Engineers at the UCLA Samueli School of Engineering have introduced a universal framework for point spread function (PSF) engineering, enabling the synthesis of arbitrary, spatially varying 3D PSFs using diffractive optical processors. The research is published in the journal Light: Science & Applications.
This framework allows for advanced imaging capabilities—such as snapshot 3D multispectral imaging—without the need for spectral filters, axial scanning, or digital reconstruction.
PSF engineering plays a significant role in modern microscopy, spectroscopy and computational imaging. Conventional techniques typically employ phase masks at the pupil plane, which constrain the complexity and mathematical representation of the achievable PSF structures.
The approach developed at UCLA enables arbitrary, spatially varying 3D PSF engineering through a series of passive surfaces optimized using deep learning algorithms, forming a physical diffractive optical processor.
Through extensive analyses, the researchers showed that these diffractive processors can approximate any linear transformation between 3D optical intensity distributions in the input and output volumes. This enables precise, diffraction-limited control of light in three dimensions, paving the way for highly customized and sophisticated optical functions for 3D optical information processing.
By jointly engineering the spatial and spectral properties of 3D PSFs, the framework supports powerful imaging modalities such as snapshot 3D multispectral imaging—achieved without mechanical scanning, spectral filters, or computational postprocessing. This all-optical approach offers unmatched versatility for high-speed, high-throughput optical systems.
This work marks a significant stepping-stone for future advances in computational imaging, optical sensing and spectroscopy, as well as 3D optical information processing. Potential applications include compact multispectral imagers, high-throughput 3D microscopy platforms, and novel optical data encoding and transmission systems.
The study was conducted by Dr. Md Sadman Sakib Rahman and Dr. Aydogan Ozcan in the UCLA Electrical and Computer Engineering Department and the California NanoSystems Institute (CNSI).
Researchers developed a single-photon Raman lidar system that operates underwater and can remotely distinguish various substances They demonstrated the system by using it to detect varying thicknesses of gasoline oil in a quartz cell that was 12 meters away from the system in a large pool. Credit: Mingjia Shangguan, Xiamen University
Researchers report a new single-photon Raman lidar system that operates underwater and can remotely distinguish various substances. They also show that the new system can detect the thickness of the oil underwater up to 12 m away, which could be useful for detecting oil spills.
“Differentiating substances in water and detecting their distribution characteristics in the ocean are of great significance for marine monitoring and scientific research,” said research team leader Mingjia Shangguan from Xiamen University in China. “For instance, the remote sensing of underwater oil that we demonstrated could be useful for monitoring leaks in underwater oil pipelines.”
Although lidar approaches based on Raman signals have been previously used for detection of underwater substances, existing systems are impractical because they are bulky and require large amounts of power.
In the journal Applied Optics, the researchers describe their new lidar system, which uses just 1 μJ of pulse energy and 22.4 mm of receiver aperture. The entire lidar system is 40 cm long with a diameter of 20 cm and can be operated up to 1 km underwater. To boost sensitivity, the researchers incorporated single-photon detection into their compact underwater Raman lidar system.
“Mounting an underwater Raman lidar system on an autonomous underwater vehicle or remotely operated vehicle could enable monitoring for leaks in underwater oil pipelines,” said Shangguan. “It could potentially also be used to explore oceanic resources or be applied in detecting seafloor sediment types, such as coral reefs.”
Single-photon sensitivity in underwater lidar
Traditional lidar systems designed to operate above water on ships, aircraft or satellites can achieve large-scale ocean profiling, but their detection depth is limited, especially during rough sea conditions. Raman lidar systems, however, can be used for analysis underwater at different depths without being affected by sea conditions.
Raman lidar works by emitting a pulse of green laser light into the water that interacts with substances such as oil. This excites inelastic Raman signals that can be used to identify substances. By measuring the intensity of Raman signals at specific wavelengths, lidar can provide information about the oil content in the water.
“Traditional Raman lidar systems rely on increasing laser power and telescope aperture to achieve remote sensing detection, which leads to a large system size and high-power consumption that make it difficult to integrate lidar systems onto underwater vehicles,” said Shangguan. “The use of single-photon detection technology made this work possible by improving detection sensitivity to the level of single photons.”
The researchers demonstrated their new lidar system by using it to detect varying thicknesses of gasoline oil in a quartz cell that was 12 m away from the system. Both the lidar system and the quartz cell were submerged at a depth of 0.6 m underwater in a large pool. The lidar system was able to detect and distinguish all thicknesses of gasoline, which ranged from 1 mm to 15 mm.
The researchers are now working to increase the number of detection channels and the Raman spectral resolution of the single-photon lidar system to enhance its ability to distinguish different substances in water. This would allow it to be used to analyze underwater bubble types and to detect corals and manganese nodules.
More information: Mingjia Shangguan et al, Remote sensing oil in water with an all-fiber underwater single-photon Raman lidar, Applied Optics (2023). DOI: 10.1364/AO.488872
Students in the laboratory presenting rotation of Schrödinger cat states. No actual cats were hurt during the project. Credit: S. Kurzyna and B. Niewelt, source: University of Warsaw
Students at the Faculty of Physics of the University of Warsaw (UW) and researchers from the QOT Center for Quantum Optical Technologies have developed an innovative method that allows the fractional Fourier Transform of optical pulses to be performed using quantum memory. This achievement is unique on the global scale, as the team was the first to present an experimental implementation of the said transformation in this type of system.
The results of the research were published in the journal Physical Review Letters. In their work, the students tested the implementation of the fractional Fourier Transform using a double optical pulse, also known as a “Schrödinger’s cat” state.
The spectrum of the pulse and temporal distribution
Waves, such as light, have their own characteristic properties—pulse duration and frequency (corresponding, in the case of light, to its color). It turns out that these characteristics are related to each other through an operation called the Fourier Transform, which makes it possible to switch from describing a wave in time to describing its spectrum in frequencies.
The fractional Fourier Transform is a generalization of the Fourier Transform that allows a partial transition from a description of a wave in time to a description in frequency. Intuitively, it can be understood as a rotation of a distribution (for example, the chronocyclic Wigner function) of the considered signal by a certain angle in the time-frequency domain.
It turns out that transforms of this type are exceptionally useful in the design of special spectral-temporal filters to eliminate noise and enable the creation of algorithms that make it possible to use the quantum nature of light to distinguish pulses of different frequencies more precisely than traditional methods. This is especially important in spectroscopy, which helps study the chemical properties of matter, and telecommunications, which requires the transmission and processing of information with high precision and speed.
An ordinary glass lens is capable of focusing a monochromatic beam of light falling on it to almost a single point (focus). Changing the angle of incidence of light on the lens results in a change in the position of the focus. This allows us to convert angles of incidence into positions, obtaining the analogy of the Fourier Transform, in the space of directions and positions. A classical spectrometer based on a diffraction grating uses this effect to convert the wavelength information of light into positions, allowing us to distinguish between spectral lines.
Time and frequency lenses
Similarly to the glass lens, time and frequency lenses allow the conversion of a pulse’s duration into its spectral distribution, or effectively, perform a Fourier transform in time and frequency space. The right selection of powers of such lenses makes it possible to perform a fractional Fourier Transform. In the case of optical pulses, the action of time and frequency lenses corresponds to applying quadratic phases to the signal.
To process the signal, the researchers used a quantum memory—or more precisely a memory equipped with quantum light processing capabilities—based on a cloud of rubidium atoms placed in a magneto-optical trap. The atoms were cooled to a temperature of tens of millions of degrees above absolute zero. The memory was placed in a changing magnetic field, allowing components of different frequencies to be stored in different parts of the cloud. The pulse was subjected to a time lens during writing and reading, and a frequencylens acted on it during storage.
The device developed at the UW allows the implementation of such lenses over a very wide range of parameters and in a programmable way. A double pulses is very prone to decoherence, hence it is often compared to the famous Schrödinger cat—a macroscopic superposition of being dead and alive, almost impossible to achieve experimentally. Still, the team was able to implement faithful operations on those fragile dual-pulse states.
Before direct application in telecommunications, the method must first be mapped to other wavelengths and parameter ranges. Fractional Fourier transform, however, could prove crucial for optical receivers in state-of-the-art networks, including optical satellite links. A quantum light processor developed at the UW makes it possible to find and test such new protocols in an efficient way.
More information: Bartosz Niewelt et al, Experimental Implementation of the Optical Fractional Fourier Transform in the Time-Frequency Domain, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.240801
A spinning neutron disintegrates into a proton, electron, and antineutrino when a down quark in the neutron emits a W boson and converts into an up quark. The exchange of quanta of light (γ) among charged particles changes the strength of this transition. Credit: Vincenzo Cirigliano, Institute for Nuclear Theory
Outside atomic nuclei, neutrons are unstable particles, with a lifetime of about fifteen minutes. The neutron disintegrates due to the weak nuclear force, leaving behind a proton, an electron, and an antineutrino. The weak nuclear force is one of the four fundamental forces in the universe, along with the strong force, the electromagnetic force, and the gravitational force.
Comparing experimental measurements of neutron decay with theoretical predictions based on the weak nuclear force can reveal as-yet undiscovered interactions. To do so, researchers must achieve extremely high levels of precision. A team of nuclear theorists has uncovered a new, relatively large effect in neutron decay that arises from the interplay of the weak and electromagnetic forces.
This research identified a shift in the strength with which a spinning neutron experiences the weak nuclear force. This has two major implications. First, scientists have known since 1956 that due to the weak force, a system and one built like its mirror image do not behave in the same way. In other words, mirror reflection symmetry is broken. This research affects the search for new interactions, technically known as “right-handed currents,” that, at very short distances of less than one hundred quadrillionths of a centimeter, restore the universe’s mirror-reflection symmetry. Second, this research points to the need to compute electromagnetic effects with higher precision. Doing so will require the use of future high-performance computers.
A team of researchers computed the impact of electromagnetic interactions on neutron decay due to the emission and absorption of photons, the quanta of light. The team included nuclear theorists from the Institute for Nuclear Theory at the University of Washington, North Carolina State University, the University of Amsterdam, Los Alamos National Laboratory, and Lawrence Berkeley National Laboratory and their results have been published in Physical Review Letters.
The calculation was performed with a modern method, known as “effective field theory,” that efficiently organizes the importance of fundamental interactions in phenomena involving strongly interacting particles. The team identified a new percent-level shift to the nucleon axial coupling, gA, which governs the strength of decay of a spinning neutron. The new correction originates from the emission and absorption of electrically charged pions, which are mediators of the strong nuclear force. While effective field theory provides an estimate of the uncertainties, improving on the current precision will require advanced calculations on Department of Energy supercomputers.
The researchers also assessed the impact on searches of right-handed current. They found that after including the new correction, experimental data and theory are in good agreement and current uncertainties still allow for new physics at a relatively low mass scale.
More information: Vincenzo Cirigliano et al, Pion-Induced Radiative Corrections to Neutron β Decay, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.121801
Two-time correlation functions of the ce-based MG measured by HP-XPCS at different pressures during compression. At each pressure, the width of the reddish diagonal contour is proportional to the relaxation time, which broadens below 2.9 GPa and then narrows during further compression. Credit: Dr. Qiaoshi Zeng of HPSTAR
A major stumbling block in our understanding of glass and glass phenomena is the elusive relationship between relaxation dynamics and glass structure. A team led by Dr. Qiaoshi Zeng from HPSTAR recently developed a new in situ high-pressure wide-angle X-ray photon correlation spectroscopy method to enable atomic-scale relaxation dynamics studies in metallic glass systems under extreme pressures. The study is published in Proceedings of the National Academy of Sciences (PNAS).
Metallic glasses (MGs), with many superior properties to both conventional metals and glasses, have been the focus of worldwide research. As thermodynamically metastable materials, like typical glasses, MGs spontaneously evolve into their more stable states all the time through various relaxation dynamic behaviors.
These relaxation behaviors have significant effects on the physical properties of MGs. Still, until now, scientists’ ability to deepen the understanding of glass relaxation dynamics and especially its relationships with atomic structures has been limited by the available techniques.
“Thanks to the recent improvements in synchrotron X-ray photon correlation spectroscopy (XPCS), measuring the collective particle motions of glassy samples with a high resolution and broad coverage in the time scale is possible, and thus, various microscopic dynamic processes otherwise inaccessible have been explored in glasses,” said Dr. Zeng.
“However, the change in atomic structures is subtle in previous relaxation process measurements, which makes it still difficult to probe the relationship between the structure and relaxation behavior. To overcome this problem, we decided to employ high pressure because it can effectively alternate the structure of various materials, including MG.”
To this end, the team developed in situ high-pressure synchrotron wide-angle XPCS to probe a cerium-based MG material during compression. In situ high-pressure wide-angle XPCS revealed that the collective atomic motion initially slows down, as generally expected with increasing density. Then, counter-intuitively it accelerates with further compression, showing an unusual non-monotonic pressure-induced steady relaxation dynamics crossover at ~3 GPa.
Furthermore, by combining these results with in situ high-pressure synchrotron X-ray diffraction, the relaxation dynamics anomaly closely correlates with the dramatic changes in local atomic structures during compression, rather than monotonically scaling with either the sample density or overall stress level.
“With density increases, atoms in glasses generally get more difficult to move or diffuse, slowing down its relaxation dynamics. This is what we normally expect from hydrostatic compression,” Dr. Zeng explained.
“So the non-monotonic relaxation behavior observed here in the cerium-based MG under pressure is quite unusual, which indicates besides density, structural details could also play an important role in glass relaxation dynamics,” Dr. Zeng explained.
These findings demonstrate that there is a close relationship between glass relaxation dynamics and atomic structures in MGs. The technique Dr. Qiaoshi Zeng’s group developed here can also be extended to explore the relationship between relaxation dynamics and atomic structures in various glasses, especially those significantly tunable by compression, offering new opportunities for glassrelaxation dynamics studies at extreme conditions.
More information: Qiaoshi Zeng et al, Pressure-induced nonmonotonic cross-over of steady relaxation dynamics in a metallic glass, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.230228112
Bosonic correlated insulator.(A) Illustration of a bosonic correlated insulator consisting of interlayer excitons. Magenta spheres indicate holes and cyan spheres, electrons. (Inset) Type II band alignment of WSe2/WS2 heterostructure. (B) Schematics of continuous-wave pump probe spectroscopy. The exciton and electron density are independently controlled by pump light and electrostatic gate. Red and green shading correspond to wide-field pump light and focused probe light, respectively. (C and E) Gate-dependent PL (C) and absorption (E) spectra of a 60°-aligned WSe2/WS2 moiré bilayer (device D1) at zero pump intensity. The PL peak shows a sudden blue shift at electron filling νe= 1 and 2 (yellow arrows), where the absorption spectrum shows kinks and splitting. (D and F) Pump intensity–dependent PL (D) and absorption (F) spectra of device D1 at charge neutrality. Right axes show dipolar-interaction–induced interlayer exciton energy shift Δdipole, which is approximately proportional to νex. The dominant PL peak in (D) at low and high pump intensity are labeled as peak I and II, respectively. All measurements are performed at a base temperature of 1.65 K. Credit: Science (2023). DOI: 10.1126/science.add5574
Take a lattice—a flat section of a grid of uniform cells, like a window screen or a honeycomb—and lay another, similar lattice above it. But instead of trying to line up the edges or the cells of both lattices, give the top grid a twist so that you can see portions of the lower one through it. This new, third pattern is a moiré, and it’s between this type of overlapping arrangement of lattices of tungsten diselenide and tungsten disulfide where UC Santa Barbara physicists found some interesting material behaviors.
“We discovered a new state of matter—a bosonic correlated insulator,” said Richen Xiong, a graduate student researcher in the group of UCSB condensed matter physicist Chenhao Jin, and the lead author of a paper that appears in the journal Science.
According to Xiong, Jin and collaborators from UCSB, Arizona State University and the National Institute for Materials Science in Japan, this is the first time such a material—a highly ordered crystal of bosonic particles called excitons—has been created in a “real” (as opposed to synthetic) matter system.
“Conventionally, people have spent most of their efforts to understand what happens when you put many fermions together,” Jin said. “The main thrust of our work is that we basically made a new material out of interacting bosons.”
Bosonic, correlated, insulator
Subatomic particles come in one of two broad types: fermions and bosons. One of the biggest distinctions is in their behavior, Jin said.
“Bosons can occupy the same energy level; fermions don’t like to stay together,” he said, “Together, these behaviors construct the universe as we know it.”
Fermions, such as electrons, underlie the matter with which we are most familiar as they are stable and interact through the electrostatic force. Meanwhile bosons, such as photons (particles of light), tend to be more difficult to create or manipulate as they are either fleeting or do not interact with each other.
A clue to their distinct behaviors is in their different quantum mechanical characteristics, Xiong explained. Fermions have half-integer “spins” such as 1/2 or 3/2 et cetera, while bosons have whole integer spins (1, 2, etc.). An exciton is a state in which a negatively charged electron (a fermion) is bound to its positively charged opposite “hole” (another fermion), with the two half-integer spins together becoming a whole integer, creating a bosonic particle.
To create and identify excitons in their system, the researchers layered the two lattices and shone strong lights on them in a method they call “pump-probe spectroscopy.” The combination of particles from each of the lattices (electrons from the tungsten disulfide and the holes from the tungsten diselenide) and the light created a favorable environment for the formation of and interactions between the excitons while allowing the researchers to probe these particles’ behaviors.
“And when these excitons reached a certain density, they could not move anymore,” Jin said. Thanks to strong interactions, the collective behaviors of these particles at a certain density forced them into a crystalline state, and created an insulating effect due to their immobility.
“What happened here is that we discovered the correlation that drove the bosons into a highly ordered state,” Xiong added. Generally, a loose collection of bosons under ultracold temperatures will form a condensate, but in this system, with both light and increased density and interaction at relatively higher temperatures, they organized themselves into a symmetric solid and charge-neutral insulator.
The creation of this exotic state of matter proves that the researchers’ moiré platform and pump-probe spectroscopy could become an important means for creating and investigating bosonic materials.
“There are many-body phases with fermions that result in things like superconductivity,” Xiong said. “There are also many-body counterparts with bosons that are also exotic phases. So what we’ve done is create a platform, because we did not really have a great way to study bosons in real materials.” While excitons are well-studied, he added, there hadn’t until this project been a way to coax them to interacting strongly with one another.
With their method, according to Jin, it could be possible to not only study well-known bosonic particles like excitons but also open more windows into the world of condensed matter with new bosonic materials.
“We know that some materials have very bizarre properties,” he said. “And one goal of condensed matter physics is to understand why they have these rich properties and find ways to make these behaviors come out more reliably.”
More information: Richen Xiong et al, Correlated insulator of excitons in WSe 2 /WS 2 moiré superlattices, Science (2023). DOI: 10.1126/science.add5574
Illustration of a unidirectional flow as investigated in the new study using a Lennard-Jones fluid as an example. The three-dimensional nonequilibrium system is set in motion (red arrows) by a force field (blue arrows) acting along the x-axis. Credit: Matthias Schmidt
Living organisms, ecosystems and the planet Earth are, from a physics point of view, examples of extraordinarily large and complex systems that are not in thermal equilibrium. To physically describe non-equilibrium systems, dynamic density functional theory has been used to date.
However, this theory has weaknesses, as physicists from the University of Bayreuth have now shown in an article published in the Journal of Physics: Condensed Matter. Power functional theory proves to perform substantially better—in combination with artificial intelligence methods, it enables more reliable descriptions and predictions of the dynamics of non-equilibrium systems over time.
Many-particle systems are all kind of systems composed of atoms, electrons, molecules, and other particles invisible to the eye. They are in thermal equilibrium when the temperature is balanced and no heat flow occurs. A system in thermal equilibrium changes its state only when external conditions change. Density functional theory is tailor-made for the study of such systems.
For more than half a century, it has proven its unrestricted value in chemistry and materials science. Based on a powerful classical variant of this theory, states of equilibrium systems can be described and predicted with high accuracy. Dynamic density functional theory (DDFT) extends the scope of this theory to non-equilibrium systems. This involves the physical understanding of systems whose states are not fixed by their external boundary conditions.
These systems have a momentum of their own: they have the ability to change their states without external influences acting on them. Findings and application methods of DDFT are therefore of great interest, for example, for the study of models for living organisms or microscopic flows.
The error potential of dynamic density functional theory
However, DDFT uses an auxiliary construction to make non-equilibrium systems accessible to physical description. It translates the continuous dynamics of these systems into a temporal sequence of equilibrium states. This results in a potential for errors that should not be underestimated, as the Bayreuth team led by Prof. Dr. Matthias Schmidt shows in the new study.
The investigations focused on a comparatively simple example—the unidirectional flow of a gas known in physics as a “Lennard-Jones fluid.” If this nonequilibrium system is interpreted as a chain of successive equilibrium states, one aspect involved in the time-dependent dynamics of the system is neglected, namely the flow field. As a result, DDFT may provide inaccurate descriptions and predictions.
“We do not deny that dynamic density functional theory can provide valuable insights and suggestions when applied to nonequilibrium systems under certain conditions. The problem, however, and we want to draw attention to this in our study using fluid flow as an example, is that it is not possible to determine with sufficient certainty whether these conditions are met in any particular case. The DDFT does not provide any control over whether the restricted framework conditions are given under which it enables reliable calculations. This makes it all the more worthwhile to develop alternative theoretical concepts for understanding nonequilibrium systems,” says Prof. Dr. Daniel de las Heras, first author of the study.
Power functional theory proves to perform substantially better
For ten years, the research team around Prof. Dr. Matthias Schmidt has been making significant contributions to the development of a still young physical theory, which has so far proven to be very successful in the physical study of many-particle systems: power functional theory (PFT). The physicists from Bayreuth are pursuing the goal of being able to describe the dynamics of non-equilibrium systems with the same precision and elegance with which classical density functional theory enables the analysis of equilibrium systems.
In their new study, they now use the example of a fluid flow to show that power functional theory is significantly superior to DDFT when it comes to understanding non-equilibrium systems. PFT allows the dynamics of these systems to be described without having to take a detour via a chain of successive equilibrium states in time. The decisive factor here is the use of artificial intelligence. Machine learning opens up the time-dependent behavior of the fluid flow by including all factors relevant to the system’s inherent dynamics—including the flow field. In this way, the team has even succeeded in controlling the flow of the Lennard-Jones fluid with high precision.
“Our investigation provides further evidence that power function theory is a very promising concept that can be used to describe and explain the dynamics of many-particle systems. In Bayreuth, we intend to further elaborate this theory in the coming years, applying it to nonequilibrium systems that have a much higher degree of complexity than the fluid flow we studied. In this way, the PFT will be able to supersede the dynamic density functional theory, whose systemic weaknesses it avoids according to our findings so far. The original density functional theory, which is tailored to equilibrium systems and has proven its worth, is retained as an elegant special case of PFT,” says Prof. Dr. Matthias Schmidt, who is chair of theoretical physics II at the University of Bayreuth.
More information: Daniel de las Heras et al, Perspective: How to overcome dynamical density functional theory, Journal of Physics: Condensed Matter (2023). DOI: 10.1088/1361-648X/accb33
Schematic top-view layout of the NA61/SHINE experiment in the configuration used during the 2017 proton data taking. In 2016 the forward time projection chambers were not present. The S5 scintillator was not used in this trigger configuration. Credit: Physical Review D (2023). DOI: 10.1103/PhysRevD.107.072004
At the time of the Big Bang, 13.8 billion years ago, every particle of matter is thought to have been produced together with an antimatter equivalent of opposite electrical charge. But in the present-day universe, there is much more matter than antimatter. Why this is the case is one of physics’ greatest questions.
The answer may lie, at least partly, in particles called neutrinos, which lack electrical charge, are almost massless and change their identity—or “oscillate”—from one of three types to another as they travel through space. If neutrinos oscillated in a different way to their antimatter equivalents, antineutrinos, they could help explain the matter–antimatter imbalance in the universe.
Experiments across the world, such as the NOvA experiment in the US, are investigating this possibility, as will next-generation experiments including DUNE. In these long-baseline neutrino-oscillation experiments, a beam of neutrinos is measured after it has traveled a long distance—the long baseline. The experiment is then run with a beam of antineutrinos, and the outcome is compared with that of the neutrino beam to see if the two twin particles oscillate in a similar or different way.
This comparison depends on an estimation of the numbers of neutrinos in the neutrino and antineutrino beams before they travel. These beams are produced by firing beams of protons onto fixed targets. The interactions with the target create other hadrons, which are focused using magnetic “horns” and directed into long tunnels in which they transform into neutrinos and other particles. But in this multi-step process, it isn’t easy to work out the particle content of the resulting beams—including the number of neutrinos they contain—which depends directly on the proton–target interactions.
Enter the NA61 experiment at CERN, also known as SHINE. Using high-energy proton beams from the Super Proton Synchrotron and appropriate targets, the experiment can recreate the relevant proton–target interactions. NA61/SHINE has previously made measurements of electrically charged hadrons that are produced in the interactions and yield neutrinos. These measurements helped improve estimations of the content of neutrino beams used at existing long-baseline experiments.
The NA61/SHINE collaboration has now released new hadron measurements that will help improve these estimations further. This time around, using a proton beam with an energy of 120 GeV and a carbon target, the collaboration measured three kinds of electrically neutral hadrons that decay into neutrino-yielding charged hadrons.
This 120-GeV proton–carbon interaction is used to produce NOvA’s neutrino beam, and it will probably also be used to create DUNE’s beam. Estimations of the numbers of the different neutrino-yielding neutral hadrons that the interaction produces rely on computer simulations, the output of which varies significantly depending on the underlying physics details.
“Up to now, simulations for neutrino experiments that use this interaction have relied on uncertain extrapolations from older measurements with different energies and target nuclei. This new direct measurement of particle production from 120-GeV protons on carbon reduces the need for these extrapolations,” explains NA61/SHINE deputy spokesperson Eric Zimmerman.
The paper is published in the journal Physical Review D.
More information: H. Adhikary et al, Measurements of KS0 , Λ , and Λ¯ production in 120 GeV/c p+C interactions, Physical Review D (2023). DOI: 10.1103/PhysRevD.107.072004
An international research team led by the Paul Scherrer Institute PSI has measured the radius of the nucleus of muonic helium-3 with unprecedented precision. The results are an important stress test for theories and future experiments in atomic physics.
1.97007 femtometer (quadrillionths of a meter): That’s how unimaginably tiny the radius of the atomic nucleus of helium-3 is. This is the result of an experiment at PSI that has now been published in the journal Science.
More than 40 researchers from international institutes collaborated to develop and implement a method that enables measurements with unprecedented precision. This sets new standards for theories and further experiments in nuclear and atomic physics.
This demanding experiment is only possible with the help of PSI’s proton accelerator facility. There Aldo Antognini’s team generates so-called muonic helium-3, in which the two electrons of the helium atom are replaced by an elementary particle called a muon. This allows the nuclear radius to be determined with high precision.
With the measurement of helium-3, the experiments on light muonic atoms have now been completed for the time being. The researchers had previously measured muonic helium-4 and, a few years ago, the atomic nucleus of muonic hydrogen and deuterium.
Muonic helium-3: Twice as slimmed-down
Helium-3 is the lighter cousin of ordinary helium, helium-4. Its atomic nucleus has two protons and two neutrons (hence the 4 after the abbreviation for the element); in helium-3, one of the neutrons is missing. The simplicity of this slimmed-down atomic nucleus is very interesting to Aldo Antognini and other physicists.
The helium-3 that PSI physicist and ETH Zurich professor Antognini is using in the current experiment lacks not only a neutron in the nucleus, but also both electrons that orbit this nucleus. The physicists replace the electrons with a negatively charged muon—hence the name muonic helium-3.
The muon is around 200 times heavier and gets close to the nucleus. Thus, the nucleus and the muon “sense” each other much more intensely, and the wave functions overlap more strongly, as they say in physics.
That makes the muon the perfect probe for measuring the nucleus and its charge radius. This indicates the area over which the positive charge of the nucleus is distributed. Ideal for the researchers, this charge radius of the nucleus does not change when the electrons are replaced by a muon.
Antognini has experience in measuring muonic atoms. A few years ago, he carried out the same experiment with muonic hydrogen, which contains only one proton in the nucleus and whose one electron was replaced by a negatively charged muon. The results caused quite a commotion at the time, because the deviation from measurements based on other methods was surprisingly large. Some critics even considered them wrong. It has now been confirmed many times over: The results were correct.
Worldwide-unique facility enables experiments
This time Antognini will not need to exercise as much persuasive power. For one thing, he has established himself as the leading expert in this area of research. Another factor is that there was no big surprise this time. The current results from muonic helium-3 fit well with those from previous experiments in which other methods were used. However, the PSI team’s measurements are around 15 times more precise.
Negatively charged muons, and plenty of them, are the most important ingredient for the experiment. These must have a very low energy—that is, they must be very slow, at least by the standards of particle physics.
At PSI, around 500 muons per second with energies of one kiloelectron-volt can be generated. This makes the PSI proton accelerator facility, with its beamline developed in-house, the only one in the world that can deliver such slow negative muons in such large numbers.
PSI physicist Aldo Antognini is pleased that he and his team, within an international collaboration, have achieved yet another fundamental result in atomic physics. Credit: Scanderbeg Sauer Photography
Laser developed in-house was crucial for success
A crucial share of the success is due to a laser system that the researchers themselves developed. There the challenge is that the laser must fire immediately when a muon flies into the experimental setup.
To make this possible, Antognini and his team installed an extremely thin foil detector in front of the airless experimental chamber. This detects when a muon passes through the foil and signals the laser to emit a pulse of light immediately and at full power.
The researchers determine the charge radius indirectly by measuring the frequency of the laser light. When the laser frequency precisely matches the resonance of a specific atomic transition, the muon is briefly excited to a higher energy state before decaying to the ground state within picoseconds; at that point it will emit a photon in the form of an X-ray.
Finding the resonance frequency at which this transition occurs requires a lot of patience, but the reward is an extremely accurate value for the charge radius of the nucleus.
New benchmark for theoretical modeling
The charge radii obtained from muonic helium-3 and helium-4 serve as important reference values for modern ab initio theories—that is, physical models that calculate the properties of complex physical systems directly from the fundamental laws of physics, without resorting to experimental data.
In the context of nuclear physics, these models offer detailed insights into the structure of light atomic nuclei and the forces between their building blocks, the protons and neutrons.
Precise knowledge of these nuclear radii is also crucial for comparisons with ongoing experiments on conventional helium ions with one electron and on neutral helium atoms with two electrons. Such comparisons provide stringent tests of quantum electrodynamics (QED) in few-body systems—the fundamental theory that describes how charged particles interact through the exchange of photons. They allow researchers to test the predictive power of our most fundamental understanding of atomic structure.
These efforts could lead to new insights into QED in for bound systems—that is, in systems such as atoms, in which particles are not free but bound to each other by forces—or perhaps even to indications of physical effects outside beyond the Standard Model of Particle Physics.
Follow-up experiments are currently being conducted by research teams in Amsterdam, Garching, and China, as well as in Switzerland by the Molecular Physics and Spectroscopy group led by Frédéric Merkt at ETH Zurich.
Antognini also has additional ideas for future experiments aimed at testing the theories of atomic and nuclear physics with even greater precision. One idea is to measure hyperfine splitting in muonic atoms. This refers to energy transitions between split energy levels that reveal deeper details about effects in the atomic nucleus that involve spin and magnetism.
An experiment with muonic hydrogen is currently being prepared, and an experiment with muonic helium is planned. “Many people who work in nuclear physics are very interested in it and are eagerly awaiting our results,” Antognini says.
But the energy density of the laser must be increased significantly, which will require an enormous advance in laser technology. This development is currently under way at PSI and ETH Zurich.
Scientists at Paderborn University have made a further step forward in the field of quantum research: for the first time ever, they have demonstrated a cryogenic circuit (i.e. one that operates in extremely cold conditions) that allows light quanta—also known as photons—to be controlled more quickly than ever before.
Specifically, these scientists have discovered a way of using circuits to actively manipulate light pulses made up of individual photons. This milestone could substantially contribute to developing modern technologies in quantum information science, communication and simulation. The results have now been published in the journal Optica.
Photons, the smallest units of light, are vital for processing quantum information. This often requires measuring a photon’s state in real time and using this information to actively control the luminous flux—a method known as a “feedforward operation.”
However, thus far this has butted up against technical limitations: light was measured, processed and controlled at a delay, limiting its use for complex applications. With their new method, these scientists have managed to significantly reduce the delay—to less than a quarter of a billionth of a second.
“We have managed to actively interconnect light pulses with detectors, adapted electronics and optical circuits at cryogenic temperatures. This enabled us to manipulate individual photons significantly more quickly than other research groups. The ability allows us to create new active circuits for quantum optics that can be used for a variety of applications,” explains Dr. Frederik Thiele, who is spearheading the project with Niklas Lamberty, both members of the “Mesoscopic Quantum Optics” research group at Paderborn’s Department of Physics.
The researchers used state-of-the-art technologies such as superconducting detectors for this development. These devices measure individual light quanta with extremely high precision.
The electronics were deployed in a cryogenic environment: the amplifier and modulators were operated at temperatures of around -270 degrees Celsius in order to process signals without any significant delay. Integrated modulators are optical components that control the light based on measurement data—virtually loss-free and at high speeds.
The process is based on measuring light pairs, or “correlated photons.” Based on the number of particles measured, the electronic circuit decides in a fraction of a second whether the light should be forwarded or blocked. What makes the integrated design special is that physical losses and delays can be reduced to a minimum.
As well as a fast response, the circuit also produces less heat, which is vital when working in cryostats (extreme cooling systems) in very small spaces.
“Our demonstration shows that we can use superconducting and semiconducting technology to achieve a new level of photonic quantum control. This opens up opportunities for fast and complex quantum circuits, which could be vitally important for quantum information science and communication,” Thiele summarizes.
