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Hrvoje Petek Writes a News and Views Article in Nature Nanotechnology

  • By Aude Marjolin
  • 10 January 2017

Photovoltaics in action: Electron motion in a type-II InSe/GaAs semiconductor heterostructure has been recorded in a movie immediately after photoexcitation with high spatial and temporal resolution.

Electrons are the lifeblood of semiconductor devices, from transistors that power computers and smart phones, and semiconductor diodes that light up the night, to photovoltaic cells that harvest solar energy to power it all. Under the influence of applied voltages or light stimulations, electrons flow through nanoscale channels and plummet potential gradients at interfaces of disparate materials. In a semiconductor device this ebb and flow occurs several times every nanosecond within billions of transistors on a single microchip, unseen by human eye, but creating text, images and movies in strings of 0s and 1s. But the true time and spatial scales on which electrons are energized and transported span a range of hundreds of femtoseconds and tens of nanometres. Thus, to capture in a movie the physical phenomena of electrons in a device requires a truly extraordinary camera. Writing in Nature Nanotechnology, Man et al. report an experiment that performs just that. Specifically, they record a movie of electron flow in energy, space and time within a semiconductor heterojunction composed of GaAs in physical contact with InSe by imaging electrons emitted into vacuum through the joint action of femtosecond duration IR generation and UV electron emission laser pulses.

David Snoke's PRL Article Highlighted in Physics Viewpoint

  • By Aude Marjolin
  • 9 January 2017

Matter-Light Condensates Reach Thermal Equilibrium

Making use of improved microcavities, hybrid condensates of matter and light can be tuned to reach a thermal equilibrium state, despite their finite lifetime.

In a laser, coherent light is created by stimulated emission of photons from an “inverted” state of matter that is significantly out of thermal equilibrium. “Inverted” means that excited states of the matter are more occupied than lower energy states, so that emission is more likely than absorption. The coherence of laser light is closely related to a quite different, and less commonly encountered, state of matter—a Bose-Einstein condensate (BEC). In the textbook description of a BEC, at low enough temperatures or high enough densities, a large number of particles occupy the same state, producing a coherent state of matter. In contrast to laser light, the textbook BEC is in thermal equilibrium. Condensates of polaritons—half-light, half-matter quasiparticles—have so far been found in conditions halfway between those of an equilibrium BEC and those of a laser. Work by David Snoke and colleagues now shows that such polariton condensates can be tuned to reach a thermal equilibrium state. With this tunability between an equilibrium and nonequilibrium state, researchers can explore how the character of phase transitions evolves between the two limits.

Peng Liu receives CAREER Award

  • By Aude Marjolin
  • 16 December 2016

Peng Liu has been selected to receive a National Science Foundation CAREER award based upon his proposal, entitled "Computational Studies of Transition-Metal-Catalyzed Reactions in Organic Synthesis." 

In this CAREER project funded by the Chemical Structure, Dynamic & Mechanism B Program of the Chemistry Division, Professor Peng Liu of the Department of Chemistry at the University of Pittsburgh is developing new strategies to use computational tools to investigate mechanisms and effects of ancillary ligands in transition-metal-catalyzed reactions of unactivated starting materials, such as C-C and C-H bonds, and unactivated olefins. The goal of this research is to reveal the fundamental reactivity rules of common organometallic intermediates in these transformations and to develop new models to interpret ligand effects on reactivity and selectivity. This proposal’s educational and outreach plan aims to maximize the power of computations to enhance learning of organic chemistry concepts and to facilitate synthetic organic chemistry research. Professor Liu’s team will develop virtual reality (VR) software and educational materials to visualize three-dimensional molecular structures and reaction mechanism videos in an interactive and immersive environment.

Watching

Topological nonsymmorphic metals, a new type of quantum critical point, and atomic scale nanoribbons

Storified by PQI Communication ·
Fri, Jan 20 2017 15:32:01

Intrinsic superconductivity of graphene unleashed, first observation of vacuum fluctuations, spontaneous symmetry breaking in microcavity, towards topological photonic crystals, and ultrafast nonthermal photo-magnetic recording

Storified by PQI Communication ·
Thu, Jan 19 2017 15:26:54

A molecular fountain, fracture in 2D materials, and optical transformation of Dirac fermions into Weyl fermions

Storified by PQI Communication ·
Wed, Jan 18 2017 14:28:14

Prediction and compensation of qubit decoherence, opto-valleytronic imaging of ultrathin seminconductors, and gold-enhanced graphene photodetector

Storified by PQI Communication ·
Tue, Jan 17 2017 14:34:40

Quantum Computing Is Real, and D-Wave Just Open-Sourced It

Storified by PQI Communication ·
Fri, Jan 13 2017 15:45:18

QUANTUM COMPUTING IS real. But it’s also hard. So hard that only a few developers, usually trained in quantum physics, advanced mathematics, or most likely both, can actually work with the few quantum computers that exist. Now D-Wave, the Canadian company behind the quantum computer that Google and NASA have been testing since 2013, wants to make quantum computing a bit easier through the power of open source software.
Traditional computers store information in “bits,” which can represent either a “1” or a “0.” Quantum computing takes advantage of quantum particles in a strange state called “superposition,” meaning that the particle is spinning in two directions at once. Researchers have learned to take advantage of these particles to create what they call “qubits,” which can represent both a 1 and a 0 at the same time. By stringing qubits together, companies like D-Wave hope to create computers that are exponentially faster than today’s machines.
IBM demonstrated a working quantum computer in 2000 and continues to improve on its technology. Google is working on its own quantum computer and also teamed up with NASA to test D-Wave’s system in 2013. Lockheed Martin and the Los Alamos National Laboratory are also working with D-Wave machines. But today’s quantum computers still aren’t practical for most real-world applications. qubits are fragile and can be easily knocked out of the superposition state. Meanwhile, quantum computers are extremely difficult to program today because they require highly specialized knowledge.
“D-Wave is driving the hardware forward,” says D-Wave International president Bo Ewald. “But we need more smart people thinking about applications, and another set thinking about software tools.”
That’s where the company’s new software tool Qbsolv comes in. Qbsolv is designed to help developers program D-Wave machines without needing a background in quantum physics. A few of D-Wave’s partners are already using the tool, but today the company released Qbsolv as open source, meaning anyone will be able to freely share and modify the software.
“Not everyone in the computer science community realizes the potential impact of quantum computing,” says Fred Glover, a mathematician at the University of Colorado, Boulder who has been working with Qbsolv. “Qbsolv offers a tool that can make this impact graphically visible, by getting researchers and practitioners involved in charting the future directions of quantum computing developments.”
Qubits for All
Qbsolv joins a small but growing pool of tools for would-be quantum computer programmers. Last year Scott Pakin of Los Alamos National Laboratory–and one of Qbsolv’s first users–released another free tool called Qmasm, which also eases the burden of writing code for D-Wave machines by freeing developers from having to worry about addressing the underlying hardware. The goal, Ewald says, is to kickstart a quantum computing software tools ecosystem and foster a community of developers working on quantum computing problems. In recent years, open source software has been the best way to build communities of both independent developers and big corporate contributors.
Of course to actually run the software you create with these tools, you’ll need access to one of the very few existing D-Wave machines. In the meantime, you can download a D-Wave simulator that will let you test the software on your own computer. Obviously this won’t be the same as running it on a piece of hardware that uses real quantum particles, but it’s a start.
Last year, IBM launched a cloud-based service that enables people to run their own programs on the company’s quantum computer. But at least for the moment, Qbsolv and Qmasm will only be useful for creating applications for D-Wave’s hardware. D-Wave’s machines take a radically different approach to computing than traditional computers, or even other quantum computing prototypes. While most computers—ranging from your smartphone to IBM’s quantum computer—are general purpose, meaning they can be programmed to solve all sort of problems, D-Wave’s machines are designed for a single purpose: solving optimization problems. The classic example is known as the traveling salesman problem: calculating the shortest route that passes through a list of specific locations.
In the early days, critics wondered whether D-Wave’s expensive machines were even quantum computers at all, but most researchers now seem to agree that the machines do exhibit quantum behavior. “There are very few doubts left that there are indeed quantum effects at work and that they play a meaningful computational role,” University of Southern California researcher Daniel Lidar told us in 2015 after Google and NASA released a research paper detailing some of their work with the D-Wave. The big question now is whether D-Waves are actually any faster than traditional computers, and if its unique approach is better than that taken by IBM and other researchers.
Pakin says his team are believers in D-Wave’s potential, even though they admit its systems might not yet offer performance improvements except in very narrow cases. He also explains that D-Wave’s computers don’t necessarily provide the most efficient answers to an optimization problem—or even a correct one. Instead, the idea is to provide solutions that are probably good, if not perfect solutions, and to do it very quickly. That narrows the D-Wave machines’ usefulness to optimization problems that need to be solved fast but don’t need to be perfect. That could include many artificial intelligence applications.
Ideally, however, the hardware and software will improve to the point that other types of computing problems can be translated into optimization problems, and Qbsolv and Qmasm are steps towards building exactly that. But to get there, they’ll need more than just open source software. They’ll need an open source community.