Category: Physics

By using ultrafast laser flashes, scientists at the University of Rostock in collaboration with researchers of the Max Planck Institute for Solid State Research in Stuttgart have generated and measured the shortest electron pulse to date. The electron pulse was created by using lasers to remove electrons from a tiny metal tip and lasted only 53 attoseconds, that is, 53 billionths of a billionth of a second. The event has set a new speed record in man-made control of electric currents in solid materials.

The research opens new avenues for advancing the performance of electronics and information technologies, as well as developing new scientific methodologies for visualizing phenomena in the microcosm at ultimate speeds.

Ever wondered what makes your computer and your other electronic gadgets slow or fast in their performance? It is the time it takes electrons, some of the tiniest particles of our microcosm, to stream out from minute leads inside the transistors of electronic microchips and to form pulses. Methods to speed up this process are central for advancing electronics and their applications to ultimate performance limits. But what is the shortest possible streaming time of electrons from a tiny metal lead in an electronic circuit?

By using extremely short laser flashes, a team of researchers led by Professor Eleftherios Goulielmakis, head of the group Extreme Photonics of the institute for Physics at the University of Rostock, and collaborators at the Max Planck Institute of Solid State Research in Stuttgart used state-of-the art laser pulses to eject electrons from a tungsten nanotip to generate the shortest electron burst to date. This work is published in Nature.

Whereas it has long been known that light can release electrons from metals—Einstein was the first to explain how—the process is extremely hard to manipulate. The electric field of light changes its direction about a million billion times per second making it challenging to control the way it rips off electrons from the surface of metals.

To overcome this challenge, the Rostock scientists and their co-workers used a modern technology that had been previously developed by their group—light field synthesis—which allowed them to shorten a light flash to less than a full swing of its own field. In turn, they used these flashes to illuminate the tip of a tungsten needle to knock electrons free into vacuum.

“Using light pulses that comprise merely a single cycle of its field, it is now possible to give electrons a precisely controlled kick to set them free from the tungsten tip within a very short time interval,” explains Eleftherios Goulielmakis, head of the research group.

But the challenge could not be overcome unless the scientists also found a way to measure the brevity of these electron bursts. To deal with this hurdle, the team developed a new type of camera that can take snapshots of the electrons during the short time the laser is pushing them out from the nanotip and into the vacuum.

“The trick was to use a second, very weak, light flash” said Dr. Hee-Yong Kim, the leading author of the new study. “This second laser flash can gently perturb the energy of the electron burst to find out how it looks like in time. It is like the game ‘What’s in the box?’ where players try to identify an object without looking at it but just by turning it around to feel its shape with their hands.”

But how could this technology be used in electronics? “As technology advances rapidly, it is reasonable to expect the development of microscopic electronic circuits in which electrons travel in a vacuum space among closely packed leads to prevent obstacles that slow them down,” says Goulielmakis. “Using light to eject electrons and drive them among these leads could speed up future electronics by several thousand times of today’s performance.”

But the researchers believe that their newly developed methodology will be used directly for scientific purposes. “Ejecting electrons from a metal within a fraction of a light’s field cycle dramatically simplifies the experiments and allows us to use advanced theoretical methods to understand the emission of electrons in ways that were not previously possible,” says Professor Thomas Fennel, a coauthor in the new publication.

“Since our electron bursts provide excellent resolution for taking snapshots of electronic and atomic motions in materials, we plan to use them to acquire a deep understanding of complex materials to facilitate their applications in technology,” Goulielmakis says.

More information: H. Y. Kim et al, Attosecond field emission, Nature (2023). DOI: 10.1038/s41586-022-05577-1

Journal information: Nature 

Provided by University of Rostock 

Researchers have developed a new detector that can precisely measure single photons at very high rates. The new device could help make high-speed quantum communication practical.

Quantum communication uses light at the single photon level to send encoded quantum information such as encryption keys in quantum key distribution. Because of the laws of physics, data transmitted in this way is guaranteed to remain secure. Sending information at faster speeds requires a single photon detector that can not only detect photons quickly but also precisely measure their arrival times.

In an article published in Optica, researchers led by Matthew D. Shaw at NASA’s Jet Propulsion Laboratory describe and demonstrate their new detector for measuring the arrival times of photons, which they call PEACOQ (performance-enhanced array for counting optical quanta) detector.

“Our new detector is made of 32 niobium nitride superconducting nanowires on a silicon chip, which enables high count rates with high precision,” said research team member Ioana Craiciu, a postdoctoral scholar. “The detector was designed with quantum communication in mind, as this is a technological area that has been limited by the performance of available detectors.”

The detector was developed as part of a NASA program to enable new technology for space-to-ground quantum communication, which can allow sharing of quantum information across intercontinental distances in the future. This work builds upon technology developed for the NASA Deep Space Optical Communication project, which will be the first demonstration of free-space optical communication from interplanetary space.

“There is not currently another detector that can count single photons this quickly with the same timing resolution,” said Craiciu. “We know this detector will be useful for quantum communication, but we also hope that it could enable other applications that we haven’t considered.”

Faster quantum communication

Speeding up quantum communication transmission rates requires a detector on the receiving end that can make quick measurements and exhibits a short dead time so that it can contend with a high rate of arriving photons. The detector must also precisely measure the arrival time of the photons.

“Although there are detectors that can measure photon arrival times with high precision they struggle to keep up when the photons are arriving in quick succession and can miss some of the photons or get their arrival times wrong,” said Craiciu. “We designed the PEACOQ detector to precisely measure the arrival times of single photons even as they are hitting the detector at a high rate. It is also efficient—it doesn’t miss many of the photons.”

The PEACOQ detector is made of nanowires just 7.5 nm thick, or about 10,000 times thinner than a human hair. Operating it at very cold temperatures—around 1 Kelvin, or -458 °F—makes the nanowires superconducting, which means they have no electrical resistance. Under superconducting conditions, any photon that hits a wire has a good chance of being absorbed by that wire. Any absorbed photons create a hot spot that increases the wire’s electrical resistance in a detectable way. A computer along with a time-to-digital converter is used to record when the resistance changed and thus when a photon arrived at the detector.

“When the detector measures a photon, it outputs an electric pulse, and the time-to-digital converter measures the arrival time of this electrical pulse very precisely, with a resolution below 100 picoseconds or 70 million times faster than a snap of the fingers,” said Craiciu. “We developed a new time-to-digital converter that can measure up to 128 channels at once with this timing resolution, which is important because our detector needs 32 channels.”

To demonstrate the new detector, the researchers cooled it down to 1 Kelvin by installing it in a cryostat. They used a custom-built testing setup to send light into the cryostat to the detector and a chain of electronics to transmit the detector’s output signal out of the cryostat, amplify it and record it. Because there are 32 nanowires, the researchers had to use 32 sets of each component, including 32 cables and 32 of each kind of amplifier.

Unprecedented count rates

“We were very pleased with how well the detector worked,” said Craiciu. “The rate at which it can measure photons was the highest we have seen. It requires a complex setup because each of the 32 nanowires is read out individually, but for applications where you really need to measure photons at a high rate with high precision, it is worth the trouble.”

Typically, quantum information being transmitted is set to a clock, with each piece of information encoded into one photon and sent on a tick. How precisely you can measure the arrival time of the photons at the receiver determines how close together the ticks can be without making a mistake, and therefore it determines how quickly you can send the information. The new detector makes it practical to perform quantum communication with a state-of-the-art clock frequency of 10-GHz.

The researchers are still working to make improvements to the PEACOQ detector, which is currently about 80% efficient—meaning 20% of photons that hit the detector are not measured. They also plan to build a portable receiver unit that can be used for quantum communication experiments. It will contain several PEACOQ detectors along with optics, readout electronics and a cryostat.

More information: Ioana Craiciu et al, High-speed detection of 1550 nm single photons with superconducting nanowire detectors, Optica (2022). DOI: 10.1364/OPTICA.478960

Journal information: Optica 

Provided by Optica 

The standard model of particle physics tells us that most particles we observe are made up of combinations of just six types of fundamental entities called quarks. However, there are still many mysteries, one of which is an exotic, but very short-lived, Lambda resonance known as Λ(1405). For a long time, it was thought to be a particular excited state of three quarks—up, down, and strange—and understanding its internal structure may help us learn more about the extremely dense matter that exists in neutron stars.

Investigators from Osaka University were part of a team that has now succeeded in synthesizing Λ(1405) for the first time by combining a K^– meson and a proton and determining its complex mass (mass and width). The K⁻ meson is a negatively charged particle containing a strange quark and an up antiquark. The much more familiar proton that makes up the matter that we are used to has two up quarks and a down quark. The researchers showed that Λ(1405) is best thought of as a temporary bound state of the K^– meson and the proton, as opposed to a three-quark excited state.

In their study published recently in Physics Letters B, the group describes the experiment they carried out at the J-PARC accelerator. K⁻ mesons were shot at a deuterium target, each of which had one proton and one neutron. In a successful reaction, a K⁻ meson kicked out the neutron, and then merged with the proton to produce the desired Λ(1405).

“The formation of a bound state of a K^– meson and a proton was only possible because the neutron carried away some of the energy,” says Kentaro Inoue, an author of the study. One of the aspects that had been perplexing scientists about Λ(1405) was its very light overall mass, even though it contains a strange quark, which is nearly 40 times as heavy as an up quark. During the experiment, the team of researchers was able to successfully measure the complex mass of Λ(1405) by observing the behavior of the decay products.

• (Top) Measured reaction cross-section. The horizontal axis is the K^– and proton collision recoil energy converted into a mass value. Large reaction events occur at mass values lower than the sum of the K^– and proton masses, which itself suggests the existence of Λ(1405). The measured data were reproduced by scattering theory (solid lines). (Bottom) Distribution of K^– and proton scattering amplitudes. When squared, these correspond to the reaction cross-section, and are generally complex numbers. The calculated values match with the measured data. When the real part (solid line) crosses 0, the value of the imaginary part reaches its maximum value. This is a typical distribution for a resonance state, and determines the complex mass. The arrows indicate the real part. Credit: 2023, Hiroyuki Noumi, Pole position of Λ(1405) measured in d(K^-,n)πΣ reactions, Physics Letters B
• Schematic illustration of the reaction used to synthesize Λ(1405) by fusing a K^– (green circle) with a proton (dark blue circle), which takes place inside a deuteron nucleus. Credit: Hiroyuki Noumi
• (Top) Measured reaction cross-section. The horizontal axis is the K^– and proton collision recoil energy converted into a mass value. Large reaction events occur at mass values lower than the sum of the K^– and proton masses, which itself suggests the existence of Λ(1405). The measured data were reproduced by scattering theory (solid lines). (Bottom) Distribution of K^– and proton scattering amplitudes. When squared, these correspond to the reaction cross-section, and are generally complex numbers. The calculated values match with the measured data. When the real part (solid line) crosses 0, the value of the imaginary part reaches its maximum value. This is a typical distribution for a resonance state, and determines the complex mass. The arrows indicate the real part. Credit: 2023, Hiroyuki Noumi, Pole position of Λ(1405) measured in d(K^-,n)πΣ reactions, Physics Letters B
• Schematic illustration of the reaction used to synthesize Λ(1405) by fusing a K^– (green circle) with a proton (dark blue circle), which takes place inside a deuteron nucleus. Credit: Hiroyuki Noumi

“We expect that progress in this type of research can lead to a more accurate description of ultra-high-density matter that exists in the core of a neutron star,” says Shingo Kawasaki, another study author.

This work implies that Λ(1405) is an unusual state consisting of four quarks and one antiquark, making a total of five quarks, and does not fit the conventional classification in which particles have either three quarks or one quark and one antiquark. This research may lead to a better understanding of the early formation of the universe, shortly after the Big Bang, as well as what happens when matter is subject to pressures and densities well beyond what we see under normal conditions.

More information: S. Aikawa et al, Pole position of Λ(1405) measured in d(K⁻,n)πΣ reactions, Physics Letters B (2022). DOI: 10.1016/j.physletb.2022.137637

Journal information: Physics Letters B 

Provided by Osaka University 

Building off a breakthrough “anti-laser,” a team of researchers has developed a system that can direct light and other electromagnetic waves for signal processing without any unwanted signal reflections—an innovation that could advance local area networks, the field of photonics, and other applications.

The results, led by A. Douglas Stone of Yale and Philipp del Hougne of University of Rennes in France, are published in Science Advances.

A little more than a decade ago, Stone led a team in the creation of the anti-laser, or “coherent perfect absorber” (CPA). Instead of emitting a beam as a laser does, an anti-laser absorbs input light with the same precision.

In a laser, light bounces back and forth between two mirrors, each time passing through an amplifying material—known as the “gain medium”—such as gallium arsenide. Because the light is of a specific wavelength, it creates a feedback that increasingly gains in intensity. In a typical light source—an everyday lightbulb, for instance—atoms radiate independently and create light of many different wavelengths, and light goes in many directions as a result. In lasers, though, atoms radiate at the same frequency and in the same direction, creating a concentrated beam of a single wavelength.

The difference in the anti-laser is that instead of using an amplifying material, it uses one that absorbs the light—that is, a “loss medium.” In its simplest version, the anti-laser splits a single laser beam into two and directs the two beams into each other, meeting at a paper-thin silicon wafer. The light’s waves are precisely tuned to interlock with each other and become trapped. They then dissipate into heat.

For their most recent work, the researchers built off this concept and developed a device based on what they call “reflectionless scattering modes” (RSMs).

“We asked if there is some principle like this where we can guide light instead of transducing it into another form of energy,” said Stone, the Carl A. Morse Professor of Applied Physics and Physics. “Because with optical fibers and modern photonic circuits, guiding light and not having any of it reflect back is extremely valuable.”

From there, they developed the device that, instead of absorbing the waves, redirected them to specific channels. Stone worked on the theoretical side of the project, while Philipp del Hougne of University of Rennes in France built the actual device.

“Instead of having it all transduced, it could either all go into our chosen output channels or some of it could be absorbed and the rest go into the output channels,” Stone said. “In the next step, we want to make a similar device where the absorption is negligible, so that all of the energy is efficiently routed to perform its information or sensing function. There is great interest in such technologies to reduce the power consumption of cell phone networks, for example.”

The device eliminated signal reflections, which have long been a problem for signal routers, a pivotal ingredient of modern nanophotonic and radiofrequency networks. Besides causing a loss in signal power, such reflections can cause devastating unwanted reflected-signal-power echoes in the network.

More information: Jérôme Sol et al, Reflectionless programmable signal routers, Science Advances (2023). DOI: 10.1126/sciadv.adf0323

Journal information: Science Advances 

Provided by Yale University 

Quantum sensors can be used to reveal a surprising new mechanism for converting light into electricity in Weyl semimetals, Boston College (BC) Assistant Professor of Physics Brian Zhou and colleagues report in the journal Nature Physics.

A number of modern technologies, such as cameras, fiber optic networks, and solar cells rely on the conversion of light into electrical signals. But with most materials, shining a light onto their surface will not generate any electricity because there is no preferred direction for the electricity to flow. The unique properties of electrons in Weyl semimetals have made them a focus of researchers trying to overcome those limits and develop novel optoelectronic devices.

“Most photoelectrical devices require two different materials to create an asymmetry in space,” said Zhou, who worked with eight BC colleagues and two researchers from the Nanyang Technological University in Singapore. “Here, we showed that the spatial asymmetry within a single material—in particular the asymmetry in its thermoelectric transport properties—can give rise to spontaneous photocurrents.”

The team studied the materials tungsten ditelluride and tantalum iridium tetratelluride, which both belong to the class of Weyl semimetals. Researchers have suspected that these materials would be good candidates for photocurrent generation because their crystal structure is inherently inversion asymmetric; that is to say, the crystal does not map onto itself by reversing directions about a point.

Zhou’s research group set out to understand why Weyl semimetals are efficient at converting light into electricity. Previous measurements could only determine the amount of electricity coming out of a device, like measuring how much water flows from a sink into a drainpipe. To better understand the origin of the photocurrents, Zhou’s team sought to visualize the flow of electricity within the device—similar to making a map of the swirling water currents in the sink.

“As part of the project, we developed a new technique using quantum magnetic field sensors called nitrogen-vacancy centers in diamond to image the local magnetic field produced by the photocurrents and reconstruct the full streamlines of the photocurrent flow,” graduate student Yu-Xuan Wang, lead author on the manuscript, said.

The team found the electrical current flowed in a four-fold vortex pattern around where the light shined on the material. The team further visualized how the circulating flow pattern is modified by the edges of the material and revealed that the precise angle of the edge determines whether the total photocurrent flowing out of the device is positive, negative, or zero.

“These never-before-seen flow images allowed us to explain that the photocurrent generation mechanism is surprisingly due to an anisotropic photothermoelectric effect—that is to say, differences in how heat is converted to current along the different in-plane directions of the Weyl semimetal,” Zhou said.

Surprisingly, the appearance of anisotropic thermopower is not necessarily related to the inversion asymmetry displayed by Weyl semimetals, and hence, may be present in other classes of materials.

“Our findings open a new direction for searching for other highly photoresponsive materials,” Zhou said. “It showcases the disruptive impact of quantum-enabled sensors on open questions in materials science.”

Zhou said future projects will use the unique photocurrent flow microscope to understand the origins of photocurrents in other exotic materials and to push the limits in detection sensitivity and spatial resolution.

More information: Yu-Xuan Wang et al, Visualization of bulk and edge photocurrent flow in anisotropic Weyl semimetals, Nature Physics (2023). DOI: 10.1038/s41567-022-01898-0

Journal information: Nature Physics 

Provided by Boston College 

An international team of scientists have demonstrated a leap in preserving the quantum coherence of quantum dot spin qubits as part of the global push for practical quantum networks and quantum computers.

These technologies will be transformative to a broad range of industries and research efforts: from the security of information transfer, through the search for materials and chemicals with novel properties, to measurements of fundamental physical phenomena requiring precise time synchronization among the sensors.

Spin-photon interfaces are elementary building blocks for quantum networks that allow converting stationary quantum information (such as the quantum state of an ion or a solid-state spin qubit) into light, namely photons, that can be distributed over large distances. A major challenge is to find an interface that is both good at storing quantum information and efficient at converting it into light.

Optically active semiconductor quantum dots are the most efficient spin-photon interface known to date but extending their storage time beyond a few microseconds has puzzled physicists in spite of decade-long research efforts. Now, researchers at the University of Cambridge, the University of Linz and the University of Sheffield have shown that there is a simple material’s solution to this problem that improves the storage of quantum information beyond hundred microseconds.

Quantum Dots are crystalline structures made out of many thousands of atoms. Each of these atoms’ nuclei has a magnetic dipole moment that couples to the quantum dot electron and can cause the loss of quantum information stored in the electron qubit. The research team’s finding, reported in Nature Nanotechnology, is that in a device constructed with semiconductor materials that have the same lattice parameter, the nuclei ‘felt’ the same environment and behaved in unison. As a result, it is now possible to filter out this nuclear noise and achieve a near two-order magnitude improvement in storage time.

“This is a completely new regime for optically active quantum dots where we can switch off the interaction with nuclei and refocus the electron spin over and over again to keep its quantum state alive,” said Claire Le Gall from Cambridge’s Cavendish Laboratory, who led the project.

“We demonstrated hundreds of microseconds in our work, but really, now we are in this regime, we know that much longer coherence times are within reach. For spins in quantum dots, short coherence times were the biggest roadblock to applications, and this finding offers a clear and simple solution to that.”

While exploring the hundred-microsecond timescales for the first time, the researchers were pleasantly surprised to find that the electron only sees noise from the nuclei as opposed to, say, electrical noise in the device. This is really a great position to be in because the nuclear ensemble is an isolated quantum system, and the coherent electron will be a gateway to quantum phenomena in large nuclear spin ensemble.

Another thing that surprised the researchers was the ‘sound’ that was picked up from the nuclei. It was not quite as harmonious as was initially anticipated, and there is room for further improvement in the system’s quantum coherence through further material engineering.

“When we started working with the lattice-matched material system employed in this work, getting quantum dots with well-defined properties and good optical quality wasn’t easy,” says Armando Rastelli, co-author of this paper at the University of Linz.

“It is very rewarding to see that an initially curiosity-driven research line on a rather ´exotic´ system and the perseverance of skilled team members Santanu Manna and Saimon Covre da Silva led to the devices at the basis of these spectacular results. Now we know what our nanostructures are good for, and we are thrilled by the perspective of further engineering their properties together with our collaborators.”

“One of the most exciting things about this research is taming a complex quantum system: a hundred thousand nuclei coupling strongly to a well-controlled electron spin,” explains Cavendish Ph.D. student, Leon Zaporski—the first author of the paper.

“Most researchers approach the problem of isolating qubit from the noise by removing all the interactions. Their qubits become a bit like sedated Schrödinger’s cats, that can barely react to anyone pulling on their tail. Our ‘cat’ is on strong stimulants, which—in practice—means we can have more fun with it.”

“Quantum dots now combine high photonic quantum efficiency with long spin coherence times,” explains Professor Mete Atatüre, co-author of this paper. “In the near future, we envisage these devices to enable the creation of entangled light states for all-photonic quantum computing and allow foundational quantum control experiments of the nuclear spin ensemble.”

More information: Leon Zaporski et al, Ideal refocusing of an optically active spin qubit under strong hyperfine interactions, Nature Nanotechnology (2023). DOI: 10.1038/s41565-022-01282-2

Journal information: Nature Nanotechnology 

Provided by University of Cambridge 

Researchers at EPFL have developed a new technique that can visualize and control the rotation of a handful of spins arranged in a vortex-like texture at the fastest speed ever achieved. The breakthrough can advance “spintronics,” a technology that includes new types of computer memory, logic gates, and high-precision sensors.

“Technological advancements in computation, data storage and sensing all require new techniques to control the nanoscaled magnetic properties of materials,” says Professor Fabrizio Carbone at EPFL’s School of Basic Sciences. One of these properties is “spin,” which refers to the magnetic orientation of individual atoms.

Spin has attracted a lot of interest in recent years, giving rise to the field of spin electronics or “spintronics.” Apart from the fundamental study of spin, the more practical aim of spintronics is to exploit not just the charge of electrons—as in traditional electronics—but also their spin, adding and extra degree of freedom that can improve the efficiency of data storage and transfer.

However, this first requires that we can control small numbers of spins. “The visualization and deterministic control of very few spins has not yet been achieved at the ultrafast timescales,” says Dr. Phoebe Tengdin, a postdoc in Carbone’s lab, pointing out the very tight timeframes that this control needs to happen for spintronics to ever make the leap into applications.

Now, Tengdin along with Ph.D. student Benoit Truc and fellow postdoc Dr. Alexey Sapozhnik have developed a new technique that can visualize and control the rotation of a handful of spins arranged in a vortex-like texture, a kind of spin “nano-whirlpool” called a skyrmion.

To do this, the scientists used sequences of laser pulses at a femtosecond timeframe (10^-15 or a quadrillionth of a second). By arranging the laser pulses apart just right, they were able to control the rotation of spins in a selenium-copper mineral known in the field by its chemical composition, Cu₂OSeO₃. The mineral is quite popular in the field of spintronics, as it provides an ideal testbed for studying spins.

Controlling the spins with laser pulses, the researchers found that they could even switch their orientation at will by simply changing the delay time between successive driving pulses and adjusting the laser polarization.

But the study didn’t stop there. By using using a type of transmission electron microscope that can “see” nanoscale dimensions, the team were also able to actually image the spin changes. The breakthrough has enormous implications for the fundamental aspects of spintronics.

The research is published in the journal Physical Review X.

The work offers the field a new protocol for controlling magnetic textures at ultrafast timescales, and opens up exciting new opportunities for spin switches in next-generation information storage devices.

“Our experiments demonstrate that it is possible to manipulate and image a handful of spins at very high speed using a moderate intensity light beam,” says Tengdin. “Such an effect can be exploited in low-consumption ultrafast devices operating on spins. New types of memories or logic gates are possible candidates, as are high-precision sensors.”

More information: Phoebe Tengdin et al, Imaging the Ultrafast Coherent Control of a Skyrmion Crystal, Physical Review X (2022). DOI: 10.1103/PhysRevX.12.041030

Journal information: Physical Review X 

Provided by Ecole Polytechnique Federale de Lausanne 

In a new breakthrough, researchers at the University of Copenhagen, in collaboration with Ruhr University Bochum, have solved a problem that has caused quantum researchers headaches for years. The researchers can now control two quantum light sources rather than one. Trivial as it may seem to those uninitiated in quantum, this colossal breakthrough allows researchers to create a phenomenon known as quantum mechanical entanglement. This in turn, opens new doors for companies and others to exploit the technology commercially.

Going from one to two is a minor feat in most contexts. But in the world of quantum physics, doing so is crucial. For years, researchers around the world have strived to develop stable quantum light sources and achieve the phenomenon known as quantum mechanical entanglement—a phenomenon, with nearly sci-fi-like properties, where two light sources can affect each other instantly and potentially across large geographic distances.

Entanglement is the very basis of quantum networks and central to the development of an efficient quantum computer.

Today, researchers from the Niels Bohr Institute published a new result in the journal Science, in which they succeeded in doing just that. According to Professor Peter Lodahl, one of the researchers behind the result, it is a crucial step in the effort to take the development of quantum technology to the next level and to “quantize” society’s computers, encryption and the internet.

“We can now control two quantum light sources and connect them to each other. It might not sound like much, but it’s a major advancement and builds upon the past 20 years of work. By doing so, we’ve revealed the key to scaling up the technology, which is crucial for the most ground-breaking of quantum hardware applications,” says Professor Peter Lodahl, who has conducted research the area since 2001.

The magic all happens in a so-called nanochip—which is not much larger than the diameter of a human hair—that the researchers also developed in recent years.

Quantum sources overtake the world’s most powerful computer

Peter Lodahl’s group is working with a type of quantum technology that uses light particles, called photons, as micro transporters to move quantum information about.

While Lodahl’s group is a leader in this discipline of quantum physics, they have only been able to control one light source at a time until now. This is because light sources are extraordinarily sensitive to outside “noise”, making them very difficult to copy. In their new result, the research group succeeded in creating two identical quantum light sources rather than just one.

“Entanglement means that by controlling one light source, you immediately affect the other. This makes it possible to create a whole network of entangled quantum light sources, all of which interact with one another, and which you can get to perform quantum bit operations in the same way as bits in a regular computer, only much more powerfully,” explains postdoc Alexey Tiranov, the article’s lead author.

This is because a quantum bit can be both a 1 and 0 at the same time, which results in processing power that is unattainable using today’s computer technology. According to Professor Lodahl, just 100 photons emitted from a single quantum light source will contain more information than the world’s largest supercomputer can process.

By using 20-30 entangled quantum light sources, there is the potential to build a universal error-corrected quantum computer—the ultimate “holy grail” for quantum technology, that large IT companies are now pumping many billions into.

Other actors will build upon the research

According to Lodahl, the biggest challenge has been to go from controlling one to two quantum light sources. Among other things, this has made it necessary for researchers to develop extremely quiet nanochips and have precise control over each light source.

With the new research breakthrough, the fundamental quantum physics research is now in place. Now it is time for other actors to take the researchers’ work and use it in their quests to deploy quantum physics in a range of technologies including computers, the internet and encryption.

“It is too expensive for a university to build a setup where we control 15-20 quantum light sources. So, now that we have contributed to understanding the fundamental quantum physics and taken the first step along the way, scaling up further is very much a technological task,” says Professor Lodahl.

More information: Alexey Tiranov et al, Collective super- and subradiant dynamics between distant optical quantum emitters, Science (2023). DOI: 10.1126/science.ade9324. www.science.org/doi/10.1126/science.ade9324

Journal information: Science 

Provided by University of Copenhagen 

Starting with the emergence of quantum mechanics, the world of physics has been divided between classical and quantum physics. Classical physics deals with the motions of objects we typically see every day in the macroscopic world, while quantum physics explains the exotic behaviors of elementary particles in the microscopic world.

Many solids or liquids are composed of particles interacting with one another at close distances, which sometimes results in the rise of “quasiparticles.” Quasiparticles are long-lived excitations that behave effectively as weakly interacting particles. The idea of quasiparticles was introduced by the Soviet physicist Lev Landau in 1941, and ever since has been highly fruitful in quantum matter research. Some examples of quasiparticles include Bogoliubov quasiparticles (i.e. “broken Cooper pairs”) in superconductivity, excitons in semiconductors, and phonons.

Examining emergent collective phenomena in terms of quasiparticles provided insight into a wide variety of physical settings, most notably in superconductivity and superfluidity, and recently in the famous example of Dirac quasiparticles in graphene. But so far, the observation and use of quasiparticles have been limited to quantum physics: in classical condensed matter, the collision rate is typically much too high to allow long-lived particle-like excitations.

However, The standard view that quasiparticles are exclusive to quantum matter has been recently challenged by a group of researchers at the Center for Soft and Living Matter (CSLM) within the Institute for Basic Science (IBS), South Korea. They examined a classical system made of microparticles driven by viscous flow in a thin microfluidic channel. As the particles are dragged by the flow, they perturb the streamlines around them, thereby exerting hydrodynamic forces on each other.

Remarkably, the researchers found that these long-range forces make the particles organize in pairs. This is because the hydrodynamic interaction breaks Newton’s third law, which states that the forces between two particles must be equal in magnitude and opposite in direction. Instead, the forces are ‘anti-Newtonian’ because they are equal and in the same direction, thus stabilizing the pair.

The large population of particles coupled in pairs hinted that these are the long-lived elementary excitations in the system—its quasiparticles. This hypothesis was proven right when the researchers simulated a large two-dimensional crystal made of thousands of particles and examined its motion. The hydrodynamic forces among the particles make the crystal vibrate, much like the thermal phonons in a vibrating solid body.

These pair quasiparticles propagate through the crystal, stimulating the creation of other pairs through a chain reaction. The quasiparticles travel faster than the speed of phonons, and thus every pair leaves behind an avalanche of newly-formed pairs, just like the Mach cone generated behind a supersonic jet plane. Finally, all those pairs collide with each other, eventually leading to the melting of the crystal.

The melting induced by pairs is observed in all crystal symmetries except for one particular case: the hexagonal crystal. Here, the three-fold symmetry of hydrodynamic interaction matches the crystalline symmetry and, as a result, the elementary excitations are extremely slow low-frequency phonons (and not pairs as usual). In the spectrum, one sees a “flat band” where these ultra-slow phonons condense. The interaction among the flat-band phonons is highly collective and correlated, which shows in the much sharper, different class of melting transition.

Notably, when analyzing the spectrum of the phonons, the researchers identified conical structures typical of Dirac quasiparticles, just like the structure found in the electronic spectrum of graphene. In the case of the hydrodynamic crystal, the Dirac quasiparticles are simply particle pairs, which form thanks to the ‘anti-Newtonian’ interaction mediated by the flow. This demonstrates that the system can serve as a classical analog of the particles discovered in graphene.

“The work is a first-of-its-kind demonstration that fundamental quantum matter concepts—particularly quasiparticles and flat bands—can help us understand the many-body physics of classical dissipative systems,” explains Tsvi Tlusty, one of the corresponding authors of the paper.

Moreover, quasiparticles and flat bands are of special interest in condensed matter physics. For example, flat bands were recently observed in double layers of graphene twisted by a specific “magic angle”, and the hydrodynamic system studied at the IBS CSLM happens to exhibit an analogous flat band in a much simpler 2D crystal.

“Altogether, these findings suggest that other emergent collective phenomena that have been so far measured only in quantum systems may be revealed in a variety of classical dissipative settings, such as active and living matter,” says Hyuk Kyu Pak, one of the corresponding authors of the paper.

More information: Imran Saeed, Quasiparticles, flat bands and the melting of hydrodynamic matter, Nature Physics (2023). DOI: 10.1038/s41567-022-01893-5. www.nature.com/articles/s41567-022-01893-5

Journal information: Nature Physics 

Provided by Institute for Basic Science 

Fluid dynamics researchers use many techniques to study turbulent flows like ocean currents, or the swirling atmosphere of other planets. Arezoo Adrekani’s team has discovered that a mathematical construct used in these fields provides valuable information about stress in complex flow geometries.

Ardekani, a Purdue University professor of mechanical engineering, studies complex flows: from the transport processes related to biopharmaceuticals, to the behavior of microorganisms around an oil spill. “Newtonian fluids like water are simple to understand, because they have no microstructure,” she said. “But complex fluids have macromolecules that stretch and relax, and that changes many properties of the fluid, leading to very exciting fluid dynamics.”

Viscoelastic flows occur frequently in nature, in biomedical settings, and in industrial applications—such as solutions used in groundwater remediation. “When groundwater becomes contaminated, remediators use certain polymer-based solutions to disperse chemicals designed to break down the contaminants,” Ardekani said. “But what type of polymer should they use, how much, and where should they inject it? The only way to answer those questions is by understanding the behavior of these flows, which comes down to measuring stresses.”

Currently, the only way to quantify the stresses of polymeric fluids is a technique called birefringence, which measure specific optical properties of the fluid. But it’s very difficult to perform, often inaccurate, and doesn’t apply to all types of macromolecules.

Ardekani’s team has discovered a new technique. The researchers created a mathematical framework that takes input from flow velocity, obtained from particle image velocimetry (a common technique in fluid dynamics), and outputs stress and stretching field topologies for complex fluids. Their research has been featured in Proceedings of the National Academy of Sciences (PNAS).

In particle image velocimetry (PIV), tracer particles are injected into a fluid. By using the movement of those particles, researchers can extrapolate information about the overall flow kinematics. While this can be readily used to evaluate stress in Newtonian fluids, Ardekani’s team has discovered a mathematical correlation between these measurements and the stresses in viscoelastic flows.

It all connects through something called Lagrangian coherent structures (LCSs). “Lagrangian coherent structures are mathematical constructs used to predict the dynamics of fluid flows,” Ardekani said. “They are used by oceanographers to predict how currents will move; biologists who are tracking microorganisms; and even astrophysicists, who are observing the turbulent clouds on places like Jupiter.”

While LCSs are often used by turbulence researchers, they have never been applied to polymeric stress until now. “We have united two disparate branches of continuum mechanics,” Ardekani said. “Using Lagrangian stretching, and applying it to Eulerian stress fields. And this applies to a wide range of scales, from the mesoscale all the way up to industry scale measurements.”

The paper is a collaboration between Ardekani, her Ph.D. student Manish Kumar and Jeffrey Guasto, Associate Professor of Mechanical Engineering at Tufts University. They presented their findings in November at the 75th Annual Meeting of the APS (American Physical Society) Division of Fluid Dynamics in Indianapolis, which Ardekani co-organized.

While the research is largely mathematical, Ardekani is excited to see how experimentalists will use the technique in the lab and in the real world. “Let’s use our groundwater remediation example again,” Ardekani said. “Researchers typically use tracer analysis on the injected fluids to measure the velocity field. But now, they can also identify the stress fields, so they can more accurately predict the transport of that fluid.”

More information: Manish Kumar et al, Lagrangian stretching reveals stress topology in viscoelastic flows, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2211347120

Journal information: Proceedings of the National Academy of Sciences 

Provided by Purdue University