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Researchers shrink double-slit experiment to atomic scale

Shrinking Young’s double-slit experiment
Shrinking Young’s experiment (a) The classical double-slit experiment using light (slit spacing ~1 mm, propagation distance ~1 m). (b) Schematic of atomic-scale interference in which a pair of neighbouring silicon atomic columns act as a “double slit” for a focused electron probe (spacing 136 pm; crystal thickness ~10 nm; distance to detector ~10 cm). (Courtesy: Koudai Tabata/University of Tokyo)

For the first time, physicists in Japan have carried out an atomic-scale version of the double-slit experiment. Using an ultraprecise electron beam channelled along two adjacent columns of atoms in a crystal of silicon, Naoya Shibata and colleagues at the University of Tokyo observed clear interference fringes in electrons scattered by the atoms: fringes that persisted even at high temperature, thanks to correlations in the atoms’ thermal vibrations.

The double-slit experiment is perhaps the most iconic demonstration in quantum mechanics: showing how a single particle’s wavefunction interferes with itself as it passes through two slits at once, with a tangible impact on where the particle is ultimately observed.

In practical research, this effect is now routinely harnessed in electron, neutron and atom interferometers. By observing the interference fringes that it creates, researchers can extract valuable information about both the quantum matter wave and whatever object is doing the diffracting.

Recently, physicists have attempted to push double-slit interferometry all the way down to the atomic scale, replacing conventional slits with crystal lattices. When a moving electron encounters an atom in a crystal, it scatters across a range of angles – producing an outgoing spherical wave that’s mathematically identical to the diffraction pattern from a classical slit.

Since atoms in a crystal sit at such well-defined separations, any neighbouring pair of them could, in principle, act as a double-slit interferometer. In reality, however, the many repeated unit cells in a crystal mean the resulting interference fringes are washed out, making it difficult to isolate the signal from a single atomic pair.

To address this challenge, Shibata’s team used the ultraprecise probe of a 4D scanning transmission electron microscope: an instrument designed to capture electron diffraction patterns with a pixellated detector. Fired into a pure silicon crystal at just the right orientation, the microscope’s precision let the researchers target two adjacent columns of aligned silicon atoms, separated by just 136 pm. As a result, the electron wave overlapped both columns, and no others.

“Our key idea was to use these two atomic columns as the two slits,” describes team member Takehito Seki. “By channelling the probe along both columns at once, we turned them into two coherent scattering sources – effectively making the crystal itself into an atomic-scale double-slit interferometer.”

At this scale, Shibata and Seki’s team created electron interference fringes some seven orders of magnitude smaller than in Thomas Young’s original double-slit experiment with light, first carried out in 1801.

Just as in a larger-scale experiment, these patterns encoded information about the crystal and the electron beam. But at this scale, the researchers could also detect the signature of individual phonon modes in the columns. Rather than destroying coherence as uncorrelated atomic motion would in a classical picture, these correlated vibrations between the two atomic columns kept them moving in sync, preserving the interference.

“In a which-path picture, relative motion of the two atomic columns makes the two electron pathways more distinguishable and reduces the interference, whereas correlated in-phase motion preserves coherence,” Shibata explains. As a result, distinct interference fringes remained visible across temperatures ranging from 300 to 900 K.

The team’s results already point to how an atomic-scale double-slit experiment could offer new ways to measure phonon correlations directly between single pairs of atoms. But as Shibata explains, the possible applications could stretch much further.

“It also opens a route to probing local lattice dynamics at the scale of individual atomic bonds, including at interfaces and defects, where local lattice dynamics can strongly influence thermal transport,” he says. “In the long run, such atomic-scale insight could help inform better thermal management strategies for future chip technologies.”

The study is published in Nature.

Noise that entangles may accelerate the dawn of quantum technologies

Qubits are delicate and often misbehave. A qubit’s advantage over an ordinary bit is that in addition to “0” or “1”, it can hold values of 0 and 1 at once, in a specific combination; that combination is what quantum algorithms actually run on. But any stray interaction with the surrounding world (a vibration, a stray field, a single errant photon) leaks out a bit of information about which state the qubit is in – and that’s enough to collapse the combination back into an ordinary, classical either/or state. Coupling to the environment leads to high decoherence, meaning high error rates, and is normally how we lose good quality qubits. As such, most of the engineering effort in quantum computing goes into avoiding this coupling.

Yet recently, a team at the Institute of Science and Technology Austria (ISTA), working with collaborators in Munich and Madrid, has defied this norm.

Writing in Physical Review X, Alejandro Andrés-Juanes, Johannes Fink and colleagues show that two superconducting transmon qubits, separated by a metre of coaxial cable, can settle into an entangled state by being exposed to the same quantum-correlated microwave field. Avoiding synchronized pulses, heralding, post-selection and feedback (techniques usually used to actively generate and verify entanglement, rather than to fix decoherence), the entanglement can be generated and maintained for as long as the field is on. In other words, the researchers have managed to exploit noise to generate entanglement, rather than losing their qubits to it.

This idea of entangling by using fields belongs to a family of techniques known as dissipation engineering. “The common understanding is that the environment is bad, and it will decohere your qubits,” Andrés-Juanes tells Physics World. “The main idea [here] is that one can engineer a specific interaction with the environment that is not detrimental but – like in this case – prepares your qubits in an interesting state without you having to actively apply any action on the system.”

This proposal dates back more than 20 years, to work performed by Barbara Kraus and Ignacio Cirac, but this is its first experimental realization.

The correlated environment comes from a superconducting device (a Josephson parametric converter) that splits each pump photon into a pair of photons at gigahertz frequencies. The two photons of this pair are entangled with one another and each is sent down its own coaxial cable to one of the qubits, half a metre away in either direction.

Quantum research at ISTA
Quantum research The lab at the Institute of Science and Technology Austria. (Courtesy: © ISTA)

Because the noise reaching the two qubits is correlated, relaxing into it does not scramble them (specifically, it does not wash out the fixed phase relationship between them, as independent noise on each qubit would). They settle instead into a superposition, for which absorbing a photon from the field and emitting one into it interfere destructively: “the destructive interference between a photon emission event in the first waveguide and a photon absorption process at the location of the second qubit,” as the authors explain.

Once there, the qubits stop evolving and the field passes through unchanged: the entangled state is dark, invisible to the very field that created it. What the experiment does, in effect, is convert the continuous-variable (CV) entanglement carried by the two microwave beams into ordinary entanglement between two conventional transmon qubits – “we obtain qubit entanglement from CV entanglement,” Andrés-Juanes confirms – with roughly a tenth of it inherited.

Most entanglement-distribution schemes deliver entanglement as events: a pulse sequence runs, a detector clicks, a pair is announced, and the state then decays until the sequence runs again. The author’s autonomous version could enable the elimination of this machinery.

“It removes a lot of overhead on the pulse sequences you need to run to get the entanglement,” says Andrés-Juanes, an advantage that’s expected to matter most at scale, since one correlated photon source can drive many pairs at once. “Another differential feature of this protocol is that the entanglement is ‘always on’ for when you need to use it. In active protocols, you would have to reinitialize the entangled state because it would decohere after a while.”

The practical limit of this setup is temperature: “We use microwave frequencies, which are only quantum at very low temperatures,” Andrés-Juanes explains. “The real limit in the separation is how big your dilution refrigerator is”, with the longest such link so far being 30 m, at ETH Zurich. He positions the work as an intermediate-distance technology rather than a long-distance one. At optical frequencies, the constraint would lift, though achieving the same strong qubit–waveguide coupling in an atomic system would be its own problem.

For now, the numbers are modest: two qubits, entangled with a concurrence of just 0.10 (on a scale where 1 is a perfect pair), inheriting only about a tenth of the entanglement carried by the microwave beams. The group is working on a module with more than one qubit per node.

But critically, the idea is no longer theoretical. The team has now realised an entangled link that is always “there”, bringing us one step closer to distributed quantum computing (linking separate processors into one machine via entanglement), networked sensing (detectors sharing entanglement to jointly see what none could alone) and quantum repeaters (relay stations for long-distance quantum links, needed because a quantum state can’t be copied and amplified like a classical signal).

Magnetism likely played a key role in early solar system formation

The oldest known solids in the solar system contain evidence that the protoplanetary disc that went on to form the planets around our Sun experienced a magnetic field stronger than the field on Earth today, new research suggests. Study of inclusions in a primordial meteorite called a carbonaceous chondrite indicates that magnetism needs to be considered in simulations of planet formation, and may even be the dominant factor.

When planetary systems condense, they initially form high-temperature molecular nebulae. These later cool and accrete into planets surrounding central stars – which, at some point, begin to undergo nuclear fusion. Even before this happens, however, the gas is highly ionized. “A good portion of the ionization is coming from the molecular cloud itself,” explains planetary scientist Cauê Borlina of Purdue University. “It’s a combination of radiation from space and high temperature.”

In any ionized system there is the potential for magnetic fields, but simulations of disc formation often take no account of them, partly as they remain poorly constrained: “It’s a lot easier to just crank up the gravity and use that as a way to match the accretion rate,” says Borlina.

The new study from Borlina and colleagues in the US, China and the UK provides an unprecedented peek at fields in the terrestrial planet-forming region in the first 500,000 years of the Solar System’s existence – before the Earth had formed – using calcium-aluminium-rich inclusions. These are the oldest known solar system solids, comprising minerals with extremely high melting points that are inclined to condense early. “It is very unlikely to get them at later stages,” says Borlina. Previous researchers have confirmed their ages using isotopic dating.

Millions of years after these inclusions formed, they agglomerated with other protoplanetary material into planetesimals, some of which went on to form planets. Planetary scientists think the remaining material accreted to form the chondrites – which astronomers distinguish into several different types depending on their chemical composition. When they fall to Earth as chondrite meteorites, therefore, they provide a valuable glimpse into the solar system’s raw material.

Carbonaceous chondrites hold particular interest as they are thought to have the most primitive chemical compositions of all meteorites. The researchers studied calcium-aluminium inclusions in the 667 g Dominion Range 08006 carbonaceous chondrite discovered in Antarctica in 2008. Such inclusions had, until around 2010, been thought to contain no magnetic minerals. Borlina and colleagues, however, found that some – although not all – of the inclusions contained nanometre-scale amounts of ferromagnetic iron-nickel.

The researchers measured the thermoremanent magnetization of these iron-nickel grains (the field strength they experienced at the time when they cooled down through their “Curie temperature”, thereby fixing their own magnetism). In theory, this could be achieved by heating the sample above the Curie temperature, which for iron-nickel is about 1050 K, and measuring the field required to demagnetize it. Unfortunately, says Borlina, heating meteorites up in the laboratory tends to oxidize the materials extremely quickly, making reliably calibrated experiments very difficult.

Instead, the researchers used an established method called anhysteric remanent magnetization, concluding that the inclusions were exposed to fields of 150–600 µT when they cooled through the Curie temperature (the field on Earth today is around 30–60 µT). “This is good evidence that…you can’t just ignore magnetic fields,” says Borlina. “They need to be present in your simulations.”

The researchers are now studying other carbonaceous chondrites and other types of chondrite meteorites – some of which are thought to have formed in different regions of the protoplanetary disc – hoping to gain a fuller picture of how magnetism shapes disc evolution. “The idea was to put it out there that there’s this new inclusion that might hold magnetic carriers, and there’s a lot of work to be done,” says Borlina.

Meenakshi Wadhwa of University of California, San Diego – who formerly directed the Center for Meteorite Studies at Arizona State University – describes the work as “genuinely significant”. First, she says, it provides the first definitive paleomagnetic signal from a carbonaceous chondrite. Second, the estimated field is multiple times previous estimates.

“Yes, the sample size is small (five inclusions from a single chondrite), but I think the authors make a reasonably solid argument for their conclusions,” she says. The next step, she concludes, “is to measure inclusions from other chondrite groups to see if the signal is reproducible, and to test whether it’s a real nebular record versus something very localized, specific to DOM 08006 or the carbonaceous chondrite reservoir. Beyond that…pinning down when and where inclusions acquired their magnetization will also be important.”

The research is published in Proceedings of the National Academy of Sciences.

Quiz of the week: how much data will NASA’s Nancy Grace Roman Telescope produce each day?

Fancy some more? Check out our puzzles page.

Astronomers discover unusual 10-sided atmospheric wave on Saturn

While Earth’s atmosphere is constantly changing, many spectacular storms and atmospheric waves on Saturn remain stable for decades or even centuries.

Images from NASA’s Voyager craft as it flew-by the planet in the early 1980s revealed a large and long-lived hexagonal atmospheric wave around Saturn’s north pole.

Yet despite decades of observations from other craft such as the Cassini mission, no similar wave has been reported in the southern hemisphere or elsewhere.

Now, Agustín Sánchez-Lavega from the University of the Basque Country in Bilbao and colleagues have spotted a growing decagon-shaped wave around the planet’s south pole.

The researchers discover the phenomenon thanks to images from NASA’s Hubble Space Telescope as well as ground-based telescopes as the planet’s south pole came back into view from Earth due to Saturn’s axial tilt.

It seems that the 10-sided wave developed between 2023 and 2025 and may still be evolving, in contrast to the 40-plus years of steadiness shown by the hexagon.

The longitudes of the decagon’s vertices oscillate over a period of 32 days, and its movements suggest it may be trapped by the curvature of an atmospheric jet.

The researchers suggest the wave could have been created by a spatially periodic disturbance in the jet peak or driven by an “anticyclonic vortex” to its north.

However, further study is required to better understand how it formed and how it compares to the hexagon structure in the north.

“The most intriguing part to me is that this seems to have just formed recently,” notes study co-author Amy Simon from NASA’s Goddard Space Flight Center in Maryland. “The question is, why did it suddenly form now when we haven’t seen one before?”

‘Lid’ of asteroid dust smothered and burned the dinosaurs, say scientists

As wildfires and extreme heat events rage across the world, “apocalyptic” has become an increasingly common descriptor. These climate disasters are not unprecedented, however. New research suggests they may have been the main mechanisms behind the mass extinction event that killed the dinosaurs – and much other life on Earth – 66 million years ago.

Our modern understanding of this extinction dates back to 1980, when the father-and-son, physicist-and-geologist team of Luis and Walter Alvarez presented evidence that it was triggered by an asteroid strike. Subsequent work identified the Chicxulub crater in Mexico’s Yucatán Peninsula as the likely impact site, but debate has persisted – including over how a single asteroid could cause worldwide destruction.

In the latest work, Brandon and Alexandria Johnson, a husband-and-wife duo at Purdue University in the US, fit another piece into this puzzle. By combining Brandon’s research on impact cratering with Alexandria’s expertise on the behaviour of atmospheric particulates, they found that the asteroid impact sent up a near-impermeable layer of dust that smothered the entire planet. Their research, published in Journal of Geophysical Research: Biogeosciences, breathes new life into a theory that had fallen out of favour: that extreme heat and wildfires were the main killers on the day the asteroid hit. 

The key to ignition

At some level, this dust layer likely produced a cooling effect, much as large volcanic eruptions do today. Indeed, a previous study showed that condensed droplets of rock known as spherules would have blocked the Sun’s radiation from entering the atmosphere.

At the Earth’s surface, however, the Johnsons think the dinosaurs had the opposite problem. The impact also sent a massive plume containing over 1000 km3 of vaporized material high into the atmosphere. When this material rained down as spherules, the dust cloud’s potential cooling effect was reversed.

“[The spherules] are essentially adding energy back to the atmosphere by travelling through it to the surface. That energy has to go somewhere,” Alexandria Johnson explains. “What we found is that this dust layer over the planet is essentially acting like a lid, and it’s keeping almost all the radiation that should try to find its way back out to space and reradiating it back down to the surface. So it’s enough to not only kill the dinosaurs but actually start widespread wildfires.”

Both researchers were surprised at just how opaque this dust layer was. By their reckoning, it trapped almost all radiation within the atmosphere, allowing a staggeringly tiny upwards transmittance of just 10-286. 

“I asked [Brandon] to redo my calculation, I was like ‘make sure these numbers are right’, because this is absolutely nothing,” says Alexandria. “I expected more than what we got to be reradiated out, but the fact that it doesn’t just enhances the whole idea of the heat pulse killing things off in a very quick manner.” 

Based on the Johnsons’ estimated heat fluxes, any creatures that couldn’t shelter or burrow – dinosaurs included – would have suffered temperature-induced deaths within an hour or two of the impact. The radiation was also more than enough to ignite lichens, grasses and pine needles, destroying vast amounts of plant life through wildfires. 

Never-ending night

Anything that survived the heat then had to face the darkness. “You would have had darkness lasting for a long time which then can cause collapse of the food chain in the oceans and cause oceanic extinctions,” says Brandon Johnson. The dust particles were about as small as those in wildfire smoke, and they may have blocked out the Sun for as long as decades before settling into what we now see as the K-Pg boundary: a rock layer a few millimetres thick that contains high levels of iridium, identified by the Alvarezes as coming from an asteroid.

Although some previous works suggested that the impact couldn’t have ejected enough material to trigger such large-scale effects, Brandon says his analysis of rock vaporization thresholds indicates that the plume’s mass was big enough to facilitate the transport of material across the planet’s atmosphere. Without this transport, he adds, the wreckage would have been more localized to the Chicxulub impact site. 

Given the interdisciplinary nature of the field, Brandon thinks there are many ways this line of research could continue. Possibilities include investigating the geological record in more detail and combustion experiments to better understand the wildfires’ impacts. He himself is hoping to carry out 3D impact simulations which can more accurately incorporate the vaporization of rock. “Essentially, [we want] to see what happens to that vapour and see if it is going to expand and be deposited globally as we expect,” he says.

Finding careers in STEM and the weirdness of AI

This episode of the Physics World Weekly podcast features two conversations. The first is with career consultant Alaina G Levine who gives advice on how to build a “unicorn career” in science, technology, engineering, and mathematics. Our second guest is the optics and artificial-intelligence researcher Janelle Shane who writes extensively about the absurd, and often hilarious, side of AI.

Levine’s latest book is called Create Your Unicorn Career and Shane writes about artificial intelligence on her blog  AI Weirdness.

This podcast is supported by American Elements, the world’s leading manufacturer of engineered and advanced materials. The company’s ability to scale laboratory breakthroughs to industrial production has contributed to many of the most significant technological advancements since 1990 – including LED lighting, smartphones, and electric vehicles.

 

Antineutrino detector could monitor spent nuclear fuel for clandestine activity

Scientists have measured antineutrino emissions from spent nuclear fuel for the first time, demonstrating that monitoring efforts need not stop when the reactors do. This is important because although independent nuclear safeguarding agencies have long used antineutrino emissions to glean information on reactor operations, previous detection schemes only worked when the reactors were running. Now, however, members of the Double Chooz collaboration have shown that it is possible to detect residual antineutrino emissions during a complete reactor shutdown – including from spent fuel stored in cooling pools.

Like neutrinos, antineutrinos are tiny, nearly massless fundamental particles that come in three flavours: electron, muon and tau. The largest human-made sources of electron antineutrinos are the reactor cores of nuclear power plants, which produce them via beta decay of neutron-rich fragments that are primarily generated during the fission of heavy elements such as uranium and plutonium.

Because antineutrinos interact so rarely with other particles, their emissions cannot be shielded and their signatures cannot be altered. They can also be detected some distance from the power plant itself. All these features make them valuable for nonintrusive nuclear reactor monitoring. Indeed, the idea of using antineutrinos to provide real-time information about the fissile content of a reactor core – and thereby detect illicit production of material that could be used in nuclear weapons manufacturing – dates back to the 1970s.

When a reactor is shut down, long-lived fission products (found in burnt fuel assemblies that remain in the reactor core, as well as assemblies that were previously removed and stored in nearby cooling pools) continue to decay, producing a residual neutrino flux. However, this residual signal is only around 1% as strong as the signal from an operating reactor. It also lies in a region of the energy spectrum that is strongly affected by background activities.

Measuring the residual neutrino flux and its energy spectrum

In the new work, which is detailed in Physical Review Letters, a team led by Thierry Lasserre and Anthony Onillon from the Max-Planck-Institut für Kernphysik (MPIK) in Heidelberg, Germany, succeeded in measuring the residual neutrino flux and its energy spectrum despite these difficulties. They obtained their data from the Double Chooz neutrino detection experiment, which is located near the two 4.25 GWth cores of the Chooz B nuclear power plant in the French Ardennes. This experiment uses a pair of detectors – labelled “near” and “far” and located around 400 m and 1.05 km from the cores – to study a fundamental parameter called the 𝜃13 neutrino mixing angle that describes how neutrinos transform between different types as they travel.

The Chooz plant uses a pair of pressurized water reactors, each of which contains 205 fuel assemblies consisting of roughly 600 kg of enriched uranium dioxide (mainly 238U with a few percent of 235U). During operation, these reactors also produce additional fissile isotopes, 239Pu and 241Pu, through neutron capture and subsequent decay processes involving 238U. In a typical cycle, they operate at full power for over a year and are then shut down for six to eight weeks for refuelling. During this time, some of the spent fuel assemblies are removed and transferred to storage pools in a nearby building.

The Double Chooz neutrino detector consists of more than 30 m3 of liquid scintillator, which is a material that emits short, double flashes of light when an antineutrino hits it. The outer part of the detector is shielded from the background radiation of the surrounding mountain rocks by 15 cm of demagnetized steel for the far detector and 1 m of water for the near detector. The inner part of the detector is shielded by a thick layer of mineral oil. Finally, an outer “muon veto” consisting of segmented scintillator modules positioned above the detector eliminates contributions from cosmic muons that can mask the signal from the antineutrinos.

Over 100 events observed

The researchers collected their data in 2017 when both of Chooz’s reactors were simultaneously shut down for 24.4 days for refuelling and maintenance. This unusual double shutdown was long enough for the team to obtain the statistics and low background signals needed to extract, for the first time, a quantitative residual reactor antineutrino spectrum.

After accounting for muon veto-induced dead time, the researchers clocked up 17.2 days of measurements for the near detector and 22.2 days for the far detector. While they analysed data from both, they mainly focused on the near detector because it is closer to the reactor cores and spent fuel pools and is therefore more sensitive to the residual antineutrino flux.

In the 1–3 MeV range, where the residual neutrino signal is strongest, the researchers observed 106 ±18 events, which represents a 5.9 σ excess over the background. This value, they say, is in very good agreement with the 88 ±7 events predicted by detailed simulations of the remaining nuclear fuel inventory and the decay of long-lived fission products.

According to the researchers, this result had a long gestation period: “In 2003, the IAEA safeguards met with the neutrino community and raised a question: could antineutrinos also provide information on spent nuclear fuel?

“The main difficulty has been detecting the faint residual antineutrino signal. This meant obtaining data during periods when both Chooz reactors were simultaneously off, very low and well-controlled detector backgrounds, and a detailed simulation of the irradiation and cooling history of relevant fuel assemblies.”

The researchers hope that their result will become a robust benchmark for agencies and experimenters designing detectors for specific spent-fuel monitoring applications. “Such monitoring is feasible and could provide an independent, non-intrusive complement to reactor-status and spent fuel inventory,” they tell Physics World, though they add that “it will not necessarily be easy, compact or inexpensive to implement.”

Plasmonic metamaterial makes a photonic time crystal

Researchers in France and Germany have created the first all-optical photonic time crystal. The nanostructure could be used to make ultrafast optical computers and amplifiers, frequency converters and perhaps even new types of lasers that work in the terahertz range.

Photonic crystals are nanostructured materials with a refractive index that varies on a length scale comparable to the wavelength of light, producing a phenomenon known as a photonic bandgap. This gap affects how photons propagate through the material in a way that resembles how a periodic potential in semiconductors affects the flow of electrons by defining allowed and forbidden energy bands. In the case of photonic crystals, light in certain wavelength ranges can pass through the material, but other wavelengths cannot.

Photonic time crystals (PhTCs) are similar to this, except their properties vary periodically in time, not space. Among other adjustments, this means that instead of photonic bandgaps, they host momentum bandgaps. These gaps are special in that light waves whose momenta fall within them grow exponentially in time.

Optical modulation at THz frequencies

PhTCs show much promise for applications like new types of tuneable lasers and ultrafast frequency converters, but controlling their photonic properties has proven very difficult. This is because these properties need to be modulated very strongly and on ultrashort timescales – on the order of the temporal period of the light itself.

Producing such extremely fast modulations is a key goal for scientists working in a branch of highly nonlinear optics known as time-domain photonics. Previously, their efforts have paid off in the microwave frequency range using PhTCs containing electrical circuits, but all-optical systems have proven elusive.

A team of physicists led by Yannis Laplace of the Ecole Polytechnique in France has now made a PhTC with properties that can be optically modulated at terahertz (THz, 1012 Hz) frequencies. The new PhTC is made from a type of photonic crystal known as a plasmonic metamaterial: an artificially engineered nanostructure consisting of micron-sized cavities made of gold atop an insulating layer and a semiconductor material based on indium and antimony. The cavities trap photons between the gold and semiconductor layers, and surface plasmons – collective, coherent oscillations of conduction electrons that interact very strongly with light – then keep them on the surface of the semiconductor.

Working with scientists from the Collège de France and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany, the researchers applied pulses of THz light from the TELBE light source at the HZDR’s ELBE accelerator to their structure. They found that they could use this intense multi-cycle THz light field to modulate the material’s optical properties over timescales of just picoseconds.

Towards new types of lasers in the THz range

“This study builds on previous work in my group related to the development and study of tuneable plasmonic metamaterials in the THz range as a means to create functional devices to control light-matter interactions in this range,” Laplace says. “One of the central questions in our work was to determine if the temporal modulation of the metamaterial would be strong and fast enough.

“By developing a theoretical model in collaboration with our colleague Marco Schiró and his team at the Collège de France that reproduced the experimental observations remarkably well, we showed that the system presented the characteristic spectroscopic signatures expected for a PhTC and provided an explanation for the behaviour of the photons therein.”

The researchers, who detail their work in Nature, also found that the ultrafast temporal modulation reduced the dissipation of photons within the metamaterial by half and they are now looking to reduce these optical losses even further.

“The THz frequency range lies at the junction between electronic and photonic technologies and is historically less developed than these two,” Laplace explains. “With the THz PhTCs that are now achievable, we hope we can soon develop devices like THz amplifiers, frequency converters and maybe new types of THz lasers in this range, hence contributing to the closing of the so-called ‘THz gap,’” he tells Physics World.

Two-dimensional material shows promise for long-term memory, thanks to slow electrons

Two-dimensional (2D) materials have attracted a lot of attention in recent years for their excellent electronic, thermal and mechanical properties, as well as their interesting quantum effects. While graphene has gathered the most commercial attention, there are hundreds of 2D materials, all of which have different combinations of properties for different applications. A research team headed up at the University of Chicago has discovered some unusual behaviour in the 2D material Fe5GeTe2.

Some 2D materials exhibit superconductivity properties and charge orders (in which the electrons are confined in a specific arrangement rather than being able to move freely). Charge orders arise from the reshaping of ground states in the electronic bands of a 2D materials to create flat electronic bands. And these flat bands cause the electrons to move very slowly in the lattice, at speeds much slower than expected.

Extremely flat electronic bands can often become incoherent, due to an ultrastrong electronic interaction strength between charge carriers. In research published in Science Advances, Shuolong Yang and colleagues found that Fe5GeTe2 exhibits a charge-ordered state where electrons move both collectively and very slowly, while remaining quantumly coherent.

“I was originally motivated by the fact that Fe5GeTe2 was discovered to host multiple structural phases with nearly the same stoichiometry. Each structural phase is associated with its unique electronic and magnetic phase,” says Yang. “These different structural phases have almost degenerate energies, which means that they can all be stabilized at room temperature.”

According to Yang, this discovery prompted two key research questions: What is the nature of each phase? Can we utilize these nearly energy-degenerate phases to encode information?

Fe5GeTe2 is part of specific class of 2D materials called van der Waals magnets, which could be used to develop new memory technology. The researchers probed the electronic structure and magnetic states of Fe5GeTe2 using angle-resolved photoemission spectroscopy (ARPES). “We were able to surgically probe the electronic band structure of each individual phase region (tens of microns in size) and figure out the physics,” Yang tells Physics World. “This immediately allows us to answer the question of ‘What is the nature of each phase?’”

After discovering that the electronic band was flat in Fe5GeTe2, further studies into the material showed that the charge order in Fe5GeTe2 was caused by the folding of electronic bands in the Brillouin zone within 30 meV of the Fermi level.

“We revealed a so-called Kondo-like phase, so the localized electronic states strongly interact with itinerant states, resulting in very peculiar, quantum coherent flat bands right at the Fermi level which dictates low-temperature transport,” says Yang. “The discovery of an effective Kondo lattice in a ferromagnetic phase [instead of magnetically non-ordered phase] is quite a surprise from a theoretical point of view, which forced us to go back to the drawing board to understand its physics. We think that that our surprising finding originates from a many-body localized state interacting with the conduction electrons.”

Because of the flat bands, the electronic system wants to spontaneously break the space-translation symmetry. It then forms a new superlattice order commensurate with the original lattice. “Our results provide the first experimental evidence that an interaction-driven flat band can itself drive electronic ordering through flat-band nesting, without relying on Moiré or geometrically frustrated flat-band engineering,” Yang explains. He notes that this “peculiar physics” used to be something obtained in delicately designed, twisted 2D materials. This finding, however, provides a new avenue to study complex many-body physics enabled by flat bands in stoichiometric and strongly correlated materials.

There’s the potential that the material’s different magnetic states could encode information for memory systems. This idea is being trialled by the team by using a micro-focused laser to switch between this observed quantum many-body phase and other phases.

“We’re using carefully designed laser pulses of different frequencies to switch between the multiple phases of Fe5GeTe2, including our discovered Kondo-like phase. If successful, we will utilize the many-body physics for some real memory operations,” Yang explains. This follow-on aims to tackle the second stated research question. However, to be practical, it will have to work at room temperature, and these quantum effects have so far only been realised at ultralow temperatures.

“Fundamental work is underway to understand the exact nature of this Kondo-like phase, whether it has any topological properties, and whether it connects to structural defects,” says Yang. “Eventually we want to understand how to microscopically describe such an exotic system.”

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