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A Graphene Superconductor That Plays More Than One Tune

Researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a graphene device that’s thinner than a human hair but has a depth of special traits. It easily switches from a superconducting material that conducts electricity without losing any energy, to an insulator that resists the flow of electric current, and back again to a superconductor — all with a simple flip of a switch.

“Usually, when someone wants to study how electrons interact with each other in a superconducting quantum phase versus an insulating phase, they would need to look at different materials. With our system, you can study both the superconductivity phase and the insulating phase in one place,”

said Guorui Chen, the study’s lead author and a postdoctoral researcher in the lab of Feng Wang, who led the study. Wang, a faculty scientist in Berkeley Lab’s Materials Sciences Division, is also a UC Berkeley physics professor.

Trilayer Graphene Superlattice

The graphene device is composed of three atomically thin (2-D) layers of graphene. When sandwiched between 2-D layers of boron nitride, it forms a repeating pattern called a moiré superlattice.

The material could help other scientists understand the complicated mechanics behind a phenomenon known as high-temperature superconductivity, where a material can conduct electricity without resistance at temperatures higher than expected, though still hundreds of degrees below freezing.

In a previous study, the researchers reported observing the properties of a Mott insulator in a device made of trilayer graphene. A Mott insulator is a class of material that somehow stops conducting electricity at hundreds of degrees below freezing despite classical theory predicting electrical conductivity.

But it has long been believed that a Mott insulator can become superconductive by adding more electrons or positive charges to make it superconductive, Chen explained.

Superconductivity

For the past 10 years, researchers have been studying ways to combine different 2-D materials, often starting with graphene — a material known for its ability to efficiently conduct heat and electricity. Out of this body of work, other researchers had discovered that moiré superlattices formed with graphene exhibit exotic physics such as superconductivity when the layers are aligned at just the right angle.

“So for this study we asked ourselves, ‘If our trilayer graphene system is a Mott insulator, could it also be a superconductor?'”

said Chen.

Working with David Goldhaber-Gordon of Stanford University and the Stanford Institute for Materials and Energy Sciences at SLAC National Accelerator Laboratory, and Yuanbo Zhang of Fudan University, the researchers used a dilution refrigerator, which can reach intensely cold temperatures of 40 millikelvins — or nearly minus 460 degrees Fahrenheit — to cool the graphene/boron nitride device down to a temperature at which the researchers expected superconductivity to appear near the Mott insulator phase, said Chen.

Mott Insulator

Once the device reached a temperature of 4 kelvins (minus 452 degrees Fahrenheit), the researchers applied a range of electrical voltages to the tiny top and bottom gates of the device.

As they expected, when they applied a high vertical electrical field to both the top and bottom gates, an electron filled each cell of the graphene/boron nitride device. This caused the electrons to stabilize and stay in place, and this “localization” of electrons turned the device into a Mott insulator.

The graphene/boron nitride moiré superlattice
The graphene/boron nitride moiré superlattice material is composed of three atomically thin (2D) layers of graphene (gray) sandwiched between 2D layers of boron nitride (red and blue) to form a repeating pattern called a moiré superlattice. Superconductivity is indicated by the light-green circles.
Credit: Guorui Chen et al./Berkeley Lab

Then, they applied an even higher electrical voltage to the gates. To their delight, a second reading indicated that the electrons were no longer stable. Instead, they were shuttling about, moving from cell to cell, and conducting electricity without loss or resistance.

In other words, the device had switched from the Mott insulator phase to the superconductor phase.

Chen explained that the boron nitride moiré superlattice somehow increases the electron-electron interactions that take place when an electrical voltage is applied to the device, an effect that switches on its superconducting phase. It’s also reversible — when a lower electrical voltage is applied to the gates, the device switches back to an insulating state.

A Tiny Playground

The multitasking device offers scientists a tiny, versatile playground for studying the exquisite interplay between atoms and electrons in exotic new superconducting materials with potential use in quantum computers — computers that store and manipulate information in qubits, which are typically subatomic particles such as electrons or photons — as well as new Mott insulator materials that could one day make tiny 2-D Mott transistors for microelectronics a reality.

“This result was very exciting for us. We never imagined that the graphene/boron nitride device would do so well,” Chen said. “You can study almost everything with it, from single particles to superconductivity. It’s the best system I know of for studying new kinds of physics,”

Chen said.

This study was supported by the Center for Novel Pathways to Quantum Coherence in Materials (NPQC), an Energy Frontier Research Center led by Berkeley Lab and funded by the DOE Office of Science. NPQC brings together researchers at Berkeley Lab, Argonne National Laboratory, Columbia University, and UC Santa Barbara to study how quantum coherence underlies unexpected phenomena in new materials such as trilayer graphene, with an eye toward future uses in quantum information science and technology.

Original Study: Signatures of tunable superconductivity in a trilayer graphene moiré superlattice

Top Image: Guorui Chen/Berkeley Lab

Quantum Control With Light Paves Way For Ultra-Fast Computers

Terahertz light can control some of the essential quantum properties of superconducting states, report researchers.

Jigang Wang patiently explains his latest discovery in quantum control that could lead to superfast computing based on quantum mechanics: He mentions light-induced superconductivity without energy gap. He brings up forbidden supercurrent quantum beats. And he mentions terahertz-speed symmetry breaking.

Then he backs up and clarified all that. After all, the quantum world of matter and energy at terahertz and nanometer scales — trillions of cycles per second and billionths of meters — is still a mystery to most of us.

“I like to study quantum control of superconductivity exceeding the gigahertz, or billions of cycles per second, bottleneck in current state-of-the-art quantum computation applications. We’re using terahertz light as a control knob to accelerate supercurrents,”

says Wang, a professor of physics and astronomy at Iowa State University.

Terahertz Light

Superconductivity is the movement of electricity through certain materials without resistance.

It typically occurs at very, very cold temperatures. Think -400 Fahrenheit for “high-temperature” superconductors.

Terahertz light is light at very, very high frequencies. Think trillions of cycles per second. It’s essentially extremely strong and powerful microwave bursts firing at very short time frames.

It all sounds esoteric and strange. But the new method could have very practical applications.

“Light-induced supercurrents chart a path forward for electromagnetic design of emergent materials properties and collective coherent oscillations for quantum engineering applications,”

Wang and his coauthors write in a paper in Nature Photonics.

In other words, the discovery could help physicists “create crazy-fast quantum computers by nudging supercurrents,”

Wang writes in a summary of the research team’s findings.

Quantum World Control

Finding ways to control, access, and manipulate the special characteristics of the quantum world and connect them to real-world problems is a major scientific push these days. The National Science Foundation has included the “Quantum Leap” in its “10 big ideas” for future research and development.

“By exploiting interactions of these quantum systems, next-generation technologies for sensing, computing, modeling, and communicating will be more accurate and efficient. To reach these capabilities, researchers need understanding of quantum mechanics to observe, manipulate, and control the behavior of particles and energy at dimensions at least a million times smaller than the width of a human hair,”

says a summary of the science foundation’s support of quantum studies.

A Universal Tool

The researchers are advancing the quantum frontier by finding new macroscopic supercurrent flowing states and developing quantum controls for switching and modulating them.

A summary of the research team’s study says experimental data they obtained from a terahertz spectroscopy instrument indicates terahertz light-wave tuning of supercurrents is a universal tool “and is key for pushing quantum functionalities to reach their ultimate limits in many cross-cutting disciplines” such as those mentioned by the science foundation.

“We believe that it is fair to say that the present study opens a new arena of light-wave superconducting electronics via terahertz quantum control for many years to come,”

the researchers conclude.

Original Study: Lightwave-driven gapless superconductivity and forbidden quantum beats by terahertz symmetry breaking

Image: Denisse Leon/Unsplash

Quantum Computer And Mezzo Soprano To Duet In World First Performance

What happens when you combine the pure tones of an internationally renowned mezzo soprano and the complex technology of a $15 million quantum supercomputer?

The answer will be exclusively revealed to audiences at the Port Eliot Festival when Superposition, created by Plymouth University composer Alexis Kirke, receives its world premiere later this summer.

Combining the arts and sciences, as Dr Kirke has done with many of his previous works, the 15-minute piece will begin dark and mysterious with celebrated performer Juliette Pochin singing a low-pitched slow theme.

But gradually the quiet sounds of electronic ambience will emerge over or beneath her voice, as the sounds of her singing are picked up by a microphone and sent over the internet to the D-Wave 2X quantum computer at the University of Southern California.

It then reacts with behaviors in the quantum realm that are turned into sounds back in the performance venue, the Round Room at Port Eliot, creating a unique and ground-breaking duet.

And when the singer ends, the quantum processes are left to slowly fade away naturally, making their final sounds as the lights go to black.

First Creative Performance Of Quantum Computer

Dr Kirke, a member of the Interdisciplinary Centre for Computer Music Research at Plymouth University, said:

“There are only a handful of these computers accessible in the world, and this is the first time one has been used as part of a creative performance. So while it is a great privilege to be able to put this together, it is an incredibly complex area of computing and science and it has taken almost two years to get to this stage. For most people, this will be the first time they have seen a quantum computer in action and I hope it will give them a better understanding of how it works in a creative and innovative way.”

The three-part performance will tell the story of Niobe, one of the more tragic figures in Greek mythology, but in this case a nod to the fact the heart of the quantum computer contains the metal named after her, niobium. It will also feature a monologue from Hamlet, interspersed with terms from quantum computing.

This is the latest of Dr Kirke’s pioneering performance works, with previous productions including an opera based on the financial crisis and a piece using a cutting edge wave-testing facility as an instrument of percussion.

D-Wave 2X

Geordie Rose, CTO and Founder, D-Wave Systems, said:

“D-Wave’s quantum computing technology has been investigated in many areas such as image recognition, machine learning and finance. We are excited to see Dr Kirke, a pioneer in the field of quantum physics and the arts, utilising a D-Wave 2X in his next performance.

Quantum computing is positioned to have a tremendous social impact, and Dr Kirke’s work serves not only as a piece of innovative computer arts research, but also as a way of educating the public about these new types of exotic computing machines.”

Image: A D-Wave 1000 Qubit Quantum Processor. Credit: D-Wave Systems Inc

Reliable Quantum Computing Superconducting Qubit Array

Qubit architectureA new level of reliability in a five-qubit array has been achieved by a team of physicists at UC Santa Barbara. This moves us a step closer to making a quantum computer a reality.

A functional quantum computer is a dream of many physicists. Contrasted with regular computers, the quantum computer would use quantum bits, or qubits, which make use of the multiple states of quantum phenomena.

When built, a quantum computer would have millions of times power at certain computations than today’s supercomputers.

Quantum computing relies on complex facets of quantum mechanics such as superposition. This idea holds that any physical object, such as an atom or electron, what quantum computers use to store information, may exist in all of its theoretical states simultaneously. This could raise parallel computing to new levels.

Qubit Error Correction

“Quantum hardware is very, very unreliable compared to classical hardware,” said UCSB staff scientist Austin Fowler. “Even the best state-of-the-art hardware is unreliable. Our paper shows that for the first time reliability has been reached.”

“Qubits are faulty, so error correction is necessary,” said co-lead author Julian Kelly.

Although the team has shown logic operations at the threshold, the array must operate below the threshold to provide an acceptable margin of error.

“We need to improve and we would like to scale up to larger systems,” said lead author Rami Barends. “The intrinsic physics of control and coupling won’t have to change but the engineering around it is going to be a big challenge.”

Xmon Power

Qubit control signalsThe novel configuration of the group’s array stems from the flexibility of geometry at the superconductive level, which allowed the scientists to create cross-shaped qubits they named Xmons.

Superconductivity comes when certain materials are cooled to a critical level that eliminates electrical resistance and eliminates magnetic fields. The team chose to place five Xmons in a single row, with each qubit talking to its nearest neighbor, a simple but effective arrangement.

“Motivated by theoretical work, we started really thinking seriously about what we had to do to move forward,” said physics professor John Martinis. “It took us a while to figure out how simple it was, and simple, in the end, was really the best.”

“If you want to build a quantum computer, you need a two-dimensional array of such qubits, and the error rate should be below 1 percent,” said Fowler. “If we can get one order of magnitude lower — in the area of 10-3 or 1 in 1,000 for all our gates — our qubits could become commercially viable. But there are more issues that need to be solved. There are more frequencies to worry about and it’s certainly true that it’s more complex. However, the physics is no different.”

Reference:

R. Barends, J. Kelly, A. Megrant, A. Veitia, D. Sank, E. Jeffrey, T. C. White, J. Mutus, A. G. Fowler, B. Campbell, Y. Chen, Z. Chen, B. Chiaro, A. Dunsworth, C. Neill, P. O’Malley, P. Roushan, A. Vainsencher, J. Wenner, A. N. Korotkov, A. N. Cleland, John M. Martinis.
Superconducting quantum circuits at the surface code threshold for fault tolerance.
Nature, 2014; 508 (7497): 500 DOI:10.1038/nature13171

Images courtesy of Erik Lucero, UCSB