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Tag Archives: Physicists
Physicists Have Manipulated ‘Quantum Light’ For The First Time, in a Huge Breakthrough – ScienceAlert
- Physicists Have Manipulated ‘Quantum Light’ For The First Time, in a Huge Breakthrough ScienceAlert
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Physicists Discover a New Approach for Solving the Bizarre Dark Energy Mystery
Physicists have proposed a new interpretation of dark energy. It could shed insight on the interconnection between quantum field theory and general relativity theory, as two perspectives on the universe and its elements.
What is behind dark energy — and what connects it to the cosmological constant introduced by Albert Einstein? Two physicists from the University of Luxembourg point the way to answering these open questions of physics.
The universe has a number of bizarre properties that are difficult to understand with everyday experience. For example, the matter we know, consisting of elementary and composite particles building molecules and materials, apparently makes up only a small part of the energy of the universe. The largest contribution, about two-thirds, comes from “dark energy” – a hypothetical form of energy whose background physicists are still puzzling over. Moreover, the universe is not only expanding steadily, but also doing so at an ever-faster pace.
Both characteristics seem to be connected, because dark energy is also considered a driver of accelerated expansion. Moreover, it could reunite two powerful physical schools of thought: quantum field theory and the general theory of relativity developed by Albert Einstein. But there is a catch: calculations and observations have so far been far from matching. Now two researchers from Luxembourg have shown a new way to solve this 100-year-old riddle in a paper published by the journal Physical Review Letters.
The trail of virtual particles in a vacuum
“Vacuum has energy. This is a fundamental result of quantum field theory,” explains Prof. Alexandre Tkatchenko, Professor of Theoretical Physics at the Department of Physics and Materials Science at the University of Luxembourg. This theory was developed to bring together quantum mechanics and special relativity, but quantum field theory seems to be incompatible with general relativity. Its essential feature: in contrast to quantum mechanics, the theory considers not only particles but also matter-free fields as quantum objects.
“In this framework, many researchers regard dark energy as an expression of the so-called vacuum energy,” says Tkatchenko: a physical quantity that, in a vivid image, is caused by a constant emergence and interaction of pairs of particles and their antiparticles — such as electrons and positrons — in what is actually empty space.
Cosmic microwave background seen by Planck. Credit: ESA and the Planck Collaboration
Physicists speak of this coming and going of virtual particles and their quantum fields as vacuum or zero-point fluctuations. While the particle pairs quickly vanish into nothingness again, their existence leaves behind a certain amount of energy.
“This vacuum energy also has a meaning in general relativity,” the Luxembourg scientist notes: “It manifests itself in the cosmological constant Einstein included into his equations for physical reasons.”
A colossal mismatch
Unlike vacuum energy, which can only be deduced from the formulae of quantum field theory, the cosmological constant can be determined directly by astrophysical experiments. Measurements with the Hubble space telescope and the Planck space mission have yielded close and reliable values for the fundamental physical quantity. Calculations of dark energy on the basis of quantum field theory, on the other hand, yield results that correspond to a value of the cosmological constant that is up to 10120 times larger – a colossal discrepancy, although in the world view of physicists prevailing today, both values should be equal. The discrepancy found instead is known as the “cosmological constant enigma.”
“It is undoubtedly one of the greatest inconsistencies in modern science,” says Alexandre Tkatchenko.
Unconventional way of interpretation
Together with his Luxembourg research colleague Dr. Dmitry Fedorov, he has now brought the solution to this puzzle, which has been open for decades, a significant step closer. In a theoretical work, the results of which they recently published in Physical Review Letters, the two Luxembourg researchers propose a new interpretation of dark energy. It assumes that the zero-point fluctuations lead to a polarizability of the vacuum, which can be both measured and calculated.
“In pairs of virtual particles with an opposite electric charge, it arises from electrodynamic forces that these particles exert on each other during their extremely short existence,” Tkatchenko explains. The physicists refer to this as a vacuum self-interaction. “It leads to an energy density that can be determined with the help of a new model,” says the Luxembourg scientist.
Together with his research colleague Fedorov, they developed the basic model for atoms a few years ago and presented it for the first time in 2018. The model was originally used to describe atomic properties, in particular the relation between polarizability of atoms and the equilibrium properties of certain non-covalently bonded molecules and solids. Since the geometric characteristics are quite easy to measure experimentally, polarizability can also be determined via their formula.
“We transferred this procedure to the processes in the vacuum,” explains Fedorov. To this end, the two researchers looked at the behavior of quantum fields, in particular representing the “coming and going” of electrons and positrons. The fluctuations of these fields can also be characterized by an equilibrium geometry which is already known from experiments. “We inserted it into the formulas of our model and in this way ultimately obtained the strength of the intrinsic vacuum polarization,” Fedorov reports.
The last step was then to quantum mechanically calculate the energy density of the self-interaction between fluctuations of electrons and positrons. The result obtained in this way agrees well with the measured values for the cosmological constant. This means: “Dark energy can be traced back to the energy density of the self-interaction of quantum fields,” emphasizes Alexandre Tkatchenko.
Consistent values and verifiable forecasts
“Our work thus offers an elegant and unconventional approach to solving the riddle of the cosmological constant,” sums up the physicist. “Moreover, it provides a verifiable prediction: namely, that quantum fields such as those of electrons and positrons do indeed possess a small but ever-present intrinsic polarization.”
This finding points the way for future experiments to detect this polarization in the laboratory as well, say the two Luxembourg researchers. “Our goal is to derive the cosmological constant from a rigorous quantum theoretical approach,” emphasizes Dmitry Fedorov. “And our work contains a recipe on how to realize this.”
He sees the new results obtained together with Alexandre Tkatchenko as the first step toward a better understanding of dark energy — and its connection to Albert Einstein’s cosmological constant.
Finally, Tkatchenko is convinced: “In the end, this could also shed light on the way in which quantum field theory and general relativity theory are interwoven as two ways of looking at the universe and its components.”
Reference: “Casimir Self-Interaction Energy Density of Quantum Electrodynamic Fields” by Alexandre Tkatchenko and Dmitry V. Fedorov, 24 January 2023, Physical Review Letters.
DOI: 10.1103/PhysRevLett.130.041601
Why More Physicists Are Starting to Think Space and Time Are ‘Illusions’
This past December, the physics Nobel Prize was awarded for the experimental confirmation of a quantum phenomenon known for more than 80 years: entanglement. As envisioned by Albert Einstein and his collaborators in 1935, quantum objects can be mysteriously correlated even if they are separated by large distances. But as weird as the phenomenon appears, why is such an old idea still worth the most prestigious prize in physics?
Coincidentally, just a few weeks before the new Nobel laureates were honored in Stockholm, a different team of distinguished scientists from Harvard, MIT, Caltech, Fermilab and Google reported that they had run a process on Google’s quantum computer that could be interpreted as a wormhole. Wormholes are tunnels through the universe that can work like a shortcut through space and time and are loved by science fiction fans, and although the tunnel realized in this recent experiment exists only in a 2-dimensional toy universe, it could constitute a breakthrough for future research at the forefront of physics.
But why is entanglement related to space and time? And how can it be important for future physics breakthroughs? Properly understood, entanglement implies that the universe is “monistic”, as philosophers call it, that on the most fundamental level, everything in the universe is part of a single, unified whole. It is a defining property of quantum mechanics that its underlying reality is described in terms of waves, and a monistic universe would require a universal function. Already decades ago, researchers such as Hugh Everett and Dieter Zeh showed how our daily-life reality can emerge out of such a universal quantum-mechanical description. But only now are researchers such as Leonard Susskind or Sean Carroll developing ideas on how this hidden quantum reality might explain not only matter but also the fabric of space and time.
Entanglement is much more than just another weird quantum phenomenon. It is the acting principle behind both why quantum mechanics merges the world into one and why we experience this fundamental unity as many separate objects. At the same time, entanglement is the reason why we seem to live in a classical reality. It is—quite literally—the glue and creator of worlds. Entanglement applies to objects comprising two or more components and describes what happens when the quantum principle that “everything that can happen actually happens” is applied to such composed objects. Accordingly, an entangled state is the superposition of all possible combinations that the components of a composed object can be in to produce the same overall result. It is again the wavy nature of the quantum domain that can help to illustrate how entanglement actually works.
Picture a perfectly calm, glassy sea on a windless day. Now ask yourself, how can such a plane be produced by overlaying two individual wave patterns? One possibility is that superimposing two completely flat surfaces results again in a completely level outcome. But another possibility that might produce a flat surface is if two identical wave patterns shifted by half an oscillation cycle were to be superimposed on one another, so that the wave crests of one pattern annihilate the wave troughs of the other one and vice versa. If we just observed the glassy ocean, regarding it as the result of two swells combined, there would be no way for us to find out about the patterns of the individual swells. What sounds perfectly ordinary when we talk about waves has the most bizarre consequences when applied to competing realities. If your neighbor told you she had two cats, one live cat and a dead one, this would imply that either the first cat or the second one is dead and that the remaining cat, respectively, is alive—it would be a strange and morbid way of describing one’s pets, and you may not know which one of them is the lucky one, but you would get the neighbor’s drift. Not so in the quantum world. In quantum mechanics, the very same statement implies that the two cats are merged in a superposition of cases, including the first cat being alive and the second one dead and the first cat being dead while the second one lives, but also possibilities where both cats are half alive and half dead, or the first cat is one-third alive, while the second feline adds the missing two-thirds of life. In a quantum pair of cats, the fates and conditions of the individual animals get dissolved entirely in the state of the whole. Likewise, in a quantum universe, there are no individual objects. All that exists is merged into a single “One.”
“I’m almost certain that space and time are illusions. These are primitive notions that will be replaced by something more sophisticated.”
— Nathan Seiberg, Princeton University
Quantum entanglement reveals to us a vast and entirely new territory to explore. It defines a new foundation of science and turns our quest for a theory of everything upside down—to build on quantum cosmology rather than on particle physics or string theory. But how realistic is it for physicists to pursue such an approach? Surprisingly, it is not just realistic—they are actually doing it already. Researchers at the forefront of quantum gravity have started to rethink space-time as a consequence of entanglement. An increasing number of scientists have come to ground their research in the nonseparability of the universe. Hopes are high that by following this approach they may finally come to grasp what space and time, deep down at the foundation, really are.
Whether space is stitched together by entanglement, physics is described by abstract objects beyond space and time or the space of possibilities represented by Everett’s universal wave function, or everything in the universe is traced back to a single quantum object—all these ideas share a distinct monistic flavor. At present it is hard to judge which of these ideas will inform the future of physics and which will eventually disappear. What’s interesting is that while originally ideas were often developed in the context of string theory, they seem to have outgrown string theory, and strings play no role anymore in the most recent research. A common thread now seems to be that space and time are not considered fundamental anymore. Contemporary physics doesn’t start with space and time to continue with things placed in this preexisting background. Instead, space and time themselves are considered products of a more fundamental projector reality. Nathan Seiberg, a leading string theorist at the Institute for Advanced Study at Princeton, is not alone in his sentiment when he states, “I’m almost certain that space and time are illusions. These are primitive notions that will be replaced by something more sophisticated.” Moreover, in most scenarios proposing emergent space-times, entanglement plays the fundamental role. As philosopher of science Rasmus Jaksland points out, this eventually implies that there are no individual objects in the universe anymore; that everything is connected with everything else: “Adopting entanglement as the world making relation comes at the price of giving up separability. But those who are ready to take this step should perhaps look to entanglement for the fundamental relation with which to constitute this world (and perhaps all the other possible ones).” Thus, when space and time disappear, a unified One emerges.
Courtesy Hachette Book Group
Conversely, from the perspective of quantum monism, such mind-boggling consequences of quantum gravity are not far off. Already in Einstein’s theory of general relativity, space is no static stage anymore; rather it is sourced by matter’s masses and energy. Much like the German philosopher Gottfried W. Leibniz’s view, it describes the relative order of things. If now, according to quantum monism, there is only one thing left, there is nothing left to arrange or order and eventually no longer a need for the concept of space on this most fundamental level of description. It is “the One,” a single quantum universe that gives rise to space, time, and matter.
“GR=QM,” Leonard Susskind claimed boldly in an open letter to researchers in quantum information science: general relativity is nothing but quantum mechanics—a hundred-year-old theory that has been applied extremely successfully to all sorts of things but never really entirely understood. As Sean Carroll has pointed out, “Maybe it was a mistake to quantize gravity, and space-time was lurking in quantum mechanics all along.” For the future, “rather than quantizing gravity, maybe we should try to gravitize quantum mechanics. Or, more accurately but less evocatively, ‘find gravity inside quantum mechanics,’” Carroll suggests on his blog. Indeed, it seems that if quantum mechanics had been taken seriously from the beginning, if it had been understood as a theory that isn’t happening in space and time but within a more fundamental projector reality, many of the dead ends in the exploration of quantum gravity could have been avoided. If we had approved the monistic implications of quantum mechanics—the heritage of a three-thousand-year-old philosophy that was embraced in antiquity, persecuted in the Middle Ages, revived in the Renaissance, and tampered with in Romanticism—as early as Everett and Zeh had pointed them out rather than sticking to the influential quantum pioneer Niels Bohr’s pragmatic interpretation that reduced quantum mechanics to a tool, we would be further on the way to demystifying the foundations of reality.
Adapted from The One: How an Ancient Idea Holds the Future of Physics by Heinrich Päs. Copyright © 2023. Available from Basic Books, an imprint of Hachette Book Group, Inc.
Physicists Break Record Firing a Laser Down Their University Corridor : ScienceAlert
Physicists have just set a new record confining a self-focused laser pulse to a cage of air, down the length of a 45 meter-long (148 foot-long) university corridor.
With previous results falling well short of a meter, this newest experiment led by physicist Howard Milchberg of the University of Maryland (UMD) breaks new ground for confining light to channels known as air waveguides.
A paper describing the research has been accepted into the journal Physical Review X, and can in the meantime be found on the preprint server arXiv . The results could inspire new ways to achieve long-range laser-based communications or even advanced laser-based weapons technology.
“If we had a longer hallway, our results show that we could have adjusted the laser for a longer waveguide,” says UMD physicist Andrew Tartaro.
“But we got our guide right for the hallway we have.”
Lasers can be useful for a range of applications, but the coherent rays of neatly-arranged light need to be corralled and focused in some way. Left to its own devices, a laser will scatter, losing power and effectiveness.
One such focusing technique is the waveguide, and it’s exactly what it sounds like: it guides electromagnetic waves down a specific path, preventing them from scattering.
Optical fiber is one example. This consists of a glass tube along which electromagnetic waves are directed. Because the cladding around the outside of the tube has a lower refractive index than the center of the tube, light that tries to scatter instead bends back into the tube, maintaining the beam along its length.
In 2014, Milchberg and his colleagues successfully demonstrated what they called an air waveguide. Rather than using a physical construct such as a tube, they used laser pulses to corral their laser light. They found that pulsed laser creates a plasma that heats the air in its wake, leaving behind a path of lower-density air. It’s like lightning and thunder in miniature: the expanding lower-density air creates a sound like a tiny thunderclap following the laser, creating what’s known as a filament.
The lower density air has a lower refractive index than the air around it – like the cladding around an optical fiber tube. So firing these filaments in a specific configuration that “cages” a laser beam in their center effectively creates a waveguide out of the air.
The initial experiments described in 2014 created an air waveguide of about 70 centimeters (2.3 feet) long, using four filaments. To scale the experiment up, they needed more filaments – and a much longer tunnel down which to shine their lights, preferably without having to move their heavy equipment. Hence, a long corridor at UMD’s Energy Research Facility, altered to allow the safe propagation of lasers beamed through a hole in the lab wall.
Corridor entry points were blocked, shiny surfaces covered, laser-absorbing curtains deployed.
“It was a really unique experience,” says UMD electrical engineer Andrew Goffin, the first author on the team’s paper.
“There’s a lot of work that goes into shooting lasers outside the lab that you don’t have to deal with when you’re in the lab – like putting up curtains for eye safety. It was definitely tiring.”
Finally, the team was able to create a waveguide capable of traversing a 45 meter corridor – accompanied by crackling, popping noises, the tiny thunderclaps created by their laser filament “lightning”. At the end of the air waveguide, the laser pulse in the center had retained about 20 percent of the light that would have been otherwise lost without a waveguide.
Back in the lab, the team also studied a shorter, 8-meter air waveguide, to take measurements of the processes that occurred in the hallway, where they didn’t have the equipment to do so. These shorter tests were able to retain 60 percent of the light that would have been lost. The tiny thunderclaps were also useful: the more energetic the waveguide, the louder the pop.
Their experiments revealed that the waveguide is extremely fleeting, lasting just hundredths of a second. To guide something that’s traveling the speed of light, however, that time is ample.
The research suggests where improvements can be made; for example, higher guiding efficiency and length should result in even less light lost. The team also wants to try different colors of laser light, and a faster filament pulse rate, to see if they can guide a continuous laser beam.
“Reaching the 50-meter scale for air waveguides literally blazes the path for even longer waveguides and many applications,” Milchberg says.
“Based on new lasers we are soon to get, we have the recipe to extend our guides to one kilometer and beyond.”
The research has been accepted in Physical Review X, and is available on arXiv.
Physicists Discover a New Way to ‘See’ Objects Without Looking at Them : ScienceAlert
Ordinarily, to measure an object we must interact with it in some way. Whether it’s by a prod or a poke, an echo of sound waves, or a shower of light, it’s near impossible to look without touching.
In the world of quantum physics, there are some exceptions to this rule.
Researchers from Aalto University in Finland propose a way to ‘see’ a microwave pulse without the absorption and re-emission of any light waves. It’s an example of a special interaction-free measurement, where something is observed without being rattled by a mediating particle.
The fundamental concept of ‘looking without touching’ isn’t new. Physicists have shown it’s possible to use the wave-like nature of light to explore spaces without evoking its particle-like behavior by splitting neatly aligned waves of light through different paths and then comparing their journeys.
Instead of lasers and mirrors, the team used microwaves and semiconductors, making it a separate achievement. The setup used what’s known as a transmon device to detect an electromagnetic wave pulsed into a chamber.
While relatively large by quantum standards, these devices mimic the quantum behavior of individual particles on multiple levels using a superconducting circuit.
“The interaction-free measurement is a fundamental quantum effect whereby the presence of a photosensitive object is determined without irreversible photon absorption,” write the researchers in their published paper.
“Here we propose the concept of coherent interaction-free detection and demonstrate it experimentally using a three-level superconducting transmon circuit.”
The team relied on the quantum coherence produced by their bespoke system – the ability for objects to occupy two different states at the same time, like Schrödinger’s cat – in order to make the complex setup successful.
“We had to adapt the concept to the different experimental tools available for superconducting devices,” says quantum physicist Gheorghe Sorin Paraoanu, from Aalto University in Finland.
“Because of that, we also had to change the standard interaction-free protocol in a crucial way: we added another layer of quantumness by using a higher energy level of the transmon. Then, we used the quantum coherence of the resulting three-level system as a resource.”
The experiments run by the team were backed up with theoretical models confirming the results. It’s an example of what scientists call the quantum advantage, the ability for quantum devices to go beyond what’s possible with classical devices.
In the delicate landscape of quantum physics, touching things is akin to breaking them. Nothing ruins a neat wave of probability like the crunch of reality. For cases where detection needs a more gentle touch, alternative methods of sensing – like this one – could come in handy.
Areas in which this protocol can be applied include quantum computing, optical imaging, noise detection and cryptographic key distribution. In each case, the efficiency of the systems involved would be significantly improved.
“In quantum computing, our method could be applied for diagnosing microwave-photon states in certain memory elements,” says Paraoanu. “This can be regarded as a highly efficient way of extracting information without disturbing the functioning of the quantum processor.”
The research has been published in Nature Communications.
Danish physicists give the gift of world’s smallest Christmas record—in stereo
The first 25 seconds of a classic Christmas song was inscribed into polymer film using the Nanofrazor 3D lithography system.
Physicists at the Technical University of Denmark (DTU) are bringing the Christmas cheer by using a 3D nanolithography tool called the Nanofrazor to cut the smallest record ever. The tune they “recorded,” in full stereo no less: the first 25 seconds of “Rocking Around the Christmas Tree.”
”I have done lithography for 30 years, and although we’ve had this machine for a while, it still feels like science fiction,” said Peter Bøggild, a physicist at DTU. “To get an idea of the scale we are working at, we could write our signatures on a red blood cell with this thing. The most radical thing is that we can create free-form 3D landscapes at that crazy resolution.”
Back in 2015, the same DTU group created a microscopic color image of the Mona Lisa, some 10,000 times smaller than Leonardo da Vinci’s original painting. To do so, they created a nanoscale surface structure consisting of rows of columns, covered by a 20-nm thick layer of aluminum. How much a column was deformed determined which colors of light were reflected, and the deformation in turn was determined by the intensity of the pulsed laser beam. For instance, low-intensity pulses only deformed the columns slightly, producing blue and purple tones, while strong pulses significantly deformed the columns, producing orange and yellow tones. The resulting image fit in a space smaller than the footprint taken up by a single pixel on an iPhone Retina display.
Enlarge / In 2015, the DTU physics group made a nanoscale Mona Lisa with a pixel size of ten nanometers.
DTU Physics
The DTU physics group acquired the Nanofrazor in order to sculpt precisely detailed 3D nanostructures quickly and relatively cheaply. The Christmas record was simply a fun holiday project for postdoc Nolan Lassaline to demonstrate the capability of shaping a surface with nanoscale precision. Instead of adding material to a surface, the Nanofrazor precisely removes material to sculpt the surface into the desired pattern or shape—a kind of gray-scale nanolithography.
“The Nanofrazor was put to work as a record-cutting lathe—converting an audio signal into a spiralled groove on the surface of the medium,” said Bøggild, who is also an amateur musician and vinyl record enthusiast. “In this case, the medium is a different polymer than vinyl. We even encoded the music in stereo—the lateral wriggles is the left channel, whereas the depth modulation contains the right channel. It may be too impractical and expensive to become a hit record. To read the groove, you need a rather costly atomic force microscope or the Nanofrazor, but it is definitely doable.”
The initial goal is to use the Nanofrazor to develop new kinds of magnetic sensors capable of detecting the currents in living brains. Lassaline plans to create “quantum soap bubbles” in graphene in hopes of discovering new ways of precisely manipulating the electrons in that and other atomically thin materials. “The fact that we can now accurately shape the surfaces with nanoscale precision at pretty much the speed of imagination is a game changer for us,” said DTU physicist Tim Booth. “We have many ideas for what to do next and believe that this machine will significantly speed up the prototyping of new structures.”
No, physicists didn’t make a real wormhole. What they did was still pretty cool
Enlarge / Artist’s illustration of a quantum experiment that studies the physics of traversable wormholes.
Wormholes are a classic trope of science fiction in popular media, if only because they provide such a handy futuristic plot device to avoid the issue of violating relativity with faster-than-light travel. In reality, they are purely theoretical. Unlike black holes—also once thought to be purely theoretical—no evidence for an actual wormhole has ever been found, although they are fascinating from an abstract theoretical physics perceptive. You might be forgiven for thinking that undiscovered status had changed if you only read the headlines this week announcing that physicists had used a quantum computer to make a wormhole, reporting on a new paper published in Nature.
Let’s set the record straight right away: This isn’t a bona fide traversable wormhole—i.e., a bridge between two regions of spacetime connecting the mouth of one black hole to another, through which a physical object can pass—in any real, physical sense. “There’s a difference between something being possible in principle and possible in reality,” co-author Joseph Lykken of Fermilab said during a media briefing this week. “So don’t hold your breath about sending your dog through a wormhole.” But it’s still a pretty clever, nifty experiment in its own right that provides a tantalizing proof of principle to the kinds of quantum-scale physics experiments that might be possible as quantum computers continue to improve.
“It’s not the real thing; it’s not even close to the real thing; it’s barely even a simulation of something-not-close-to-the-real-thing,” physicist Matt Strassler wrote on his blog. “Could this method lead to a simulation of a real wormhole someday? Maybe in the distant future. Could it lead to making a real wormhole? Never. Don’t get me wrong. What they did is pretty cool! But the hype in the press? Wildly, spectacularly overblown.”
So what is this thing that was “created” in a quantum computer if it’s not an actual wormhole? An analog? A toy model? Co-author Maria Spiropulu of Caltech referred to it as a novel “wormhole teleportation protocol” during the briefing. You could call it a simulation, but as Strassler wrote, that’s not quite right either. Physicists have simulated wormholes on classical computers, but no physical system is created in those simulations. That’s why the authors prefer the term “quantum experiment” because they were able to use Google’s Sycamore quantum computer to create a highly entangled quantum system and make direct measurements of specific key properties. Those properties are consistent with theoretical descriptions of a traversable wormhole’s dynamics—but only in a special simplified theoretical model of spacetime.
Lykken described it to The New York Times as “the smallest, crummiest wormhole you can imagine making.” Even then, perhaps a “collection of atoms with certain wormhole-like properties” might be more accurate. What makes this breakthrough so intriguing and potentially significant is how the experiment draws on some of the most influential and exciting recent work in theoretical physics. But to grasp precisely what was done and why it matters, we need to go on a somewhat meandering journey through some pretty heady abstract ideas spanning nearly a century.
Enlarge / Diagram of the so-called AdS/CFT correspondence (aka the holographic principle) in theoretical physics.
APS/Alan Stonebraker
Revisiting the holographic principle
Let’s start with what’s popularly known as the holographic principle. As I’ve written previously, nearly 30 years ago, theoretical physicists introduced the mind-bending theory positing that our three-dimensional universe is actually a hologram. The holographic principle began as a proposed solution to the black hole information paradox in the 1990s. Black holes, as described by general relativity, are simple objects. All you need to describe them mathematically is their mass and their spin, plus their electric charge. So there would be no noticeable change if you threw something into a black hole—nothing that would provide a clue as to what that object might have been. That information is lost.
But problems arise when quantum gravity enters the picture because the rules of quantum mechanics hold that information can never be destroyed. And in quantum mechanics, black holes are incredibly complex objects and thus should contain a great deal of information. Jacob Bekenstein realized in 1974 that black holes also have entropy. Stephen Hawking tried to prove him wrong but wound up proving him right instead, concluding that black holes, therefore, had to produce some kind of thermal radiation.
So black holes must also have entropy, and Hawking was the first to calculate that entropy. He also introduced the notion of “Hawking radiation”: The black hole will emit a tiny bit of energy, decreasing its mass by a corresponding amount. Over time, the black hole will evaporate. The smaller the black hole, the more quickly it disappears. But what then happens to the information it contained? Is it truly destroyed, thereby violating quantum mechanics, or is it somehow preserved in the Hawking radiation?
Per the holographic principle, information about a black hole’s interior could be encoded on its two-dimensional surface area (the “boundary”) rather than within its three-dimensional volume (the “bulk”). Leonard Susskind and Gerard ‘t Hooft extended this notion to the entire universe, likening it to a hologram: our three-dimensional universe in all its glory emerges from a two-dimensional “source code.”
Juan Maldacena next discovered a crucial duality, technically known as the AdS/CFT correspondence—which amounts to a mathematical dictionary that allows physicists to go back and forth between the languages of two theoretical worlds (general relativity and quantum mechanics). Dualities in physics refer to models that appear to be different but can be shown to describe equivalent physics. It’s a bit like how ice, water, and vapor are three different phases of the same chemical substance, except a duality looks at the same phenomenon in two different ways that are inversely related. In the case of AdS/CFT, the duality is between a model of spacetime known as anti-de Sitter space (AdS)—which has constant negative curvature, unlike our own de Sitter universe—and a quantum system called conformal field theory (CFT), which lacks gravity but has quantum entanglement.
It’s this notion of duality that accounts for the wormhole confusion. As noted above, the authors of the Nature paper didn’t make a physical wormhole—they manipulated some entangled quantum particles in ordinary flat spacetime. But that system is conjectured to have a dual description as a wormhole.
Physicists Create Theoretical Wormhole Using Quantum Computer
Artwork depicting a quantum experiment that observes traversable wormhole behavior. Credit: inqnet/A. Mueller (Caltech)
Physicists observe wormhole dynamics using a quantum computer in a step toward studying quantum gravity in the lab.
For the first time, scientists have developed a quantum experiment that allows them to study the dynamics, or behavior, of a special kind of theoretical wormhole. The experiment allows researchers to probe connections between theoretical wormholes and quantum physics, a prediction of so-called quantum gravity. Quantum gravity refers to a set of theories that seek to connect gravity with quantum physics, two fundamental and well-studied descriptions of nature that appear inherently incompatible with each other. Note that the experiment has not created an actual wormhole (a rupture in space and time known as an Einstein-Rosen bridge).
“We found a quantum system that exhibits key properties of a gravitational wormhole yet is sufficiently small to implement on today’s quantum hardware,” says Maria Spiropulu, the principal investigator of the U.S. Department of Energy Office of Science research program Quantum Communication Channels for Fundamental Physics (QCCFP) and the Shang-Yi Ch’en Professor of Physics at Caltech.
“This work constitutes a step toward a larger program of testing quantum gravity physics using a quantum computer. It does not substitute for direct probes of quantum gravity in the same way as other planned experiments that might probe quantum gravity effects in the future using quantum sensing, but it does offer a powerful testbed to exercise ideas of quantum gravity.”
The research was published in the journal Nature on December 1. Daniel Jafferis of Harvard University and Alexander Zlokapa (BS ’21), a former undergraduate student at Caltech who started on this project for his bachelor’s thesis with Spiropulu and has since moved on to graduate school at
This illustration of a wormhole (Einstein-Rosen bridge) depicts a tunnel with two ends at separate points in spacetime. A wormhole is a speculative structure connecting disparate points in spacetime, and is based on a special solution of the Einstein field equations.
Wormholes are bridges between two remote regions in spacetime. They have not been observed experimentally, but scientists have theorized about their existence and properties for close to 100 years. In 1935, Albert Einstein and Nathan Rosen described wormholes as tunnels through the fabric of spacetime in accordance with Einstein’s general theory of relativity, which describes gravity as a curvature of spacetime. Researchers call wormholes Einstein–Rosen bridges after the two physicists who invoked them, while the term “wormhole” itself was coined by physicist John Wheeler in the 1950s.
The notion that wormholes and quantum physics, specifically entanglement (a phenomenon in which two particles can remain connected across vast distances), may have a connection was first proposed in theoretical research by Juan Maldacena and Leonard Susskind in 2013. The physicists speculated that wormholes (or “ER”) were equivalent to entanglement (also known as “EPR” after Albert Einstein, Boris Podolsky [PhD ’28], and Nathan Rosen, who first proposed the concept). In essence, this work established a new kind of theoretical link between the worlds of gravity and quantum physics. “It was a very daring and poetic idea,” says Spiropulu of the ER = EPR work.
Later, in 2017, Jafferis, along with his colleagues Ping Gao and Aron Wall, extended the ER = EPR idea to not just wormholes but traversable wormholes. The scientists concocted a scenario in which negative repulsive energy holds a wormhole open long enough for something to pass through from one end to the other. The researchers showed that this gravitational description of a traversable wormhole is equivalent to a process known as quantum teleportation. In quantum teleportation, a protocol that has been experimentally demonstrated over long distances via optical fiber and over the air, information is transported across space using the principles of quantum entanglement.
The present work explores the equivalence of wormholes with quantum teleportation. The Caltech-led team performed the first experiments that probe the idea that information traveling from one point in space to another can be described in either the language of gravity (the wormholes) or the language of quantum physics (quantum entanglement).
A key finding that inspired possible experiments occurred in 2015, when Caltech’s Alexei Kitaev, the Ronald and Maxine Linde Professor of Theoretical Physics and Mathematics, showed that a simple quantum system could exhibit the same duality later described by Gao, Jafferis, and Wall, such that the model’s quantum dynamics are equivalent to quantum gravity effects. This Sachdev–Ye–Kitaev, or SYK model (named after Kitaev, and Subir Sachdev and Jinwu Ye, two other researchers who worked on its development previously) led researchers to suggest that some theoretical wormhole ideas could be studied more deeply by doing experiments on quantum processors.
Furthering these ideas, in 2019, Jafferis and Gao showed that by entangling two SYK models, researchers should be able to perform wormhole teleportation and thus produce and measure the dynamical properties expected of traversable wormholes.
In the new study, the team of physicists performed this type of experiment for the first time. They used a “baby” SYK-like model prepared to preserve gravitational properties, and they observed the wormhole dynamics on a quantum device at Google, namely the Sycamore quantum processor. To accomplish this, the team had to first reduce the SYK model to a simplified form, a feat they achieved using machine learning tools on conventional computers.
“We employed learning techniques to find and prepare a simple SYK-like quantum system that could be encoded in the current quantum architectures and that would preserve the gravitational properties,” says Spiropulu. “In other words, we simplified the microscopic description of the SYK quantum system and studied the resulting effective model that we found on the quantum processor. It is curious and surprising how the optimization on one characteristic of the model preserved the other metrics! We have plans for more tests to get better insights on the model itself.”
In the experiment, the researchers inserted a qubit—the quantum equivalent of a bit in conventional silicon-based computers—into one of their SYK-like systems and observed the information emerge from the other system. The information traveled from one quantum system to the other via quantum teleportation—or, speaking in the complementary language of gravity, the quantum information passed through the traversable wormhole.
“We performed a kind of quantum teleportation equivalent to a traversable wormhole in the gravity picture. To do this, we had to simplify the quantum system to the smallest example that preserves gravitational characteristics so we could implement it on the Sycamore quantum processor at Google,” says Zlokapa.
Co-author Samantha Davis, a graduate student at Caltech, adds, “It took a really long time to arrive at the results, and we surprised ourselves with the outcome.”
“The near-term significance of this type of experiment is that the gravitational perspective provides a simple way to understand an otherwise mysterious many-particle quantum phenomenon,” says John Preskill, the Richard P. Feynman Professor of Theoretical Physics at Caltech and director of the Institute for Quantum Information and Matter (IQIM). “What I found interesting about this new Google experiment is that, via machine learning, they were able to make the system simple enough to simulate on an existing quantum machine while retaining a reasonable caricature of what the gravitation picture predicts.”
In the study, the physicists report wormhole behavior expected both from the perspectives of gravity and from quantum physics. For example, while quantum information can be transmitted across the device, or teleported, in a variety of ways, the experimental process was shown to be equivalent, at least in some ways, to what might happen if information traveled through a wormhole. To do this, the team attempted to “prop open the wormhole” using pulses of either negative repulsive energy pulse or the opposite, positive energy. They observed key signatures of a traversable wormhole only when the equivalent of negative energy was applied, which is consistent with how wormholes are expected to behave.
“The high fidelity of the quantum processor we used was essential,” says Spiropulu. “If the error rates were higher by 50 percent, the signal would have been entirely obscured. If they were half we would have 10 times the signal!”
In the future, the researchers hope to extend this work to more complex quantum circuits. Though bona fide quantum computers may still be years away, the team plans to continue to perform experiments of this nature on existing
“The relationship between quantum entanglement, spacetime, and quantum gravity is one of the most important questions in fundamental physics and an active area of theoretical research,” says Spiropulu. “We are excited to take this small step toward testing these ideas on quantum hardware and will keep going.”
Reference: “Traversable wormhole dynamics on a quantum processor” by Daniel Jafferis, Alexander Zlokapa, Joseph D. Lykken, David K. Kolchmeyer, Samantha I. Davis, Nikolai Lauk, Hartmut Neven and Maria Spiropulu, 30 November 2022, Nature.
DOI: 10.1038/s41586-022-05424-3
The study was funded by the U.S. Department of Energy Office of Science via the QCCFP research program. Other authors include: Joseph Lykken of Fermilab; David Kolchmeyer, formerly at Harvard and now a postdoc at MIT; Nikolai Lauk, formerly a postdoc at Caltech; and Hartmut Neven of Google.
Did physicists create a wormhole in a quantum computer?
Physicists have used a quantum computer to perform a new kind of quantum teleportation, the ability of quantum states to be transported between distant places, as though information could travel instantly. Although teleportation is an established technique in quantum technology, the purpose of the latest experiment was to simulate the behaviour of a passage called a ‘wormhole’ through a virtual universe.
The researchers behind the experiment, described in Nature on 30 November1, say that it is a step towards using ordinary quantum physics to explore ideas about abstract universes where gravity and quantum mechanics seem to work harmoniously together. Quantum computers could help to develop a quantum theory of gravity in these ‘toy’ universes (developing a quantum theory of gravity for our own Universe is one of the biggest open questions in physics). “It’s a test of quantum-gravity ideas on a real lab experimental testbed,” says Maria Spiropulu, a particle physicist at the California Institute of Technology who led the study.
Tunnels in space-time
Physicists Albert Einstein and Nathan Rosen proposed the idea of wormholes — passages through space-time that could connect the centres of black holes — in 1935. They calculated that, in principle, wormholes were allowed by Einstein’s general theory of relativity, which explains gravity as an effect of the curvature of space-time. (Physicists soon realized that even if wormholes exist, they are unlikely to allow anything like the interstellar travel that feature in science fiction.)
Because they were working with an exotic toy universe, the latest research didn’t simulate anything resembling the kind of wormhole envisioned by Einstein and Rosen that could conceivably exist in our Universe. But their teleportation experiment can be interpreted as analogous to a wormhole in their virtual system — quantum information fed into one side of the researchers’ ‘wormhole’ reappeared on the other side.
“The surprise is not that the message made it across in some form, but that it made it across unscrambled,” write the authors of an accompanying News and Views article. “However, this is easily understood from the gravitational description: the message arrives unscrambled on the other side because it has traversed the wormhole.”
Exotic physics
The experiment was inspired by earlier research linking the physics of exotic universes and their own version of gravity to more-standard — but still virtual — quantum system. The main idea is that some abstract versions of space-time emerge from the collective behaviour of ordinary quantum particles living in a sort of ‘shadow world’ — similar to how a two-dimensional hologram can create the illusion of a three-dimensional image. That ‘holographic’ behaviour dictates how the emergent space-times curve upon themselves, producing the effects of gravity.
Although physicists do not yet know how to write quantum theories of gravity for emergent universes directly, they know that such phenomena should be fully encapsulated in the physics of the shadow world. This means that gravitational phenomena such as black holes — which still pose riddles to theoretical physicists — or wormholes must be compatible with quantum theory.
The latest experiment follows a scheme that co-author Daniel Jafferis, a theoretical physicist at Harvard University in Cambridge, Massachusetts, and his collaborators proposed in 20172. That work focused on the simplest such holographic correspondence, known as SYK after the initials of its creators. In this toy model universe, space has only one dimension rather than three.
In the latest study, Jafferis and colleagues simulated an even more stripped-down version of such a hologram using the quantum bits, or qubits, of Google’s Sycamore processor. They expected their simulated quantum particles to reproduce some behaviours of gravity in the virtual universe — but they were limited by the capabilities of today’s quantum computers. “We had to find a model that kind of preserves the gravity properties and that we can code on a quantum processor that has a limited amount of qubits,” says Maria Spiropulu, a particle physicist at the California Institute of Technology who led the study. “We shrunk it down to a baby model, and we checked that it preserves gravitational dynamics.”
“Before we worked on this project, it wasn’t obvious that a system with such a small number of qubits could exhibit this phenomenon,” Jafferis adds.
Some researchers believe that this line of research is a promising pathway for developing a quantum theory of gravity for our own Universe, although others see it as a dead end. The theory tested at the Google lab “only has a very tangential relationship to any possible theories of quantum gravity in our Universe”, says Peter Shor, a mathematician at the Massachusetts Institute of Technology in Cambridge.