Research News - UMD Physics

LHCb Experiment Discovers CP Violation in the Charm Quark System

Details: Published: Thursday, December 12 2019 02:53

A major unsolved puzzle in particle physics and cosmology is the apparent asymmetry in the abundance of matter and antimatter in the universe. One condition for such an asymmetry to arise is the breaking of charge-parity (CP) symmetry in laws of interactions amongst particles, resulting in differences in reactions involving partiPhoebe Hamilton, Will Parker, Hassan Jawahery, Yipeng Sun, Manuel Franco Sevilla and Zishuo Yang. cles and antiparticles. CP violation has been observed in particles containing the strange and bottom quarks--kaons and B mesons--and the results conform to the predictions of the Standard Model at the current level of experimental accuracy. However, CP violation in the Standard Model is not sufficient to explain the matter-antimatter asymmetry in the universe. A key goal of the LHCb experiment is to search for additional sources of CP violation beyond the Standard Model. The Standard Model predicts negligible CP violation in decays of particles containing the charm quark, thus any significant effect in charm mesons could be due to new sources of CP violation. Using the data recorded in 2011-2018, the LHCb experiment has observed CP violation in decays of D mesons – mesons made of a charm quark and anti-up quark.
(see https://arxiv.org/abs/1903.08726 and https://home.cern/news/press-release/physics/lhcb-sees-new-flavour-matter-antimatter-asymmetry.) This work was rated a Physics World Breakthrough of the Year finalist for 2019.

Theorists are hard at work to explain the observed CP violation, which is very small but larger than most of the existing predictions within the Standard Model. Failing any Standard Model explanation, this result could open the door to exploring new sources of CP violation beyond the Standard Model.

The LHCb collaboration is now preparing to complete and install the new upgraded detector elements that will significantly enhance the precision of the spectrometer, thus allowing more precise measurements of CP violation and other very rare effects that may arise from particles and interactions that are not present in the Standard Model. The new upgraded detector elements would enable the experiment to operate at higher intensities of the large hadron collider (LHC) and read out the entire detector--millions of sensor signals--at beam collisions that occur every 25 nanoseconds. Over the past six years, the Maryland team in LHCb has been deeply engaged in the design and construction of a charge particle detector, based on silicon sensors, focusing on the development of the electronics that would enable the readout and transfer of the sensor signals. The team is now preparing to complete the construction of the nearly 600 main electronics boards and 2000 auxiliary boards, which will be shipped to CERN for installation in the LHCb experiment starting in Spring of 2020. The experiment is expected to resume operation in 2021.

Hybrid Device among First to Meld Quantum and Conventional Computing

Details: Published: Friday, October 18 2019 07:02

The result, which was published Oct. 18, 2019 in the journal Science Advances, is among the first demonstrations of quantum hardware teaming up with conventional computing power—in this case to do generative modeling, a machine learning task in which a computer learns to mimic the structure of a given dataset and generate examples that capture the essential character of the data.

“We combined one of the highest performance quantum computers with one of the most powerful AI programs—over the internet—to form a unique kind of hybrid machine,” says Joint Quantum Institute (JQI) Fellow Norbert Linke, an assistant professor of physics at UMD and a co-author of the new paper.

The researchers used four trapped atomic ions for the quantum half of their hybrid computer, with each ion representing a quantum bit, or qubit—the basic unit of information in a quantum computer. To manipulate the qubits, researchers punch commands into an ordinary computer, which interprets them and orchestrates a sequence of laser pulses that zap the qubits.Close-up photo of an ion trap. Credit: S. Debnath and E. Edwards/JQI

The UMD quantum computer is fully programmable, with connections between every pair of qubits. “We can implement any quantum function by executing a standard set of gates between the qubits,” says JQI and Joint Center for Quantum Information and Computer Science (QuICS) Fellow Christopher Monroe, a physics professor at UMD who was also a co-author of the new paper. “We just needed to optimize the parameters of each gate to train our machine learning algorithm. This is how quantum optimization works.”

Monroe, Linke and their colleagues trained their computer to produce an output that matched the “bars-and-stripes” set, a collection of images with blocks of color arranged vertically or horizontally to look like bars or stripes—a standard dataset in generative modeling because of its simplicity.

“Machine learning is generally categorized into two types,” says Daiwei Zhu, the lead author of the paper and a graduate student in physics at JQI. “One enables you to tell whether something is a cat or dog, and the other lets you generate an image of a cat or dog. We’re performing a scaled-back version of the latter task.”

Turning the hybrid system into a properly trained generative model meant finding the laser sequence that would turn a simple input state into an output capable of capturing the patterns in the bars-and-stripes set—something that qubits could do more efficiently than regular bits. “In essence, the power of this lies in the nature of quantum superposition,” says Zhu, referring to the ability of qubits to store multiple states—in this case, the entire set of bars-and-stripes images with four pixels—simultaneously.

Through a series of iterative steps, the researchers attempted to nudge the output of their hybrid computer closer and closer to the quantum bars-and-stripes state. They began by preparing the input qubits, subjecting them to a random sequence of laser pulses and measuring the resulting output. Those measurement results were then fed to a conventional, or “classical,” computer, which crunched the numbers and suggested adjustments to the laser pulses to make the output look more like the bars-and-stripes state.

By adjusting the laser parameters and repeating the procedure, the team could test whether the output eventually converged on the desired quantum state. They found that in some cases it did, and in some cases it didn’t.

The researchers studied the convergence using two different patterns of connectivity between qubits. In one, each qubit was able to interact with all the others, a situation that the team called all-to-all connectivity. In a second, a central qubit interacted with the other three, none of which interacted directly with one another. They called this star connectivity. (This was an artificial constraint, as the four ions are naturally able to interact in the all-to-all fashion. But it could be relevant to experiments with a larger number of ions.)

The all-to-all interactions produced states closer to bars-and-stripes after training short sequences of pulses. But the experimenters had another setting to play with: They also studied the performance of two different number crunching methods used on the conventional half of the hybrid computer.

One method, called particle swarm optimization, worked well when all-to-all interactions were available, but it failed to converge on the bars-and-stripes output for star connectivity. A second method, which was suggested by three researchers at the Oxford, UK AI company Mind Foundry Limited, proved much more successful across the board.

The second method, called Bayesian optimization, was made available over the internet, which enabled the researchers to train sequences of laser pulses that could produce the bars-and-stripes state for both all-to-all and star connectivity. Not only that, but it significantly reduced the number of steps in the iterative training process, effectively cutting in half the time it took to converge on the correct output.

“What our experiment shows is that a quantum-classical hybrid machine, while in principle more powerful than either of the components individually, still needs the right classical piece to work,” says Linke. “Using these schemes to solve problems in chemistry or logistics will require both a boost in quantum computer performance and tailored classical optimization strategies.”

Story by Chris Cesare

In addition to Linke, Monroe and Zhu, co-authors of the research paper include University College London computer science student Marcello Benedetti; JQI physics graduate students Nhung Hong Nguyen, Cinthia Huerta Alderete and Laird Egan and recent JQI Ph.D. graduate Kevin Landsman; Zapata Computing scientist Alejandro Perdomo-Ortiz; Mind Foundry Limited scientists Nathan Korda, Alistair Garfoot and Charles Brecque; and Central Connecticut State University Mathematical Sciences Professor Oscar Perdomo.

Reference Publication

"Training of quantum circuits on a hybrid quantum computer," D. Zhu, N. M. Linke, M. Benedetti, K. A. Landsman, N. H. Nguyen, C. H. Alderete, A. Perdomo-Ortiz, N. Korda, A. Garfoot, C. Brecque, L. Egan, O. Perdomo, C. Monroe, Science Advances, 5, (2019)

Research Contacts:

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Stretched Photons Recover Lost Interference

Details: Published: Tuesday, October 15 2019 07:02

The smallest pieces of nature—individual particles like electrons, for instance—are pretty much interchangeable. An electron is an electron is an electron, regardless of whether it’s stuck in a lab on Earth, bound to an atom in some chalky moon dust or shot out of an extragalactic black hole in a superheated jet. In practice, though, differences in energy, motion or location can make it easy to tell two electrons apart.

One way to test for the similarity of particles like electrons is to bring them together at the same time and place and look for interference—a quantum effect that arises when particles (which can also behave like waves) meet. This interference is important for everything from fundamental tests of quantum physics to the speedy calculations of quantum computers, but creating it requires exquisite control over particles that are indistinguishable.

Researchers recorded these patterns of quantum interference between three photons that started out as separate, distinguishable particles.

With an eye toward easing these requirements, researchers at the Joint Quantum Institute (JQI) and the Joint Center for Quantum Information and Computer Science (QuICS) have stretched out multiple photons—the quantum particles of light—and turned three distinct pulses into overlapping quantum waves. The work, which was published recently in the journal Physical Review Letters, restores the interference between photons and may eventually enable a demonstration of a particular kind of quantum supremacy—a clear speed advantage for computers that run on the rules of quantum physics.

“While photons do not directly interact with each other, when they meet they can exhibit a purely quantum feature absent from classical, non-quantum waves,” says JQI Fellow Mohammad Hafezi, a co-author of the paper and an associate professor of physics and electrical and computer engineering at the University of Maryland.

These days, testing the similarity of photons is routine. It involves bringing them together at a device called a beam splitter and measuring the light coming out the other side.

When a single photon hits a balanced beam splitter, there’s a 50 percent chance that it will travel straight through and a 50 percent chance that it will reflect off at an angle. By placing detectors in these two possible paths, scientists can measure which way individual photons end up going.

If two identical photons meet at the beam splitter, with one traveling to the east and the other to the north, it’s tempting to apply the same treatment to each particle individually. It’s true that both photons have an equal chance to travel through or reflect, but because the photons are indistinguishable, it’s impossible to tell which one goes where.

The upshot of this identity confusion is that two of the possible combinations—those in which both photons travel straight through the beam splitter and both photons reflect—cancel each other out, leaving behind a distinctly quantum result: The photons team up and travel as a pair, always ending up at one of the two detectors together.

Now Hafezi and his colleagues from UMD and the University of Portsmouth have observed a similar interference effect with distinguishable photons—pulses of light just two picoseconds long (a picosecond is a trillionth of a second) that are separated by tens of picoseconds. The essential trick was finding a way to make the pulses less distinguishable so that they could interfere.

“We used a single optical element that’s basically a fiber,” says Sunil Mittal, a postdoctoral researcher at JQI and a co-author of the new paper. “It emulates the equivalent of about 150 kilometers of fiber, which stretches the photons. It acts a bit like a lens in reverse, causing different frequencies in the pulses to disperse and defocus.”

By lengthening each photon by a factor of about 1000, the researchers could effectively erase the time delay between pulses and create large sections of overlap. That overlap made it more likely that photons would arrive to detectors at the same time and interfere with one another.

Prior experiments (including by JQI and QuICS Fellow Christopher Monroe and collaborators) have successfully interfered distinguishable photons, but those results required multiple channels for the incoming light—one for each photon. The new work uses just a single channel that carries light at standard telecom frequencies, which the authors say allows their system to easily scale to include many more photons.

Having more photons would allow researchers to study boson sampling, a computational problem that’s thought to be too hard for ordinary computers (similar to the problem Google is rumored to have solved). In its standard form, boson sampling concerns photons—which are members of a family of particles called bosons—making their way through a big network of beam splitters. The photons enter the network through different channels and exit to detectors, with one detector per channel.

The boson sampling “problem” amounts to doing a complicated coin flip, since each experiment samples from the underlying chance that (say) three photons entering the network at ports 1, 2 and 5 will end up at outputs 2, 3 and 7. The interference inside the network is complex and impossible to track with a regular computer—even for modest numbers of photons—and it gets harder the more photons you add. But with real photons in a real network, the problem would solve itself.

“The connection of this experiment to boson sampling is a great example of how the growing synergy between quantum many-body physics and computational complexity theory can lead to great progress in both fields,” says JQI and QuICS Fellow Alexey Gorshkov, an adjunct associate professor of physics at UMD and another co-author of the paper.

But up until now, boson sampling experiments have suffered from the problem of scalability: Solving the problem for more photons meant adding more channels, which meant taking up more space and timing the arrival of yet more photons to ensure their interference. Mittal says that their technique potentially solves both of these problems.

“In our system, the inputs don’t need to be in different fibers,” Mittal says. “All the photons can travel in a single fiber and the time differences can be erased by the same method we’ve already demonstrated.” Another off-the-shelf device could mimic the network of beam splitters, with the added benefit of allowing for easy reconfiguration, Mittal says. “We’re not doing boson sampling now, but it would be relatively easy to go in that direction.”

Story by Chris Cesare This email address is being protected from spambots. You need JavaScript enabled to view it.

In addition to Hafezi, Mittal and Gorshkov, co-authors of the research paper include electrical and computer engineering graduate student Venkata Vikram Orre; JQI Research Scientist Elizabeth Goldschmidt, who is now an assistant professor of physics at the University of Illinois at Urbana-Champaign; physics graduate student Abhinav Deshpande; and Vincenzo Tamma, a physicist at the University of Portsmouth.

Reference Publication

"Interference of Temporally Distinguishable Photons Using Frequency-Resolved Detection," Venkata Vikram Orre, Elizabeth A. Goldschmidt, Abhinav Deshpande, Alexey V. Gorshkov, Vincenzo Tamma, Mohammad Hafezi, Sunil Mittal, Phys. Rev. Lett., 123, 123603 (2019)

Rare “Lazarus Superconductivity” Observed in Rediscovered Material

Details: Published: Tuesday, October 08 2019 07:02

Researchers from the University of Maryland, the National Institute of Standards and Technology (NIST), the National High Magnetic Field Laboratory (National MagLab) and the University of Oxford have observed a rare phenomenon called re-entrant superconductivity in the material uranium ditelluride. The discovery furthers the case for uranium ditelluride as a promising material for use in quantum computers.

A team of researchers has observed a rare phenomenon called re-entrant superconductivity in the material uranium ditelluride. Nicknamed “Lazarus superconductivity,” the phenomenon occurs when a superconducting state arises, breaks down, then re-emerges in a material due to a change in a specific parameter—in this case, the application of a very strong magnetic field. The discovery furthers the case for uranium ditelluride as a promising material for use in quantum computers. Image credit: Emily Edwards/JQI

Nicknamed “Lazarus superconductivity” after the biblical figure who rose from the dead, the phenomenon occurs when a superconducting state arises, breaks down, then re-emerges in a material due to a change in a specific parameter—in this case, the application of a very strong magnetic field. The researchers published their results on October 7, 2019, in the journal Nature Physics.

Once dismissed by physicists for its apparent lack of interesting physical properties, uranium ditelluride is having its own Lazarus moment. The current study is the second in as many months (both published by members of the same research team) to demonstrate unusual and surprising superconductivity states in the material.

“This is a very recently discovered superconductor with a host of other unconventional behavior, so it's already weird,” said Adjunct Assistant Professor Nicholas Butch, a physicist at the NIST Center for Neutron Research. “[Lazarus superconductivity] almost certainly has something to do with the novelty of the material. There's something different going on in there.”

The previous research, published on August 16, 2019 in the journal Science, described the rare and exotic ground state known as spin-triplet superconductivity in uranium ditelluride. The discovery marked the first clue that uranium ditelluride is worth a second look, due to its unusual physical properties and its high potential for use in quantum computers.

“This is indeed a remarkable material and it’s keeping us very busy,” said Johnpierre Paglione, a professor of physics at UMD, the director of UMD’s Center for Nanophysics and Advanced Materials (CNAM; soon to be renamed the Quantum Materials Center) and a co-author of the paper. “Uranium ditelluride may very well become the ‘textbook’ spin-triplet superconductor that people have been seeking for dozens of years and it likely has more surprises in store. It could be the next strontium ruthenate—another proposed spin-triplet superconductor that has been studied for more than 25 years.”

Superconductivity is a state in which electrons travel through a material with perfect efficiency. By contrast, copper—which is second only to silver in terms of its ability to conduct electrons—loses roughly 20% power over long-distance transmission lines, as the electrons bump around within the material during travel.

Lazarus superconductivity is especially strange, because strong magnetic fields usually destroy the superconducting state in the vast majority of materials. In uranium ditelluride, however, a strong magnetic field coupled with specific experimental conditions caused Lazarus superconductivity to arise not just once, but twice.

For Butch, Paglione and their team, the discovery of this rare form of superconductivity in uranium ditelluride was serendipitous; the study’s lead author, CNAM Research Associate Sheng Ran, synthesized the crystal accidentally while attempting to produce another uranium-based compound. The team decided to try some experiments anyway, even though previous research on the compound hadn’t yielded anything unusual.

The team’s curiosity was soon rewarded many times over. In the earlier Science paper, the researchers reported that uranium ditelluride’s superconductivity involved unusual electron configurations called spin triplets, in which pairs of electrons are aligned in the same direction. In the vast majority of superconductors, the orientations—called spins—of paired electrons point in opposite directions. These pairs are (somewhat counterintuitively) called singlets. Magnetic fields can more easily disrupt singlets, killing superconductivity.

Spin triplet superconductors, however, can withstand much higher magnetic fields. The team’s early findings led them to the National MagLab, where a unique combination of very high-field magnets, capable instrumentation and resident expertise allowed the researchers to push uranium ditelluride even further.

At the lab, the team tested uranium ditelluride in some of the highest magnetic fields available. By exposing the material to magnetic fields up to 65 teslas—more than 30 times the strength of a typical MRI magnet—the team attempted to find the upper limit at which the magnetic fields crushed the material’s superconductivity. Butch and his team also experimented with orienting the uranium ditelluride crystal at several different angles in relation to the direction of the magnetic field.

At about 16 teslas, the material’s superconducting state abruptly changed. While it died in most of the experiments, it persisted when the crystal was aligned at a very specific angle in relation to the magnetic field. This unusual behavior continued until about 35 teslas, at which point all superconductivity vanished and the electrons shifted their alignment, entering a new magnetic phase.

As the researchers increased the magnetic field while continuing to experiment with angles, they found that a different orientation of the crystal yielded yet another superconducting phase that persisted to at least 65 teslas, the maximum field strength the team tested. It was a record-busting performance for a superconductor and marked the first time two field-induced superconducting phases have been found in the same compound.

Instead of killing superconductivity in uranium ditelluride, high magnetic fields appeared to stabilize it. While it is not yet clear exactly what is happening at the atomic level, Butch said the evidence points to a phenomenon fundamentally different than anything scientists have seen to date.

“I'm going to go out on a limb and say that these are probably different—quantum mechanically different—from other superconductors that we know about,” Butch said. “It is sufficiently different, I think, to expect it will take a while to figure out what's going on.”

On top of its convention-defying physics, uranium ditelluride shows every sign of being a topological superconductor, as are other spin-triplet superconductors, Butch added. Its topological properties suggest it could be a particularly accurate and robust component in the quantum computers of the future.

“The discovery of this Lazarus superconductivity at record-high fields is likely to be among the most important discoveries to emerge from this lab in its 25-year history,” said National MagLab Director Greg Boebinger. “I would not be surprised if unraveling the mysteries of uranium ditelluride leads to even stranger manifestations of superconductivity in the future.”

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This release was adapted from text provided by the National High Magnetic Field Laboratory.

In addition to Butch, Paglione and Ran, UMD-affiliated co-authors of the research paper include physics postdoctoral researcher Yun Suk Eo; physics graduate students I-Lin Liu, Daniel Campbell and Christopher Eckberg; undergraduate physics major Paul Neves, physics faculty assistant Wesley Fuhrman; CNAM (QMC) Assistant Research Scientist Hyunsoo Kim and CNAM (QMC) Associate Research Scientist Shanta Saha.

The research paper, “Extreme magnetic field-boosted superconductivity,” Sheng Ran, I-Lin Liu, Yun Suk Eo, Daniel Campbell, Paul Neves, Wesley Fuhrman, Shanta Saha, Christopher Eckberg, Hyunsoo Kim, Johnpierre Paglione, David Graf, Fedor Balakirev, John Singleton and Nicholas Butch, was published in the journal Nature Physics on October 7, 2019.

This work was supported by the Schmidt Science Fellows program (in partnership with the Rhodes Trust), the National Science Foundation (Award Nos. DMR-1610349, DMR-1157490, DMR-1644779), the U.S. Department of Energy (Award No. DE-SC-0019154), the Gordon and Betty Moore Foundation’s EPiQS Initiative (Award No. GBMF4419), and the State of Florida. The content of this article does not necessarily reflect the views of these organizations.

Ions Clear Another Hurdle Toward Scaled-up Quantum Computing

Details: Published: Friday, August 23 2019 10:02

Scientists at the Joint Quantum Institute (JQI) have been steadily improving the performance of ion trap systems, a leading platform for future quantum computers. Now, a team of researchers led by JQI Fellows Norbert Linke and Christopher Monroe has performed a key experiment on five ion-based quantum bits, or qubits. They used laser pulses to simultaneously create quantum connections between different pairs of qubits—the first time these kinds of parallel operations have been executed in an ion trap. The new study, which is a critical step toward large-scale quantum computation, was published on July 24 in the journal Nature.

“When it comes to the scaling requirements for a quantum computer, trapped ions check all of the boxes,” says Monroe, who is also the Bice-Sechi Zorn professor in the UMD Department of Physics and co-founder of the quantum computing startup IonQ. “Getting these parallel operations to work further illustrates that advancing ion trap quantum processors is not limited by the physics of qubits and is instead tied to engineering their controllers.”

Ion traps are devices for capturing charged atoms and molecules, and they are commonly deployed for chemical analysis. In recent decades, physicists and engineers have combined ion traps with sophisticated laser systems to exert control over single atomic ions. Today, this type of hardware is one of the most promising for building a universal quantum computer.

The JQI ion trap used in this study is made from gold-coated electrodes, which carry the electric fields that confine ytterbium ions. The ions are caught in the middle of the trap where they form a line, each one separated from its neighbor by a few microns. This setup enables researchers to have fine control over individual ions and configure them as qubits.

Each ion has internal energy levels or quantum states that are naturally isolated from outside influences. This feature makes them ideal for storing and controlling quantum information, which is notoriously delicate. In this experiment, the research team uses two of these states, called “0” and “1”, as the qubit.

The researchers aim laser pulses at a string of qubits to execute programs on this small-scale quantum computer. The programs, also called circuits, are broken down into a set of single- and two-qubit gates. A single-qubit gate can, for instance, flip the state of an ion from 1 to 0. This is a straightforward task for a laser pulse. A two-qubit gate requires more sophisticated pulses because it involves tailoring the interactions between qubits. Certain two-qubit operations can create entanglement—a quantum connection necessary for quantum computation—between two qubits.

Until now, circuits in ion trap quantum computers have been limited to a sequence of individual gates, one after another. With this new demonstration, researchers can now do two-qubit gates in parallel, creating entanglement between different pairs of ions simultaneously. The research team achieved this by optimizing the laser pulse sequences used to perform operations, making sure to cancel out unwanted laser-qubit interactions. In this way, they were able to successfully implement simultaneous entangling gates on two separate ion pairs.

According to the authors, parallel entangling gates will enable programs to correct errors during a quantum computation—a near-certain requirement in quantum computers with many more qubits. In addition, a quantum computer that factors large numbers or simulates quantum physics will likely need parallel entangling operations to achieve a speed advantage over conventional computers.

Story by E. Edwards

In addition to Monroe and Linke, Caroline Figgatt, former JQI graduate student and scientist at Honeywell, was lead author on this research paper and provided background material for this news story. The research paper was published simultaneous to similar work done by former JQI postdoctoral researcher and Tsinghua University professor Kihwan Kim.

REFERENCE PUBLICATION

"Parallel entangling operations on a universal ion-trap quantum computer," Caroline Figgatt, A Ostrander, Norbert M. Linke, Kevin A. Landsman, D Zhu, Dmitri Maslov, Christopher Monroe, Nature, , (2019)

LHCb Experiment Discovers CP Violation in the Charm Quark System

Hybrid Device among First to Meld Quantum and Conventional Computing

Stretched Photons Recover Lost Interference

Rare “Lazarus Superconductivity” Observed in Rediscovered Material

Ions Clear Another Hurdle Toward Scaled-up Quantum Computing

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