring is the first to fully lay it bare.
