Evolution from paramagnetism to ferromagnetism. Credit: Springer
Researchers have explored the evolution of systems of interacting spins, as they transition from random to orderly alignments. Through new simulations, they show that this evolution can be investigated by measuring the changing strength of the system’s magnetism.
The Ising model describes systems of interacting atomic spins relaxing from a “paramagnetic” state—whose spins point in random directions, to a “ferromagnetic” state—whose spins spontaneously align with each other. So far, the nonequilibrium dynamics of this transition has been studied by measuring the growth of regions, or “domains” of aligned spins.
In new research published in The European Physical Journal Special Topics, researchers led by Wolfhard Janke at the University of Leipzig, Germany, show how this can be done far more easily by measuring the strength of the system’s magnetization. The team’s discovery could help researchers to better understand the atomic-scale interactions underlying many different phenomena in nature: from electrostatic forces, to neuroscience and economics.
As a system evolves from a paramagnetic to a ferromagnetic state, it is driven to minimize its energy to reach a stable state of thermodynamic equilibrium. This occurs through a reduction in area of domain walls, where the alignment direction of the spins abruptly changes.
In the past, this evolution was typically quantified by directly measuring the growth of a system’s domain sizes over time, which is a technically demanding job. Through their simulations of the Ising model, Janke’s team showed that this can be done just as accurately by measuring the strength of the system’s magnetization, an easily measurable quantity in experiments as well.
According to the researchers, this quantity was largely ignored in previous studies considering that in the thermodynamic limit of infinite systems the magnetization is vanishing. In contrast, the team’s simulations revealed that in finite systems the signature of the growing length scale is encoded in the amplitude of the leading finite-size scaling correction.
This outcome held both for nearest-neighbor interactions between spins, and long-range interactions—which haven’t been widely studied thus far. As a result, Janke and colleagues now hope their new approach could lead to new discoveries in the many areas of nature where long-range spin interactions can be found.
More information: Wolfhard Janke et al, The role of magnetization in phase-ordering kinetics of the short-range and long-range Ising model, The European Physical Journal Special Topics (2023). DOI: 10.1140/epjs/s11734-023-00882-w
Schematic of an MHI (HD+): It comprises a hydrogen (p) and a deuteron nucleus (d) that can rotate around and vibrate against each other. In addition, there is an electron (e). The movements of p and d are expressed in the appearance of spectral lines. Credit: HHU/Soroosh Alighanbari
Using ultra-high-precision laser spectroscopy on a simple molecule, a group of physicists led by Professor Stephan Schiller Ph.D. from Heinrich Heine University Düsseldorf (HHU) has measured the wave-like vibration of atomic nuclei with an unprecedented level of precision.
In the journal Nature Physics, the physicists report that they can thus confirm the wave-like movement of nuclear material more precisely that ever before and that they have found no evidence of any deviation from the established force between atomic nuclei.
Simple atoms have been the subjects of precision experimental and theoretical investigations for nearly 100 years, with pioneering work carried out on the description and measurement of the hydrogen atom, the simplest atom with just one electron.
Currently, hydrogen atom energies—and thus their electromagnetic spectrum—are the most precisely computed energies of a bound quantum system. As extremely precise measurements of the spectrum can also be made, the comparison of theoretical predictions and measurements enables testing of the theory on which the prediction is based.
Such tests are very important. Researchers around the world are seeking—albeit unsuccessfully to date—evidence of new physical effects that could occur as a result of the existence of Dark Matter. These effects would lead to a discrepancy between measurement and prediction.
By contrast with the hydrogen atom, the simplest molecule was not a subject for precision measurements for a long time. However, the research group headed by Professor Stephan Schiller Ph.D. from the Chair of Experimental Physics at HHU has dedicated itself to this topic. In Düsseldorf, the group has conducted pioneering work and developed experimental techniques that are among the most accurate in the world.
The simplest molecule is the molecular hydrogen ion (MHI): a hydrogen molecule, which is missing an electron and comprises three particles. One variant, H2+, comprises two protons and an electron, while HD+ comprises a proton, a deuteron—a heavier hydrogen isotope—and an electron. Protons and deuterons are charged “baryons,” i.e. particles which are subject to the so-called strong force.
Within the molecules, the components can behave in various ways: The electrons move around the atomic nuclei, while the atomic nuclei vibrate against or rotate around each other, with the particles acting like waves. These wave motions are described in detail by quantum theory.
The different modes of motion determine the spectra of the molecules, which are reflected in different spectral lines. The spectra arise in a similar way to atom spectra, but are significantly more complex.
The art of current physics research now involves measuring the wavelengths of the spectral lines extremely precisely and—with the help of quantum theory—also calculating these wavelengths extremely precisely. A match between the two results is interpreted as proof of the accuracy of the predictions, while a mismatch could be a hint for “new Physics.”
Over the years, the team of physicists at HHU has refined the laser spectroscopy of the MHI, developing techniques that have improved the experimental resolution of the spectra by multiple orders of magnitude. Their objective: the more precisely the spectra can be measured, the better the theoretical predictions can be tested. This enables the identification of any potential deviations from the theory and thus also starting points for how the theory might need to be modified.
Professor Schiller’s team has improved experimental precision to a level better than theory. To achieve this, the physicists in Düsseldorf confine a moderate number of around 100 MHI in an ion trap in an ultra-high vacuum container, using laser cooling techniques to cool the ions down to a temperature of 1 milli Kelvin.
This enables extremely precise measurement of the molecular spectra of rotational and vibrational transitions. Following earlier investigations of spectral lines with wavelengths of 230 μm and 5.1 μm, the authors now present measurements for a spectral line with the significantly shorter wavelength of 1.1 μm in Nature Physics.
Professor Schiller says, “The experimentally determined transition frequency and the theoretical prediction agree. In combination with previous results, we have established the most precise test of the quantum motion of charged baryons: Any deviation from the established quantum laws must be smaller than 1 part in 100 billion, if it exists at all.”
The result can also be interpreted in an alternative way: Hypothetically, a further fundamental force could exist between the proton and deuteron in addition to the well-known Coulomb force (the force between electrically charged particles). Lead author Dr. Soroosh Alighanbari says, “Such a hypothetical force may exist in connection with the phenomenon of Dark Matter. We have not found any evidence for such a force in the course of our measurements, but we will continue our search.”
More information: S. Alighanbari et al, Test of charged baryon interaction with high-resolution vibrational spectroscopy of molecular hydrogen ions, Nature Physics (2023). DOI: 10.1038/s41567-023-02088-2
The muon system of the CMS experiment. Credit: CERN
The CMS collaboration has recently presented new results in searches for long-lived heavy neutral leptons (HNLs). Also known as “sterile neutrinos”, HNLs are interesting hypothetical particles that could solve three major puzzles in particle physics: they could explain the smallness of neutrino masses via the so-called “see-saw” mechanism, they could explain the matter-antimatter asymmetry of the universe, and at the same time they could provide a candidate for dark matter.
They are however very difficult to detect since they interact very weakly with known particles. The current analysis is an example of researchers having to use increasingly creative methods to detect particles that the detectors were not specifically designed to measure.
Most of the particles studied in the large LHC experiments have one thing in common: they are unstable and decay almost immediately after being produced. The products of these decays are usually electrons, muons, photons and hadrons—well-known particles that the big particle detectors were designed to observe and measure.
Studies of the original short-lived particles are performed based on careful analysis of the observed decay products. Many of the flagship LHC results were obtained this way, from the Higgs boson decaying into photon pairs and four leptons to studies of the top quark and discoveries of new exotic hadrons.
The HNLs studied in this analysis require a different approach. They are neutral particles with comparatively long lifetimes that allow them to fly for meters undetected, before decaying somewhere in the detector. The analysis presented here focuses on cases where an HNL would appear after the decay of a W boson in a proton-proton collision, and would then itself decay somewhere in the muon system of the CMS detector.
The muon system constitutes the outermost part of CMS and was designed—as its name suggests—to detect muons. Muons produced in the LHC proton-proton collisions traverse the whole detector, leaving a trace in the inner tracking system and then another one in the muon system. Combining these two traces into the full muon track lets physicists identify muons and measure their properties. In the HNL search, a muon is replaced by a weakly interacting heavy particle that leaves no trace—until it decays.
If it decays in the muon system it can produce a shower of particles clearly visible in the muon detectors. But—unlike a muon—it leaves no trace in the inner tracking detector, and no other activity in the muon system. This analysis is based on looking for “out-of-nowhere” clusters of tracks in the muon detectors.
The analysis started by selecting collision events with a reconstructed electron or muon from the decay of the W boson and an isolated cluster of traces in the muon system. Then, the analysis required the removal of cases where standard processes could imitate the HNL signal. After the full analysis, no excess of signal above expectation has been observed. As a result, a range of possible HNL parameters was excluded, setting the most stringent limits to date for HNLs with masses of 2-3 GeV.
MIT scientists and colleagues have created a superconducting device that could dramatically cut energy use in computing, among other important applications. In one design the diode consists of a ferromagnetic strip (pink) atop a superconducting thin film (grey). The team also identified the key factors behind the resulting current that travels in only one direction with no resistance. Credit: A. Varambally, Y-S. Hou and H. Chi
MIT scientists and colleagues have created a simple superconducting device that could transfer current through electronic devices much more efficiently than is possible today. As a result, the new diode, a kind of switch, could dramatically cut the amount of energy used in high-power computing systems, a major problem that is estimated to become much worse.
Even though it is in the early stages of development, the diode is more than twice as efficient as similar ones reported by others. It could even be integral to emerging quantum computing technologies. The work, which is reported in the July 13 online issue of Physical Review Letters, is also the subject of a news story in Physics Magazine.
“This paper showcases that the superconducting diode is an entirely solved problem from an engineering perspective,” says Philip Moll, Director of the Max Planck Institute for the Structure and Dynamics of Matter in Germany. Moll was not involved in the work. “The beauty of [this] work is that [Moodera and colleagues] obtained record efficiencies without even trying [and] their structures are far from optimized yet.”
“Our engineering of a superconducting diode effect that is robust and can operate over a wide temperature range in simple systems and potentially opening the door for novel technologies,” says Jagadeesh Moodera, leader of the current work and a senior research scientist in MIT’s Department of Physics. Moodera is also affiliated with the Materials Research Laboratory, the Francis Bitter Magnet Laboratory, and the Plasma Science and Fusion Center (PSFC).
The nanoscopic rectangular diode—about 1,000 times thinner than the diameter of a human hair—is easily scalable. Millions could be produced on a single silicon wafer.
Toward a superconducting switch
Diodes, devices that allow current to travel easily in one direction but not in the reverse, are ubiquitous in computing systems. Modern semiconductor computer chips contain billions of diode-like devices known as transistors. However, these devices can get very hot due to electrical resistance, requiring vast amounts of energy to cool the high-power systems in the data centers behind myriad modern technologies, including cloud computing.
According to a 2018 news feature in Nature, these systems could use nearly 20% of the world’s power in 10 years.
As a result, work toward creating diodes made of superconductors has been a hot topic in condensed matter physics. That’s because superconductors transmit current with no resistance at all below a certain low temperature (the critical temperature), and are therefore much more efficient than their semiconducting cousins, which have noticeable energy loss in the form of heat.
Until now, however, other approaches to the problem have involved much more complicated physics. “The effect we found is due [in part] to a ubiquitous property of superconductors that can be realized in a very simple, straightforward manner. It just stares you in the face,” says Moodera.
Says Moll of the Max Planck Institute for the Structure and Dynamics of Matter, “the work is an important counterpoint to the current fashion to associate superconducting diodes [with] exotic physics, such as finite-momentum pairing states. While in reality, a superconducting diode is a common and wide-spread phenomenon present in classical materials, as a result of certain broken symmetries.”
A somewhat serendipitous discovery
In 2020 Moodera and colleagues observed evidence of an exotic particle pair known as Majorana fermions. These particle pairs could lead to a new family of topological qubits, the building blocks of quantum computers. While pondering approaches to creating superconducting diodes, the team realized that the material platform they developed for the Majorana work might also be applied to the diode problem.
They were right. Using that general platform, they developed different iterations of superconducting diodes, each more efficient than the last. The first, for example, consisted of a nanoscopically thin layer of vanadium, a superconductor, which was patterned into a structure common to electronics (the Hall bar). When they applied a tiny magnetic field comparable to the Earth’s magnetic field, they saw the diode effect—a giant polarity dependence for current flow.
They then created another diode, this time layering a superconductor with a ferromagnet (a ferromagnetic insulator in their case), a material that produces its own tiny magnetic field. After applying a tiny magnetic field to magnetize the ferromagnet so that it produces its own field, they found an even bigger diode effect that was stable even after the original magnetic field was turned off.
Ubiquitous properties
The team went on to figure out what was happening.
In addition to transmitting current with no resistance, superconductors also have other, less well-known but just as ubiquitous properties. For example, they don’t like magnetic fields getting inside. When exposed to a tiny magnetic field, superconductors produce an internal supercurrent that induces its own magnetic flux that cancels the external field, thereby maintaining their superconducting state.
This phenomenon, known as the Meissner screening effect, can be thought of as akin to our bodies’ immune system releasing antibodies to fight the infection of bacteria and other pathogens. This works, however, only up to some limit. Similarly, superconductors cannot entirely keep out large magnetic fields.
The diodes the team created make use of this universal Meissner screening effect. The tiny magnetic field they applied—either directly, or through the adjacent ferromagnetic layer—activates the material’s screening current mechanism for expelling the external magnetic field and maintaining superconductivity.
The team also found that another key factor in optimizing these superconductor diodes is tiny differences between the two sides or edges of the diode devices. These differences “create some sort of asymmetry in the way the magnetic field enters the superconductor,” Moodera says.
By engineering their own form of edges on diodes to optimize these differences—for example, one edge with sawtooth features, while the other edge not intentionally altered—the team found that they could increase the efficiency from 20% to more than 50%. This discovery opens the door for devices whose edges could be “tuned” for even higher efficiencies, Moodera says.
In sum, the team discovered that the edge asymmetries within superconducting diodes, the ubiquitous Meissner screening effect found in all superconductors, and a third property of superconductors known as vortex pinning all came together to produce the diode effect.
“It is fascinating to see how inconspicuous yet ubiquitous factors can create a significant effect in observing the diode effect,” says Yasen Hou, first author of the paper and a postdoctoral associate at the Francis Bitter Magnet Laboratory and the PSFC. “What’s more exciting is that [this work] provides a straightforward approach with huge potential to further improve the efficiency.”
Christoph Strunk is a professor at the University of Regensburg in Germany. Strunk, who was not involved in the research, says, “the present work demonstrates that the supercurrent in simple superconducting strips can become non-reciprocal. Moreover, when combined with a ferromagnetic insulator, the diode effect can even be maintained in the absence of an external magnetic field.”
“The rectification direction can be programmed by the remanent magnetization of the magnetic layer, which may have high potential for future applications. The work is important and appealing both from the basic research and from the applications point of view.”
Moodera noted that the two researchers who created the engineered edges did so while still in high school during a summer at Moodera’s lab. They are Ourania Glezakou-Elbert of Richland, Washington, who will be going to Princeton this fall, and Amith Varambally of Vestavia Hills, Alabama, who will be entering the California Institute of Technology.
More information: Yasen Hou et al, Ubiquitous Superconducting Diode Effect in Superconductor Thin Films, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.027001
Simulating an evolving quantum system. Credit: The European Physical Journal D (2023). DOI: 10.1140/epjd/s10053-023-00659-9
Quantum walks have been widely studied for their ability to simulate real physical phenomena. Physicists have previously studied two distinct types of quantum walk, but so far, they haven’t widely considered how their mathematical descriptions could be linked.
Through new research published in The European Physical Journal D, a pair of physicists in France— Nicolas Jolly at ENS de Lyon, and Giuseppe Di Molfetta at Aix-Marseille University—show how “discrete-time” and “continuous-time” quantum walks can be described using more general mathematical language. Their results could allow researchers to simulate an even broader range of phenomena using quantum walks.
In a classical walk, a particle moves by taking steps in different random directions—each of which has a certain probability attached to it. In contrast, a particle on a quantum walk can choose to travel in multiple directions simultaneously, according to the principles of quantum superposition.
In the past, physicists exclusively studied two different types of quantum walker. In a discrete-time walk, a system evolves in discrete steps, in which the particle performs an operation that updates its state. In contrast, the state of a continuous-time quantum walker is constantly evolving. So far, both types of quantum walker have mostly been studied through separate lines of research. Recently, however, researchers showed how both types of walker can be described using the same mathematical language. These calculations led them to a new family named “plastic” quantum walkers, which allow for both discrete-time and continuous-time behaviors.
In their latest study, Jolly and Di Molfetta introduce a more general family named “twisted” quantum walkers, named for the alteration in the choices available to them, which assume the form of plastic quantum walkers under the right conditions. In their paper, the researchers describe several important consequences for their updated theory. They ultimately hope that it could further expand the range of phenomena that can be simulated by quantum phenomena to areas including the behaviors of graphene electrons in electromagnetic fields.
More information: Nicolas Jolly et al, Twisted quantum walks, generalised Dirac equation and Fermion doubling, The European Physical Journal D (2023). DOI: 10.1140/epjd/s10053-023-00659-9
Schematic view of the final gas sensing microsystem based on a low-cost polymer microresonator and the use of a vertical laser diode (VCSEL) as a probing source. Credit: Journal of Optical Microsystems (2023). DOI: 10.1117/1.JOM.3.3.033501
There is a growing demand for portable gas sensors, from both environmental and health sciences users as well as industry. Resonant optical sensors, in particular planar micro-resonators, combine high sensitivity and small footprint, which makes them good candidates for these applications.
The sensing principle of these guided-wave sensors is based on a variation in their spectral response in the presence of the target molecules. The laser source to be used for probing such spectral shifts should emit a single-mode and polarization-stable beam and should be spectrally tunable over at least a few nanometers.
A team of researchers based at the University of Toulouse in France aimed at fabricating such a compact optical microsystem for ammonia gas detection using a near infra-red single-mode laser diode source, namely a vertical cavity surface emitting laser, or VCSEL.
This kind of semiconductor laser diode is very compact and can be spectrally tuned over few nanometers by simply adjusting the operating current. Moreover, the specific VCSEL chip used in their work includes a grating relief etched at its surface that ensures a good polarization stability of the emitted beam. However, although it is smaller than for a LED or for a standard edge-emitting laser diode, the beam divergence of this VCSEL chip is too large for most practical uses in optical microsystems.
In this research, the spot size at the aimed working distance (2 mm) is indeed larger than 250µm. It should be reduced to less than 100µm to ensure an optimal coupling with the detection area. Polarization-stable single-mode VCSEL chips having a reduced divergence are unfortunately not yet commercially available. The challenge lies therefore in finding an accurate method to directly integrate a collimation microlens on a small-sized VCSEL chip (200x200x150 µm3) that is already mounted on a printed circuit board.
In this work, published in the Journal of Optical Microsystems, the researchers demonstrate that 2-photon-polymerization 3-D printing can be exploited to fabricate such a microlens in a single step and with a writing time of only 5 minutes. To this aim, they optimized the lens design and fabrication conditions to obtain a sufficient surface quality as well as a suitable focal length.
The beam divergence of the laser chip could be reduced from 14.4° to 3°, corresponding to a beam spot size at a distance of 2 mm of only 55µm. They also studied experimentally and theoretically the effects of lens addition on the device spectral properties and proposed a new design to avoid a reduction of the tuning range.
The team’s work demonstrates the interest of 2-photon-polymerization 3-D-printing as a fast and accurate technique for VCSEL collimation at a post-mounting stage and paves the way toward the development of optimized laser chips directly integrable in portable optical sensing systems.
More information: Qingyue Li et al, Direct 3D-printing of microlens on single mode polarization-stable VCSEL chip for miniaturized optical spectroscopy, Journal of Optical Microsystems (2023). DOI: 10.1117/1.JOM.3.3.033501
Illustration of an ultra-peripheral collision where the two lead ion beams at the LHC pass by close to each other without colliding. Photons emitted from one beam strike the other, producing electromagnetic interactions. The structure of the gluonic matter in the nucleus gets further exposed when probed by higher energy photons. Credit: CERN
In the Large Hadron Collider (LHC), proton and lead beams travel close to the speed of light. They carry a strong electromagnetic field that acts like a flux of photons as the beam moves through the accelerator. When the two beams at the LHC pass by close to each other without colliding, one of the beams may emit a photon of very high energy that strikes the other beam. This can result in photon—nucleus, photon—proton, and even photon—photon collisions.
The ALICE collaboration studies these collisions to investigate protons and the inner structure of nuclei, and has recently released new results on this topic at the LHCP 2023 conference.
Photons are ideal tools to study the interior of nuclei. Usually when a photon collides with a nucleus, two gluons (force carriers of the strong interaction) are exchanged, which results in the production of a quark-antiquark pair. Researchers further distinguish two different classes of these collisions: when a photon interacts with the whole nucleus (a coherent collision), and when a photon interacts with a single nucleon inside the nucleus (an incoherent collision).
Inside nuclei, scientists look for high numbers of gluons, which indicate high levels of gluon density. Theoretical models suggest that the gluon density inside nuclei increases when they approach the speed of light.
If the density increases enough, the nucleus will become saturated with gluonic matter, meaning that the number of gluons in the nucleus cannot increase any further. Directly probing gluonic saturated matter is one of the main outstanding challenges in the field of strong interactions, and observing it could lead to further insight into the inner structure of protons and nuclei.
If a charm quark-antiquark pair is produced in a photon—nucleus collision, this is known as J/ψ meson production. Scientists study how coherent J/ψ production varies with photon energy in order to look for gluon saturation effects. As the photon energy increases, it becomes easier and easier to detect the gluonic matter inside the nuclei. The new ALICE results on J/ψ production using LHC Run 2 data cover a larger momentum range than previous measurements from Run 1, and are in line with expectations of gluon-saturation models.
Incoherent collisions offer the opportunity to study geometrical configurations of the quantum fluctuations in the internal structure of the proton. The ALICE collaboration achieves this by studying the distribution of momentum that is transferred to the J/ψ meson. In a new study, the collaboration has been able to show that this momentum transfer can only be described when areas of saturated gluonic matter, called gluonic hotspots, are introduced into the models.
The ALICE collaboration will continue to investigate these phenomena in LHC Runs 3 and 4, where high-precision measurements with larger data samples will provide more powerful tools to better understand the role of saturation and gluonic hotspots.
View through the OMEGA laser’s 20-cm disk amplifiers at the Laboratory for Laser Energetics. In a scaled-down, proof-of-principle experiment, Rochester researchers used the laser to demonstrate a critical step in the dynamic shell concept. Credit: University of Rochester photo / J. Adam Fenster
Fusion, which replicates the same reaction that powers the sun, has long been viewed as an ideal energy source due to its potential to be safe, clean, cheap, and reliable.
Since the early 1960s, scientists have pursued the possibility of using high-powered lasers to compress thermonuclear material long enough and at high enough temperatures to trigger ignition—the point at which the resultant output of inertial fusion energy is greater than the energy delivered to the target.
Scientists achieved ignition in December 2022 at the National Ignition Facility at Lawrence Livermore National Laboratory, but many hurdles remain in making fusion energy technically and commercially viable for mass production and consumption.
Researchers at the University of Rochester’s Laboratory for Laser Energetics (LLE) have, for the first time, experimentally demonstrated a method called dynamic shell formation, which may help achieve the goal of creating a fusion power plant.
The researchers, including Igor Igumenshchev, a senior scientist at LLE, and Valeri Goncharov, a distinguished scientist and theory division director at LLE and an assistant professor (research) in the Department of Mechanical Engineering, discuss their findings in a paper published in Physical Review Letters.
“This experiment has demonstrated feasibility of an innovative target concept suitable for affordable, mass production for inertial fusion energy,” Igumenshchev says.
The conventional approach to inertial fusion energy
In the conventional approach to inertial fusion energy, a target consisting of a small amount of hydrogen fuel—in the form of the hydrogen isotopes deuterium and tritium—is frozen solid into a spherical shell. The shell is then bombarded by lasers, heating the central fuel to extremely high pressures and temperatures. When these conditions are achieved, the shell collapses and ignites, undergoing fusion.
The process releases an enormous amount of energy that has the potential to drive a carbon-free power plant. But a fusion power plant, still hypothetical, would require nearly a million targets per day. The current methods for fabricating targets using a frozen preparation process are costly and the targets are difficult to produce.
Dynamic shell formation: More feasible, less costly
Dynamic shell formation is an alternative method to create targets in which a liquid droplet of deuterium and tritium is injected into a foam capsule. When bombarded by laser pulses, the capsule develops into a spherical shell, then implodes and collapses, resulting in ignition. Dynamic shell formation does not require the costly cryogenic layering that conventional methods of generating inertial fusion energy employ, because it uses liquid targets. These targets will also be easier to make.
Goncharov first described dynamic shell formation in a paper in 2020, but the concept hadn’t been demonstrated experimentally. In a scaled-down, proof-of-principle experiment, Igumenshchev, Goncharov, and their colleagues used LLE’s OMEGA laser to shape a sphere of plastic foam that had the same density as deuterium-tritium liquid fuel into a shell, demonstrating a critical step in the dynamic shell concept.
To actually generate fusion using the dynamic shell formation technique, future research will require lasers with longer and more energetic pulses, but the current experiment suggests that dynamic shell formation could be feasible as a path toward more practical fusion energy reactors.
“Combining this target concept with a highly efficient laser system that is currently under development at LLE will provide a very attractive path to fusion energy,” Igumenshchev says.
More information: I. V. Igumenshchev et al, Proof-of-Principle Experiment on the Dynamic Shell Formation for Inertial Confinement Fusion, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.015102
Diagram of the scheme. Alice sends a message to Bob while Eve tries to retrieve the message. In this scheme, the original message is only retrieved after passing through both PUFs, meaning Eve can’t get the message. Credit: University of Twente
Researchers from the Complex Photonic Systems (COPS) group used two layers of random materials to encrypt and decrypt a message sent via light communication. With that, they hid the sender and receiver simultaneously, and only when the light passed through both layers was the message received.
The research team published their findings in the journal Optics Express, and believes this proof of concept has applications in visible light communication systems, light fidelity (LiFi), and optical fiber communications.
In an era when digital information is the lifeblood of our interconnected world, ensuring its security is paramount. Encryption plays a vital role in safeguarding our data, turning a simple message into a complex pattern and then converting it back, rendering it indecipherable if the message is intercepted in the middle.
As technology advances at an unprecedented pace, the future of communication lies in the realm of visible light. But how can we guarantee the security of this cutting-edge form of communication?
Surprisingly, the solution may be found in everyday objects. Researchers from the University of Twente, in collaboration with experts from the Technical University of Eindhoven and the innovative Signify company (formerly known as Philips Lighting), have shown that random materials—such as a layer of paint, a piece of paper, or a glass diffuser—enhance the secrecy of light communication by scrambling the message.
When light traverses through these random materials, it scatters in multiple directions, creating an intricate pattern known as a speckle pattern. This very pattern becomes the foundation for encryption.
Distribution of 50,000 random patterns at the receiver (Bob). With this distribution, Alice and Bob agree on which patterns are a binary 0 or a binary 1. Bob just needs to measure spots A and B to receive the message. Credit: University of Twente
This encryption follows the concept of physical unclonable function (PUF). A PUF is an object so complex that it cannot be copied with current technologies. If a PUF is used as an encryption key, only the correct key—which is unclonable—can access the information. In this case, the key is the random object, and the information is the speckle pattern.
Researchers from the Complex Photonic Systems (COPS) group took this concept even further. Instead of using a single key to encrypt the message, they use two layers of random media as dual keys. With that, they hide the sender and receiver simultaneously, and only when the light passes through both keys is the message received. Any malicious eavesdropper attempting to intercept the message in transit would be confronted with a meaningless mixture of random patterns, utterly unrelated to the original message.
Furthermore, the secrecy is enhanced by the redundancy of the system. The proposed system is based on modulating the incident light using a device similar to a screen projector (or beamer) at the sender. Because the random materials are so complex, there are thousands of different ways to modulate the light, resulting in the same message, while changing the random pattern between the two keys. If the sender is constantly switching between the different modulations, an attacker in the middle is overwhelmed with random patterns, while the receiver is unaffected by it.
The paper, “Enhanced secrecy in optical communication using speckle from multiple scattering layers,” by Alfredo Rates, Joris Vrehen, Bert Mulder, Wilbert L. Ijzerman, and Willem L. Vos, appears in Optics Express. The data used for the publication are available in the Zenodo database
More information: Alfredo Rates et al, Enhanced secrecy in optical communication using speckle from multiple scattering layers, Optics Express (2023). DOI: 10.1364/OE.493479
Rates Alfredo et al, From Noise to Signal: Multi-layer Speckle Correlation with Applications in Visible Light Communication, Zenodo (2022). DOI: 10.5281/zenodo.6397330
Operation principle of the two-domain fiber laser. (A) Schematic of the two-domain (phonon and photon) laser based on forward intermodal SBS in a TMF ring cavity. Dispersion diagrams with energy and momentum conservation conditions for (B) backward SBS and (C) forward SBS in optical fiber. (D) Working principle of simultaneous phonon and phonon lasing illustrated in the frequency domain. Credit: Science Advances (2023). DOI: 10.1126/sciadv.adg7841
Lasers are a significant historical invention with ubiquitous impact in society. The concept also has interdisciplinary applications as phonon lasers and atom lasers. A laser in one physical domain can be pumped by energy in another. Nevertheless, all lasers demonstrated in practice have only lased in one physical domain thus far.
In a new report published in Science Advances, Ning Wang and a research team at the College of Optics and Photonics at the University of Central Florida, U.S., and the Prysmian group in France demonstrated the simultaneous process of photon and phonon lasing. The two-domain laser has multiple applications as optical and acoustic tweezers to conduct mechanical sensing to generate microwaves and perform quantum processing. The team expects this demonstration to open new paths for multidomain laser-related applications.
Developing a two-domain laser
Lasers are an extension of electronic oscillators at radiofrequencies and masers at micro-frequencies in the optical region. Lasers have tremendous applications with new extensions of the concept across domains such as acoustic oscillators, also known as sasers, and oscillators at atom or matter waves. The concept of laser traditionally describes an optical oscillator based on stimulated emission, although the term phonon laser and atom/matter laser are also quite common.
There are a few applications in which the process of simultaneous photon and phonon lasing can be useful. These include the development of acoustic tweezers at the sub-millimeter scale. Combined ultrasonic and photonic biological imaging for improved imaging quality and two-domain lasers have scope across quantum information processing and sensing. Existing demonstrations have shown the Stokes optical acoustic wave to be a byproduct in a phonon laser. In this work, Wang and colleagues developed a system of coupled oscillators that lased in two distinct physical domains pumped from the same source to show how the two-domain concurrent photon and phonon lasing enhanced output powers of both photon and phonon lasers.
Experimental setup. (A) Experimental setup for demonstrating two-domain, simultaneous phonon and photon lasing based on forward intermodal SBS in a 10-m TMF ring cavity. (B) Measured refractive index profile of the reduced-cladding TMF used as the gain media in the two-domain (phonon and photon) laser. (C) Microscope cross-sectional image of the TMF. (D) measured mode profiles of the three guided optical modes of the TMF at λ = 976 nm. Credit: Science Advances (2023). DOI: 10.1126/sciadv.adg7841
Principle-of-action
The team generated the low-frequency flexural acoustic wave using forward stimulated Brillouin scattering; the interaction of photons and phonons within a two-mode fiber. The low-frequency phonons were confined in the silica fiber with a long lifetime of 10 milliseconds. The propagation length approximated 10 meters, allowing phonons to lase as well. In the experimental setup, the coherent oscillation of the optical wave enhanced the gain of acoustic phonons and vice versa, to generate lasing in two domains.
The team noted four states of function in the device by increasing the optical pump power to produce photon and phonon lasing for which the gains for both Stokes optical wave and the acoustic wave had to exceed their losses. The experimentalists devised a method to allow phonon energy within the ring cavity to facilitate phonon lasing. While the phonon laser power was confined inside the cavity, the Stokes optical laser was seen at the output of the coupler.
The experiments
During the experiments, the researchers used a 976 nm fiber coupled pump diode with a maximum output power of 400 mW. They used a thermoelectric cooler to regulate the functional temperature of the system. The pump launched into a two-mode fiber coupled into the outer diameter ring cavity.
The scientists used a reduced cladding 2-mode fiber made of pure silica cladding and a germanium-oxide doped, silica core. Since the acoustic fields extended into the entire cladding, the process of reducing the two-mode fiber cladding size improved the overlap between acoustic and optical fields to increase the stimulated Brillouin scattering gain coefficient.
Experimental results. (A) Measured optical power of the LP11 Stokes photon laser versus injected pump power, (inset) optical power in log scale. (B) RF power of the beat note between the pump and the Stokes photon laser at each pump power. (C) RF power in linear scale versus squared pump power and beat-note electrical spectra at a pump power of (D) 100 mW, (E) 161 mW, (F) 271 mW, and (G) 367 mW, color-matched to the circles in (A) and (B). Credit: Science Advances (2023). DOI: 10.1126/sciadv.adg7841
Laser power
The team measured the phonon laser output power as a function of pump power injected into the ring cavity to obtain two thresholds corresponding to the photon laser and phonon laser. The threshold pump power of the photon laser was 180 mW. When they increased the pump power to 308 mW, the phonon laser started lasing, as well.
The measured threshold pump power and output laser power agreed with the outcomes of the numerical simulation results. The photon–phonon laser represented an inverted dissipative hierarchy, where the acoustic emission linewidth was much narrower than the pump laser linewidth when compared to existing standards.
Outlook
In this way, Ning Wang and colleagues showed how two coherently coupled lasers in different physical domains performed a variety of practical tasks. Light and sound have distinct space and time properties, and interact differently with materials; therefore, their availability can be explored differently. This phenomenon of coupled lasing in two different physical domains within the same cavity is a first-in-study outcome. This outcome goes beyond already established methods which include coherently coupled lasers such as laser-diode arrays.
The two-domain laser explored forward intermodal stimulated Brillouin scattering to enable coupling for concurrent photon and phonon lasing within the same cavity. The team did not directly observe phonon laser power in this study due to the absence of high-resolution and high frame-rate cameras. The scientists observed several regimes of laser functionality relative to spontaneous Brillouin scattering, photon lasing, and photon-phonon lasing, which aligned with the theoretical model of 2-domain lasing. The outcomes can lead to future advances in optomechanics and will usher in multidomain lasers and related applications.
More information: Ning Wang et al, Laser 2 : A two-domain photon-phonon laser, Science Advances (2023). DOI: 10.1126/sciadv.adg7841
