TL;DR

• Nuclear physicists have found a new way to use the Relativistic Heavy Ion Collider (RHIC) to see the shape and details inside atomic nuclei. The method relies on particles of light that surround gold ions as they speed around the collider and a new type of quantum entanglement that’s never been seen before.
• Researchers have demonstrated an architecture that can enable high fidelity and scalable communication between superconducting quantum processors. Their technique can generate and route photons, which carry quantum information, in a user-specified direction. This method could be used to develop a large-scale network of quantum processors that could efficiently communicate with one another.
• Researchers have created visible lasers of very pure colors from near-ultraviolet to near-infrared that fit on a fingertip. The colors of the lasers can be precisely tuned and extremely fast — up to 267 petahertz per second, which is critical for applications such as quantum optics. The team is the first to demonstrate chip-scale narrow-linewidth and tunable lasers for colors of light below red — green, cyan, blue, and violet.
• Researchers have found a way to create much stronger interactions between photons and electrons, in the process producing a hundredfold increase in the emission of light from a phenomenon called Smith-Purcell radiation. The finding has potential implications for both commercial applications and fundamental scientific research.
• Much of modern electronic and computing technology is based on one idea: add chemical impurities, or defects, to semiconductors to change their ability to conduct electricity. These altered materials are then combined in different ways to produce the devices that form the basis for digital computing, transistors, and diodes. Indeed, some quantum information technologies are based on a similar principle: adding defects and specific atoms within materials can produce qubits, the fundamental information storage units of quantum computing.
• Researchers developed a new graphene-based nanoelectronics platform compatible with conventional microelectronics manufacturing, paving the way for a successor to silicon.
• In a recent experimental breakthrough researchers demonstrated the ability to control the quantum states of individual molecules with an electrically controllable substrate. Their experiment showed how a specific two-dimensional material, known as SnTe, provides the instrumental strategy needed to control molecular states.
• A team of scientists has reviewed the basic concepts and optical responses of Weyl semimetals.
• A research team experimentally observed the phase transitions between triply degenerate points (TDPs) with different topological charges through highly controllable quantum simulations.
• Researchers have connected, on a single microchip, quantum dots — artificial atoms that generate individual photons rapidly and on-demand when illuminated by a laser — with miniature circuits that can guide the light without significant loss of intensity.
• And more!

Quantum Computing Market

According to the recent market research report ‘Quantum Computing Market with COVID-19 impact by Offering (Systems and Services), Deployment (On Premises and Cloud Based), Application, Technology, End-use Industry and Region — Global Forecast to 2026’, published by MarketsandMarkets, the Quantum Computing market is expected to grow from USD 472 million in 2021 to USD 1,765 million by 2026, at a CAGR of 30.2%. The early adoption of quantum computing in the banking and finance sector is expected to fuel the growth of the market globally. Other key factors contributing to the growth of the quantum computing market include rising investments by governments of different countries to carry out research and development activities related to quantum computing technology. Several companies are focusing on the adoption of QCaaS post-COVID-19. This, in turn, is expected to contribute to the growth of the quantum computing market. However, stability and error correction issues are expected to restrain the growth of the market.

According to ‘Quantum Computing Market Research Report: By Offering, Deployment Type, Application, Technology, Industry — Industry Share, Growth, Drivers, Trends and Demand Forecast to 2030’ report, the quantum computing market is projected to reach $64,988 million by 2030. Machine learning (ML) is expected to progress at the highest CAGR, during the forecast period, among all application categories, owing to the fact that quantum computing is being integrated in ML for improving the latter’s use case.

Latest Research

Tomography of ultrarelativistic nuclei with polarized photon-gluon collisions

by STAR Collaboration in Science Advances

Nuclear physicists have found a new way to use the Relativistic Heavy Ion Collider (RHIC) — a particle collider at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory — to see the shape and details inside atomic nuclei. The method relies on particles of light that surround gold ions as they speed around the collider and a new type of quantum entanglement that’s never been seen before.

Through a series of quantum fluctuations, the particles of light (a.k.a. photons) interact with gluons — gluelike particles that hold quarks together within the protons and neutrons of nuclei. Those interactions produce an intermediate particle that quickly decays into two differently charged “pions” (π). By measuring the velocity and angles at which these π+ and π- particles strike RHIC’s STAR detector, the scientists can backtrack to get crucial information about the photon — and use that to map out the arrangement of gluons within the nucleus with higher precision than ever before.

“This technique is similar to the way doctors use positron emission tomography (PET scans) to see what’s happening inside the brain and other body parts,” said former Brookhaven Lab physicist James Daniel Brandenburg, a member of the STAR collaboration who joined The Ohio State University as an assistant professor in January 2023. “But in this case, we’re talking about mapping out features on the scale of femtometers — quadrillionths of a meter — the size of an individual proton.”

Illustration of the processes used to study polarized photon-gluon collisions.

Even more amazing, the STAR physicists say, is the observation of an entirely new kind of quantum interference that makes their measurements possible.

“We measure two outgoing particles and clearly their charges are different — they are different particles — but we see interference patterns that indicate these particles are entangled, or in sync with one another, even though they are distinguishable particles,” said Brookhaven physicist and STAR collaborator Zhangbu Xu.

That discovery may have applications well beyond the lofty goal of mapping out the building blocks of matter. For example, many scientists, including those awarded the 2022 Nobel Prize in Physics, are seeking to harness entanglement — a kind of “awareness” and interaction of physically separated particles. One goal is to create significantly more powerful communication tools and computers than exist today. But most other observations of entanglement to date, including a recent demonstration of interference of lasers with different wavelengths, have been between photons or identical electrons.

“This is the first-ever experimental observation of entanglement between dissimilar particles,” Brandenburg said.

2D momentum distribution of ρ0.

RHIC operates as a DOE Office of Science user facility where physicists can study the innermost building blocks of nuclear matter — the quarks and gluons that make up protons and neutrons. They do this by smashing together the nuclei of heavy atoms such as gold traveling in opposite directions around the collider at close to the speed of light. The intensity of these collisions between nuclei (also called ions) can “melt” the boundaries between individual protons and neutrons so scientists can study the quarks and gluons as they existed in the very early universe — before protons and neutrons formed. But nuclear physicists also want to know how quarks and gluons behave within atomic nuclei as they exist today — to better understand the force that holds these building blocks together.

A recent discovery using “clouds” of photons that surround RHIC’s speeding ions suggests a way to use these particles of light to get a glimpse inside the nuclei. If two gold ions pass one another very closely without colliding, the photons surrounding one ion can probe the internal structure of the other.

“In that earlier work, we demonstrated that those photons are polarized, with their electric field radiating outward from the center of the ion. And now we use that tool, the polarized light, to effectively image the nuclei at high energy,” Xu said.

The quantum interference observed between the π+ and π- in the newly analyzed data makes it possible to measure the photons’ polarization direction very precisely. That in turn lets physicists look at the gluon distribution both along the direction of the photon’s motion and perpendicular to it. That two-dimensional imaging turns out to be very important.

“All past measurements, where we didn’t know the polarization direction, measured the density of gluons as an average — as a function of the distance from the center of the nucleus,” Brandenburg said. “That’s a one-dimensional image.”

Those measurements all came out making the nucleus look too big when compared with what was predicted by theoretical models and measurements of the distribution of charge in the nucleus.

“With this 2D imaging technique, we were able to solve the 20-year mystery of why this happens,” Brandenburg said.

The house-size STAR detector at the Relativistic Heavy Ion Collider (RHIC) acts like a giant 3D digital camera to track particles emerging from particle collisions at the center of the detector.

The new measurements show that the momentum and energy of the photons themselves gets convoluted with that of the gluons. Measuring just along the photon’s direction (or not knowing what that direction is) results in a picture distorted by these photon effects. But measuring in the transverse direction avoids the photon blurring.

“Now we can take a picture where we can really distinguish the density of gluons at a given angle and radius,” Brandenburg said. “The images are so precise that we can even start to see the difference between where the protons are and where the neutrons are laid out inside these big nuclei.”

The new pictures match up qualitatively with the theoretical predictions using gluon distribution, as well as the measurements of electric charge distribution within the nuclei, the scientists say.

Dependence of the apparent nuclear shape on the polarization angle ϕ.

To understand how the physicists make these 2D measurements, let’s step back to the particle generated by the photon-gluon interaction. It’s called a rho, and it decays very quickly — in less than four septillionths of a second — into the π+ and π-. The sum of the momenta of those two pions gives physicists the momentum of the parent rho particle — and information that includes the gluon distribution and the photon blurring effect.

To extract just the gluon distribution, the scientists measure the angle between the path of either the π+ or π- and the rho’s trajectory. The closer that angle is to 90 degrees, the less blurring you get from the photon probe. By tracking pions that come from rho particles moving at a range of angles and energies, the scientists can map out the gluon distribution across the entire nucleus.

Now for the quantum quirkiness that makes the measurements possible — the evidence that the π+ and π- particles striking the STAR detector result from interference patterns produced by the entanglement of these two dissimilar oppositely charged particles. Keep in mind that all the particles we are talking about exist not just as physical objects but also as waves. Like ripples on the surface of a pond radiating outward when they strike a rock, the mathematical “wavefunctions” that describe the crests and troughs of particle waves can interfere to reinforce or cancel one another out.

When the photons surrounding two near-miss speeding ions interact with gluons inside the nuclei, it’s as if those interactions actually generate two rho particles, one in each nucleus. As each rho decays into a π+ and π-, the wavefunction of the negative pion from one rho decay interferes with the wavefunction of the negative pion from the other. When the reinforced wavefunction strikes the STAR detector, the detector sees one π-. The same thing happens with the wavefunctions of the two positively charged pions, and the detector sees one π+.

“The interference is between two wavefunctions of the identical particles, but without the entanglement between the two dissimilar particles — the π+ and π- — this interference would not materialize,” said Wangmei Zha, a STAR collaborator at the University of Science and Technology of China, and one of the original proponents of this explanation. “This is the weirdness of quantum mechanics!”

Could the rhos simply be entangled? The scientists say no. The rho particle wavefunctions originate at a distance 20 times the distance they could travel within their short lifetime, so they cannot interact with each other before they decay to π+ and π-. But the wavefunctions of the π+ and π- from each rho decay retain the quantum information of their parent particles; their crests and troughs are in phase, “aware of each other,” despite striking the detector meters apart.

“If the π+ and π- were not entangled, the two π+ (or π-) wavefunctions would have a random phase, without any detectable interference effect,” said Chi Yang, a STAR collaborator from Shandong University in China, who also helped lead the analysis for this result. “We wouldn’t see any orientation related to the photon polarization — or be able to make these precision measurements.”

Future measurements at RHIC with heavier particles and different lifetimes — and at an Electron-Ion Collider (EIC) being built at Brookhaven — will probe more detailed distributions of gluons inside nuclei and test other possible quantum interference scenarios.

Widely tunable and narrow-linewidth chip-scale lasers from near-ultraviolet to near-infrared wavelengths

by Mateus Corato-Zanarella, Andres Gil-Molina, Xingchen Ji, Min Chul Shin, Aseema Mohanty, Michal Lipson in Nature Photonics

As technologies keep advancing at exponential rates and demand for new devices rises accordingly, miniaturizing systems into chips has become increasingly important. Microelectronics has changed the way we manipulate electricity, enabling sophisticated electronic products that are now an essential part of our daily lives. Similarly, integrated photonics has been revolutionizing the way we control light for applications such as data communications, imaging, sensing, and biomedical devices. By routing and shaping light using micro- and nanoscale components, integrated photonics shrinks full optical systems into the size of tiny chips.

Despite its success, integrated photonics has been missing a key component to achieve complete miniaturization: high-performance chip-scale lasers. While some progress has been done on near-infrared lasers, the visible-light lasers that currently feed photonic chips are still benchtop and expensive. Since visible light is essential for a wide range of applications including quantum optics, displays, and bioimaging, there is a need for tunable and narrow-linewidth chip-scale lasers emitting light of different colors.

Researchers at Columbia Engineering’s Lipson Nanophotonics Group have created visible lasers of very pure colors from near-ultraviolet to near-infrared that fit on a fingertip. The colors of the lasers can be precisely tuned and extremely fast — up to 267 petahertz per second, which is critical for applications such as quantum optics. The team is the first to demonstrate chip-scale narrow-linewidth and tunable lasers for colors of light below red — green, cyan, blue, and violet. These inexpensive lasers also have the smallest footprint and shortest wavelength (404 nm) of any tunable and narrow-linewidth integrated laser emitting visible light.

“What’s exciting about this work is that we’ve used the power of integrated photonics to break the existing paradigm that high-performance visible lasers need to be benchtop and cost tens of thousands of dollars,” says the study’s lead author Mateus Corato Zanarella, a PhD student who works with Michal Lipson, Higgins Professor of Electrical Engineering and professor of applied physics. “Until now, it’s been impossible to shrink and mass-deploy technologies that require tunable and narrow-linewidth visible lasers. A notable example is quantum optics, which demands high-performance lasers of several colors in a single system. We expect that our findings will enable fully integrated visible light systems for existing and new technologies.”

Illustration of the integrated laser platform created by the Lipson Nanophotonics Group, where a single chip generates narrow linewidth and tunable visible light covering all colors. Credit: Myles Marshall/Columbia Engineering

The importance of lasers emitting wavelengths shorter than red is clear when you consider some important applications. Displays, for example, require red, green, and blue light simultaneously to compose any color. In quantum optics, green, blue, and violet lasers are used for trapping and cooling atoms and ions. In underwater Lidar (Light Detection and Ranging), green or blue light is needed to avoid water absorption. However, at wavelengths shorter than red, the coupling and propagation losses of photonic integrated circuits increase significantly, which has prevented the realization of high-performance lasers at these colors.

The researchers solved the coupling loss problem by choosing Fabry-Perot (FP) diodes as the light sources, which minimizes the impact of the losses on the performance of the chip-scale lasers. Unlike other strategies that use different types of sources, the team’s approach enables the realization of lasers at record-short wavelengths (404 nm) while also providing scalability to high optical powers, FP laser diodes are inexpensive and compact solid-state lasers widely used in research and industry. However, they emit light of several wavelengths simultaneously and are not easily tunable, preventing them to be directly used for applications requiring pure and precise lasers. By combining them with the specially designed photonic chip, the researchers are able to modify the laser emission to be single-frequency, narrow-linewidth, and widely tunable.

The team overcame the propagation loss issue by designing a platform that minimizes both the material absorption and surface scattering losses simultaneously for all the visible wavelengths. To guide the light, they used silicon nitride, a dielectric widely used in the semiconductor industry that is transparent for visible light of all colors. Even though there is minimal absorption, the light still experiences loss due to unavoidable roughness from the fabrication processes. The team solved this problem by designing a photonic circuit with a special type of ring resonator. The ring has a variable width along its circumference, allowing for single-mode operation characteristic of narrow waveguides, and low loss characteristic of wide waveguides. The resulting photonic circuit provides a wavelength-selective optical feedback to the FP diodes that forces the laser to emit at a single desired wavelength with very narrow linewidth.

“By combining these intricately designed pieces, we were able to build a robust and versatile platform that is scalable and works for all colors of light,” said Corato Zanarella.

“As a laser manufacturer we recognize that integrated photonics will have a tremendous impact on our industry and will enable a new generation of applications that have so far been impossible,” said Chris Haimberger, Director of Laser Technology, TOPTICA Photonics, Inc. “This work represents an important step forward in the pursuit of compact and tunable visible lasers that will power future developments in computing, medicine, and industry.”

Fine frequency tuning speeds via microheater and laser-current modulations.

The study’s findings could revolutionize a broad range of applications, including:

• Quantum information. Most quantum bits for quantum computation use atoms or ions that are trapped and probed using visible light. The light must be very pure (narrow linewidth) and have very specific wavelengths to address atomic transitions. Currently, the lasers available for these applications are expensive and benchtop. This new study shows that these bulky sources can be replaced by tiny and inexpensive chips, which will enable quantum systems to be scaled down and eventually become part of technologies accessible by the general public.
• Atomic Clocks. The most precise clocks are based on strontium atoms, which need to be trapped and probed by lasers of many different colors at the same time. Similarly to quantum optics systems, the massive size of the currently available lasers confines this technology to research labs. The chip-scale lasers will make it possible to shrink these systems with the goal of making portable atomic clocks.
• Biosensing. Several neural probes use a technology called optogenetics to measure, modify, and understand the neural response. In this technology, neurons are genetically modified to produce a type of protein called opsin that is sensitive to visible light. By shining visible light, typically blue, into these cells, scientists can turn on specific neurons at will. Similarly, in fluorescent imagining, fluorophores need to be excited with visible light in order to generate the desired images. These high-performance, compact lasers open the doors for miniaturizing these systems.
• Underwater ranging. Underwater ranging requires blue or green light because ocean water strongly absorbs light of all the other colors. In addition, for the popular ranging strategy called Frequency-Modulated Continuous Wave LiDAR, the laser needs to be speedily tunable for accurate sensing of the distance and velocity of objects. These lasers could be used for portable underwater ranging systems employing this technology.
• Li-Fi. As the demand for bandwidth in communication systems increases, networks have become saturated. Li-Fi, or visible light communications, is a rapidly growing technology that promises to supplement the traditional microwave links at the user end to overcome this bottleneck. The high modulation speeds of the lasers are ideal for enabling extremely fast optical wireless communication links.

The researchers, who have filed a provisional patent for their technology, are now exploring how to optically and electrically package the lasers to turn them into standalone units and use them as sources in chip-scale visible light engines, quantum experiments, and optical clocks.

“In order to move forward, we have to be able to miniaturize and scale these systems, enabling them to eventually be incorporated in mass-deployed technologies,” said Lipson, a pioneer in silicon photonics whose research has strongly shaped the field from its inception decades ago, with foundational contributions in the active and passive devices that are part of any current photonic chip. She added, “Integrated photonics is an exciting field that is truly revolutionizing our world, from optical telecommunications to quantum information to biosensing.”

On-demand directional microwave photon emission using waveguide quantum electrodynamics

by Bharath Kannan, Aziza Almanakly, Youngkyu Sung, et al in Nature Physics

Quantum computers hold the promise of performing certain tasks that are intractable even on the world’s most powerful supercomputers. In the future, scientists anticipate using quantum computing to emulate materials systems, simulate quantum chemistry, and optimize hard tasks, with impacts potentially spanning finance to pharmaceuticals.

However, realizing this promise requires resilient and extensible hardware. One challenge in building a large-scale quantum computer is that researchers must find an effective way to interconnect quantum information nodes — smaller-scale processing nodes separated across a computer chip. Because quantum computers are fundamentally different from classical computers, conventional techniques used to communicate electronic information do not directly translate to quantum devices. However, one requirement is certain: Whether via a classical or a quantum interconnect, the carried information must be transmitted and received.

To this end, MIT researchers have developed a quantum computing architecture that will enable extensible, high-fidelity communication between superconducting quantum processors. In work, MIT researchers demonstrate step one, the deterministic emission of single photons — information carriers — in a user-specified direction. Their method ensures quantum information flows in the correct direction more than 96 percent of the time. Linking several of these modules enables a larger network of quantum processors that are interconnected with one another, no matter their physical separation on a computer chip.

“Quantum interconnects are a crucial step toward modular implementations of larger-scale machines built from smaller individual components,” says Bharath Kannan PhD ’22, co-lead author of a research paper describing this technique.

“The ability to communicate between smaller subsystems will enable a modular architecture for quantum processors, and this may be a simpler way of scaling to larger system sizes compared to the brute-force approach of using a single large and complicated chip,” Kannan adds.

Kannan wrote the paper with co-lead author Aziza Almanakly, an electrical engineering and computer science graduate student in the Engineering Quantum Systems group of the Research Laboratory of Electronics (RLE) at MIT. The senior author is William D. Oliver, a professor of electrical engineering and computer science and of physics, an MIT Lincoln Laboratory Fellow, director of the Center for Quantum Engineering, and associate director of RLE.

This image shows a module composed of superconducting qubits that can be used to directionally emit microwave photons. Credits: Image: Krantz NanoArt

In a conventional classical computer, various components perform different functions, such as memory, computation, etc. Electronic information, encoded and stored as bits (which take the value of 1s or 0s), is shuttled between these components using interconnects, which are wires that move electrons around on a computer processor. But quantum information is more complex. Instead of only holding a value of 0 or 1, quantum information can also be both 0 and 1 simultaneously (a phenomenon known as superposition). Also, quantum information can be carried by particles of light, called photons. These added complexities make quantum information fragile, and it can’t be transported simply using conventional protocols.

A quantum network links processing nodes using photons that travel through special interconnects known as waveguides. A waveguide can either be unidirectional, and move a photon only to the left or to the right, or it can be bidirectional. Most existing architectures use unidirectional waveguides, which are easier to implement since the direction in which photons travel is easily established. But since each waveguide only moves photons in one direction, more waveguides become necessary as the quantum network expands, which makes this approach difficult to scale. In addition, unidirectional waveguides usually incorporate additional components to enforce the directionality, which introduces communication errors.

“We can get rid of these lossy components if we have a waveguide that can support propagation in both the left and right directions, and a means to choose the direction at will. This ‘directional transmission’ is what we demonstrated, and it is the first step toward bidirectional communication with much higher fidelities,” says Kannan.

Using their architecture, multiple processing modules can be strung along one waveguide. A remarkable feature the architecture design is that the same module can be used as both a transmitter and a receiver, he says. And photons can be sent and captured by any two modules along a common waveguide.

“We have just one physical connection that can have any number of modules along the way. This is what makes it scalable. Having demonstrated directional photon emission from one module, we are now working on capturing that photon downstream at a second module,” Almanakly adds.

To accomplish this, the researchers built a module comprising four qubits. Qubits are the building blocks of quantum computers, and are used to store and process quantum information. But qubits can also be used as photon emitters. Adding energy to a qubit causes the qubit to become excited, and then when it de-excites, the qubit will emit the energy in the form of a photon. However, simply connecting one qubit to a waveguide does not ensure directionality. A single qubit emits a photon, but whether it travels to the left or to the right is completely random. To circumvent this problem, the researchers utilize two qubits and a property known as quantum interference to ensure the emitted photon travels in the correct direction.

The technique involves preparing the two qubits in an entangled state of single excitation called a Bell state. This quantum-mechanical state comprises two aspects: the left qubit being excited and the right qubit being excited. Both aspects exist simultaneously, but which qubit is excited at a given time is unknown.

When the qubits are in this entangled Bell state, the photon is effectively emitted to the waveguide at the two qubit locations simultaneously, and these two “emission paths” interfere with each other. Depending on the relative phase within the Bell state, the resulting photon emission must travel to the left or to the right. By preparing the Bell state with the correct phase, the researchers choose the direction in which the photon travels through the waveguide. They can use this same technique, but in reverse, to receive the photon at another module.

“The photon has a certain frequency, a certain energy, and you can prepare a module to receive it by tuning it to the same frequency. If they are not at the same frequency, then the photon will just pass by. It’s analogous to tuning a radio to a particular station. If we choose the right radio frequency, we’ll pick up the music transmitted at that frequency,” Almanakly says.

The researchers found that their technique achieved more than 96 percent fidelity — this means that if they intended to emit a photon to the right, 96 percent of the time it went to the right. Now that they have used this technique to effectively emit photons in a specific direction, the researchers want to connect multiple modules and use the process to emit and absorb photons. This would be a major step toward the development of a modular architecture that combines many smaller-scale processors into one larger-scale, and more powerful, quantum processor.

Photonic flatband resonances for free-electron radiation

by Yi Yang, Charles Roques-Carmes, Steven E. Kooi, Haoning Tang, Justin Beroz, Eric Mazur, Ido Kaminer, John D. Joannopoulos, Marin Soljačić in Nature

The way electrons interact with photons of light is a key part of many modern technologies, from lasers to solar panels to LEDs. But the interaction is inherently a weak one because of a major mismatch in scale: A wavelength of visible light is about 1,000 times larger than an electron, so the way the two things affect each other is limited by that disparity.

Now, researchers at MIT and elsewhere have come up with an innovative way to make much stronger interactions between photons and electrons possible, in the process producing a hundredfold increase in the emission of light from a phenomenon called Smith-Purcell radiation. The finding has potential implications for both commercial applications and fundamental scientific research, although it will require more years of research to make it practical. The findings are reported in a paper by MIT postdocs Yi Yang (now an assistant professor at the University of Hong Kong) and Charles Roques-Carmes, MIT professors Marin Soljačić and John Joannopoulos, and five others at MIT, Harvard University, and Technion-Israel Institute of Technology.

In a combination of computer simulations and laboratory experiments, the team found that using a beam of electrons in combination with a specially designed photonic crystal — a slab of silicon on an insulator, etched with an array of nanometer-scale holes — they could theoretically predict stronger emission by many orders of magnitude than would ordinarily be possible in conventional Smith-Purcell radiation. They also experimentally recorded a one hundredfold increase in radiation in their proof-of-concept measurements.

Unlike other approaches to producing sources of light or other electromagnetic radiation, the free-electron-based method is fully tunable — it can produce emissions of any desired wavelength, simply by adjusting the size of the photonic structure and the speed of the electrons. This may make it especially valuable for making sources of emission at wavelengths that are difficult to produce efficiently, including terahertz waves, ultraviolet light, and X-rays.

Measurement of radiation from flatbands.

The team has so far demonstrated the hundredfold enhancement in emission using a repurposed electron microscope to function as an electron beam source. But they say that the basic principle involved could potentially enable far greater enhancements using devices specifically adapted for this function.

The approach is based on a concept called flatbands, which have been widely explored in recent years for condensed matter physics and photonics but have never been applied to affecting the basic interaction of photons and free electrons. The underlying principle involves the transfer of momentum from the electron to a group of photons, or vice versa. Whereas conventional light-electron interactions rely on producing light at a single angle, the photonic crystal is tuned in such a way that it enables the production of a whole range of angles.

The same process could also be used in the opposite direction, using resonant light waves to propel electrons, increasing their velocity in a way that could potentially be harnessed to build miniaturized particle accelerators on a chip. These might ultimately be able to perform some functions that currently require giant underground tunnels, such as the 30-kilometer-wide Large Hadron Collider in Switzerland.

“If you could actually build electron accelerators on a chip,” Soljačić says, “you could make much more compact accelerators for some of the applications of interest, which would still produce very energetic electrons. That obviously would be huge. For many applications, you wouldn’t have to build these huge facilities.”

The new system could also potentially provide a highly controllable X-ray beam for radiotherapy purposes, Roques-Carmes says. And the system could be used to generate multiple entangled photons, a quantum effect that could be useful in the creation of quantum-based computational and communications systems, the researchers say.

“You can use electrons to couple many photons together, which is a considerably hard problem if using a purely optical approach,” says Yang. “That is one of the most exciting future directions of our work.”

Much work remains to translate these new findings into practical devices, Soljačić cautions. It may take some years to develop the necessary interfaces between the optical and electronic components and how to connect them on a single chip, and to develop the necessary on-chip electron source producing a continuous wavefront, among other challenges.

“The reason this is exciting,” Roques-Carmes adds, “is because this is quite a different type of source.” While most technologies for generating light are restricted to very specific ranges of color or wavelength, and “it’s usually difficult to move that emission frequency. Here it’s completely tunable. Simply by changing the velocity of the electrons, you can change the emission frequency. … That excites us about the potential of these sources. Because they’re different, they offer new types of opportunities.”

But, Soljačić concludes, “in order for them to become truly competitive with other types of sources, I think it will require some more years of research. I would say that with some serious effort, in two to five years they might start competing in at least some areas of radiation.”

Self-Induced Dirac Boundary State and Digitization in a Nonlinear Resonator Chain

by Gengming Liu, Jiho Noh, Jianing Zhao, Gaurav Bahl in Physical Review Letters

Much of modern electronic and computing technology is based on one idea: add chemical impurities, or defects, to semiconductors to change their ability to conduct electricity. These altered materials are then combined in different ways to produce the devices that form the basis for digital computing, transistors, and diodes. Indeed, some quantum information technologies are based on a similar principle: adding defects and specific atoms within materials can produce qubits, the fundamental information storage units of quantum computing.

Gaurav Bahl, professor of mechanical science and engineering at the University of Illinois Urbana-Champaign and member of the Illinois Quantum Information Sciences and Technology Center, is exploring how special non-linear properties in engineered materials can achieve similar functionalities without the need to add intentional defects. As his research group reports, a metamaterial can change its functionality on its own depending on the power level of the input.

A metamaterial is an artificial system that replicates the behavior of real materials made of natural atoms. The researchers constructed a whose behavior is analogous to a special kind of semiconductor called a Dirac material. It consisted of a chain of magnetic-mechanical resonators, where the magnetic interactions acted like bonds between atoms in a one-dimensional crystal. When any of these “atoms” was mechanically excited, that is, was made to move periodically, the excitation spread to the rest of the crystal, just like electrons injected into a semiconductor.

Engineering Dirac boundary mode in linear and nonlinear diatomic arrays.

After demonstrating that a completely uniform Dirac metamaterial does not allow mechanical excitations to pass through (just like electrons are forbidden from flowing through insulating semiconductor), the researchers introduced a specific set of nonlinearities into the system. This new property added sensitivity to the level of the mechanical excitation and could subtly change the resonance energy of the magneto-mechanical atoms. With the right choice of nonlinearity, the researchers observed a sharp transition from insulating to conducting behavior depending on how strong an input was provided.

This intriguing behavior resulted from the spontaneous appearance of a new boundary where the effective mass of the mechanical excitation, an invisible internal property of Dirac materials, underwent a change of sign depending on the level of the excitation. The researchers were surprised to find that this boundary was accompanied by a new state that “popped in” at the boundary and allowed input energy to transmit through the material. This effect was very similar to how a defect atom acts within a semiconductor

“In photonics and electronics,” Bahl said, “nonlinear properties like this could be engineered to form the foundation of new computational systems that don’t rely on the conventional semiconductor approach.”

An epitaxial graphene platform for zero-energy edge state nanoelectronics

by Vladimir S. Prudkovskiy, Yiran Hu, Kaimin Zhang, et al in Nature Communications

A pressing quest in the field of nanoelectronics is the search for a material that could replace silicon. Graphene has seemed promising for decades. But its potential faltered along the way, due to damaging processing methods and the lack of a new electronics paradigm to embrace it. With silicon nearly maxed out in its ability to accommodate faster computing, the next big nanoelectronics platform is needed now more than ever.

Walter de Heer, Regents’ Professor in the School of Physics at the Georgia Institute of Technology, has taken a critical step forward in making the case for a successor to silicon. De Heer and his collaborators developed a new nanoelectronics platform based on graphene — a single sheet of carbon atoms. The technology is compatible with conventional microelectronics manufacturing, a necessity for any viable alternative to silicon. In the course of their research, the team may have also discovered a new quasiparticle. Their discovery could lead to manufacturing smaller, faster, more efficient, and more sustainable computer chips, and has potential implications for quantum and high-performance computing.

The epigraphene edge state.

“Graphene’s power lies in its flat, two-dimensional structure that is held together by the strongest chemical bonds known,” de Heer said. “It was clear from the beginning that graphene can be miniaturized to a far greater extent than silicon — enabling much smaller devices, while operating at higher speeds and producing much less heat. This means that, in principle, more devices can be packed on a single chip of graphene than with silicon.”

In 2001, de Heer proposed an alternative form of electronics based on epitaxial graphene, or epigraphene — a layer of graphene that was found to spontaneously form on top of silicon carbide crystal, a semiconductor used in high power electronics. At the time, researchers found that electric currents flow without resistance along epigraphene’s edges, and that graphene devices could be seamlessly interconnected without metal wires. This combination allows for a form of electronics that relies on the unique light-like properties of graphene electrons.

“Quantum interference has been observed in carbon nanotubes at low temperatures, and we expect to see similar effects in epigraphene ribbons and networks,” de Heer said. “This important feature of graphene is not possible with silicon.”

To create the new nanoelectronics platform, the researchers created a modified form of epigraphene on a silicon carbide crystal substrate. In collaboration with researchers at the Tianjin International Center for Nanoparticles and Nanosystems at the University of Tianjin, China, they produced unique silicon carbide chips from electronics-grade silicon carbide crystals. The graphene itself was grown at de Heer’s laboratory at Georgia Tech using patented furnaces.

The researchers used electron beam lithography, a method commonly used in microelectronics, to carve the graphene nanostructures and weld their edges to the silicon carbide chips. This process mechanically stabilizes and seals the graphene’s edges, which would otherwise react with oxygen and other gases that might interfere with the motion of the charges along the edge. Finally, to measure the electronic properties of their graphene platform, the team used a cryogenic apparatus that allows them to record its properties from a near-zero temperature to room temperature.

Neutral epigraphene characterization.

The electric charges the team observed in the graphene edge state were similar to photons in an optical fiber that can travel over large distances without scattering. They found that the charges traveled for tens of thousands of nanometers along the edge before scattering. Graphene electrons in previous technologies could only travel about 10 nanometers before bumping into small imperfections and scattering in different directions.

“What’s special about the electric charges in the edges is that they stay on the edge and keep on going at the same speed, even if the edges are not perfectly straight,” said Claire Berger, physics professor at Georgia Tech and director of research at the French National Center for Scientific Research in Grenoble, France.

In metals, electric currents are carried by negatively charged electrons. But contrary to the researchers’ expectations, their measurements suggested that the edge currents were not carried by electrons or by holes (a term for positive quasiparticles indicating the absence of an electron). Rather, the currents were carried by a highly unusual quasiparticle that has no charge and no energy, and yet moves without resistance. The components of the hybrid quasiparticle were observed to travel on opposite sides of the graphene’s edges, despite being a single object.

The unique properties indicate that the quasiparticle might be one that physicists have been hoping to exploit for decades — the elusive Majorana fermion predicted by Italian theoretical physicist Ettore Majorana in 1937.

“Developing electronics using this new quasiparticle in seamlessly interconnected graphene networks is game changing,” de Heer said.

Control of Molecular Orbital Ordering Using a van der Waals Monolayer Ferroelectric

by Mohammad Amini et al in Advanced Materials

Controlling the internal states of quantum systems is one of the biggest challenges in quantum materials. At the deepest level, single molecules can display different quantum states, even while possessing the same number of electrons. These states are associated with different electron configurations, which can lead to dramatically different properties.

The capability of controlling the electronic configuration of single molecules could lead to major developments in both fundamental science and technology. On the one hand, controlling the internal states of molecules may allow for the development of new artificial materials with exotic properties. On the other hand, it might also make possible the ultimate miniaturization of classical computer memories, with the two configurations could make it possible to encode a 0 and a 1 in a classical memory unit at the molecular level. However, controlling the internal states of molecules still remains a challenge, and realistic, scalable strategies for overcoming it have not been proposed.

a) Schematics of the FePc molecules on SnTe (green and red area show the ferroelectric domains and the arrows show the direction of electric polarization in each domain).

In a recent experimental breakthrough researchers from Aalto University and the University of Jyväskylä demonstrated the ability to control the quantum states of individual molecules with an electrically controllable substrate. Their experiment showed how a specific two-dimensional material, known as SnTe, provides the instrumental strategy needed to control molecular states.

The mechanism demonstrated by the researchers is based on the ability of a substrate to tune the internal state of molecules due to internal electric fields. This mechanism, known as ferroelectric molecular switching, enables researchers to control individual molecules merely by applying a voltage to the substrate. The strategy relies on the strong tunability of SnTe by external voltages, which stems from a unique quantum property known as ferroelectricity. The research team involved the groups of Professors Peter Liljeroth, Adam Foster, and Jose Lado from Aalto University, and the team was led by Professor Shawulienu Kezilebieke from the University of Jyväskylä.

a) FePc island on SnTe (image size 80 × 60 nm2, V = 1.5 V, I = 300 pA). b) Line spectra over a distance of 12.5 nm inside a single FE domain (blue arrow). c) Line spectra over 27 nm crossing two domains (green arrow. Black arrow shows the position of boundary between two domains).

“Our results demonstrate how we can control individual molecules using electrically-tunable two-dimensional materials. From a practical point of view, two-dimensional ferroelectrics have been instrumental, as its ultraclean interface allows realizing this strategy of quantum control. These experiments put forward a strategy to engineer quantum states at the molecular level, opening exciting possibilities in artificial materials and single-molecule electronics,” Kezilebieke says.

“In our experiments, we demonstrated how two-dimensional ferroelectrics allow us to realize electrically switchable quantum states. Controlling quantum states electrically is a major milestone in quantum materials, and here we demonstrated one strategy for doing it at the deepest level of individual molecules,” says Ph.D. researcher Mohammad Amini, the first author of the study.

Light control with Weyl semimetals

by Cheng Guo et al in eLight

Weyl semimetals are topological materials whose low-energy excitations obey the Weyl equation. In a Weyl semimetal, the conduction and valence bands touch at discrete points in momentum space called Weyl nodes. Weyl nodes are monopoles of the Berry curvature and are robust under generic perturbations. The quasiparticles near the Weyl nodes are analogous to Weyl fermions in high-energy physics; they exhibit linear dispersion and well-defined chirality.

In a new paper, a team of scientists led by Professor Shanhui Fan at Stanford University has reviewed the basic concepts and optical responses of Weyl semimetals. The non-trivial topology of Weyl semimetals leads to many unusual electronic, magnetic, thermal, and optical properties. These intriguing features have been extensively studied in the literature. Besides these fundamental interests, Weyl semimetals may also enable new opportunities in practical applications. For example, photonic applications include compact optical isolators and circulators, orbital angular momentum detectors, higher-order harmonic generation, and non-reciprocal thermal emitters. However, such an application-oriented exploration is still at an early stage, which requires more joint efforts from scientists and engineers.

(a) Conventional semimetals, (b) Weyl semimetals.

Weyl semimetals are a special class of semimetals. They exhibit common properties of semimetals as well as some unique characteristics. According to band theory, solids can be classified as insulators, semiconductors, semimetals, and metals. An insulator or a semiconductor has a band gap between the valence and conduction bands; the band gap is more significant for an insulator than for a semiconductor. A semimetal has a minimal overlap between the conduction and valence bands and a negligible density of states at the Fermi level. A metal has a partially filled conduction band and an appreciable density of states at the Fermi level.

(a) Optical isolation. (b) Direct OAM detection. (c) High harmonic generation.

The researchers provide their outlook on future works on the emerging topic of photonics based on Weyl semimetals. So far, most works on Weyl semimetals focus on novel physics. Engineers have tremendous challenges and opportunities to make these physical effects practically useful.

There are many opportunities, including the synthesis of high-quality and large-area Weyl semimetals and the fabrication of photonic devices based on Weyl semimetal materials. Other options include the design of photonic structures to enhance the light-matter interactions in Weyl semimetals and photon management to enhance the light absorption and photocurrents in Weyl semimetals. Indeed, many efforts must be undertaken to construct practical devices from Weyl semimetals.

Observation of Spin-Tensor Induced Topological Phase Transitions of Triply Degenerate Points with a Trapped Ion

by Mengxiang Zhang et al in Physical Review Letters

Topology refers to the overall property that remains unchanged despite continuous local modifications. A coffee cup and a donut are no different to mathematicians because they have the same topological charge. Materials with various topological charges display diverse properties. Exploring phase transitions between different topological states brings prospects for novel materials and new physics.

Recently, a research team led by Prof. Du Jiangfeng, Prof. Lin Yiheng, and Prof. Luo Xiwang from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) experimentally observed the phase transitions between triply degenerate points (TDPs) with different topological charges through highly controllable quantum simulations.

The researchers performed quantum simulations of topological phase transitions between TDPs in fermionic systems. Through a three-level trapped Be+ ion driven by ratio frequency and microwave fields, a spin-1 quantum state is obtained. By tuning spin-tensor-momentum coupling strengths, the researchers observed the topological phase transitions of the quantum states and illuminated the important roles played by the spin tensors.

Phase transition characterized by the jump of spin vector and tensor.

While building the multilevel trapped ion systems, the research team developed various technologies to study high-spin physics. They prolonged coherence time by an order of magnitude via dynamical decoupling. Furthermore, they also realized swift quantum control techniques on a four-level trapped ion system through analytical models. These previous efforts laid the foundation for the current research.

The study paved the way for future exploration of novel topological phenomena. The reviewer paid high praise and noted that “…importantly, the spin-tensor-momentum-coupling could be generated for spin-1 systems and induce intriguing quantum phenomena different from spin-1/2 ones. This work is of interest and importance.”

Ultra-low loss quantum photonic circuits integrated with single quantum emitters

by Ashish Chanana et al in Nature Communications

The ability to transmit and manipulate, with minimal loss, the smallest unit of light — the photon — plays a pivotal role in optical communications as well as designs for quantum computers that would use light rather than electric charges to store and carry information.

Now, researchers at the National Institute of Standards and Technology (NIST) and their colleagues have connected, on a single microchip, quantum dots — artificial atoms that generate individual photons rapidly and on-demand when illuminated by a laser — with miniature circuits that can guide the light without significant loss of intensity.

To create the ultra-low-loss circuits, the researchers fabricated silicon-nitride waveguides — the channels through which the photons traveled — and buried them in silicon dioxide. The channels were wide but shallow, a geometry that reduced the likelihood that photons would scatter out of the waveguides. Encapsulating the waveguides in silicon dioxide also helped to reduce scattering.

The scientists reported that their prototype circuits have a loss of intensity equal to only one percent of similar circuits — also using quantum dots — that were fabricated by other teams. Ultimately, devices that incorporate this new chip technology could take advantage of the strange properties of quantum mechanics to perform complex computations that classical (non-quantum) circuits may not be capable of doing.

Illustration shows some of the steps in creating the new ultra-low-loss photonic circuit on a chip. A microprobe lifts a gallium arsenide device containing a quantum dot — artificial atoms that generate single photons — from one chip. Then the probe places the quantum-dot device atop a low-loss silver-nitride waveguide built on another chip. Credit: S. Kelley/NIST

For instance, according to the laws of quantum mechanics, a single photon has a probability of residing in two different places, such as two different waveguides, at the same time. Those probabilities can be used to store information; an individual photon can act as a quantum bit, or qubit, which carries much more information than the binary bit of a classical computer, which is limited to a value of 0 or 1.

To perform operations necessary to solve computational problems, these photon qubits — all of which travel at the same speed and are indistinguishable from each other — must simultaneously arrive at specific processing nodes in the circuit. That poses a challenge because photons originating from different locations — and traveling along different waveguides — across the circuit may lie at significantly different distances from processing points. To ensure simultaneous arrival, photons emitted closer to the designated destination must delay their journey, giving those that lie in more distant waveguides a head start.

The circuit devised by NIST researchers including Ashish Chanana and Marcelo Davanco, along with an international team of colleagues, allows for significant time delays because it employs waveguides of various lengths that can store photons for relatively long periods of time. For instance, the researchers calculate that a 3-meter-long waveguide (tightly coiled so its diameter on a chip is only a few millimeters) would have a 50 percent probability of transmitting a photon with a time delay of 20 nanoseconds (billionths of a second). By comparison, previous devices, developed by other teams and operating under similar conditions, were limited to inducing time delays only one one-hundredth as long.

The longer delay times achieved with the new circuit are also important for operations in which photons from one or more quantum dots need to arrive at a specific location at equally spaced time intervals. In addition, the low-loss quantum-dot circuit could dramatically increase the number of single photons available for carrying quantum information on a chip, enabling larger, speedier, and more reliable computational and information-processing systems

Laser light shining on the quantum dots triggers them to produce a series of single photons that travel through the silicon nitride waveguide. Credit: S. Kelley/NIST

The hybrid circuit consists of two components, each initially built on a separate chip. One, a gallium arsenide semiconductor device designed and fabricated at NIST, hosts the quantum dots and directly funnels the single photons they generate into a second device — a low-loss silicon nitride waveguide developed at UCSB.

To marry the two components, researchers at MIT first used the fine metal tip of a pick-and-place microprobe, acting like a miniature crowbar, to pry the gallium arsenide device from the chip built at NIST. They then placed it atop the silicon nitride circuit on the other chip.

The researchers face several challenges before the hybrid circuit can be routinely employed in a photonic device. At present, only about 6 percent of the individual photons generated by the quantum dots can be funneled into the circuit. However, simulations suggest that if the team changes the angle at which the photons are funneled, in tandem with improvements in the positioning and orientation of the quantum dots, the rate could rise above 80 percent.

Another issue is that the quantum dots do not always emit single photons at exactly the same wavelength, a requirement for creating the indistinguishable photons necessary for the quantum computational operations. The team is exploring several strategies, including applying a constant electric field to the dots, that may alleviate that problem.

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