Simulating the Quantum World with Electron Traps

Quantum behavior plays a crucial role in novel and emergent material properties, such as superconductivity and magnetism. Unfortunately, it is still impossible to calculate the underlying quantum behavior, let alone fully understand it. Scientists of QuTech, the Kavli Institute of Nanoscience in Delft and TNO, in collaboration with ETH Zurich and the University of Maryland, have now succeeded in building an "artificial material" that mimics this type of quantum behavior on a small scale. In doing so, they have laid the foundations for new insights and potential applications. Their work is published today in Nature.

Over the past century, an increased understanding of semiconductor materials has led to many technological improvements, such as computer chips becoming ever faster and smaller. We are, however, gradually reaching the limits of Moore's Law, the trend that predicts a doubling in computing power for half the price every two years. But this prediction ignores the possibility that computers might harness quantum physics.

"There is so much physics left to discover if we truly want to understand materials on the very smallest scale," says Lieven Vandersypen, a professor at TU Delft in the Netherlands and the lead experimentalist on the new paper. And that new physics is set to bring even more new technology with it. "The difficulty is that, at this scale, quantum theory determines the behavior of electrons and it is virtually impossible to calculate this behavior accurately even for just a handful of electrons, using even the most powerful supercomputers," Vandersypen says.

Scientists are now combining the power of the semiconductor industry with their knowledge of quantum technology in order to mimic the behavior of electrons in materials—a technique known as quantum simulation. "I hope that, in the near future, this will enable us to learn so much about materials that we can open some important doors in technology, such as the design of superconductors at room temperature, to make possible loss-free energy transport over long distances, for example," Vandersypen says.

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Mimicking nature

It has long been known that individual electrons can be confined to small regions on a chip, known as quantum dots. There are, in principle, suitable for researching the behavior and interactions of electrons in materials. The captured electrons can move, or tunnel, between the quantum dots in a controlled way, while they interact through the repulsion of their negative charges. "Processes like these in quantum dots, cooled to a fraction of a degree above absolute zero, are perfectly suitable for simulating the electronic properties of new materials," says Toivo Hensgens, a graduate student at TU Delft and the lead author of the paper.

In practice, it is a major challenge to control the electrons in quantum dots so precisely that the underlying physics becomes visible. Imperfections in the quantum chips and inefficient methods of controlling the electrons in the dots have made this a particularly hard nut to crack.

Quantum equipment

Researchers have now demonstrated a method that is both effective and can be scaled up to larger numbers of quantum dots. The number of electrons in each quantum dot can be set from 0 to 4 and the chance of tunnelling between neighbouring dots can be varied from negligible to the point at which neighbouring dots actually become one large dot. "We use voltages to distort the (potential) landscape that the electrons sense," explains Hensgens. "That voltage determines the number of electrons in the dots and the relative interactions between them."

In a quantum chip with three quantum dots, the QuTech team has demonstrated that they are capable of simulating a series of material processes experimentally. But the most important result is the method that they have demonstrated. "We are now easily able to  add more quantum dots with electrons and control the potential landscape in such a way that we can ultimately simulate very large and interesting quantum processes," Hensgens says.

The Vandersypen team aims to progress towards more quantum dots as soon as possible. To achieve that, he and his colleagues have entered a close collaboration with chipmaker Intel. "Their knowledge and expertise in semiconductor manufacturing combined with our deep understanding of quantum control offers opportunities that are now set to bear fruit," he says.

This story was prepared by the Delft University of Technology (TU Delft) and adapted with permission. The experiments described were performed at TU Delft, with theoretical and numerical contributions from JQI Fellow and Condensed Matter Theory Center Director Sankar Das Sarma and JQI postdoctoral researcher Xiao Li.

 

Research Contacts:

Professor Lieven Vandersypen
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Dr. Xiao Li
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Proposed LHC Experiment Would Spot Invisible, Long-lived Particles

More than 300 feet underground, looping underneath both France and Switzerland on the outskirts of Geneva, a 16-mile-long ring called the Large Hadron Collider (LHC) smashes protons together at nearly the speed of light. Sifting through the wreckage, scientists have made some profound discoveries about the fundamental nature of our universe.

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But what if all that chaos underground is shrouding subtle hints of new physics? David Curtin, a postdoctoral researcher at the Maryland Center for Fundamental Physics here at UMD, has an idea for a detector that could be built at the surface—far away from the noise and shrapnel of the main LHC experiments. The project, which he and his collaborators call MATHUSLA, may resolve some of the mysteries that are lingering behind our best theories.

This episode of the Relatively Certain podcast was produced by Chris Cesare, Emily Edwards, Sean Kelley and Kate Delossantos. It features music by Dave Depper, Podington Bear, Broke for Free, Chris Zabriskie and the LHCsound project. Relatively Certain is a production of the Joint Quantum Institute, a research partnership between the University of Maryland and the National Institute of Standards and Technology, and you can find it on iTunes, Google Play or Soundcloud.

 

To listen now, click here and a player will open.

For more on this topic, read this Quanta Magazine article. 

 

Atomic cousins team up in early quantum networking node

Large-scale quantum computers, which are an active pursuit of many university labs and tech giants, remain years away. But that hasn’t stopped some scientists from thinking ahead, to a time when quantum computers might be linked together in a network or a single quantum computer might be split up across many interconnected nodes.

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Tiny magnetic tremors unlock exotic superconductivity

tiny magnetic

Deep within solids, individual electrons zip around on a nanoscale highway paved with atoms. For the most part, these electrons avoid one another, kept in separate lanes by their mutual repulsion. But vibrations in the atomic road can blur their lanes and sometimes allow the tiny particles to pair up. The result is smooth and lossless travel, and it’s one way to create superconductivity.

But there are other, less common ways to achieve this effect. Scientists from the University of Maryland (UMD), the University of California, Irvine (UCI) and Fudan University have now shown that tiny magnetic tremors lead to superconductivity in a material made from metallic nano-layers. And, beyond that, the resulting electron pairs shatter a fundamental symmetry between past and future. Although the material is a known superconductor, these researchers provide a theoretical model and measurement, which, for the first time, unambiguously reveals the material’s exotic nature.

In quantum materials, breaking the symmetry between the past and the future often signifies unconventional phases of matter. The nickel-bismuth (Ni-Bi) sample studied here is the first example of a 2D material where this type of superconductivity is intrinsic, meaning that it happens without the help of external agents, such as a nearby superconductor. These findings, recently published in Science Advances, make Ni-Bi an appealing choice for use in future quantum computers. This research may also assist scientists in their search for other similarly strange superconductors.

Mehdi Kargarian*, a postdoctoral researcher at UMD and a co-author of the paper, explains that even after a century of study, superconductivity remains a vibrant area of research. “It is a rather old problem, so it is surprising that people are still discovering types of superconductivity in the lab that are unprecedented,” Kargarian says, adding that there are typically two questions scientists ask of a new superconductor. “First, we want to understand the underlying electron pairing—what is causing the superconductivity,” he says. “The second thing, related to applications, is to see if superconductivity is possible at higher temperatures.”

Superconductors, particularly the exotic types, largely remain shackled to unwieldy cryogenic equipment. Scientists are searching for ways to push superconducting temperatures higher, thus making these materials easier to use for things like improved electricity distribution and building quantum devices. In this new research, the team tackles Kargarian’s first question and the material hints at a positive outlook for the second question. Its exotic superconductivity, although still cryogenic, occurs at a higher temperature compared to other similar systems.

Ni-Bi superconductivity was first observed in the early 1990s. But later, when Fudan University scientists published studies of an ultrapure, ultra-thin sample, they noticed something unusual happening.

The strangeness starts with the superconductivity itself. Bismuth alone is not a superconductor, except under extraordinarily low temperatures and high pressure—conditions that are not easy to achieve. Nickel is magnetic and not a superconductor. In fact, strong magnets are known to suppress the effect. This means that too much nickel destroys the superconductivity, but a small amount induces it.

UMD theorists* proposed that fluctuations in nickel’s magnetism are at the heart of this peculiar effect. These tiny magnetic tremors help electrons to form pairs, thus doing the work performed by vibrations in conventional superconductors. If there is too much nickel, magnetism dominates and the effect of the fluctuations diminishes. If there is too much bismuth, then the top surface, where superconductivity takes place, is too far away from the source of magnetic fluctuations.

The goldilocks zone occurs when a twenty-nanometer-thick bismuth layer is grown on top of two nanometers of nickel. For this layer combination, superconductivity happens at around 4 degrees above absolute zero. While this is about as cold as deep space, it is actually quite lab-friendly and reachable using standard cryogenic equipment.

The idea that magnetic fluctuations can promote superconductivity is not new and dates back to the end of the 20th century. However, most earlier examples of such behavior require strict operating conditions, such as high pressure. The researchers explain that Ni-Bi is different because straightforward cooling is enough to achieve this type of exotic superconductivity, which breaks time symmetry.

The researchers employed a highly customized apparatus to search for signs of the broken symmetry. Light should rotate when reflected from samples that have this property. For Ni-Bi, the expected amount of light rotation is tens of nanoradians, which is about 100 billionths of a tick on a watch face. Jing Xia*, a co-author of the paper and a professor at UCI, has one of the only devices in the world capable of measuring such an imperceptible light rotation.

In order to measure this rotation for Ni-Bi, light waves are first injected into one end of a single special-purpose optical fiber. The two waves travel through the fiber, as if on independent paths. They hit the sample and then retrace their paths. Upon return, the waves are combined and form a pattern. Rotations of the light waves—from, say, symmetry breaking—will show up in the analyzed pattern as small translations. Xia and his colleagues at UCI measured around 100 nanoradians of rotation, confirming the broken symmetry. Importantly, the effect appeared just as the Ni-Bi sample became a superconductor, suggesting that the broken time symmetry and the appearance of superconductivity are strongly linked.

This form of superconductivity is rare and researchers say that there is still no recipe for making it happen. But, as Xia points out, there is guidance in the math behind the electron behavior. “We know mathematically how to make electron pairs break time-reversal symmetry,” Xia says. Practically, how do you achieve this formulaically? That is the million-dollar question. But my instinct is that when you do get magnetic fluctuation-mediated superconductivity, like in this material, then it is highly likely you get break that symmetry.”

* M. Kargarian is a postdoctoral researcher at the Condensed Matter Theory Center (CMTC), University of Maryland (UMD) and is affiliated with the Joint Quantum Institute. V. Yakovenko and V. Galitski are members of CMTC, fellows of the Joint Quantum Institute, and UMD professors. J. Xia is a professor at the University of California, Irvine.

Written by E. Edwards/JQI

REFERENCE PUBLICATION
"Time-reversal symmetry-breaking superconductivity in epitaxial bismuth/nickel bilayers," X. Gong, M. Kargarian, A. Stern, D. Yue, H. Zhou, X. Jin, V.M. Galitski, V.M. Yakovenko, J. Xia, Science Advances, 3, (2017)

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