JQI researchers modified a Josephson junction to include a sliver of topological crystalline insulator (TCI). Using this circuit, they detected signs of exotic quantum states lurking on the TCI's surface. (Credit: E. Edwards/JQI)

Electrons tend to avoid one another as they go about their business carrying current. But certain devices, cooled to near zero temperature, can coax these loner particles out of their shells. In extreme cases, electrons will interact in unusual ways, causing strange quantum entities to emerge.

At the Joint Quantum Institute (JQI), a group, led by Jimmy Williams, is working to develop new circuitry that could host such exotic states. “In our lab, we want to combine materials in just the right way so that suddenly, the electrons don’t really act like electrons at all,” says Williams, a JQI Fellow and an assistant professor in the University of Maryland Department of Physics. “Instead the surface electrons move together to reveal interesting quantum states that collectively can behave like new particles.”

These states have a feature that may make them useful in future quantum computers: They appear to be inherently protected from the destructive but unavoidable imperfections found in fabricated circuits. As described recently in Physical Review Letters, Williams and his team have reconfigured one workhorse superconductor circuit—a Josephson junction—to include a material suspected of hosting quantum states with boosted immunity.

Josephson junctions are electrical synapses comprised of two superconductors separated by a thin strip of a second material. The electron movement across the strip, which is usually made from an insulator, is sensitive to the underlying material characteristics as well as the surroundings. Scientists can use this sensitivity to detect faint signals, such as tiny magnetic fields. In this new study, the researchers replaced the insulator with a sliver of topological crystalline insulator (TCI) and detected signs of exotic quantum states lurking on the circuit’s surface.

Physics graduate student Rodney Snyder, lead author on the new study, says this area of research is full of unanswered questions, down to the actual process for integrating these materials into circuits. In the case of this new device, the research team found that beyond the normal level of sophisticated material science, they needed a bit of luck.

“I'd make like 16 to 25 circuits at a time. Then, we checked a bunch of those and they would all fail, meaning they wouldn’t even act like a basic Josephson junction,” says Snyder. “We eventually found that the way to make them work was to heat the sample during the fabrication process. And we only discovered this critical heating step because one batch was accidentally heated on a fluke, basically when the system was broken.”

Once they overcame the technical challenges, the team went hunting for the strange quantum states. They examined the current through the TCI region and saw dramatic differences when compared to an ordinary insulator. In conventional junctions, the electrons are like cars haphazardly trying to cross a single lane bridge. The TCI appeared to organize the transit by opening up directional traffic lanes between the two locations. 

The experiments also indicated that the lanes were helical, meaning that the electron’s quantum spin, which can be oriented either up or down, sets its travel direction. So in the TCI strip, up and down spins move in opposite directions. This is analogous to a bridge that restricts traffic according to vehicle colors—blue cars drive east and red cars head west. These kinds of lanes, when present, are indicative of exotic electron behaviors.

Just as the careful design of a bridge ensures safe passage, the TCI structure played a crucial role in electron transit. Here, the material’s symmetry, a property that is determined by the underlying atom arrangement, guaranteed that the two-way traffic lanes stayed open. “The symmetry acts like a bodyguard for the surface states, meaning that the crystal can have imperfections and still the quantum states survive, as long as the overall symmetry doesn’t change,” says Williams.

Physicists at JQI and elsewhere have previously proposed that built-in bodyguards could shield delicate quantum information. According to Williams, implementing such protections would be a significant step forward for quantum circuits, which are susceptible to failure due to environmental interference.

In recent years, physicists have uncovered many promising materials with protected travel lanes, and researchers have begun to implement some of the theoretical proposals. TCIs are an appealing option because, unlike more conventional topological insulators where the travel lanes are often given by nature, these materials allow for some lane customization. Currently, Williams is working with materials scientists at the Army Research Laboratory to tailor the travel lanes during the manufacturing process. This may enable researchers to position and manipulate the quantum states, a step that would be necessary for building a quantum computer based on topological materials.

In addition to quantum computing, Williams is driven by the exploration of basic physics questions. “We really don't know yet what kind of quantum matter you get from collections of these more exotic states,” Williams says. “And I think, quantum computation aside, there is a lot of interesting physics happening when you are dealing with these oddball states.”

Written by E. Edwards and S. Elbeshbishi

State-of-the-art quantum devices are not yet large enough to be called full-scale computers. The biggest comprise just a few dozen qubits—a meager count compared to the billions of bits in an ordinary computer’s memory. But steady progress means that these machines now routinely string together 10 or 20 qubits and may soon hold sway over 100 or more.

In the meantime, researchers are busy dreaming up uses for small quantum computers and mapping out the landscape of problems they’ll be suited to solving. A paper by researchers from the Joint Quantum Institute (JQI) and the Joint Center for Quantum Information and Computer Science (QuICS), published recently in Physical Review Letters, argues that a novel non-quantum perspective may help sketch the boundaries of this landscape and potentially even reveal new physics in future experiments.

The new perspective involves a mathematical tool—a standard measure of computational difficulty known as sampling complexity—that gauges how easy or hard it is for an ordinary computer to simulate the outcome of a quantum experiment. Because the predictions of quantum physics are probabilistic, a single experiment could never verify that these predictions are accurate. You would need to perform many experiments, just like you would need to flip a coin many times to convince yourself that you’re holding an everyday, unbiased nickel.

If an ordinary computer takes a reasonable amount of time to mimic one run of a quantum experiment—by producing samples with approximately the same probabilities as the real thing—the sampling complexity is low; if it takes a long time, the sampling complexity is high.

Few expect that quantum computers wielding lots of qubits will have low sampling complexity—after all, quantum computers are expected to be more powerful than ordinary computers, so simulating them on your laptop should be hard. But while the power of quantum computers remains unproven, exploring the crossover from low complexity to high complexity could offer fresh insights about the capabilities of early quantum devices, says Alexey Gorshkov, a JQI and QuICS Fellow who is a co-author of the new paper.

“Sampling complexity has remained an underappreciated tool,” Gorshkov says, largely because small quantum devices have only recently become reliable. “These devices are now essentially doing quantum sampling, and simulating this is at the heart of our entire field.”

To demonstrate the utility of this approach, Gorshkov and several collaborators proved that sampling complexity tracks the easy-to-hard transition of a task that small- and medium-sized quantum computers are expected to perform faster than ordinary computers: boson sampling.

Bosons are one of the two families of fundamental particles (the other being fermions). In general two bosons can interact with one another, but that’s not the case for the boson sampling problem. “Even though they are non-interacting in this problem, bosons are sort of just interesting enough to make boson sampling worth studying,” says Abhinav Deshpande, a graduate student at JQI and QuICS and the lead author of the paper.

In the boson sampling problem, a fixed number of identical particles are allowed to hop around on a grid, spreading out into quantum superpositions over many grid sites. Solving the problem means sampling from this smeared-out quantum probability cloud, something a quantum computer would have no trouble doing.

Deshpande, Gorshkov and their colleagues proved that there is a sharp transition between how easy and hard it is to simulate boson sampling on an ordinary computer. If you start with a few well-separated bosons and only let them hop around briefly, the sampling complexity remains low and the problem is easy to simulate. But if you wait longer, an ordinary computer has no chance of capturing the quantum behavior, and the problem becomes hard to simulate.

The result is intuitive, Deshpande says, since at short times the bosons are still relatively close to their starting positions and not much of their “quantumness” has emerged. For longer times, though, there’s an explosion of possibilities for where any given boson can end up. And because it’s impossible to tell two identical bosons apart from one another, the longer you let them hop around, the more likely they are to quietly swap places and further complicate the quantum probabilities. In this way, the dramatic shift in the sampling complexity is related to a change in the physics: Things don’t get too hard until bosons hop far enough to switch places.

Gorshkov says that looking for changes like this in sampling complexity may help uncover physical transitions in other quantum tasks or experiments. Conversely, a lack of ramping up in complexity may rule out a quantum advantage for devices that are too error-prone. Either way, Gorshkov says, future results arising from this perspective shift should be interesting. “A deeper look into the use of sampling complexity theory from computer science to study quantum many-body physics is bound to teach us something new and exciting about both fields,” he says.

Story by Chris Cesare

Reference Publication
"Dynamical Phase Transitions in Sampling Complexity," Abhinav Deshpande, Bill Fefferman, Minh C. Tran, Michael Foss-Feig, Alexey V. Gorshkov, Phys. Rev. Lett., 121, 030501 (2018)
Research Contact: Abhinav Deshpande, This email address is being protected from spambots. You need JavaScript enabled to view it.: Alexey Gorshkov, This email address is being protected from spambots. You need JavaScript enabled to view it.

Orginal story: https://jqi.umd.edu/news/complexity-test-offers-new-perspective-on-small-quantum-computers