Bilayer Graphene Inspires Two-Universe Cosmological Model

Details

Published: Thursday, May 05 2022 02:46

Physicists sometimes come up with crazy stories that sound like science fiction. Some turn out to be true, like how the curvature of space and time described by Einstein was eventually borne out by astronomical measurements. Others linger on as mere possibilities or mathematical curiosities.

In a new paper in Physical Review Research(link is external), Victor Galitski and graduate student Alireza Parhizkar have explored the imaginative possibility that our reality is only one half of a pair of interacting worlds. Their mathematical model may provide a new perspective for looking at fundamental features of reality—including why our universe expands the way it does and how that relates to the most miniscule lengths allowed in quantum mechanics. These topics are crucial to understanding our universe and are part of one of the great mysteries of modern physics.

The pair of scientists stumbled upon this new perspective when they were looking into research on sheets of graphene—single atomic layers of carbon in a repeating hexagonal pattern. They realized that experiments on the electrical properties of stacked sheets of graphene produced results that looked like little universes and that the underlying phenomenon might generalize to other areas of physics. In stacks of graphene, new electrical behaviors arise from interactions between the individual sheets, so maybe unique physics could similarly emerge from interacting layers elsewhere—perhaps in cosmological theories about the entire universe.A curved and stretched sheet of graphene laying over another curved sheet creates a new pattern that impacts how electricity moves through the sheets. A new model suggests that similar physics might emerge if two adjacent universes are able to interact. (Credit: Alireza Parhizkar, JQI)

“We think this is an exciting and ambitious idea,” says Galitski, who is also a Fellow of the Joint Quantum Institute (JQI). “In a sense, it's almost suspicious that it works so well by naturally ‘predicting’ fundamental features of our universe such as inflation and the Higgs particle as we described in a follow up preprint(link is external).”

Stacked graphene’s exceptional electrical properties and possible connection to our reality having a twin comes from the special physics produced by patterns called moiré patterns. Moiré patterns form when two repeating patterns—anything from the hexagons of atoms in graphene sheets to the grids of window screens—overlap and one of the layers is twisted, offset, or stretched.

The patterns that emerge can repeat over lengths that are vast compared to the underlying patterns. In graphene stacks, the new patterns change the physics that plays out in the sheets, notably the electrons' behaviors. In the special case called “magic angle graphene,” the moiré pattern repeats over a length that is about 52 times longer than the pattern length of the individual sheets, and the energy level that governs the behaviors of the electrons drops precipitously, allowing new behaviors, including superconductivity.

Galitski and Parhizkar realized that the physics in two sheets of graphene could be reinterpreted as the physics of two two-dimensional universes where electrons occasionally hop between universes. This inspired the pair to generalize the math to apply to universes made of any number of dimensions, including our own four-dimensional one, and to explore if similar phenomenon resulting from moiré patterns might pop up in other areas of physics. This started a line of inquiry that brought them face to face with one of the major problems in cosmology.

“We discussed if we can observe moiré physics when two real universes coalesce into one,” Parhizkar says. “What do you want to look for when you're asking this question? First you have to know the length scale of each universe.”

A length scale—or a scale of a physical value generally—describes what level of accuracy is relevant to whatever you are looking at. If you’re approximating the size of an atom, then a ten-billionth of a meter matters, but that scale is useless if you’re measuring a football field because it is on a different scale. Physics theories put fundamental limits on some of the smallest and largest scales that make sense in our equations.

The scale of the universe that concerned Galitski and Parhizkar is called the Planck length(link is external), and it defines the smallest length that is consistent with quantum physics. The Planck length is directly related to a constant—called the cosmological constant(link is external)—that is included in Einstein’s field equations of general relativity(link is external). In the equations, the constant influences whether the universe—outside of gravitational influences—tends to expand or contract.

This constant is fundamental to our universe. So to determine its value, scientists, in theory, just need to look at the universe, measure several details, like how fast galaxies are moving away from each other, plug everything into the equations and calculate what the constant must be.

This straightforward plan hits a problem because our universe contains both relativistic and quantum effects. The effect of quantum fluctuations across the vast vacuum of space should influence behaviors even at cosmological scales. But when scientists try to combine the relativistic understanding of the universe given to us by Einstein with theories about the quantum vacuum, they run into problems.

One of those problems is that whenever researchers attempt to use observations to approximate the cosmological constant, the value they calculate is much smaller than they would expect based on other parts of the theory. More importantly, the value jumps around dramatically depending on how much detail they include in the approximation instead of homing in on a consistent value. This lingering challenge is known as the cosmological constant problem, or sometimes the “vacuum catastrophe.”

“This is the largest—by far the largest—inconsistency between measurement and what we can predict by theory,” Parhizkar says. “It means that something is wrong.”

Since moiré patterns can produce dramatic differences in scales, moiré effects seemed like a natural lens to view the problem through. Galitski and Parhizkar created a mathematical model (which they call moiré gravity) by taking two copies of Einstein’s theory of how the universe changes over time and introducing extra terms in the math that let the two copies interact. Instead of looking at the scales of energy and length in graphene, they were looking at the cosmological constants and lengths in universes.

Galitski says that this idea arose spontaneously when they were working on a seemingly unrelated project that is funded by the John Templeton Foundation(link is external) and is focused on studying hydrodynamic flows in graphene and other materials to simulate astrophysical phenomena.

Playing with their model, they showed that two interacting worlds with large cosmological constants could override the expected behavior from the individual cosmological constants. The interactions produce behaviors governed by a shared effective cosmological constant that is much smaller than the individual constants. The calculation for the effective cosmological constant circumvents the problem researchers have with the value of their approximations jumping around because over time the influences from the two universes in the model cancel each other out.

“We don't claim—ever—that this solves cosmological constant problem,” Parhizkar says. “That's a very arrogant claim, to be honest. This is just a nice insight that if you have two universes with huge cosmological constants—like 120 orders of magnitude larger than what we observe—and if you combine them, there is still a chance that you can get a very small effective cosmological constant out of them.”

In preliminary follow up work(link is external), Galitski and Parhizkar have started to build upon this new perspective by diving into a more detailed model of a pair of interacting worlds—that they dub “bi-worlds.” Each of these worlds is a complete world on its own by our normal standards, and each is filled with matching sets of all matter and fields. Since the math allowed it, they also included fields that simultaneously lived in both worlds, which they dubbed “amphibian fields.”

The new model produced additional results the researchers find intriguing. As they put together the math, they found that part of the model looked like important fields that are part of reality. The more detailed model still suggests that two worlds could explain a small cosmological constant and provides details about how such a bi-world might imprint a distinct signature on the cosmic background radiation—the light that lingers from the earliest times in the universe.

This signature could possibly be seen—or definitively not be seen—in real world measurements. So future experiments could determine if this unique perspective inspired by graphene deserves more attention or is merely an interesting novelty in the physicists’ toy bin.

“We haven't explored all the effects—that's a hard thing to do, but the theory is falsifiable experimentally, which is a good thing,” Parhizkar says. “If it's not falsified, then it's very interesting because it solves the cosmological constant problem while describing many other important parts of physics. I personally don't have my hopes up for that— I think it is actually too big to be true.”

The research was supported by the Templeton Foundation and the Simons Foundation.

Original story by Bailey Bedford: https://jqi.umd.edu/news/bilayer-graphene-inspires-two-universe-cosmological-model?fbclid=IwAR2IS02vynZeBfnmX2tdgEr1TdLYb2OUN1E1vIXGUj1lDiLvbgFPl_LCzxs 

New Perspective Blends Quantum and Classical to Understand Quantum Rates of Change

Details

Published: Wednesday, March 09 2022 02:46

There is nothing permanent except change. This is perhaps never truer than in the fickle and fluctuating world of quantum mechanics.

The quantum world is in constant flux. The properties of quantum particles flit between discrete, quantized states without any possibility of ever being found in an intermediate state. How quantum states change defies normal intuition and remains the topic of active debate—for both scientists and philosophers.

For instance, scientists can design a quantum experiment where they find a particle’s spin—a quantum property that behaves like a magnet—pointing either up or down. No matter how often they perform the experiment they never find the spin pointing in a direction in between. Quantum mechanics is good at describing the probability of finding one or the other state and describing the state as a mix of the two when not being observed, but what actually happens between observations is ambiguous.In the figure, a path winds through an abstract landscape of possible quantum states (gray sheet). At each point along the journey, a quantum measurement could yield many different outcomes (colorful distributions below the sheet). A new theory places strict limits on how quickly (and how slowly) the result of a quantum measurement can change over time depending on the various circumstances of the experiment. For instance, how precisely researchers initially know the value of a measurement affects how quickly the value can change—a less precise value (the wider distribution on the left) can change more quickly (represented by the longer arrow pointing away from its peak) than a more certain value (the narrower peak on the right). Credit: Schuyler Nicholson

This ambiguity extends to looking at interacting quantum particles as a group and even to explaining how our everyday world can result from these microscopic quantum foundations. The rules governing things like billiards balls and the temperature of a gas look very different from the quantum rules governing things like electron collisions and the energy absorbed or released by a single atom. And there is no known sharp, defining line between these two radically different domains of physical laws. Quantum changes are foundational to our universe and understanding them is becoming increasingly important for practical applications of quantum technologies.

In a paper(link is external) published Feb. 28, 2022 in the journal Physical Review X, Adjunct Assistant Professor Alexey Gorshkov, Assistant Research Scientist Luis Pedro García-Pintos and their colleagues provide a new perspective for investigating quantum changes. They developed a mathematical description that sorts quantum behaviors in a system into two distinct parts. One piece of their description looks like the behavior of a quantum system that isn’t interacting with anything, and the second piece looks like the familiar behavior of a classical system. Using this perspective, the researchers identified limits on how quickly quantum systems can evolve based on their general features, and they better describe how those changes relate to changes in non-quantum situations.

“Large quantum systems cannot in general be simulated on classical computers,” says Gorshkov, who is a Fellow of the Joint Quantum Institute (JQI)  and the Joint Center for Quantum Information and Computer Science (QuICS). “Therefore, understanding something important about how these systems behave—such as our insights into the speed of quantum changes—is always exciting and bound to have applications in quantum technologies.”

There is a long history of researchers investigating quantum changes, with most of the research focused on transitions between quantum states. These states contain all the information about a given quantum system. But two distinct states can be as different as can be mathematically despite being extremely similar in practice. This means the state approach often offers a perspective that's too granular to generate useful experimental insights.

In this new research, the team instead focused on an approach that is more widely applicable in experiments. They didn’t focus on changes of quantum states themselves but rather on observables—the results of quantum measurements, which are what scientists and quantum computer users can actually observe. Observables can be any number of things, such as the momentum of a particle, the total magnetization of a collection of particles or the charge of a quantum battery(link is external) (a promising but still theoretical quantum technology). The researchers also chose to investigate quantum behaviors that are influenced by the outside world—a practical inevitability.

The team looked at general features of a possible quantum system, like how well known its energy is and how precisely the value they want to look at is known beforehand. They used these features to derive mathematical rules about how fast an observable can change for the given conditions.

“The spirit of the whole approach is not to go into the details of what the system may be,” says García-Pintos, who is also a QuICS postdoctoral researcher and is the lead author on the paper. “The approach is completely general. So once you have it, you can ask about a quantum battery, or anything you want, like how fast you're able to flip a qubit.”

This approach is possible because in quantum mechanics, two quantities can be intricately connected with strict mathematical rules about what you can know about them simultaneously (the most famous of these rules is the Heisenberg uncertainty principle for a quantum particle’s location and speed).

In addition to their new limits, they were able to reverse the process to show how to make a system that achieves a desired change quickly.

These new results build upon a previous work(link is external) from García-Pintos and colleagues. They studied classical changes such as how quickly energy and entropy can be exchanged between non-quantum systems. This previous result allowed the researchers to break up different behaviors into quantum-like and non-quantum-like descriptions. With this approach, they have a single theory that spans the extremes of possible outside influence—from enough interaction to allow no quantum behavior to the purely theoretical realms of quantum situations without any external influence.

“It's nice; it's elegant that we have this framework where you can include both of these extremes,” García-Pintos says. “One interesting thing is that when you combine these two bounds, we get something that is tighter, meaning better than the established bound.”

Having the two terms also allowed the researchers to describe the slowest speed at which a particular observable will change based on the details of the relevant situation. In essence, to find the slowest possible change they look at what happens when the two types of effects are completely working against each other. This is the first time that a lower bound has been put on observables in this way.

In the future, these results might provide insights into how to best design quantum computer programs or serve as a starting point for creating even more stringent limits on how quickly specific quantum situations can change.

Original story by Bailey Bedford: https://jqi.umd.edu/news/new-perspective-blends-quantum-and-classical-understand-quantum-rates-change

In addition to Gorshkov and García-Pintos, authors on the paper include Schuyler Nicholson, a postdoctoral fellow at Northwestern University; Jason R. Green, a professor of chemistry at the University of Massachusetts Boston; and Adolfo del Campo, a professor of physics at the University of Luxembourg.

Tug-of-War Unlocks Menagerie of Quantum Phases of Matter

Details

Published: Tuesday, January 25 2022 02:46

Enhancing Simulations of Curved Space with Qubits

Details

Published: Tuesday, January 18 2022 02:46

In a Smooth Move, Ions Ditch Disorder and Keep Their Memories

Details

Published: Monday, December 20 2021 10:46

A Persian adage, notably wielded by Abe Lincoln(link is external) and the band OK Go(link is external), expresses the ephemeral nature of the world: “This, too, shall pass.”

Physicists have their own version of this rule. It says that wiggles and wrinkles—really any small disturbances—tend to get ironed out over time. For instance, a couple drops of blue food coloring mixed into some cake batter will impart a blue tint to the whole batch; fresh water from a river funneled into the salty ocean will spread out and make a slightly less salty ocean; and a gush of cold wind entering your room will mingle with the air inside and reach a single, cooler temperature. The basic idea is that, given enough time, everything will reach equilibrium, regardless of where it started.

There are a few notable exceptions to this equanimous rule. In the quantum world of atoms and electrons, particles confined in a container made of electric and magnetic fields—akin to a bowl confining cake batter—can get stuck in place if the container isn’t smooth. When this “bowl” is rough, disorderly, and random, the particles can’t make up their minds about which way to go and instead stay put. Oddly, even when a bunch of these localized particles are allowed to influence each other, they can manage to stay localized, not exchanging energy and avoiding equilibrium. This effect, known as many-body localization (MBL), imparts particles with a kind of memory of where they started.

Now, scientists have found a new way to create disturbances that do not fade away. Instead of relying on disorder to freeze things in place, they tipped the quantum particles’ container to one side—a trick that is easier to conjure in the lab. A collaboration between the experimental group of College Park Professor Christopher Monroe and the theoretical group of Alexey Gorshkov, who is also a Fellow of the Joint Quantum Institute, and the Joint Center for Quantum Information and Computer Science and a physicist at the National Institute of Standards and Technology, has used trapped ions to implement this new technique, confirming that it prevents their quantum particles from reaching equilibrium. The team also measured the slowed spread of information with the new tipping technique for the first time. They published their results(link is external) recently in the journal Nature.

“One advantage of this method of many-body localization is that we don't need that disorder,” says Fangli Liu, former graduate student in physics at the University of Maryland (now a research scientist at QuEra Computing) and lead theorist on the work. “In the original system the disorder is realized in a random form. But with this method, each time you do a measurement you will have exactly the same result. It gives us the possibility to more efficiently use this many-body localization to do something interesting.”Researchers have demonstrated a new way for atomic ions to host disturbances that do not fade away. (credit: E. Edwards/JQI)

Instead of color (as in the dough example) or temperature (in the case of air in your room), the disturbance in the JQI experiment was in the ions’ spins—their little internal magnets that can point up or down (or a bit of both at the same time, as in a quantum superposition). These ion spins sit in a container shaped not like a bowl but instead like a single row of an egg carton, with each ion residing in a different dimple of the container. Normally, after some time all spins would point in the same direction uniformly, with no memory of whether each spin pointed up or down to begin with.

By controlling the ions individually, the scientists can prepare one spin that points up while the rest point down. With an egg carton container that’s flat (like it’s sitting on a table), the single spin disturbance can hop between ions, chatting with neighbors and ultimately causing all the ions to agree on a uniform configuration. In traditional many-body localization, where randomness and disorder rule the Normally, ion spins that start out pointing in opposite directions will interact and reach an equilibrium, with no trace of where they started. But when the tilt in their container is large enough, they keep pointing in their original direction, creating a many-body localized state that remembers its initial configuration. (Credit: Adapted from article by the authors/JQI)day, the egg-carton dimples become offset up or down from each other in a random way, paralyzing each spin in its spot.

Instead of adding disorder, the team tilted the egg carton, offsetting each dimple a little higher than its neighbor to the left in a smooth, consistent way. This caused the spins to get localized as well, but for a very different reason. Quantum particles have wave-like properties, and once they start rolling down in the direction of a tilt, they can get reflected by the edges of the egg carton dimples. So instead of rolling downhill forever, they roll down and bounce back up over and over again, which confines them to their small region of the container.  

For a single particle, this pinning mechanism has been known since the 1930s. But whether it would persist in the face of interactions between many particles and halt equilibration has only recently been explored. Indeed, the idea that tilting the egg carton would result in a breakdown of equilibration was only proposed in 2019.

The JQI team confirmed this in their experiment. Using tightly focused lasers, they adjusted each ion individually and prepared them in a highly disturbed state, with spins pointing in alternating directions. At the same time, they had extra lasers shining on all the ions together, allowing them to talk to each other even while far apart. If the tilt was high enough, the team found, the ions’ spins remained in their original configuration for an extended period, refusing to succumb to equilibrium.

In addition to a conceptual leap, creating MBL without disorder may come with certain practical advantages. First, it is experimentally easier to implement a smooth tilt (in fact, a small tilt was present in the JQI experiment whether they wanted it or not). Second, it makes measurements much more straightforward. And third, this method is immune to an accidental break down of MBL. In regular disorder-based MBL, the random offsets of the dimples need to be large. If they aren’t, localization can break down in some spots and infect the whole batch. With a smooth tilt, there’s no such risk.

This opens the possibility of using many-body localization to create a robust memory. MBL might help maintain quantum information in future quantum computers or help preserve curiosities like time crystals or topological phases.

In the past year, two other experiments realizing this method were reported. The team of H. Wang in Hangzhou, China set it up using superconducting qubits(link is external), and Monika Aidelburger’s team in Munich, Germany made it happen(link is external) with ultracold atoms.

“There's a lot of shared themes between our three papers,” says William Morong, a postdoctoral researcher at JQI and lead author on the work, “and I would say all of them together give a more complete picture of the phenomenon then each individually.”

The JQI group was the only one, however, to demonstrate another key property of many-body localization: the slow spread of entanglement between their ions. The team used a technique adapted from nuclear magnetic resonance imaging to measure the crawling pace with which entanglement spread across their atoms, a hallmark of MBL.

“I think that our work shows the exciting progress that has been made in modern quantum simulation platforms,” Morong says, “We are reaching the point where we have enough control over collections of quantum particles in these platforms that we can read a theoretical paper describing some interesting effect that emerges in a specific system, program in the forces that we need to create this effect for ourselves, and measure subtle signatures in the quantum entanglement between the particles that are only revealed when you can observe each particle individually. "

Original story by Dina Genkina:  https://jqi.umd.edu/news/smooth-move-ions-ditch-disorder-and-keep-their-memories

In addition to Liu, Morong, Monroe and Gorshkov, authors on the paper included former graduate student in physics Patrick Becker (now a physicist at Booz Allen Hamilton), graduate student in physics Kate Collins, postdoctoral researcher at JQI Lei Feng, former graduate student in physics (now a postdoctoral fellow at Indiana University in Bloomington) Antonis Kyprianidis, former postdoctoral fellow at JQI (now assistant professor of physics at Rice University) Guido Pagano, and undergraduate researcher Tianyu You.