Quantum Computers Do the (Instantaneous) Twist

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Published: Wednesday, August 19 2020 05:33

Regardless of what makes up the innards of a quantum computer, its speedy calculations all boil down to sequences of simple instructions applied to qubits—the basic units of information inside a quantum computer.

Whether that computer is built from chains of ions, junctions of superconductors, or silicon chips, it turns out that a handful of simple operations, which affect only one or two qubits at a time, can mix and match to create any quantum computer program—a feature that makes a particular handful “universal.” Scientists call these simple operations quantum gates, and they have spent years optimizing the way that gates fit together. They’ve slashed the number of gates (and qubits) required for a given computation and discovered how to do it all while ensuring that errors don’t creep in and cause a failure.

Now, researchers at JQI have discovered ways to implement robust, error-resistant gates using just a constant number of simple building blocks—achieving essentially the best reduction possible in a parameter called circuit depth. Their findings, which apply to quantum computers based on topological quantum error correcting codes, were reported in two papers published recently in the journals Physical Review Letters(link is external) and Physical Review B(link is external), and expanded on in a third paper published earlier in the journal Quantum(link is external).Unlike other kinds of quantum computers, quantum computers built atop topological error correction smear a single qubit’s worth of information out among a network of many qubits. (Credit: Gerd Altmann/Pixabay)

Circuit depth counts the number of gates that affect each qubit, and a constant depth means that the number of gates needed for a given operation won’t increase as the computer grows—a necessity if errors are to be kept at bay. This is a promising feature for robust and universal quantum computers, says Associate Professor Maissam Barkeshli. 

“We have discovered that a huge class of operations in topological states of matter and topological error correcting codes can be implemented via constant depth unitary circuits,” says Barkeshli, who is a member of the Joint Quantum Institute and the Condensed Matter Theory Center at UMD.

Unlike other kinds of quantum computers, quantum computers built atop topological error correction—which so far have only been studied theoretically—don’t store information in individual physical qubits. Instead, they smear a single qubit’s worth of information out among a network of many qubits—or, more exotically, across special topological materials.

This information smearing provides resilience against stray bits of light or tiny vibrations—quantum disturbances that may cause errors—and it allows small errors to be detected and then actively corrected during a computation. It’s one of the main advantages that quantum computers based on topological error correction offer. But the advantage comes at a cost: If noise can’t get to the information easily, neither can you.

Until now it seemed that operating such a quantum computer required small, sequential changes to the network that stores the information—often depicted as a grid or lattice in two dimensions. In time, these small changes add up and effectively move one region of the lattice in a loop around another region, leaving the network looking the same as when it started.

These transformations of the network are known as braids because the patterns they trace out in space and time look like braided hair or a plaited loaf of bread. If you imagine stacking snapshots of the network up like pancakes, they will form—step by step—an abstract braid. Depending on the underlying physics of the network—including the kinds of particles, called anyons, that can hop around on it—these braids can be enough to run any quantum program.

In the new work, the authors showed that braiding can be accomplished almost instantaneously. Gone are the knotted diagrams, replaced by in-situ rearrangements of the network.

“It was kind of a textbook dogma that these braids can only be done adiabatically or very slowly so as to avoid creating errors in the process,” says Guanyu Zhu, a former JQI postdoctoral researcher who is currently a research staff member at the IBM Thomas J. Watson Research Center. “However, in this work, we realized that instead of slowly moving regions with anyons around each other, we could just stretch or squeeze the space between them in a constant number of steps.”

The new recipe requires two ingredients. One is the ability to make local modifications that reconfigure the interactions between the physical qubits that make up the network. This part isn’t too different from what ordinary braiding requires, but it is assumed to happen in parallel across the region being braided. The second ingredient is the ability to swap the information on physical qubits that are not close to each other—potentially even at opposite corners of the braiding region.

Networks of qubits (represented by black dots in the image on the right) are deformed in order to braid two regions (represented by red and blue dots) around each other. These images show two intermediate stages of the process. Images provided courtesy of the authors.

Networks of qubits (represented by black dots in the image on the right) are deformed in order to braid two regions (represented by red and blue dots) around each other. These images show two intermediate stages of the process. Images provided courtesy of the authors.

This second requirement is a big ask for some quantum computing hardware, but the authors say that there are systems that could naturally support it.

“A variety of experimental platforms with long-range connectivity could support our scheme, including ion traps, circuit QED systems with long transmission-line resonators, modular architectures with superconducting cavities, and silicon photonic devices,” says Zhu. “Or you could imagine using platforms with movable qubits. One can think of such platforms as fluid quantum computers, where qubits can freely flow around via classical motion.”

In the paper in Physical Review Letters, the authors provided explicit instructions for how to achieve their instantaneous braids in a particular class of topological quantum codes. In the Physical Review B and Quantum papers, they extended this result to a more general setting and even examined how it would apply to a topological code in hyperbolic space (where, additionally, adding a new smeared out qubit requires adding only a constant number of physical qubits to the network).

The authors haven’t yet worked out how their new braiding techniques will mesh with the additional goals of detecting and correcting errors; that remains an open problem for future research.

“We hope our results may ultimately be useful for establishing the possibility of fault-tolerant quantum computation with constant space-time overhead,” says Barkeshli.

Original story by Chris Cesare: https://jqi.umd.edu/news/quantum-computers-do-instantaneous-twist

In addition to Barkeshli and Zhu, the three papers had one additional co-author: Ali Lavasani, a JQI graduate fellow and physics graduate student at UMD.

Plasma Guides Maintain Laser Focus

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Published: Friday, August 14 2020 08:17

In science fiction, firing powerful lasers looks easy—the Death Star can just send destructive power hurtling through space as a tight beam. But in reality, once a powerful laser has been fired, care must be taken to ensure it doesn’t get spread too thin.

If you’ve ever pointed a flashlight at a wall, then you’ve observed a more mundane example of this diffusion of light. The farther you are from the wall, the more the beam spreads, resulting in a larger and dimmer spot of light. Lasers generally expand much more slowly than a flashlight, but the effect is important when the laser travels a long way or must maintain a high intensity.

Whether your goal is blowing up a planet to achieve galactic domination or, more realistically, accelerating electrons to incredible speeds for physics research, you’ll want as tight and powerful a beam as possible to maximize the intensity. For terrestrial experiments, researchers can use devices called waveguides, like the optical fibers that might be carrying internet throughout your neighborhood, to transport a laser while keeping it contained to a narrow beam. The distinct core and outer shell—or cladding—of a waveguide keep the laser from spreading out. But if the laser pulse is too intense you run into a problem—it will destroy an optical fiber in a thousandth of a nanosecond.

Lasers are used to create an indestructible optical fiber out of plasma that helps researchers confine a separate laser pulse as it travels through the plasma. (Credit: Intense Laser-Matter Interactions Lab, University of Maryland)

Researchers at the University of Maryland, led by UMD Physics Professor Howard Milchberg, have developed an improved technique to make waveguides that can withstand the power of intense lasers. In a paper published on August 14, 2020 in Physical Review Letters, they demonstrated how powerful pulses can be transmitted along a waveguide that is created by firing weaker laser pulses into a cloud of hydrogen. They predict that the technique, developed with support from the US Department of Energy High Energy Physics program and the National Science Foundation, will be a powerful tool in high-energy particle acceleration experiments.

“A plasma waveguide can be a powerful tool for a variety of fields,” says Bo Miao, a co-author of the paper and UMD physics postdoctoral associate. “I’m excited that the experiment finally worked out after two years of hard work of alternating delight and frustration.”

Their technique relies on building a waveguide from a plasma—a gas where the electrons have been torn from the nuclei of the atoms.

“A plasma waveguide has all the structure of an optical fiber, the classic core, the classic cladding,” says Milchberg. “Although in this case, it's indestructible. The hydrogen plasma forming the waveguide is already ripped up into its protons and electrons, so there's not much more violence you can do to it.”

In the early 1990s, Milchberg and colleagues developed a related technique to use lasers to create plasma waveguides for other, more intense, lasers. In this earlier technique a laser beam is sent into a gas; as it travels, it rips electrons from their atoms along the beam, creating a plasma tunnel that is warmer than the surrounding gas. Due to its heat, the plasma expands, forming a low-density plasma core surrounded by a high-density wall formed by the shockwave from the plasma’s rush outward.

This structure is precisely what is needed for a waveguide, but the method has a pitfall—researchers can’t craft the core and wall independently. To get the wall to have the necessary thickness and density of electrical charges to function as a waveguide, required the core to be kept too dense for particle acceleration applications.

In the new paper, the team demonstrates an improved method that lets them craft the wall and core independently. Their insight was to use two specialized laser beams—called Bessel beams—to craft the waveguide. The first laser is a simple Bessel beam that forms the low-density core while causing less heating than the previous method.

Caption: On the left is a cross section of the intensity of the Bessel beam responsible for creating the low-density plasma core. On the right is a cross section of the intensity of the Bessel beam that creates the high-density plasma wall. The left image is 50 micrometers across and the right image is 100 micrometers across. (Credit: Intense Laser-Matter Interactions Lab, University of Maryland)But the second laser beam is more exotic. It is a hollow tube of light that allows them to build the wall of the waveguide by creating additional plasma from the gas surrounding the plasma core. Since the second laser pulse can match the shape of the high-density wall, they can tailor it without impacting the conditions of the core.

“Basically, the version of the technique that was used up until our paper is very constrained in the size of the guide, the length and other parameters,” says Linus Feder, a co-author of the paper and a UMD physics graduate student. “This new technique is highly adaptable and tunable. It just does away with a lot of the restrictions on the types of laser beams you can guide.”

The researchers demonstrated that the improvement allowed them to guide a laser for 30 centimeters in a tight beam—about 50% farther than previous experiments that used wider, 20-centimeter plasma waveguides created with a different technique.

Milchberg says their waveguide is like a long hypodermic needle and that the older method was more like a drinking straw. With the smaller guide, the laser’s energy is packed into a much smaller area, resulting in a much higher intensity.

“The only reason we were limited to 30 centimeters was lab geometry and not having enough laser energy,” says Milchberg. “But with more laser energy, there's no obstacle to us doing this for a couple of meters.

The new method may increase the practicality of using plasma waveguiding of intense laser pulses to accelerate charged particles for high energy physics experiments. The group is planning experiments to confirm their predictions of how the process will work with more powerful lasers.

The paper was selected as an editors’ suggestion and was highlighted in Physical Review Focus.

Story by Bailey Bedford: This email address is being protected from spambots. You need JavaScript enabled to view it.

In addition to Milchberg, Feder, and Miao, graduate students Andrew Goffin and Jaron Shrock were co-authors.
This research was supported by US Department of Energy (DESC0015516) and the National Science Foundation (PHY1619582).

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Bo Miao: This email address is being protected from spambots. You need JavaScript enabled to view it.
Linus Feder:This email address is being protected from spambots. You need JavaScript enabled to view it.

Scientists See Train of Photons in a New Light

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Published: Tuesday, August 04 2020 05:33

Flashlight beams don’t clash together like lightsabers because individual units of light—photons—generally don’t interact with each other. Two beams don’t even flicker when they cross paths.

But by using matter as an intermediary, scientists have unlocked a rich world of photon interactions. In these early days of exploring the resulting possibilities, researchers are tackling topics like producing indistinguishable single photons and investigating how even just three photons form into basic molecules of light. The ability to harness these exotic behaviors of light is expected to lead to advances in areas such as quantum computing and precision measurement.

In a paper recently published in Physical Review Research, Adjunct Associate Professor Alexey Gorshkov, Joint Quantum Institute (JQI) postdoctoral researcher Przemyslaw Bienias, and their colleagues describe an experiment that investigates how to extract a train of single photons from a laser packed with many photons.

In the experiment, the researchers examined how photons in a laser beam can interact through atomic intermediaries so that most photons are dissipated—scattered out of the beam—and only a single photon is transmitted at a time. They also developed an improved model that makes better predictions for more intense levels of light than previous research focused on (greater intensity is expected to be required for practical applications). The new results reveal details about the work to be done to conquer the complexities of interacting photons.A cloud of Rydberg atoms can scatter most light to whittle a laser down to train of individual photons. But photons can get re-absorbed within the larger control beam making things more complicated. (Credit: Przemyslaw Bienias, University of Maryland)

“Until recently, it was basically too difficult to study anything other than a few of these interacting photons because even when we have two or three things get extremely complicated,” says Gorshkov, whi is also a physicist at the National Institute of Standards and Technology and Fellow of the Joint Center for Quantum Information and Computer Science. “The hope with this experiment was that dissipation would somehow simplify the problem, and it sort of did.”

Trains, Blockades and Water Slides

To create the interactions, the researchers needed atoms that are sensitive to the electromagnetic influence of individual photons. Counterintuitively, the right tool for the job is a cloud of electrically neutral atoms. But not just any neutral atoms; these specific atoms—known as Rydberg atoms—have an electron with so much energy that it stays far from the center of the atom.

The atoms become photon intermediaries when these electrons are pushed to their extreme, remaining just barely tethered to the atom. With the lone, negatively charged electron so far out, the central electrons and protons are left contributing a counterbalancing positive charge. And when stretched out, these opposite charges make the atom sensitive to the influence of passing photons and other atoms. In the experiment, the interactions between these sensitive atoms and photons is tailored to turn a laser beam that is packed with photons into a well-spaced train.

The cloud of Rydberg atoms is kind of like a lifeguard at a water park. Instead of children rushing down a slide dangerously close together, only one is allowed to pass at a time. The lifeguard ensures the kids go down the slide as a steady, evenly spaced train and not in a crowded rush.

Unlike a lifeguard, the Rydberg atoms can’t keep the photons waiting in line. Instead they let one through and turn away the rest for a while. The interactions in the cloud of atoms form a blockade around each transmitted photon that scatters other photons aside, ensuring its solitary journey.

To achieve the effect, the researchers used Rydberg atoms and a pair of lasers to orchestrate a quantum mechanical balancing act. They selected the frequency of the first laser so that its photons would be absorbed by the atoms and scattered in a new direction. But this is the laser that is whittled down into the photon train, and they needed a way to let individual photons through.

That’s were the second laser comes in. It creates another possible photon absorption that quantum mechanically interferes with the first and allows a single photon to pass unabsorbed. When that single photon gets through, it disturbs the state of the nearby atoms, upsetting the delicate balance achieved with the two lasers and blocking the passage of any photons crowding too closely behind.

Ideally, if this process is efficient and the stream of photons is steady enough, it should produce a stream of individual photons each following just behind the blockade of the previous. But if the laser is not intense enough, it is like a slow day at the waterpark, when there is not always a kid eagerly awaiting their turn. In the new experiment, the researchers focused on what happens when they crowed many photons into the beam.

Model (Photon) Trains

Gorshkov and Bienias’s colleagues performed the experiment, and the team compared their results to two previous models of the blockade effect. Their measurements of the transmitted light matched the models when the number of photons was low, but as the researchers pushed the intensity to higher levels, the results and the models’ predictions started looking very different. It looked like something was building up over time and interfering with the predicted, desired formation of well-defined photon trains.

The team determined that the models failed to account for an important detail: the knock-on effects of the scattered photons. Just because those photons weren’t transmitted, doesn’t mean they could be ignored. The team suspected the models were being thrown off by some of the scattered light interacting with Rydberg atoms outside of the laser beam. These additional interactions would put the atoms into new states, which the scientists call pollutants, that would interfere with the efficient creation of a single photon train.

The researchers modified one of their models to capture the important effects of the pollutants without keeping track of every interaction in the larger cloud of atoms. While this simplified model is called a “toy model,” it is really a practical tool that will help researchers push the technique to greater heights in their larger effort to understand photon interactions. The model helped the researchers explain the behavior of the transmitted light that the older models failed to capture. It also provides a useful way to think about the physics that is preventing an ideal single photon train and might be useful in judging how effectively future experiments prevent the undesirable affects—perhaps by using cloud of atoms with different shapes.

“We are quite optimistic when it comes to removing the pollutants or trying to create less of them,” says Bienias. “It will be more experimentally challenging, but we believe it is possible.”

Original story by Bailey Bedford: https://jqi.umd.edu/news/scientists-see-train-photons-new-light

In addition to Bienias and Gorshkov, James Douglas, a Co-founder at MEETOPTICS; Asaf Paris-Mandoki, a physics researcher at Instituto de Física, Universidad Nacional Autónoma de México; JQI postdoctoral researcher Paraj Titum; Ivan Mirgorodskiy; Christoph Tresp, a research and development employee at TOPTICA Photonics; Emil Zeuthen, a physics professor at the Niels Bohr Institute; Michael J. Gullans, a former JQI postdoctoral researcher and current associate scholar at Princeton University; Marco Manzoni, a data scientist at Big Blue Analytics; Sebastian Hofferberth, a professor of physics at the University of Southern Denmark; and Darrick Chang, a professor at the Institut de Ciencies Fotoniques, were also co-authors of the paper.

Quantum Simulation Stars Light in the Role of Sound

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Published: Thursday, July 30 2020 05:33

Inside a material, such as an insulator, semiconductor or superconductor, a complex drama unfolds that determines the physical properties. Physicists work to observe these scenes and recreate the script that the actors—electrons, atoms and other particles—play out. It is no surprise that electrons are most frequently the stars in the stories behind electrical properties. But there is an important supporting actor that usually doesn’t get a fair share of the limelight.

This underrecognized actor in the electronic theater is sound, or more specifically the quantum mechanical excitations that carry sound and heat. Scientists treat these quantized vibrations as quantum mechanical particles called phonons(link is external). Similar to how photons are quantum particles of light, phonons are quantum particles of sound and other vibrations in a solid. Phonons are always pushing and pulling on electrons, atoms or molecules and producing new interactions between them.

The role that phonons play in the drama can be tricky for researchers to suss out. And sometimes when physicists identify an interesting story to study, they can’t easily find a material with all the requisite properties or of sufficient chemical purity.Cigar shaped clouds of atoms (pink) are levitated in a chamber where an experiment uses light to recreate behavior that normally is mediated by quantum particles of sound. (Credit: Yudan Guo, Stanford)

To help overcome the challenges of working directly with phonons in physical materials, Professor Victor Galitski,  Joint Quantum Institute (JQI) postdoctoral researcher Colin Rylands and their colleagues have cast photons in the role of phonons in a classic story of phonon-driven physics. In a paper published recently in Physical Review Letters(link is external), the team proposes an experiment to demonstrate photons adequacy as an understudy and describes the setup to make the show work.

“The key idea came from an interdisciplinary collaboration that led to the realization that seemingly unrelated electron-phonon systems and neutral atoms coupled to light may share the exact same mathematical description,” says Galitski. “This new approach promises to deliver a treasure trove of exciting phenomena that can be transplanted from material physics to light-matter cavity systems and vice versa."

The Stage

Galitski and colleagues propose using a very carefully designed mirrored chamber—like coauthor Benjamin Lev has in his lab at Stanford University—as the stage where photons can take on the role of phonons. This type of chamber, called an optical cavity, is designed to hold light for a long time by bouncing it between the highly-reflective walls.

“We made cavities where if you stick your head in there—of course it's only a centimeter wide—you would see yourself 50,000 times,” says Lev. “Our mirrors are very highly polished and so the reflections don't rapidly decay away and get lost.”

In an optical cavity, the bouncing light can hold a cloud of atoms in a pattern that mimics the lattice of atoms in a solid. But if a cavity is too simple and can only contain a single light pattern—a mode—the lattice is frozen in place. The light has to be able to take on many modes to simulate the natural distortions of phonons in the material.

To create more dynamic stories with phonons, the team suggests using multimode confocal cavities. “Multimode confocal” basically means the chamber is shaped with unforgiving precision so that it can contain many distinct spatial distributions of light.

“If it were just a normal single-mode cavity—two curved mirrors spaced at some arbitrary distance from one another— it would only be a Gaussian shape that could bounce back and forth and would be kind of boring; your face would be really distorted,” says Lev. “But if you stick your face in our cavities, your face wouldn’t look too different—it looks a little different, but not too different. You can support most of the different shapes of the waveform of your face, and that will bounce back and forth.”

Mirrors with a green tint can be seen inside a small experimental cavity.

View of the cavity mirrors that serve as a stage for quantum simulations where light takes on the role of sound. (Credit: LevLab, Stanford)

The variety of light distributions that these special cavities can harbor, along with the fact that the photons can interact with one atom and then get reflected back to a different atom, allows the researchers to create many different interactions in order to cast the light as phonons.

“It's that profile of light in the cavity which is playing the role of the phonons,” says Jonathan Keeling, a coauthor of the paper and a physicist at the University of St Andrews. “So the equivalent of the lattice distorting in one place is that this light is more intense in this place and less intense in another place.”

The Script

In the paper, the team proposes the first experiment to perform with these multimode confocal cavities—the first play to premier on the new stage. It’s a classic of condensed matter physics: the Peierls transition. The transition occurs in one-dimensional chains of particles with an attractive force between them. The attractive force leads the particles to pair up so that they form a density wave—two close particles and a space followed by two close particles and a space, on and on. Even a tiny pull between particles creates an instability that pulls them into pairs instead of distributing randomly.

In certain materials, the attractive pull from phonons is known to trigger a dramatic electrical effect through the Peierls transition. The creation of the density wave makes it harder for electrons to move through a material—resulting in a sudden transition from conductor to insulator.

“The Peierls transition is mathematically very similar to, but less well known than, superconductivity,” says Rylands. “And, like in superconducting systems, or many other systems that you would study in a solid-state lab, all these different phases of matter are driven by the interactions between phonons and the electrons.”

To recast the phonons as light the team has to also recast the electrons in the 1D material as cigar shaped clouds of atoms levitated in the chamber, as shown in the image above. But in this case, they don’t have a sudden cut off of an electrical current to conveniently signal that the transition occurred like in the traditional experiments with solids. Instead, they predicted what the light exiting the cavity should look like if the transition occurs.

Opening Night

The authors say that the proposed experiment will debut in cavities in Lev’s lab.

“It’s kind of nice when you have an experimentalist on a theory paper—you can hold the theorists’ feet to the fire and say, ‘Well, you know, that sounds great, but you can't actually do that,’” says Lev. “And here we made sure that you can do everything in there. So really, it's a quite literal roadmap to what we're going to do in the near future.”

If this experiment lines up with their predictions, it will give them confidence in the system’s ability to reveal new physics through other simulations.

There is competition for which shows will get the chance to take central stage, since there are many different scripts of physical situations that researchers can use the cavities to explore. The technique has promise for creating new phases of matter, investigating dynamic quantum mechanical situations, and understanding materials better. The cavities as stages for quantum simulations put researchers in the director’s chair for many quantum shows.

Original story by Bailey Bedford: https://jqi.umd.edu/news/quantum-simulation-stars-light-role-sound 

A note from the researchers: The JQI and Stanford collaborators are especially grateful for support from the US Army Research Office, and discussions with Dr. Paul Baker at US-ARO, that made this work possible.

In addition to Galitski, Rylands, Lev and Keeling, Stanford graduate student Yudan Guo was also a co-author of the paper.

Reference Publication

"Photon-Mediated Peierls Transition of a 1D Gas in a Multimode Optical Cavity," Colin Rylands, Yudan Guo, Benjamin L. Lev, Jonathan Keeling, Victor Galitski, Phys. Rev. Lett., 125, 010404 (2020)

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Colin Rylands: This email address is being protected from spambots. You need JavaScript enabled to view it.

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Diamonds Shine a Light on Hidden Currents in Graphene

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Published: Wednesday, July 22 2020 05:33