An overview of the QRNG setup. (a) The vacuum noise that is used as a source to generate random numbers. (b) A micrograph of the manufactured PIC and TIA. (c) The Gaussian distribution after digitization. (d) The distribution of the distilled random 32-bit integers, grouped into 256 bins. Credit: PRX Quantum (2023). DOI: 10.1103/PRXQuantum.4.010330
A team of physicists from Ghent University—Interuniversity Microelectronics Center, Technical University of Denmark and Politecnico & Università di Bari, reports that it is possible to use quantum fluctuations to generate random numbers faster than standard methods.
In their study, reported in the journal PRX Quantum, the group used the behavior of pairs of particles and antiparticles to create a random generator that is up to 200 times faster than conventional systems.
Random number generation is important in computer science. In addition to such applications as generating random backdrops and scenarios in video games, random numbers are used to create encryption keys for a host of sensitive applications. But generating keys that cannot be easily cracked requires computer power and time. For that reason, computer scientists are constantly looking for new ways to generate random numbers.
In this new effort, the research team turned to a new source—quantum fluctuation—which, in its most basic form, is a temporary change in the amount of energy that exists at a unique point in space. Such flickering has been widely studied due to the way it impacts chemical bonding and resulting types of light scattering. In this new effort, the research team took advantage of the randomness of such flickering to create a random number generator. In their approach, they focused on quantum flickering related to instances of particles and antiparticles forming and self-destructing and the fields of energy associated with them. Such flickering has in the past been shown to be random.
To capture the randomness of such flickering, the researchers used an integrated balanced homodyne detector—a device that is capable of measuring the electric field of a quantum state. But noting that such a device is susceptible to also capturing the less-than-random behavior of entangling particles, they added another device designed to identify this noise and ignore it while taking measurements.
The team then shrank the components used by their homodyne detector to a size that would allow incorporation on a chip installed in a computer system. They then used data from the chip to generate random numbers.
More information: Cédric Bruynsteen et al, 100-Gbit/s Integrated Quantum Random Number Generator Based on Vacuum Fluctuations, PRX Quantum (2023). DOI: 10.1103/PRXQuantum.4.010330
An illustration of how a 2D photonic time crystal can boost light waves. Credit: Xuchen Wang / Aalto University
Researchers have developed a way to create photonic time crystals, and they have shown that these bizarre, artificial materials amplify the light that shines on them. These findings, described in a paper in Science Advances, could lead to more efficient and robust wireless communications and significantly improved lasers.
Time crystals were first conceived by Nobel laureate Frank Wilczek in 2012. Mundane, familiar crystals have a structural pattern that repeats in space, but in a time crystal, the pattern repeats in time instead. While some physicists were initially skeptical that time crystals could exist, recent experiments have succeeding in creating them. Last year, researchers at Aalto University’s Low Temperature Laboratory created paired time crystals that could be useful for quantum devices.
Now, another team has made photonic time crystals, which are time-based versions of optical materials. The researchers created photonic time crystals that operate at microwave frequencies, and they showed that the crystals can amplify electromagnetic waves. This ability has potential applications in various technologies, including wireless communication, integrated circuits, and lasers.
So far, research on photonic time crystals has focused on bulk materials—that is, three-dimensional structures. This has proven enormously challenging, and the experiments haven’t gotten past model systems with no practical applications. So the team, which included researchers from Aalto University, the Karlsruhe Institute of Technology (KIT), and Stanford University, tried a new approach: building a two-dimensional photonic time crystal, known as a metasurface.
“We found that reducing the dimensionality from a 3D to a 2D structure made the implementation significantly easier, which made it possible to realize photonic time crystals in reality,” says Xuchen Wang, the study’s lead author, who was a doctoral student at Aalto and is currently at KIT.
The new approach enabled the team to fabricate a photonic time crystal and experimentally verify the theoretical predictions about its behavior. “We demonstrated for the first time that photonic time crystals can amplify incident light with high gain,” says Wang.
“In a photonic time crystal, the photons are arranged in a pattern that repeats over time. This means that the photons in the crystal are synchronized and coherent, which can lead to constructive interference and amplification of the light,” explains Wang. The periodic arrangement of the photons means they can also interact in ways that boost the amplification.
Two-dimensional photonic time crystals have a range of potential applications. By amplifying electromagnetic waves, they could make wireless transmitters and receivers more powerful or more efficient. Wang points out that coating surfaces with 2D photonic time crystals could also help with signal decay, which is a significant problem in wireless transmission. Photonic time crystals could also simplify laser designs by removing the need for bulk mirrors that are typically used in laser cavities.
Another application emerges from the finding that 2D photonic time crystals don’t just amplify electromagnetic waves that hit them in free space but also waves traveling along the surface. Surface waves are used for communication between electronic components in integrated circuits. “When a surface wave propagates, it suffers from material losses, and the signal strength is reduced. With 2D photonic time crystals integrated into the system, the surface wave can be amplified, and communication efficiency enhanced,” says Wang.
Sketch of the KISS experimental setup. The blue- and yellow-colored areas are filled with Ar and He gases, respectively. Differential pumping systems are located after the doughnut-shaped gas cell as well as before and after the GCCB. Credit: Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.132502
A team of nuclear physicists affiliated with multiple institutions in Japan, working with a colleague from Korea, has discovered a previously unknown uranium isotope with atomic number 92 and mass 241. In their study, reported in the journal Physical Review Letters, the group forced the isotope to reveal itself and tested the results of their efforts to show that what they had found was indeed uranium-241.
Over the past several decades, physicists have found that determining the properties of neutron-rich isotopes is difficult due to problems caused in creating them. For that reason, ongoing research has been looking for new ways to synthesize them under lab conditions. In this new effort, the research team tried a new approach—they fired a sample of unranium-238 nuclei at a sample of platinum-198 nuclei using an isotope separation system. Such interactions are known to result in multinucleon transfer, in which isotopes swap neutrons and protons. The collision resulted in the creation of a large number of fragments, which the researchers studied to determine their makeup.
They found evidence of 19 heavy isotopes holding from 143 to 150 neutrons. Each was measured using time-of-flight mass spectrometry, a technique that involves determining the mass of a traveling ion by tracking the time it takes to travel a given distance when its initial acceleration is known. The research team noted that most of the isotopes they measured had never been measured before. They also noted that one of them, uranium-241, had never been observed before and that it marks the first time since 1979 that a neutron-rich uranium isotope has been discovered. The researchers also calculated that uranium-241 likely has a half-life of just 40 minutes.
The technique used by the team represents a pathway to better understanding the shapes of large nuclei associated with the heavy elements, which could yield changes to models used to build nuclear power plants and weapons and to theories describing the behavior of exploding stars. The research team notes that that their method of discovery could be used to learn more about other heavy isotopes and also, perhaps, to discover new ones.
More information: T. Niwase et al, Discovery of New Isotope U241 and Systematic High-Precision Atomic Mass Measurements of Neutron-Rich Pa-Pu Nuclei Produced via Multinucleon Transfer Reactions, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.132502
Giulia Galli, Marco Govoni and fellow researchers have explored the possibility of predicting the electronic structure of complex materials using a quantum computer, an advancement in fields from materials engineering to drug design. Credit: Argonne National Laboratory
If you know the atoms that compose a particular molecule or solid material, the interactions between those atoms can be determined computationally, by solving quantum mechanical equations—at least, if the molecule is small and simple. However, solving these equations, critical for fields from materials engineering to drug design, requires a prohibitively long computational time for complex molecules and materials.
Now, researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory and the University of Chicago’s Pritzker School of Molecular Engineering (PME) and Department of Chemistry have explored the possibility of solving these electronic structures using a quantum computer.
The research, which uses a combination of new computational approaches, was published online in the Journal of Chemical Theory and Computation. It was supported by Q-NEXT, a DOE National Quantum Information Science Research Center led by Argonne, and by the Midwest Integrated Center for Computational Materials (MICCoM).
“This is an exciting step toward using quantum computers to tackle challenging problems in computational chemistry,” said Giulia Galli, who led the research with Marco Govoni, a staff scientist at Argonne and member of the UChicago Consortium for Advanced Science and Engineering (CASE).
A computational challenge
Predicting the electronic structure of a material involves solving complex equations that determine how electrons interact, as well as modeling how various possible structures compare to each other in their overall energy levels.
Unlike conventional computers that store information in binary bits, quantum computers use qubits that can exist in superposition of states, letting them solve certain problems more easily and quickly. Computational chemists have debated whether and when quantum computers might eventually be able to tackle the electronic structure problem of complex materials better than conventional computers. However, today’s quantum computers remain relatively small and produce noisy data.
Even with these weaknesses, Galli and her colleagues wondered whether they still could make progress in creating the underlying quantum computational methods required to solve electronic structure problems on quantum computers.
“The question we really wanted to address is what is possible to do with the current state of quantum computers,” Govoni said. “We asked the question: Even if the results of quantum computers are noisy, can they still be useful to solve interesting problems in materials science?”
An iterative process
The researchers designed a hybrid simulation process, using IBM quantum computers. In their approach, a small number of qubits—between four and six—perform part of the calculations, and the results are then further processed using a classical computer.
“We designed an iterative computational process that takes advantages of the strengths of both quantum and conventional computers,” said Benchen Huang, a graduate student in the Galli Group and first author of the new paper.
After several iterations, the simulation process was able to provide the correct electronic structures of several spin defects in solid-state materials. In addition, the team developed a new error mitigation approach to help control for the inherent noise generated by the quantum computer and ensure accuracy of the results.
Hints at the future
For now, the electronic structures solved using the new quantum computational approach could already be solved using a conventional computer. Therefore, the longstanding debate of whether a quantum computer can be superior to a classical one in solving electronic structure problems is not settled yet.
However, the results provided by the new method pave the way for quantum computers to address more complex chemical structures.
“When we scale this up to 100 qubits instead of 4 or 6, we think we might have an advantage over conventional computers,” Huang said. “But only time will tell for sure.”
The research group plans to keep improving and scaling up their approach, as well as using it to solve different types of electronic problems, such as molecules in the presence of solvents, and molecules and materials in excited states.
More information: Benchen Huang et al, Quantum Simulations of Fermionic Hamiltonians with Efficient Encoding and Ansatz Schemes, Journal of Chemical Theory and Computation (2023). DOI: 10.1021/acs.jctc.2c01119
From left: Martin Schultze (TU Graz), Maryna Meretska (Harvard), Marcus Ossiander (TU Graz), Hana Hampel (TU Graz). Credit: Lunghammer / TU Graz
Developed at Harvard, and successfully tested at Graz University of Technology (TU Graz), a revolutionary new meta-optics for microscopes with extremely high spatial and temporal resolution has proven its functional ability in laboratory tests at the Institute of Experimental Physics at TU Graz.
Microscopes using this kind of lens promise completely new research and development approaches, especially in semiconductor and solar cell technology. The research team from Graz and Boston currently reports on the construction and the successful laboratory experiment with this new meta-optics in the journal Science.
The lens of the microscope has made it possible to use extreme ultraviolet radiation for the first time. Its extremely short wavelength enables it to follow ultra-fast physical processes in the attosecond range. For example, real-time images from the inside of modern transistors or the interaction of molecules and atoms with light. Marcus Ossiander came up with the idea for the novel lens during his research work in Federico Capasso’s group at Harvard University, and since January 2023, the ERC Starting Grant and FWF START Award winner has been conducting research at the Institute of Experimental Physics at TU Graz.
Joint success for Boston and Graz
Attosecond physics uses extreme ultraviolet light. Because this light oscillates quickly and all the materials in the construction kit of optics development are opaque to this light, there have been no usable imaging systems for it until now. Marcus Ossiander remarks, “I asked myself whether the classical principle of optics could not be reversed. Can you use the absence of material in small areas as the basis of an optical element?”
The lens developed at Harvard on the basis of this idea and successfully tested at TU Graz implements this design principle. A precisely calculated arrangement of tiny holes in an extremely thin silicon foil conducts and focuses the incident attosecond light. A remarkable observation of the research team is that these vacuum tunnels transmit more light energy than should be possible due to the hole-covered surface. This means that the innovative meta-optics literally sucks the ultraviolet light into the focal point.
Holes of a few nanometers in diameter
Extremely small and precisely controlled structures are required for this breakthrough. Their production is close to the limits of what is technically feasible today. The technical implementation was achieved by Federico Capasso’s team at Harvard, which is the world leader in this field, after an experimental phase of around two years.
Proof of functionality was achieved in collaboration with TU Graz, where Martin Schultze’s group at the Institute of Experimental Physics is dedicated to the generation and application of ultra-short ultraviolet light flashes. “This is a great success for the cooperation between Boston and Graz. Now we want to use it to study microelectronics, nanoparticles and similar things soon,” explains Marcus Ossiander.
The meta-optics consists of an approximately 200-nanometer-thin film into which tiny hole structures have been etched. The entire lens consists of many hundreds of millions of holes; there are about ten of these structures per micrometer on the membrane. A single hole measures between 20 and 80 nanometers in diameter. For comparison: a human hair is about 60 to 100 micrometers thick, a small virus has a diameter of 15 nanometers. The diameters of the holes vary and decrease from the center of the membrane outwards. Depending on the size of the hole, the incident light radiation there is delayed and thus collapses into a tiny focal point.
Laser meets gas cloud
To measure the new type of lens, Martin Schultze and Hana Hampel from the Institute of Experimental Physics at TU Graz have unique expertise in generating the necessary extreme ultraviolet radiation. “Reliably generating short light pulses with high energy requires precise control of light-controlled atomic processes and very precise optical set-ups. For this project, we have developed a light source that is particularly efficient in generating radiation of the wavelength for which these meta-optics were designed,” says Martin Schultze.
In the experimental set-up in Graz, where a laser was focused into an inert gas jet, the extreme ultraviolet radiation could be generated and concentrated in very short pulses. The effectiveness of the meta-optics was proved by means of this light source which was optimized for attosecond physics.
Next step: A microscope with meta-optics
The development of a microscope that works with this lens is now the next step. The possible applications for the new research field of attosecond microscopy are manifold. Semiconductor and solar cell technology in particular will benefit from the possibility of being able to track the ultrafast movement of charge carriers in space and time for the first time.
In modern transistors and optoelectronic circuits, the relevant processes take place within a few nanometers of spatial expansion and in a time frame of a few attoseconds. The new meta-optics will make it possible to watch these central components of information technology at work and optimize them even further.
Defect centers in diamond nanostructures can be used as quantum bits. Via quantum operations (entanglement), quantum information can be stored in emitted single photons and transmitted in optical fibers in the future quantum internet. Credit: Humboldt-Universität zu Berlin
Diamond material is of great importance for future technologies such as the quantum internet. Special defect centers can be used as quantum bits (qubits) and emit single light particles that are referred to as single photons.
To enable data transmission with feasible communication rates over long distances in a quantum network, all photons must be collected in optical fibers and transmitted without being lost. It must also be ensured that these photons all have the same color, i.e., the same frequency. Fulfilling these requirements has been impossible until now.
Researchers in the “Integrated Quantum Photonics” group led by Prof. Dr. Tim Schröder at Humboldt-Universität zu Berlin have succeeded for the first time worldwide in generating and detecting photons with stable photon frequencies emitted from quantum light sources, or, more precisely, from nitrogen-vacancy defect centers in diamond nanostructures.
This was enabled by carefully choosing the diamond material; sophisticated nanofabrication methods carried out at the Joint Lab Diamond Nanophotonics of the Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik; and specific experimental control protocols. By combining these methods, the noise of the electrons, which previously disturbed data transmission, can be significantly reduced, and the photons are emitted at a stable (communication) frequency.
NV in the dark (regime 3). (a) Shutter experiment in which we alternate between PLE scanning for 20 s and blocking the radiation for 60 s. When PLE scans are performed, the center frequency of the ZPL resonance is extracted from Voigt fits (gray dots). Here, a data set is exemplarily presented. (b) Occurrence of spectral shifts obtained from many data sets. The extracted spectral diffusion value for “Laser on” corresponds to the spanned frequency range recorded in a period of 20 s. The spectral diffusion for “Laser off” is extracted from the spectral difference of the last PLE scan before and the first scan after blocking the laser, as illustrated in panel (a). Credit: Physical Review X (2023). DOI: 10.1103/PhysRevX.13.011042
In addition, the Berlin researchers show that the current communication rates between spatially separated quantum systems can prospectively be increased more than 1,000-fold with the help of the developed methods—an important step closer to a future quantum internet.
The scientists have integrated individual qubits into optimized diamond nanostructures. These structures are 1,000 times thinner than a human hair and make it possible to transfer emitted photons in a directed manner into glass fibers.
However, during the fabrication of the nanostructures, the material surface is damaged at the atomic level, and free electrons create uncontrollable noise for the generated light particles. Noise, comparable to an unstable radio frequency, causes fluctuations in the photon frequency, preventing successful quantum operations such as entanglement.
A special feature of the diamond material used is its relatively high density of nitrogen impurity atoms in the crystal lattice. These possibly shield the quantum light source from electron noise at the surface of the nanostructure. “However, the exact physical processes need to be studied in more detail in the future,” explains Laura Orphal-Kobin, who investigates quantum systems together with Prof. Dr. Tim Schröder.
The conclusions drawn from the experimental observations are supported by statistical models and simulations, which Dr. Gregor Pieplow from the same research group is developing and implementing together with the experimental physicists.
The paper is published in the journal Physical Review X.
More information: Laura Orphal-Kobin et al, Optically Coherent Nitrogen-Vacancy Defect Centers in Diamond Nanostructures, Physical Review X (2023). DOI: 10.1103/PhysRevX.13.011042
The scheme of high accuracy RF ranging with a quantum sensor. a Conceptual setup for the radio ranging with quantum sensor. Two RF paths with the same frequency serve as the reference (RF A) and ranging signal (RF B). The ranging signal is reflected by a target with a distance of L. Then, the free space interference signal between the two paths is confined in a microscale volume and interacts with the NV center quantum sensor. b Principle of extracting the target’s distance information. The phase of the back scattered RF pulse changes with the position of the target, as φ(L). It determines the amplitude (BRF) of the interference between the backscattered and the reference RF pulses. Subsequently, the Rabi oscillation rate of quantum sensor will change with the position of target, as Ω(L). The position of target is finally estimated by measuring the electron spin of NV center ensemble in diamond. Credit: Nature Communications (2023). DOI: 10.1038/s41467-023-36929-8
A study published in Nature Communications highlights the progress made in practical quantum sensing by a team led by academician Guo Guangcan and Prof. Sun Fangwen from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS). The team utilized micro and nano quantum sensing, coupled with local electromagnetic field enhancement at deep sub-wavelength scales, to study the detection of microwave signals and wireless ranging, achieving a positioning accuracy of 10-4 wavelengths.
Radar positioning technology based on microwave signal measurement is widely used in activities such as automatic driving, intelligent manufacturing, health monitoring, and geological exploration. In this study, the research team combined quantum sensing of solid-state systems with micro/nano resolution and deep subwavelength localization of electromagnetic fields to develop high-sensitivity microwave detection and high-precision microwave positioning technology.
The researchers designed a composite microwave antenna composed of diamond spin quantum sensors and metal nanostructures, which collects and converges microwave signals propagating in free space into nano-space. By probing the solid-state quantum probe state in the local domain, they measured the microwave signals. The method converted the detection of weak signals in free space into the detection of electromagnetic field and solid-state spin interactions at the nanoscale, improving the microwave signal measurement sensitivity of solid-state quantum sensors by 3–4 orders of magnitude.
To further utilize the high sensitivity microwave detection to achieve high-precision microwave localization, the researchers built a microwave interferometry device based on the diamond quantum sensor, and obtained the phase of the reflected microwave signal and the position information of the object through the solid-state spin detection of the interference result between the reflected microwave signal and the reference signal of the object. Based on the coherent interaction between solid-state spin quantum probes and microwave photons multiple times, they achieved quantum-enhanced position measurement with an accuracy of 10 micrometers (about one ten-thousandth of the wavelength).
Compared with traditional radar systems, this quantum measurement method does not require active devices such as amplifiers at the detection end, reducing the impact of electronic noise and other factors on the measurement limit. Subsequent research will allow further improvement of radio localization accuracy, sampling rate, and other indicators based on solid-state spin quantum sensing, and the development of practical solid-state quantum radar localization technology that exceeds the performance level of existing radars.
More information: Xiang-Dong Chen et al, Quantum enhanced radio detection and ranging with solid spins, Nature Communications (2023). DOI: 10.1038/s41467-023-36929-8
Instantaneous flow fields (a, b) and time-averaged heat flux fields (c, d) in a canonical thermal turbulence system with rectangular geometry. By applying spatial confinement through decreasing the lateral sizes of the system, the domain-sized circulatory flow is replaced by more energetic thermal coherent structures (indicated by the red/blue structures). This manipulation in the coherent structures not only leads to significant change in the global heat transfer, but also alters the spatial distribution pattern of heat flux greatly. Credit: Science China Press
This topic is reviewed by Prof. Ke-Qing Xia (Southern University of Science and Technology, Shenzhen, China) and his collaborators, mainly based on their research work over the past ten years.
Being the last unsolved problem in classical physics, fluid turbulence has attracted much attention from both academic and engineering communities. In contrast to completely disordered systems, one defining feature of turbulent flows is the existence of coherent structures, which are spatial-temporally correlated over a range of scales.
It has long been known that these coherent structures are the primary carriers for mass, momentum, and heat transport in turbulence. However, owing to the inherent characteristics of turbulent flows, such as strong nonlinearity and strong dissipation, how to manipulate coherent structures to control turbulent transport has been a long-standing issue.
In the past decade, Prof. Xia’s team have made significant progresses in this issue. By conducting a series of studies in a canonical thermal turbulence system, namely the turbulent Rayleigh-Bénard convection, they discovered a new mechanism of tuning turbulent heat transport via coherent structure manipulation through simple geometrical confinement.
Under this mechanism, the heat transfer efficiency is controlled by the coherency of thermal structures (characterized by their geometrical properties), rather than the turbulence intensity.
As a result, the heat transport efficiency can be significantly enhanced even the resultant flow is much slower. Very importantly, this mechanism is fundamentally different from the prevalent heat-management approach based on the classical view of wall-bounded turbulence, which usually centers on directly modifying the diffusion-dominant boundary layer to enhance or inhibit turbulent heat transfer.
In the review article, Prof. Xia and his collaborators introduced, and explained in detail, the physical picture behind this newly discovered mechanism, and discussed its potential applications in passive thermal management (such as electronics cooling).
Moreover, by introducing additional examples of thermal turbulence systems that are subject to various dynamical processes (including rotation, double-diffusion, magnetic field, tilting, modification by polymer additive and so on), they further demonstrate how the framework of coherent structure manipulation can be generalized to understand heat transport behaviors in seemingly different turbulence systems in a unified way. This universal mechanism is expected to be realized in other types of turbulent flows.
This review article also covers other important progresses in this research topic and outlines some future directions. These not only provide new understanding for the communities of turbulence research and heat transfer, but also promote the design and development of engineering systems with tunable transport efficiencies.
The work is published in National Science Review.
More information: Ke-Qing Xia et al, Tuning Heat Transport Via Coherent Structure Manipulation: Recent Advances in Thermal Turbulence, National Science Review (2023). DOI: 10.1093/nsr/nwad012
An international research team led by Professors Tsuneyuki Ozaki and François Légaré at the Institut national de la recherche scientifique (INRS) in Canada, has developed a unique method to enhance the power of a laser source emitting extreme ultraviolet light pulses. Credit: INRS
An international research team led by Professors Tsuneyuki Ozaki and François Légaré at the Institut national de la recherche scientifique (INRS), has developed a unique method to enhance the power of a laser source emitting extreme ultraviolet light pulses. The underlying mechanism of the newly observed phenomenon involves the unique role of dark-autoionizing states through coupling with other pertinent electronic states.
Thanks to this work, the team will be able to study the ultrafast dynamics of a single dark autoionizing state at the femtosecond timescale, which was previously impossible due to its inability to undergo single-photon emission or absorption, combined with the ultrashort lifetime of these states.
Recently published in the journal Physical Review Letters, their results allow the generation of ultrafast extreme ultraviolet light relevant for advanced ultrafast science applications such as angle-resolved photoemission spectroscopy and photoemission electron microscopy.
This work was done in collaboration with Professor Vasily Strelkov at the Prokhorov General Physics Institute of the Russian Academy of Sciences, Russia, and Research Assistant Professor Muhammad Ashiq Fareed at the University of Nebraska-Lincoln, USA.
Unraveling the mysteries of the dark-autoionizing states
In their laboratories at the Énergie Matériaux Télécommunications Research Centre, Professors Tsuneyuki Ozaki and François Légaré, along with Ph.D. student Mangaljit Singh, have been exploiting special types of electronic states, known as dark-autoionizing states. Their work was accomplished using high-order harmonic generation, an optical phenomenon unconventional to laser physics.
“The newly published results are a step forward not only in understanding the behavior of dark autoionizing states under intense ultrafast laser-matter interactions, but also in bringing intense extreme-ultraviolet laser sources from large-scale synchrotron and free-electron laser facilities to the moderate-sized laser laboratories,” says Ph.D. student Mangaljit Singh, first author of the study.
Many limitations imposed by the fundamentals of laser physics restrict most lasers used in medicine, communications, or industry. Likewise, they tend to operate only in the ultraviolet, visible (from violet to red), or the invisible near and mid-infrared wavelength range. However, many advanced scientific applications require lasers to operate at shorter wavelengths in the extreme ultraviolet range.
The state-of-the-art systems employ commercially available primary laser sources for high-order harmonic generation from noble gases to develop secondary sources of coherent extreme ultraviolet light.
In this study, instead of noble gases, Singh and colleagues used a laser-ablated plume (obtained from the laser ablation of a solid material) for the high-order harmonic generation in sync with the unique response of dark-autoionizing states.
They found that under certain resonance conditions governed by the primary laser parameters and the electronic structure of the atomic and ionic species in the laser-ablated plume, the conversion efficiency, and hence the power of the extreme ultraviolet laser source is enhanced by more than ten times. This implies that the same extreme ultraviolet power can be obtained using a primary laser with power that is one-tenth of the power required for a typical noble gas.
In addition to providing an intense extreme ultraviolet light source, this study also shows for the first time the prospect of studying the dynamics of dark autoionizing states at the femtosecond timescale using the technique of high harmonic spectroscopy. Such dark states could be the basis of several quantum technologies, especially in improving the performance of quantum computation.
More information: Mangaljit Singh et al, Ultrafast Resonant State Formation by the Coupling of Rydberg and Dark Autoionizing States, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.073201
A supersolid can be engineered in a structure comprising two conducting layers separated by an insulating barrier. The upper layer is doped with negatively-charged electrons and the lower layer with positively-charged holes. Interlayer excitons (bound pairs of an electron and a hole) form the supersolid. Credit: FLEET
A collaboration of Australian and European physicists predict that layered electronic 2D semiconductors can host a curious quantum phase of matter called the “supersolid.”
The supersolid is a very counterintuitive phase indeed. It is made up of particles that simultaneously form a rigid crystal and yet at the same time flow without friction since all the particles belong to the same single quantum state.
A solid becomes “super” when its quantum properties match the well-known quantum properties of superconductors. A supersolid simultaneously has two orders, solid and super:
Solid because of the spatially repeating pattern of particles.
Super because the particles can flow without resistance.
“Although a supersolid is rigid, it can flow like a liquid without resistance,” explains Lead author Dr. Sara Conti (University of Antwerp).
The study was conducted at UNSW (Australia), University of Antwerp (Belgium) and University of Camerino (Italy) and has been published in Physical Review Letters.
A 50-year journey toward the exotic supersolid
Geoffrey Chester, a Professor at Cornell University, predicted in 1970 that solid helium-4 under pressure should at low temperatures display:
Crystalline solid order, with each helium atom at a specific point in a regularly ordered lattice and, at the same time,
Bose-Einstein condensation of the atoms, with every atom in the same single quantum state, so they flow without resistance.
However in the following five decades the Chester supersolid has not been unambiguously detected.
Alternative approaches to forming a supersolid-like state have reported supersolid-like phases in cold-atom systems in optical lattices. These are either clusters of condensates or condensates with varying density determined by the trapping geometries. These supersolid-like phases should be distinguished from the original Chester supersolid in which each single particle is localized in its place in the crystal lattice purely by the forces acting between the particles.
The new Australia-Europe study predicts that such a state could instead be engineered in two-dimensional (2D) electronic materials in a semiconductor structure, fabricated with two conducting layers separated by an insulating barrier of thickness d.
The phase diagram’s triple point is particularly intriguing. There should be exciting physics coming from the exotic interfaces separating these domains. Credit: FLEET
One layer is doped with negatively-charged electrons and the other with positively-charged holes.
The particles forming the supersolid are interlayer excitons, bound states of an electron and hole tied together by their strong electrical attraction. The insulating barrier prevents fast self-annihilation of the exciton bound pairs. Voltages applied to top and bottom metal “gates” tune the average separation r0 between excitons.
The research team predicts that excitons in this structure will form a supersolid over a wide range of layer separations and average separations between the excitons. The electrical repulsion between the excitons can constrain them into a fixed crystalline lattice.
“A key novelty is that a supersolid phase with Bose-Einstein quantum coherence appears at layer separations much smaller than the separation predicted for the non-super exciton solid that is driven by the same electrical repulsion between excitons,” says co-corresponding author Prof. David Neilson, University of Antwerp.
“In this way, the supersolid pre-empts the non-super exciton solid. At still larger separations, the non-super exciton solid eventually wins, and the quantum coherence collapses.”
“This is an extremely robust state, readily achievable in experimental setups,” adds co-corresponding author Prof. Alex Hamilton (UNSW). “Ironically, the layer separations are relatively large and are easier to fabricate than the extremely small layer separations in such systems that have been the focus of recent experiments aimed at maximizing the interlayer exciton binding energies.”
As for detection, for a superfluid it is well known that this cannot be rotated until it can host a quantum vortex, analogous to a whirlpool. But to form this vortex requires a finite amount of energy, and hence a sufficiently strong rotational force. So up to this point, the measured rotational moment of inertia (the extent to which an object resists rotational acceleration) will remain zero. In the same way, a supersolid can be identified by detecting such an anomaly in its rotational moment of inertia.
The research team has reported the complete phase diagram of this system at low temperatures.
“By changing the layer separation relative to the average exciton spacing, the strength of the exciton-exciton interactions can be tuned to stabilize either the superfluid, or the supersolid, or the normal solid,” says Dr. Sara Conti.
“The existence of a triple point is also particularly intriguing. At this point, the boundaries of supersolid and normal-solid melting, and the supersolid to normal-solid transition, all cross. There should be exciting physics coming from the exotic interfaces separating these domains, for example, Josephson tunneling between supersolid puddles embedded in a normal-background.”
More information: Sara Conti et al, Chester Supersolid of Spatially Indirect Excitons in Double-Layer Semiconductor Heterostructures, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.057001
