TL;DR

• A new computational analysis supports the idea that photons (a.k.a. particles of light) colliding with heavy ions can create a fluid of ‘strongly interacting’ particles. In a new paper, they show that calculations describing such a system match up with data collected by the ATLAS detector at Europe’s Large Hadron Collider (LHC).
• Researchers have pioneered a new imaging method that allows the capture of the light-induced phase transition in vanadium oxide (VO2) with high spatial and temporal resolution.
• A new study exploring the connection between the quantum and classical worlds, have discovered a new and much more effective way to carry out interaction-free experiments. The team used transmon devices — superconducting circuits that are relatively large but still show quantum behavior — to detect the presence of microwave pulses generated by classical instruments.
• Researchers developed a new graphene-based nanoelectronics platform compatible with conventional microelectronics manufacturing, paving the way for a successor to silicon.
• Quantum dots are normally made in industrial settings with high temperatures and toxic, expensive solvents — a process that is neither economical nor environmentally friendly. But researchers have now pulled off the process at the bench using water as a solvent, making a stable end-product at room temperature. Their work opens the door to making nanomaterials in a more sustainable way by demonstrating that protein sequences not derived from nature can be used to synthesize functional materials.
• A single particle has no temperature. It has a certain energy or a certain speed — but it is not possible to translate that into a temperature. Only when dealing with random velocity distributions of many particles, a well-defined temperature emerges.
• The two research teams discovered that the likelihood that an electron will undergo tunneling, the phase at which the electron tunnels out and the timing of the tunneling event depend on the chirality of the molecule.
• The recent study has confirmed that the light scalar mesons contain a significant four-quark component, a feature that puts scalar mesons in the challenging category of exotic hadron spectroscopy.
• A new technique reveals changing shapes of magnetic noise in space and time.
• Mathematical analysis identifies a vortex structure that is impervious to decay.
• 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

Collectivity in Ultraperipheral Pb+Pb Collisions at the Large Hadron Collider

by Wenbin Zhao, Chun Shen, Björn Schenke in Physical Review Letters

A new computational analysis by theorists at the U.S. Department of Energy’s Brookhaven National Laboratory and Wayne State University supports the idea that photons (a.k.a. particles of light) colliding with heavy ions can create a fluid of “strongly interacting” particles. In a paper, they show that calculations describing such a system match up with data collected by the ATLAS detector at Europe’s Large Hadron Collider (LHC).

As the paper explains, the calculations are based on the hydrodynamic particle flow seen in head-on collisions of various types of ions at both the LHC and the Relativistic Heavy Ion Collider (RHIC), a DOE Office of Science user facility for nuclear physics research at Brookhaven Lab. With only modest changes, these calculations also describe flow patterns seen in near-miss collisions, where photons that form a cloud around the speeding ions collide with the ions in the opposite beam.

“The upshot is that, using the same framework we use to describe lead-lead and proton-lead collisions, we can describe the data of these ultra-peripheral collisions where we have a photon colliding with a lead nucleus,” said Brookhaven Lab theorist Bjoern Schenke, a coauthor of the paper. “That tells you there’s a possibility that, in these photon-ion collisions, we create a small dense strongly interacting medium that is well described by hydrodynamics — just like in the larger systems.”

The charged hadron pseudorapidity distributions dNch/dη in 0%–90% p+Pb and γ∗+Pb collisions from the 3D−GLAUBER+MUSIC+URQMD simulations. The theoretical calculations are compared with experimental data from the ATLAS Collaboration in the laboratory frame.

Observations of particles flowing in characteristic ways have been key evidence that the larger collision systems (lead-lead and proton-lead collisions at the LHC; and gold-gold and proton-gold collisions at RHIC) create a nearly perfect fluid. The flow patterns were thought to stem from the enormous pressure gradients created by the large number of strongly interacting particles produced where the colliding ions overlap.

“By smashing these high-energy nuclei together we’re creating such high energy density — compressing the kinetic energy of these guys into such a small space — that this stuff essentially behaves like a fluid,” Schenke said.

Spherical particles (including protons and nuclei) colliding head on are expected to generate a uniform pressure gradient. But partially overlapping collisions generate an oblong, almond-shaped pressure gradient that pushes more high-energy particles out along the short axis than perpendicular to it.

This “elliptic flow” pattern was one of the earliest hints that particle collisions at RHIC could create a quark-gluon plasma, or QGP — a hot soup of the fundamental building blocks that make up the protons and neutrons of nuclei/ions. Scientists were at first surprised by the QGP’s liquid-like behavior. But they later established elliptic flow as a defining feature of QGP, and evidence that the quarks and gluons were still interacting strongly, even when free from confinement within individual protons and neutrons. Later observations of similar flow patterns in collisions of protons with large nuclei, intriguingly suggest that these proton-nucleus collision systems can also create tiny specks of quark-gluon soup.

“Our new paper is about pushing this to even further extremes, looking at collisions between photons and nuclei,” Schenke said.

Charged hadron anisotropic flow coefficients v2{2} and v3{2} as functions of charged hadron multiplicity Nch in p+Pb (dashed lines) and γ∗+Pb (solid lines) collisions at LHC energies from the 3D−GLAUBER+MUSIC+URQMD simulations.

It has long been known that that ultra-peripheral collisions could create photon-nucleus interactions, using the nuclei themselves as the source of the photons. That’s because charged particles accelerated to high energies, like the lead nuclei/ions accelerated at the LHC (and gold ions at RHIC), emit electromagnetic waves — particles of light. So, each accelerated lead ion at the LHC is essentially surrounded by a cloud of photons.

“When two of these ions pass each other very closely without colliding, you can think of one as emitting a photon, which then hits the lead ion going the other way,” Schenke said. “Those events happen a lot; it’s easier for the ions to barely miss than to precisely hit one another!”

ATLAS scientists recently published data on intriguing flow-like signals from these photon-nucleus collisions.

“We had to set up special data collection techniques to pick out these unique collisions,” said Blair Seidlitz, a Columbia University physicist who helped set up the ATLAS trigger system for the analysis when he was a graduate student at the University of Colorado, Boulder. “After collecting enough data, we were surprised to find flow-like signals that were similar to those observed in lead-lead and proton-lead collisions, although they were a little smaller.”

Schenke and his collaborators set out to see whether their theoretical calculations could accurately describe the particle flow patterns. They used the same hydrodynamic calculations that describe the behavior of particles produced in lead-lead and proton-lead collision systems. But they made a few adjustments to account for the “projectile” striking the lead nucleus changing from a proton to a photon. According to the laws of physics (specifically, quantum electrodynamics), a photon can undergo quantum fluctuations to become another particle with the same quantum numbers. A rho meson, a particle made of a particular combination of a quark and antiquark held together by gluons, is one of the most likely results of those photon fluctuations. If you think back to the proton — made of three quarks — this two-quark rho particle is just a step down the complexity ladder.

“Instead of having a gluon distribution around three quarks inside a proton, we have the two quarks (quark-antiquark) with a gluon distribution around those to collide with the nucleus,” Schenke said.

This graphic shows the energy density at different times during the hydrodynamic evolution of the matter created in a collision of a lead nucleus (moving to the left) with a photon emitted from the other lead nucleus (moving to the right). Yellow represents the highest energy density and purple the lowest.

The calculations also had to account for the big difference in energy in these photon-nucleus collision systems, compared to proton-lead and especially lead-lead.

“The emitted photon that’s colliding with the lead won’t carry the entire momentum of the lead nucleus it came from, but only a tiny fraction of that. So, the collision energy will be much lower,” Schenke said.

That energy difference turned out to be even more important than the change of projectile. In the most energetic lead-lead or gold-gold heavy ion collisions, the pattern of particles emerging in the plane transverse to the colliding beams generally persists no matter how far you look from the collision point along the beamline (in the longitudinal direction). But when Schenke and collaborators modeled the patterns of particles expected to emerge from lower-energy photon-lead collisions, it became apparent that including the 3D details of the longitudinal direction made a difference. The model showed that the geometry of the particle distributions changes rapidly with increasing longitudinal distance; the particles become “decorrelated.”

“The particles see different pressure gradients depending on their longitudinal position,” Schenke explained. “So, for these low energy photon-lead collisions, it is important to run a full 3D hydrodynamic model (which is more computationally demanding) because the particle distribution changes more rapidly as you go out in the longitudinal direction,” he said.

When the theorists compared their predictions using this lower-energy, full 3D, hydrodynamic model with the particle flow patterns observed in photon-lead collisions by the ATLAS detector, the data and theory matched up nicely, at least for the most obvious elliptic flow pattern, Schenke said.

“From this result, it looks like it’s conceivable that, even in photon-heavy ion collisions, we have a strongly interacting fluid that responds to the initial collision geometry, as described by hydrodynamics,” Schenke said. “If the energies and temperatures are high enough,” he added, “there will be a quark-gluon plasma.”

“It’s conceivable that, in photon-heavy ion collisions, we have a strongly interacting fluid,” said Brookhaven Lab theorist Bjoern Schenke.

Ultrafast X-ray imaging of the light-induced phase transition in VO2

by Johnson, A.S. et al in Nature Physics

The use of light to produce transient phases in quantum materials is fast becoming a novel way to engineer new properties in them, such as the generation of superconductivity or nanoscale topological defects. However, visualizing the growth of a new phase in a solid is not easy, due in-part to the wide range of spatial and time scales involved in the process.

Although in the last two decades scientists have explained light-induced phase transitions by invoking nanoscale dynamics, real space images have not yet been produced and, thus, no one has seen them.

ICFO researchers Allan S. Johnson and Daniel Pérez-Salinas, led by former ICFO Prof. Simon Wall, in collaboration with colleagues from Aarhus University, Sogang University, Vanderbilt University, the Max Born Institute, the Diamond Light Source, ALBA Synchrotron, Utrecht University, and the Pohang Accelerator Laboratory, have pioneered a new imaging method that allows the capture of the light-induced phase transition in vanadium oxide (VO2) with high spatial and temporal resolution.

The new technique implemented by the researchers is based on coherent X-ray hyperspectral imaging at a free electron laser, which has allowed them to visualize and better understand, at the nanoscale, the insulator-to-metal phase transition in this very well-known quantum material.

Time-dependent X-ray holographic imaging of VO2.

The crystal VO2 has been widely used in to study light-induced phase transitions. It was the first material to have its solid-solid transition tracked by time-resolved X-ray diffraction and its electronic nature was studied by using for the first time ultrafast X-ray absorption techniques. At room temperature, VO2 is in the insulating phase. However, if light is applied to the material, it is possible to break the dimers of the vanadium ion pairs and drive the transition from an insulating to a metallic phase.

In their experiment, the authors of the study prepared thin samples of VO2 with a gold mask to define the field of view. Then, the samples were taken to the X-ray Free Electron Laser facility at the Pohang Accelerator Laboratory, where an optical laser pulse induced the transient phase, before being probed by an ultrafast X-ray laser pulse.

A camera captured the scattered X-rays, and the coherent scattering patterns were converted into images by using two different approaches: Fourier Transform Holography (FTH) and Coherent Diffractive Imaging (CDI). Images were taken at a range of time delays and X-ray wavelengths to build up a movie of the process with 150 femtosecond time resolution and 50 nm spatial resolution, but also with full hyperspectral information.

Fluence dependence of the transient dynamics.

The new methodology allowed the researchers to better understand the dynamics of the phase transition in VO2. They found that pressure plays a much larger role in light-induced phase transitions than previously expected or assumed.

“We saw that the transient phases aren’t nearly as exotic as people had believed! Instead of a truly non-equilibrium phase, what we saw was that we had been misled by the fact that the ultrafast transition intrinsically leads to giant internal pressures in the sample millions of times higher than atmospheric. This pressure changes the material properties and takes time to relax, making it seem like there was a transient phase,” says Allan Johnson, postdoctoral researcher at ICFO.

“Using our imaging method, we saw that, at least in this case, there was no link between the picosecond dynamics that we did see and any nanoscale changes or exotics phases. So, it looks like some of those conclusions will have to be revisited.”

To identify the role played by the pressure in the process, it was crucial to use the hyperspectral image. “By combining imaging and spectroscopy into one great image, we are able to retrieve much more information that permits us to actually see detailed features and decipher exactly where they come from,” continues Johnson.

“This was essential to look at each part of our crystal and determine whether it was a normal or an exotic out-of-equilibrium phase-and with this information we were able to determine that during the phase transitions all the regions of our crystal were the same, except for the pressure.”

One of the main challenges the researchers faced during the experiment was to ensure that the crystal sample of VO2 returned to its original starting phase each time and after being illuminated by the laser. To guarantee that this would occur, they conducted preliminary experiments at synchrotrons where they took several crystal samples and repeatedly shone the laser on them to test their capacity to recover back to their original state.

The second challenge resided in having access to an X-Ray free electron laser, large research facilities where the time windows to conduct the experiments are very competitive and in-demand because there are only a few in the world. “We had to spend two weeks in quarantine in South Korea due to the COVID-19 restrictions before we got our one shot of just five days to make the experiment work, so that was an intense time,” Johnson recalls.

Although the researchers describe the present work as fundamental research, the potential applications of this technique could be diverse, since they could “look at polarons moving inside catalytic materials, try imaging superconductivity itself, or even help us understand novel nanotechnologies by viewing and imaging inside nanoscale devices,” concludes Johnson.

Coherent interaction-free detection of microwave pulses with a superconducting circuit

by Shruti Dogra et al in Nature Communications

We see the world around us because light is being absorbed by specialized cells in our retina. But can vision happen without any absorption at all — without even a single particle of light? Surprisingly, the answer is yes.

Imagine that you have a camera cartridge that might contain a roll of photographic film. The roll is so sensitive that coming into contact with even a single photon would destroy it. With our everyday classical means there is no way there’s no way to know whether there’s film in the cartridge, but in the quantum world it can be done. Anton Zeilinger, one of the winners of the 2022 Nobel Prize in Physics, was the first to experimentally implement the idea of an interaction-free experiment using optics.

Now, in a study exploring the connection between the quantum and classical worlds, Shruti Dogra, John J. McCord, and Gheorghe Sorin Paraoanu of Aalto University have discovered a new and much more effective way to carry out interaction-free experiments. The team used transmon devices — superconducting circuits that are relatively large but still show quantum behavior — to detect the presence of microwave pulses generated by classical instruments.

Although Dogra and Paraoanu were fascinated by the work done by Zeilinger’s research group, their lab is centered around microwaves and superconductors instead of lasers and mirrors.

“We had to adapt the concept to the different experimental tools available for superconducting devices. Because of that, we also had to change the standard interaction-free protocol in a crucial way: we added another layer of ‘quantumness’ by using a higher energy level of the transmon. Then, we used the quantum coherence of the resulting three-level system as a resource,” Paraoanu says.

Coherent interaction-free detection.

Quantum coherence refers to the possibility that an object can occupy two different states at the same time — something that quantum physics allows for. However, quantum coherence is delicate and easily collapses, so it wasn’t immediately obvious that the new protocol would work. To the team’s pleasant surprise, the first runs of the experiment showed a marked increase in detection efficiency. They went back to the drawing board several times, ran theoretical models confirming their results, and double-checked everything. The effect was definitely there.

“We also demonstrated that even very low-power microwave pulses can be detected efficiently using our protocol,” says Dogra.

The experiment also showed a new way in which quantum devices can achieve results that are impossible for classical devices — a phenomenon known as quantum advantage. Researchers generally believe that achieving quantum advantage will require quantum computers with many qubits, but this experiment demonstrated genuine quantum advantage using a relatively simpler setup.

Histogram of events for θ = π and N = 1, and the corresponding confusion matrix and efficiency.

Interaction-free measurements based on the less effective older methodology have already found applications in specialized processes such as optical imaging, noise-detection, and cryptographic key distribution. The new and improved method could increase the efficiency of these processes dramatically.

“In quantum computing, our method could be applied for diagnosing microwave-photon states in certain memory elements. This can be regarded as a highly efficient way of extracting information without disturbing the functioning of the quantum processor,” Paraoanu says.

The group led by Paraoanu is also exploring other exotic forms of information processing using their new approach, such as counterfactual communication (communication between two parties without any physical particles being transferred) and counterfactual quantum computing (where the result of a computation is obtained without in fact running the computer).

An epitaxial graphene platform for zero-energy edge state nanoelectronics

by Vladimir S. Prudkovskiy, Yiran Hu, Kaimin Zhang, Yue Hu, 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.

“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.”

The epigraphene edge state.

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.

It will likely be another five to 10 years before we have the first graphene-based electronics, according to de Heer. But thanks to the team’s new epitaxial graphene platform, technology is closer than ever to crowning graphene as a successor to silicon.

A de novo protein catalyzes the synthesis of semiconductor quantum dots

by Leah C. Spangler, Yueyu Yao , Guangming Cheng, Nan Yao, Sarangan L. Chari, Gregory D. Scholes and Michael H. Hecht in PNAS

Nature uses 20 canonical amino acids as building blocks to make proteins, combining their sequences to create complex molecules that perform biological functions.

But what happens with the sequences not selected by nature? And what possibilities lie in constructing entirely new sequences to make novel, or de novo, proteins bearing little resemblance to anything in nature? That’s the terrain Princeton University’s Hecht Lab works in. And recently, their curiosity for designing their own sequences paid off. They discovered the first known de novo protein that catalyzes, or drives, the synthesis of quantum dots. Quantum dots are fluorescent nanocrystals used in electronic applications from LED screens to solar panels. Their work opens the door to making nanomaterials in a more sustainable way by demonstrating that protein sequences not derived from nature can be used to synthesize functional materials — with pronounced benefits to the environment.

Quantum dots are normally made in industrial settings with high temperatures and toxic, expensive solvents — a process that is neither economical nor environmentally friendly. But Hecht Lab researchers pulled off the process at the bench using water as a solvent, making a stable end-product at room temperature.

These quantum dots, seen through an electron microscope, were produced in the Hecht Lab using de novo proteins. Each dot is 2 nanometers in diameter, an important factor since size determines color. Image courtesy of the Hecht Lab

“We’re interested in making life molecules, proteins, that did not arise in life,” said Professor of Chemistry Michael Hecht, who led the research with Greg Scholes, the William S. Tod Professor of Chemistry and chair of the department. “In some ways we’re asking, are there alternatives to life as we know it? All life on earth arose from common ancestry. But if we make lifelike molecules that did not arise from common ancestry, can they do cool stuff?

“So here, we’re making novel proteins that never arose in life doing things that don’t exist in life.”

The team’s process can also tune nanoparticle size, which determines the color quantum dots glow, or fluoresce, in. That holds possibilities for tagging molecules within a biological system, like staining cancer cells in vivo.

“Quantum dots have very interesting optical properties due to their sizes,” said Yueyu Yao, co-author on the paper and a fifth-year graduate student in the Hecht Lab. “They’re very good at absorbing light and converting it to chemical energy — that makes them useful for being made into solar panels or any sort of photo sensor. “But on the other hand, they’re also very good at emitting light at a certain desired wavelength, which makes them suitable for making LED screens.”

And because they’re small — comprised of only about 100 atoms and maybe 2 nanometers across — they’re able to penetrate some biological barriers, making their utility in medicines and biological imaging especially promising.

“I think using de novo proteins opens up a way for designability,” said Leah Spangler, lead author on the research and a former postdoc in the Scholes Lab. “A key word for me is ‘engineering.’ I want to be able to engineer proteins to do something specific, and this is a type of protein you can do that with.

“The quantum dots we’re making aren’t great quality yet, but that can be improved by tuning the synthesis,” she added. “We can achieve better quality by engineering the protein to influence quantum dot formation in different ways.”

Based on work done by Sarangan Chari, Hecht Lab senior chemist and a corresponding author, the team used a de novo protein it designed named ConK to catalyze the reaction. Researchers first isolated ConK in 2016 from a large combinatorial library of proteins. It’s still made of natural amino acids, but it qualifies as “de novo” because its sequence doesn’t have any similarity to a natural protein. Researchers found that ConK enabled the survival of E. coli in otherwise toxic concentrations of copper, suggesting it might be useful for metal binding and sequestration. The quantum dots used in this research are made out of cadmium sulfide. Cadmium is a metal, so researchers wondered if ConK could be used to synthesize quantum dots. Their hunch paid off. ConK breaks down cysteine, one of the 20 amino acids, into several products, including hydrogen sulfide. That acts as the active sulfur source that will then go on to react with the metal cadmium. The result is CdS quantum dots.

“To make a cadmium sulfide quantum dot, you need the cadmium source and the sulfur source to react in solution,” said Spangler. “What the protein does is make the sulfur source slowly over time. So, we add the cadmium initially but the protein generates the sulfur, which then reacts to make distinct sizes of quantum dots.”

Canonical Density Matrices from Eigenstates of Mixed Systems

by Mahdi Kourehpaz, Stefan Donsa, Fabian Lackner, Joachim Burgdörfer, Iva Březinová in Entropy

A single particle has no temperature. It has a certain energy or a certain speed — but it is not possible to translate that into a temperature. Only when dealing with random velocity distributions of many particles, a well-defined temperature emerges.

How can the laws of thermodynamics arise from the laws of quantum physics? This is a topic that has attracted growing attention in recent years. At TU Wien (Vienna), this question has now been pursued with computer simulations, which showed that chaos plays a crucial role: Only where chaos prevails do the well-known rules of thermodynamics follow from quantum physics.

The air molecules randomly flying around in a room can assume an unimaginable number of different states: Different locations and different speeds are allowed for each individual particle. But not all of these states are equally likely.

“Physically, it would be possible for all the energy in this space to be transferred to one single particle, which would then move at extremely high speeds while all the other particles stand still,” says Prof. Iva Brezinova from the Institute of Theoretical Physics at TU Wien. “But this is so unlikely that it will practically never be observed.”

The probabilities of different allowed states can be calculated — according to a formula that the Austrian physicist Ludwig Boltzmann set up according to the rules of classical physics. And from this probability distribution, the temperature can then also be read off: it is only determined for a large number of particles.

However, this causes problems when dealing with quantum physics. When a large number of quantum particles are in play at the same time, the equations of quantum theory become so complicated that even the best supercomputers in the world have no chance of solving them.

One of the particles acts as a “thermometer”, the whole system is simulated on the computer.

In quantum physics, the individual particles cannot be considered independently of each other, as is the case with classical billiard balls. Every billiard ball has its own individual trajectory and its own individual location at every point in time. Quantum particles, on the other hand, have no individuality — they can only be described together, in a single large quantum wave function.

“In quantum physics, the entire system is described by a single large many-particle quantum state,” says Prof. Joachim Burgdörfer (TU Wien). “How a random distribution and thus a temperature should arise from this remained a puzzle for a long time.”

A team at TU Wien has now been able to show that chaos plays a key role. To do this, the team performed a computer simulation of a quantum system that consists of a large number of particles — many indistinguishable particles (the “heat bath”) and one of a different kind of particle, the “sample particle” that acts as a thermometer. Each individual quantum wave function of the large system has a specific energy, but no well-defined temperature — just like a single classical particle. But if you now pick out the sample particle from the single quantum state and measure its velocity, you can surprisingly find a velocity distribution that corresponds to a temperature that fits the well-established laws of thermodynamics.

“Whether or not it fits depends on chaos — that is what our calculations clearly showed,” says Iva Brezinova. “We can specifically change the interactions between the particles on the computer and thus create either a completely chaotic system, or one that shows no chaos at all — or anything in between.” And in doing so, one finds that the presence of chaos determines whether a quantum state of the sample particle displays a Boltzmann temperature distribution or not.

“Without making any assumptions about random distributions or thermodynamic rules, thermodynamic behavior arises from quantum theory all by itself — if the combined system of sample particle and heat bath behaves quantum chaotically. And how well this behavior fits the well-known Boltzmann formulae is determined by the strength of the chaos,” explains Joachim Burgdörfer.

This is one of the first cases in which the interplay between three important theories has been rigorously demonstrated by many-particle computer simulations: quantum theory, thermodynamics and chaos theory.

Revealing the Influence of Molecular Chirality on Tunnel-Ionization Dynamics

by E. Bloch et al in Physical Review X

Will an electron escaping a molecule through a quantum tunnel behave differently depending on the left- or right-handedness of the molecule?

Chemists have borrowed the phrases “left-handed” and “right-handed” from anatomy to describe molecules that are characterized by a particular type of asymmetry. To explore the concept of chirality, look at your hands, palms up. Clearly, the two are mirror images of one another. But try as we might to superimpose them, they will not overlap completely. Such objects, termed “chiral,” can be found at all scales in nature, from galaxies down to molecules.

Each day, we experience chirality not only when we grab an object or put on our shoes but also when we eat or breathe: our taste and smell can distinguish two mirror images of a chiral molecule. In fact, our body is so sensitive to chirality that a molecule can be a medicine and its mirror image a poison. Chirality is thus crucial in pharmacology, where 90 percent of synthesized drugs are chiral compounds.

Chiral molecules have particular symmetry properties that make them great candidates for the investigation of fundamental phenomena in physics. Recently, the research teams led by Prof. Yann Mairesse from CNRS / Bordeaux University and Prof. Nirit Dudovich of the Weizmann Institute’s Department of Physics of Complex Systems used chirality to shed new light on one of the most intriguing quantum phenomena: the tunneling process.

Schematic view of sub-barrier and continuum electron dynamics in strong-field ionization (a) and principles of chiral attoclock (b) and subcycle gated photoelectron interferometry (c) techniques. In (a), ionization occurs as part of the initial bound electron wave packet tunnels through the target potential barrier lowered by the strong laser field.

Tunneling is a phenomenon in which quantum particles cross seemingly impossible-to-cross physical barriers. Since this motion is forbidden in classical mechanics, it is very difficult to establish an intuitive picture of its dynamics. To create a tunnel in chiral molecules, the researchers exposed them to an intense laser field.

“The electrons of the molecules are naturally bound around the nuclei by an energy barrier,” explains Mairesse. “You can imagine the electrons as air trapped inside an inflatable balloon. The strong laser fields have the ability to reduce the thickness of the balloon enough for some air to tunnel through it, even though there’s no hole in the balloon.”

Mairesse, Dudovich and their teams set out to study an as-of-yet-unexplored aspect of tunneling: the moment in which a chiral molecule meets a chiral light field, and the way in which their brief encounter affects electron tunneling. “We were very excited to explore the connection between chirality and tunneling. We were keen to learn more about what tunneling would look like under these particular circumstances,” says Dudovich.

It only takes a few hundred attoseconds for an electron to escape an atom or molecule. Such minuscule time frames characterize many of the processes studied in Mairesse’s and Dudovich’s labs. The two teams asked the following question: How does the chirality of a molecule affect the escape of an electron?

“We used a laser field that rotates in time to spin the barrier around the chiral molecules,” says Mairesse. “To follow up with the balloon metaphor, if the laser field rotates horizontally, you expect the air to exit the balloon on the horizontal plane, following the direction of the laser field. What we found is that if the balloon is chiral, the air exits the balloon flying towards the floor or the ceiling, depending on the rotation direction of the laser. In other words, the electrons emerge from the chiral tunnel with a memory of the rotation direction of the barrier. This is very much like the effect of a corkscrew, but at the nanometer and attosecond scales.”

The two teams thus discovered that the likelihood that an electron will undergo tunneling, the phase at which the electron tunnels out and the timing of the tunneling event depend on the chirality of the molecule. These exciting results lay the groundwork for additional studies that will use the unique symmetry properties of chiral molecules to investigate the fastest processes occurring in light-matter interaction.

Chiral nonet mixing in pi-eta scattering

by Amir H. Fariborz et al in The European Physical Journal C

At this stage in the evolution of the universe (about 14 billion years after the big bang) there are four fundamental forces in action that cause interactions among the constituents of matter.

One of these forces is gravity that, for example, keeps us revolving around the sun and be able to enjoy the four seasons. Another one is the electromagnetic force that benefits us every day. From the light bulb that we turn on every night to the dynamics of electrons inside our electronic devices, they are all driven by the electromagnetic force. The other two forces are not as commonly noticed in daily life, but this does not mean that they are any less important. These two forces are confined within the nucleus of atoms (distances around 10–15 m or smaller) and are traditionally referred to as “nuclear forces.” One of them is the strong nuclear force (the strongest of all four forces) that is responsible for keeping the nucleus intact by bonding its protons and neutrons together.

Without the strong nuclear force, nuclei wouldn’t form, we wouldn’t exist, and the sky would be empty. The other one is the weak nuclear force that is responsible for transforming one nucleus into another and sometimes breaking them apart. We benefit from the effects of the weak nuclear force in our nuclear reactors.

Contour plot of function F(sr,si). The point at which F=0 represents the first physical isovector scalar meson pole and provides the properties of the a0(980).

The protons and neutrons are members of a large family of composite particles called hadrons, and they are all made of fundamental particles called quarks. The theory that describes the strong interaction of quarks is called Quantum Chromodynamics (QCD), according to which, the quarks engage in strong interaction by exchanging mediating particles called gluons. This is similar to how fundamental charged particles engage in electromagnetic interaction by exchanging photons in the theory of Quantum Electrodynamics (QED). However, there are some major differences between QCD and QED, such as, for example, the fact that photons cannot form bound states but gluons, in principle, can bound together and form composites called glueballs.

Theoretical understanding of the formation and interactions of glueballs with quark matter, as well as their experimental detection, are ambitious objectives of formidable complexity. Despite several Nobel prizes in physics already awarded for the remarkable discoveries in particle physics related to QCD, some aspects remain open questions and have challenged the theoretical physics for many decades. This problem is recognized by the Clay Mathematics Institute as one of the seven unsolved problems in mathematics, known as Millennium Problems in Mathematics.

The main research of Amir Fariborz is on the strong interaction of quarks and their interactions with glueballs. Models developed by Fariborz and collaborators have been widely successful in describing experimental data and received noticeable citations in the literature. Further information about Fariborz’s research can be found at the Inspire high energy physics literature database.

In this recent paper the generalized linear sigma model of QCD (developed by Fariborz et al) is applied to the scattering of two special types of hadrons called pion (π) and eta (η). This scattering is particularly important because it probes an intermediate composite state [called a0980] which is part of a family of hadrons called scalar mesons.

These composite particles of quarks play a special role in QCD by breaking a symmetry in the dynamical equations called chiral symmetry. Understanding the quark substructure of scalars sheds light on the strong interaction of quarks and gluons. This recent work has confirmed that the light scalar mesons contain a significant four-quark component, a feature that puts scalar mesons in the challenging category of exotic hadron spectroscopy.

Nanoscale covariance magnetometry with diamond quantum sensors

by Jared Rovny et al in Science

Electromagnetic noise poses a major problem for communications, prompting wireless carriers to invest heavily in technologies to overcome it. But for a team of scientists exploring the atomic realm, measuring tiny fluctuations in noise could hold the key to discovery.

“Noise is usually thought of as a nuisance, but physicists can learn many things by studying noise,” said Nathalie de Leon, an associate professor of electrical and computer engineering at Princeton University. “By measuring the noise in a material, they can learn its composition, its temperature, how electrons flow and interact with one another, and how spins order to form magnets. It is generally difficult to measure anything about how the noise changes in space or time.”

Using specially designed diamonds, a team of researchers at Princeton and the University of Wisconsin-Madison have developed a technique to measure noise in a material by studying correlations, and they can use this information to learn the spatial structure and time-varying nature of the noise. This technique, which relies on tracking tiny fluctuations in magnetic fields, represents a stark improvement over previous methods that averaged many separate measurements.

Using specially designed diamonds with nitrogen-vacancy centers, researchers at Princeton University and the University of Wisconsin-Madison have developed a technique to measure noise in a material by studying correlations, and they can use this information to learn the spatial structure and time-varying nature of the noise. In this image, a diamond with near-surface nitrogen-vacancy centers is illuminated by green laser light from a microscope objective lens. Credit: David Kelly Crow

De Leon is a leader in the fabrication and use of highly controlled diamond structures called nitrogen-vacancy (NV) centers. These NV centers are modifications to a diamond’s lattice of carbon atoms in which a carbon is replaced by a nitrogen atom, and adjacent to it is an empty space, or vacancy, in the molecular structure. Diamonds with NV centers are one of the few tools that can measure changes in magnetic fields at the scale and speed needed for critical experiments in quantum technology and condensed matter physics.

While a single NV center allowed scientists to take detailed readings of magnetic fields, it was only when de Leon’s team worked out a method to harness multiple NV centers simultaneously that they were able to measure the spatial structure of noise in a material. This opens the door to understanding the properties of materials with bizarre quantum behaviors that until now have been analyzed only theoretically, said de Leon, the senior author of a paper.

“It’s a fundamentally new technique,” said de Leon. “It’s been clear from a theoretical perspective that it would be very powerful to be able to do this. The audience that I think is most excited about this work is condensed matter theorists, now that there’s this whole world of phenomena they might be able to characterize in a different way.”

One of these phenomena is a quantum spin liquid, a material first explored in theories nearly 50 years ago that has been difficult to characterize experimentally. In a quantum spin liquid, electrons are constantly in flux, in contrast to the solid-state stability that characterizes a typical magnetic material when cooled to a certain temperature.

“The challenging thing about a quantum spin liquid is that by definition there’s no static magnetic ordering, so you can’t just map out a magnetic field” the way you would with another type of material, said de Leon. “Until now there’s been essentially no way to directly measure these two-point magnetic field correlators, and what people have instead been doing is trying to find complicated proxies for that measurement.”

By simultaneously measuring magnetic fields at multiple points with diamond sensors, researchers can detect how electrons and their spins are moving across space and time in a material. In developing the new method, the team applied calibrated laser pulses to a diamond containing NV centers, and then detected two spikes of photon counts from a pair of NV centers — a readout of the electron spins at each center at the same point in time. Previous techniques would have taken an average of these measurements, discarding valuable information and making it impossible to distinguish the intrinsic noise of the diamond and its environment from the magnetic field signals generated by a material of interest.

“One of those two spikes is a signal we’re applying, the other is a spike from the local environment, and there’s no way to tell the difference,” said study coauthor Shimon Kolkowitz, an associate professor of physics at the University of Wisconsin-Madison. “But when we look at the correlations, the one that is correlated is from the signal we’re applying and the other is not. And we can measure that, which is something people couldn’t measure before.”

Topologically protected vortex knots and links

by Toni Annala, Roberto Zamora-Zamora, Mikko Möttönen in Communications Physics

Scientists have shown how three vortices can be linked in a way that prevents them from being dismantled. The structure of the links resembles a pattern used by Vikings and other ancient cultures, although this study focused on vortices in a special form of matter known as a Bose-Einstein condensate. The findings have implications for quantum computing, particle physics and other fields.

Postdoctoral researcherToni Annala uses strings and water vortices to explain the phenomenon: ‘If you make a link structure out of, say, three unbroken strings in a circle, you can’t unravel it because the string can’t go through another string. If, on the other hand, the same circular structure is made in water, the water vortices can collide and merge if they are not protected.’

‘In a Bose-Einstein condensate, the link structure is somewhere between the two,’ says Annala, who began working on this in Professor Mikko Möttönen’s research group at Aalto University before moving back to the University of British Columbia and then to the Institute for Advanced Study in Princeton. Roberto Zamora-Zamora, a postdoctoral researcher in Möttönen’s group, was also involved in the study. The researchers mathematically demonstrated the existence of a structure of linked vortices that cannot break apart because of their fundamental properties.

‘The new element here is that we were able to mathematically construct three different flow vortices that were linked but could not pass through each other without topological consequences. If the vortices interpenetrate each other, a cord would form at the intersection, which binds the vortices together and consumes energy. This means that the structure cannot easily break down,’ says Möttönen.

Wirtinger presentation of the fundamental group of the link complement and colored link diagrams.

The structure is conceptually similar to the Borromean rings, a pattern of three interlinked circles which has been widely used in symbolism and as a coat of arms. A Viking symbol associated with Odin has three triangles interlocked in a similar way. If one of the circles or triangles is removed, the entire pattern dissolves because the remaining two are not directly connected. Each element thus links its two partners, stabilising the structure as a whole.

The mathematical analysis in this research shows how similarly robust structures could exist between knotted or linked vortices. Such structures might be observed in certain types of liquid crystals or condensed matter systems and could affect how those systems behave and develop.

‘To our surprise, these topologically protected links and knots had not been invented before. This is probably because the link structure requires vortices with three different types of flow, which is much more complex than the previously considered two-vortex systems,’ says Möttönen.

Decay of colored links via local and global vortex splitting.

These findings may one day help make quantum computing more accurate. In topological quantum computing, the logical operations would be carried out by braiding different types of vortices around each other in various ways. ‘In normal liquids, knots unravel, but in quantum fields there can be knots with topological protection, as we are now discovering,’ says Möttönen.

Annala adds that ‘the same theoretical model can be used to describe structures in many different systems, such as cosmic strings in cosmology.’ The topological structures used in the study also correspond to the vacuum structures in quantum field theory. The results could therefore also have implications for particle physics. Next, the researchers plan to theoretically demonstrate the existence of a knot in a Bose-Einstein condensate that would be topologically protected against dissolving in an experimentally feasible scenario.

‘The existence of topologically protected knots is one of the fundamental questions of nature. After a mathematical proof, we can move on to simulations and experimental research,’ says Möttönen.

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