In the whispering gallery mode of a 2D excitonic laser made from a monolayer of tungsten disulfide and a microdisk resonator, the electric field at the edges of the resonator helps promote a high Q factor with low power consumption.

An important step towards next-generation ultra-compact photonic and optoelectronic devices has been taken with the realization of a two-dimensional excitonic laser. Scientists with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) embedded a monolayer of tungsten disulfide into a special microdisk resonator to achieve bright excitonic lasing at visible light wavelengths.

“Our observation of high-quality excitonic lasing from a single molecular layer of tungsten disulfide marks a major step towards two-dimensional on-chip optoelectronics for high-performance optical communication and computing applications,” says Xiang Zhang, director of Berkeley Lab’s Materials Sciences Division and the leader of this study.

Zhang, who also holds the Ernest S. Kuh Endowed Chair at the University of California (UC) Berkeley and is a member of the Kavli Energy NanoSciences Institute at Berkeley (Kavli ENSI), is the corresponding author of a paper describing this research in the journal Nature Photonics. The paper is titled “Monolayer excitonic laser.” The lead authors are Yu Ye and Zi Jing Wong, members of Zhang’s research group, plus Xiufang Lu, Xingjie Ni, Hanyu Zhu, Xianhui Chen and Yuan Wang.

Among the most talked about class of materials in the world of nanotechnology today are two-dimensional (2D) transition metal dichalcogenides (TMDCs). These 2D semiconductors offer superior energy efficiency and conduct electrons much faster than silicon. Furthermore, unlike graphene, the other highly touted 2D semiconductor, TMDCs have natural bandgaps that allow their electrical conductance to be switched “on and off,” making them more device-ready than graphene. Tungsten disulfide in a single molecular layer is widely regarded as one of the most promising TMDCs for photonic and optoelectronic applications. However, until now, coherent light emission, or lasing, considered essential for “on-chip” applications, had not been realized in this material.

“TMDCs have shown exceptionally strong light-matter interactions that result in extraordinary excitonic properties,” Zhang says. “These properties arise from the quantum confinement and crystal symmetry effect on the electronic band structure as the material is thinned down to a monolayer. However, for 2D lasing, the design and fabrication of microcavities that provide a high optical mode confinement factor and high quality, or Q, factor is required.”

In a previous study, Zhang and his research group had developed a “whispering gallery microcavity” for plasmons, electromagnetic waves that roll across the surfaces of metals. Based on the principle behind whispering galleries – where words spoken softly beneath a domed ceiling can be clearly heard on the opposite side of the chamber – this micro-sized metallic cavity for plasmons strengthened and greatly enhanced the Q factor of light emissions. In this new study, Zhang and his group were able to adapt this microcavity technology from plasmons to excitons – photoexcited electrons/hole pairs within a single layer of molecules.

In this 2D excitonic laser, the sandwiching of a monolayer of tungsten disulfide between the two dielectric layers of a microdisk resonator creates the potential for ultralow-threshold lasing.

In this 2D excitonic laser, the sandwiching of a monolayer of tungsten disulfide between the two dielectric layers of a microdisk resonator creates the potential for ultralow-threshold lasing.

“For our excitonic laser, we dropped the metal coating and designed a microdisk resonator that supports a dielectric whispering gallery mode rather than a plasmonic mode, and gives us a high Q factor with low power consumption,” says co-lead author Ye. “When a monolayer of tungsten disulfide – serving as the gain medium – is sandwiched between the two dielectric layers of the resonator, we create the potential for ultralow-threshold lasing.”

In addition to its photonic and optoelectronic applications, this 2D excitonic laser technology also has potential for valleytronic applications, in which digital information is encoded in the spin and momentum of an electron moving through a crystal lattice as a wave with energy peaks and valleys. Valleytronics is seen as an alternative to spintronics for quantum computing.

“TMDCs such as tungsten disulfide provide unique access to spin and valley degrees of freedom,” says co-lead author Wong. “Selective excitation of the carrier population in one set of two distinct valleys can further lead to lasing in the confined valley, paving the way for easily-tunable circularly polarized lasers. The demand for circularly polarized coherent light sources is high, ranging from three-dimensional displays to effective spin sources in spintronics, and information carriers in quantum computation.”

 

Scientists at the U.S. Department of Energy’s Ames Laboratory have developed molecular modeling simulations and new theoretical formulations to help understand and optimize catalytic reactions that take place in chemical environments where the reactant “ingredients” for catalysis are not well mixed.

In catalysis a chemical reaction is accelerated by adding another component, the catalyst, which remains unchanged during the reaction. In many cases, such as when the catalytic reaction takes place in a solution, the reactants are able to mix together easily (and are sometimes stirred to ensure mixing). This mixing is assumed in standard theories of chemical kinetics that describe how quickly and efficiently catalytic reactions occur.

But not all catalytic reactions occur where ingredients are able to mix. For instance, catalytic reactions to create fuels in mesoporous particles and to remove pollutants in high-pressure surface reactions happen in unmixed systems. And for those cases, the standard analysis of chemical kinetics needed to be refined.

In papers that appeared recently in Chemical Reviews and the Journal of Chemical Physics, Ames Laboratory chemists Jim Evans, Dajiang Liu and their research team focused on modeling reactions in three classes of catalytic environments where chemical components do not easily mix. For two classes, catalysis in narrow pores of mesoporous particles and catalysis on metallic surfaces exposed to high pressure, crowding prevents reactants (and products) from moving enough to mix. The third class is surfaces under low pressure, where interactions between the adsorbed reactants themselves cause these species to organize into ordered domains or “islands” rather to than mix randomly.

 “For all of these scenarios, there are many ‘extra’ steps in the overall catalysis reaction process that must be built into chemical simulation models to reliably describe these systems that are not well stirred,” said Evans. “For example, for catalysis within crowded narrow pores, we simulate the entrance of the reactant into the pores, diffusion within the pore, conversion of the reactants to products within the pore, and diffusion out of the pore, rather than just assuming ‘well mixed’ conditions. All of these steps control the catalytic yield for these unique reaction processes. All of them must be built into the simulation models.”

The result is comprehensive molecular-level simulations that more accurately and realistically describe what is going on in non-stirred catalytic systems.

“The simulations can play the role of ‘numerical experiments,’ meaning if we do our work right, rather than having to do an experiment in a lab, our models can tell us what these types of catalytic reactions will do.” 

Evans’ team also refined existing analytic theories of catalytic reaction rates so that they can be better applied to unmixed systems. The research team included Andres Garcia, graduate research associate and Iowa State University graduate student in the Department of Physics and Astronomy; Jing Wang and Chi-Jen Wang, both graduate research associates and ISU graduate students in the Department of Mathematics; and David Ackerman, graduate research associate and ISU graduate student in the Department of Chemistry.

A team of physicists at the University of Toronto (U of T) have taken a step toward making the essential building block of quantum computers out of pure light. Their advance, described in a paper published this week in Nature Physics, has to do with a specific part of computer circuitry known as a "logic gate." 

Logic gates perform operations on input data to create new outputs. In classical computers, logic gates take the form of diodes or transistors. But quantum computer components are made from individual atoms and subatomic particles. Information processing happens when the particles interact with one another according to the strange laws of quantum physics. 

Light particles - known as "photons" - have many advantages in quantum supercomputing, but it is notoriously difficult to get them to interact with one another in useful ways. This experiment demonstrates how to create such interactions.

"We've seen the effect of a single particle of light on another optical beam," said Canadian Institute for Advanced Research (CIFAR) Senior Fellow Aephraim Steinberg, one of the paper's authors and a researcher at U of T's Centre for Quantum Information & Quantum Computing. "Normally light beams pass through each other with no effect at all. To build technologies like optical quantum computers, you want your beams to talk to one another. That's never been done before using a single photon."

The interaction was a two-step process. The researchers shot a single photon at rubidium atoms that they had cooled to a millionth of a degree above absolute zero. The photons became "entangled" with the atoms, which affected the way the rubidium interacted with a separate optical beam. The photon changes the atoms' refractive index, which caused a tiny but measurable "phase shift" in the beam. 

This process could be used as an all-optical quantum logic gate, allowing for inputs, information-processing and outputs. 

"Quantum logic gates are the most obvious application of this advance," said Steinberg. "But being able to see these interactions is the starting page of an entirely new field of optics. Most of what light does is so well understood that you wouldn't think of it as a field of modern research. But two big exceptions are, "What happens when you deal with light one particle at a time?' and "What happens when there are media like our cold atoms that allow different light beams to interact with each other?'" 

Both questions have been studied, he says, but never together until now.  

Nanodiamonds are added to the surface of a "hyperbolic metamaterial" to enhance the production of single photons, a step toward creating devices aimed at developing quantum computers and communications technologies. Purdue University is announcing a new center dedicated to quantum science and technology, which could bring advances rivaling those from integrated circuits and lasers.

Purdue University during a two-day international workshop beginning Oct. 13 will launch a new center dedicated to quantum science and technology, which could bring advances rivaling those from integrated circuits and lasers. 

The university will announce the Purdue Quantum Center during the International Workshop on Quantum Control of Light and Matter, said Vladimir M. Shalaev, scientific director of nanophotonics at Purdue's Birck Nanotechnology Centerand a distinguished professor of electrical and computer engineering.

"This kickoff workshop will bring together an illustrious group of scientists specializing in diverse areas of quantum physics," he said. "The primary goal is to brainstorm future directions for the field, which will have a profound impact on society in the years to come."

Session chairs and speakers will include senior editors from the journals Science and Nature, researchers from universities including Harvard, MIT, Caltech, UC Berkeley and Columbia, and representatives from federal agencies including the Department of Defense, Air force, Army and Navy.

Speakers will discuss topics in four sessions: Quantum Information and Computing, Quantum Nanophotonics and Metamaterials, Quantum Atomic and Molecular Optics, and Atom-like Solid State Systems.

"Quantum science and technology are likely to bring advances at least as great as those spawned by the integrated circuit and lasers," said Shalaev, who led efforts to create the center along with Chris Greene, Purdue's Albert Overhauser Distinguished Professor of Physics, and Andrew Weiner, the Scifres Family Distinguished Professor of Electrical and Computer Engineering.

"The next technological revolution will be quantum, and we want to be part of that rather than just observing what's going on," said Shalaev, who is co-director of the center with Greene.

unnamedoooooqThe potential applications include advanced quantum computers and quantum Internet technology; compact and ultra-precise sensors for a variety of purposes including medical diagnostics and homeland security; and miniature chip-based devices for positioning and navigation instruments with unprecedented precision.

Researchers from the College of Science will focus on fundamental research, whereas the College of Engineering is more focused on creating devices based on quantum technology.

The center is an extension of a "pre-eminent team" formed at Purdue in 2013 to work on quantum photonics. Pre-eminent teams are chosen because the work they do has the potential for dramatic impact and international pre-eminence. The colleges of Science and Engineering have hired seven new faculty members in the research area since then. 

"One goal of the center is to create a synergistic atmosphere for research in quantum science and technology," Shalaev said.

A group led by Shalaev, Greene and Weiner organized the workshop. A full list of Purdue faculty involved in organizing the workshop is available at http://www.conf.purdue.edu/landing_pages/pcqworkshop/committee.aspx

In quantum photonics, technologies could make possible devices that are able to harness single particles called photons, dramatically increasing the performance of computers, sensors and other devices. Conventional computers use electrons to process information. However, the performance might be ramped up considerably by employing the unique quantum properties of electrons and photons.

Quantum computers would take advantage of a phenomenon described by quantum theory called "superposition" or "entanglement." Instead of only the states of one and zero that exist in conventional computers, there are many possible "superposition quantum states." Computers based on quantum physics would have quantum bits, or "qubits," increasing the computer's capacity to process, store and transmit information.

"The challenge is how to keep this very fragile quantum superposition entangled for as long as possible," Shalaev said.

One potential solution is to use lasers to cool atoms nearly to absolute zero in a field known as atomic and molecular optics, or AMO. Another approach is "atom-like solid state systems" harnessing new "metamaterials." Metamaterials are made of engineered structures that contain features, patterns or elements, such as tiny antennas or alternating ultrathin layers of different materials that enable unprecedented control of light. Constructed of artificial atoms and molecules, the optical metamaterials owe their unusual potential to precision engineering on the scale of nanometers.

"Metamaterials could help us control this quantum superposition," Shalaev said.

Quantum technology also could be used to perfect "spintronics." Conventional computers use the presence and absence of an electric charge to represent ones and zeroes in a binary code needed to carry out computations. Spintronics, however, uses the "spin state" of electrons to represent ones and zeros and could bring circuits that resemble biological neurons and synapses to perform tasks such as facial recognition.

"One big challenge for spintronics right now is speed," Shalaev said. "It's too slow. However, we may be able to solve this problem by combining quantum nanophotonics with spintronics to speed it up dramatically."

CAPTION QUT's Dr. Stebila, along with researchers Joppe Bos from chip maker NXP Semiconductors and Craig Costello and Michael Naehrig from Microsoft Research, have developed upgrades to the Internet's core encryption protocol that will prevent quantum computer users from intercepting Internet communications. CREDIT Erika Fish

For the powerful quantum computers that will be developed in the future, cracking online bank account details and credit cards number will be a cinch

But a team of cryptographers which includes QUT's Dr Douglas Stebila is already working at future-proofing the privacy of today's Internet communications from tomorrow's powerful supercomputers.

Dr Stebila, along with researchers Joppe Bos from chip maker NXP Semiconductors and Craig Costello and Michael Naehrig from Microsoft Research, have developed upgrades to the Internet's core encryption protocol that will prevent quantum supercomputer users from intercepting Internet communications.

"Governments and the computing industry are working with scientists to try to build quantum computers. It's a very significant scientific challenge, but quantum computers could be reality in a few decades," Dr Stebila said.

"Quantum computers will be able to solve complex scientific problems, like simulating chemical reactions, much faster than today's most powerful supercomputers, but they'll also be able to break much of the public key cryptography that's used to protect Internet, mobile telephone, and other electronic communication."

"Though quantum computers don't exist yet, they could be used to retroactively decrypt past transmissions," Dr Stebila explained.

"That's why it's important that we start updating our communication infrastructure. We've tested some new techniques and found some very promising first steps towards future-proofing Internet encryption."

Dr Stebila said that Internet communication was currently protected by encryption using the Transport Layer Security (TLS) standard, which ensures that web browsers can't be tricked into sending data to the wrong web server, and that eavesdroppers can't intercept passwords or other personal information.

"The TLS Internet encryption protocol uses a variety of mathematical techniques to protect information, some of which would need to be updated to be resistant to quantum computers.

"We've developed a new quantum-proof version of TLS that incorporates a mathematical technique called the 'ring learning with errors problem', a fairly recent technique that mathematicians think has the potential to resist quantum attacks.

"We've tested our new protocol to encrypt data moving between two PCs -- the new techniques are a little slower than existing ones, but the confidentiality of the data is improved.

"The speed of the new protocol is now something we will work on, but this is a big step forward, demonstrating the practicality of these new techniques. We're optimistic this will provide a framework for developing effective ways of future-proofing our data in the world of quantum computers."

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