Category: Physics

Findings published in Nature settle the dispute: phonons can be chiral. This fundamental concept, discovered using circular X-ray light, sees phonons twisting like a corkscrew through quartz.

Throughout nature, at all scales, you can find examples of chirality—or handedness. Imagine trying to eat a sandwich with two hands that were not enantiomers—non-superimposable mirror images—of each other. Consider the pharmacological disasters caused by administering the wrong drug enantiomer or, at a subatomic scale, the importance of the concept of parity in particle physics. Now, thanks to a new study led by researchers at Paul Scherrer Institute PSI, we know that phonons can also possess this property.

A phonon is a quasiparticle that describes the collective vibrational excitations of the atoms in a crystal lattice; imagine it as the Irish Riverdance of the atoms. Physicists have predicted that if phonons can demonstrate chirality they could have important implications on the fundamental physical properties of materials. With the rapid rise in recent years of research into topological materials that exhibit curious electronic and magnetic surface properties, interest in chiral phonons has grown. Yet, experimental proof for their existence has remained elusive.

What makes phonons chiral is the steps of their dance. In the new study, the atomic vibrations dance a twist that moves forwards like a corkscrew. This corkscrew motion is one of the reasons there has been such a drive to discover the phenomenon. If phonons can revolve in this way, like the coil of wire that forms a solenoid, perhaps they could create a magnetic field in a material.

A new slant on the problem

It is this possibility that motivated the group of Urs Staub at PSI, who led the study. “It is because we are at the juncture between ultrafast X-ray science and materials research that we could approach the problem from a different angle,” he says. The researchers are interested in manipulating chiral modes of materials using chiral light—light that is circularly polarized.

It was using such light that the researchers could make their proof. Using quartz, one of the best-known minerals whose atoms—silicon and oxygen—form a chiral structure, they showed how circularly polarized light coupled to chiral phonons. To do this, they used a technique known as resonant inelastic X-ray scattering (RIXS) at the Diamond Light Source in the UK. This was complemented with supporting theoretical descriptions of how the process would create and enable the detection of chiral phonons from groups at the ETH Zurich (Carl Romao and Nicola Spaldin) and MPI Dresden (Jeroen van den Brink).

‘It doesn’t usually work like this in science’

In their experiment, circularly polarized light shines on quartz. The photons of light possess angular momentum, which they transfer to the atomic lattice, launching the vibrations into their corkscrew motion. The direction that the phonons revolve depends on the intrinsic chirality of the quartz crystal. As the phonons revolve, they release energy in the form of scattered light, which can be detected.

Imagine standing on a roundabout and throwing a Frisbee. If you throw the Frisbee with the same direction of movement as the roundabout, you would expect it to whizz. Throw it the other way and it will spin less, as the angular momentum of the roundabout and the Frisbee will cancel out. In the same way, when the circularly polarized light twists the same way as the phonon it excites, the signal is enhanced, and chiral phonons could be detected.

A well-planned experiment, careful theoretical calculations, and then something strange happened: almost everything went according to plan. As soon as they analyzed the results, the difference in response as the chirality of the light flipped was undeniable.

“The results were convincing almost immediately, especially when we compared the difference with the other quartz enantiomers,” remembers PSI scientist and publication first-author Hiroki Ueda. Sitting at his computer to analyze the data, Ueda was the first person to see the results: “I kept checking my analysis codes to make sure it was true.” Staub emphasizes, “It’s not normal! It doesn’t usually work like this in science!”

During the quest for chiral phonons, there have been several false alarms. Will this settle the debate? “Yes, I think so, that’s the beauty of this piece of work,” believes Staub, whose opinion was shared by the reviewers at Nature. “Because it’s simple, and beautiful and straightforward. It’s obvious. It’s so simple, it’s obvious that this is the chiral motion.”

More information: Hiroki Ueda et al, Chiral phonons in quartz probed by X-rays, Nature (2023). DOI: 10.1038/s41586-023-06016-5

Journal information: Nature 

Provided by Paul Scherrer Institute 

Scientists from the Department of Energy’s Ames National Laboratory made an intriguing discovery while conducting experiments to characterize magnetism in a material known as a dilute magnetic topological insulator where magnetic defects are introduced. Despite this material’s ferromagnetism, the team discovered strong antiferromagnetic interactions between some pairs of magnetic defects that play a key role in several families of magnetic topological insulators.

Topological insulators (TIs) as their name indicates, are insulators. However, because of their unique electronic band structure, they conduct electricity on the surface under the right conditions. By introducing magnetism, TIs can transmit electrical currents from one point to another without any heat generation or energy loss. This quality means that they have the potential to reduce future energy footprints for computing and electricity transmission.

According to Rob McQueeney, a scientist from Ames Lab and a member of the research team, “Finding topological insulators is not so easy. You have to find this unique situation where the electronic bands are knotted up.” He further explained that applying a magnetic field to a TI turns the surface into a unique two-dimensional insulator, while the very edges of the surface remain metallic.

An important goal is to obtain a ferromagnetic TI. Ferromagnetism is when all of the magnetic moments in the material spontaneously align in the same direction. However, the team also discovered that TIs are susceptible to antiferromagnetic interactions when defects are introduced. Antiferromagnetism is when some of the ions spontaneously align with neighboring ions. The opposing magnetic forces decrease the overall magnetism of the material.

There are two ways that scientists introduce magnetism into a TI. The first is to introduce dilute amounts of magnetic ions, such as manganese-doped bismuth telluride or antimony telluride. The second is to create an intrinsic magnetic TI by inserting a layer of magnetic ions into the material, such as manganese-bismuth-tellurium (MnBi₂Te₄) and manganese-antimony-tellurium (MnSb₂Te₄).

Since the intrinsic magnetic TIs have a full layer of magnetic ions, ideally the magnetism is not randomly distributed the way it is in the first method.

For this project, the team focused on dilute magnetic TIs, which have randomly distributed magnetic defects. “We wanted to understand the magnetic interactions at the most fundamental level. We were doping our sample using small amounts of magnetic ions to try to understand how the magnetic interactions occur,” said Farhan Islam, an Iowa State University graduate student and team member. “So basically, we’re trying to understand how microscopic interactions affect the overall magnetism of the system.”

To conduct their research, the team used a specialized method called neutron scattering. This method involves passing a beam of neutrons (sub-atomic particles with a neutral charge) through a sample of material. Data is collected by noting where and when the neutrons that scattered from the sample hit a detector.

This type of research can only be done a few places in the world. Neutron scattering for this project was conducted at the Spallation Neutron Source, a Department of Energy Office of Science User Facility operated by the Oak Ridge National Laboratory.

One challenge with neutron scattering is its weak signal. The team had concerns about studying dilute magnetism, because of the small overall number of magnetic ions. “I was very skeptical that we would see anything at all,” said McQueeney. “But we did. Actually, what we saw was pretty straightforward to observe, which was surprising.”

The team discovered that despite the overall ferromagnetism of manganese doped antimony telluride (Sb_1.94Mn_0.06Te₃), some isolated pairs of magnetic defects are coupled antiferromagnetically with opposite moment directions. Other magnetic pairs, especially those in different blocks of the layered structure, are ferromagnetically coupled with parallel moments. The competing magnetic forces decrease the overall magnetism of the material.

“The intrinsic magnetic TIs actually have defects in it,” Islam explained. “So for example, manganese can actually go into the sites of antimony where they’re not supposed to be and the way the manganese is going into those sites is random.”

This random manganese site-mixing creates magnetic defects in the intrinsic magnetic TIs. The team found that the same interactions between defects in the dilute materials also occur in the intrinsic materials (i.e. MnSb₂Te₄). The magnetic ground state of the intrinsic magnetic TIs can be either ferromagnetic or antiferromagnetic, and the team now understands how magnetic defects control this behavior.

“We determined the interactions between defects in the dilute case and realized that these interactions are transferable to the intrinsic case,” said McQueeney. “By doing so, we conclude that defects control the magnetic order for both families.”

The study is published in the journal Advanced Materials.

More information: Farhan Islam et al, Role of Magnetic Defects in Tuning Ground States of Magnetic Topological Insulators, Advanced Materials (2023). DOI: 10.1002/adma.202209951

Journal information: Advanced Materials 

Provided by Ames National Laboratory 

We’ve all played at least once with a spring toy, but did you know that light can be shaped like a spring too?

An international team of researchers, led by Marco Piccardo, a former researcher at the Italian Institute of Technology (IIT) and now a Professor in the Physics Department of Técnico Lisboa and Principal Investigator at the Engineering Institute for Microsystems and Nanotechnologies (INESC MN), has harnessed ultrafast optics and structured light to synthesize in the laboratory a new family of spatiotemporal light beams, known as light springs.

The research was done in collaboration between IIT, Politecnico di Milano and Técnico Lisboa. The discovery has a disruptive potential for applications in photonics with complex light, such as time-resolved microscopy (useful, for example, to produce movies depicting the motion of molecules and viruses), laser-plasma acceleration and free-space (e.g., in the atmosphere) optical communications.

The research is published in Nature Photonics.

In ultrafast optics, it is possible to shorten or elongate the duration of extremely short optical pulses—down to a few femtoseconds, or thousandths of billionths of a second—or even produce complex pulses, by means of a technique known as pulse shaping. A central idea to this principle is that short laser pulses are composed of a large range of colors.

Scientists separate a pulse into its constituent colors, which are then separately manipulated and recombined, resulting in a new shape of the laser pulse. While pulse shaping allows to manipulate the time profile of a pulse, there is another set of techniques—known as wavefront shaping—allowing to give light a spatial structure. Designers of light have learnt how to combine these two methods to simultaneously shape light in space and time, bridging ultrafast optics and structured light for entirely new spatiotemporal applications.

A paradigm shift in spatiotemporal light shaping

Reporting now in Nature Photonics, Piccardo and his collaborators have introduced a paradigm shift in spatiotemporal light shaping. Unlike conventional shapers that separate different colors along a colorful strip, now the researchers used a special type of diffraction grating with circular symmetry, to create a round rainbow of colors.

This is an experiment that everyone can try at home: by shining a flashlight on an old CD-ROM and taking a picture with the phone camera, a round rainbow will be captured. Now, replace the flashlight with an ultrashort laser pulse and the CD-ROM with a microstructured diffractive device fabricated in the nanofabrication cleanroom and you are halfway through the experiment. The second part of the experiment consists in using advanced holograms to structure the many colors of light into different optical vortices shaped like a corkscrew.

“This results in a novel family of spatiotemporal light beams, which evolve on an ultrashort femtosecond timescale with a twisted and widely tailorable light structure,” said Marco Piccardo. “It opens unprecedented design capabilities in photonics, with many spectral and structural components to address.”

The broadband nature of these new light beams poses new challenges for their characterization, which the team has overcome by developing a powerful reconstruction technique, called hyperspectral holography, providing the complete tomography of the complex space-time structures.

“Our technique, which combines holography with Fourier transform spectroscopy, enables full characterization of the spatiotemporal profile of complex beams, enabling radically novel applications in the study of light-matter interactions,” said Giulio Cerullo, a Professor in Politecnico di Milano and co-author of the study.

The team showcased the unprecedented control enabled by their space-time shaper by tailoring many properties of the light springs. A beautiful demonstration shows two of such springs dancing together in space and time.

“We have found extremely interesting physics using these beams, which could lead us to a whole new generation of compact accelerators and light sources in plasma. This technique is very exciting because it promises to bring these theoretical concepts to the laboratory and to trigger major advance in laser-plasma physics,” said Jorge Vieira, a Professor in Técnico Lisboa and co-author of the study.

Now that it is finally possible to synthesize these light springs with complete freedom in the laboratory, the next natural step will be to bring them in laser-plasma experiments.

“This is a very challenging goal but the nanophotonic fabrication capabilities of INESC MN in Lisbon and the excellent plasma research groups of Técnico represent an ideal ecosystem to pursue this ambitious research,” said Piccardo. “Combining these advanced space-time beams with intense nonlinear laser-matter interactions could have important fundamental and technological implications. “

More information: Marco Piccardo et al, Broadband control of topological–spectral correlations in space–time beams, Nature Photonics (2023). DOI: 10.1038/s41566-023-01223-y

Journal information: Nature Photonics 

Provided by Técnico Lisboa

Quantum information (QI) processing may be the next game changer in the evolution of technology, by providing unprecedented computational capabilities, security and detection sensitivities. Qubits, the basic hardware element for quantum information, are the building block for quantum computers and quantum information processing, but there is still much debate on which types of qubits are actually the best.

Research and development in this field is growing at astonishing paces to see which system or platform outruns the other. To mention a few, platforms as diverse as superconducting Josephson junctions, trapped ions, topological qubits, ultra-cold neutral atoms, or even diamond vacancies constitute the zoo of possibilities to make qubits.

So far, only a handful of qubit platforms have been demonstrated to have the potential for quantum computing, marking the checklist of high-fidelity controlled gates, easy qubit-qubit coupling, and good isolation from the environment, which means sufficiently long-lived coherence.

Nano-mechanical resonators may be a part of the handful of platforms. They are oscillators, like springs and strings (e.g. guitars) that when driven, create harmonic or anharmonic sounds depending on the strength of the drive. But what happens when we cool a nano resonator down to absolute zero temperature?

The energy levels of the oscillator becomes quantized and the resonator vibrates with its characteristic zero point motion. The zero-point motion arises from the Heisenberg uncertainty principle. In other words, a resonator maintains motion even when it is in the ground state. The realization of a mechanical qubit is possible if the quantized energy levels of a resonator are not evenly spaced.

The challenge is to keep the nonlinear effects big enough in the quantum regime, where the oscillator’s zero point displacement is miniscule. If this is achieved, then the system may be used as qubit by manipulating it between the two lowest quantum levels without driving it in higher energy states.

For many years, there has been a lot of interest in realizing a qubit system with a mechanical nano resonator. In 2021, Fabio Pistolesi (Univ. Bordeaux-CNRS), Andrew N. Cleland (Univ. Chicago), and ICFO Prof. Adrian Bachtold, established a solid theoretical concept of a mechanical qubit, based on a nanotube resonator coupled to a double-quantum dot under an ultrastrong coupling regime.

These theoretical results proved that these nanomechanical resonators could indeed become ideal candidates for qubits. Why? Because they have been shown to feature long coherence times, a definite “must” for quantum computing.

Taking into account that there was a theoretical framework to work with, the challenge now was to actually make a qubit out of a mechanical resonator, and find the appropriate conditions and parameters to control the non-linearities in the system.

After several years of endless work on these systems, the challenges of experimentally realizing this has given its first very welcomed green light. In a recent study published in Nature Physics, ICFO researchers Chandan Samanta, Sergio Lucio de Bonis, Christoffer Moller, Roger Tormo-Queralt, W. Yang, Carles Urgell, led by ICFO Prof. Adrian Bachtold, in collaboration with researchers B. Stamenic and B.Thibeault from University of California Santa Barbara, Y. Jin from Université Paris-Saclay-CNRS, D.A. Czaplewski from Argonne National Laboratory, and F. Pistolesi from Univ. Bordeaux-CNRS, achieved the first pre-experimental steps for the future realization of a mechanical qubit by demonstrating a new mechanism to boost the anharmonicity of a mechanical oscillator in its quantum regime.

The experiment: Engineering anharmonicity close to the ground state

The team of researchers fabricated a suspended nanotube device of approximately 1.4 micrometers in length, with its extremes hooked onto the edges of two electrodes. They defined a quantum dot which is a two level electronic system on the vibrating nanotube by electrostatically creating tunnel junctions at both ends of the suspended nanotube.

Then, by adjusting the voltage on the gate electrode, they allowed the flow of only one electron at a time onto the nanotube. The mechanical motion of the nanotube was then coupled to the single electron in the single electron tunneling regime. This electromechanical coupling created anharmonicity to the mechanical system.

Subsequently, they decreased the temperature down to mK (milikelvins, almost absolute zero) and entered into an ultra-strong coupling regime where each additional electron on the nanotube shifted the equilibrium position of the nanotube away from its zero-point amplitude. With an amplitude only a factor of 13 over the zero-point motion, they were able to notice these nonlinear vibrations.

The results are astonishing because vibrations present in other resonators, cooled to the quantum ground state, showed to only be nonlinear at amplitudes approximately 10⁶ times greater than its zero-point motion.

This new mechanism displays remarkable physics because, contrary to what was expected, the anharmonicity increases as the vibrations are cooled closer to the ground state. This is just the opposite of what has been observed in all other mechanical resonators so far. As first author Chandan Samanta says, “when researchers first began studying nanomechanical resonators, a recurrent question was whether it would be possible to achieve nonlinearities in vibrations that are in the quantum ground state.”

“Some leading researchers in the field argued that this would be a challenging feat due to technological limitations, and this view has remained the accepted paradigm until now. In this context, our work represents a significant conceptual advance because we demonstrate that nonlinear vibrations in the quantum regime are indeed achievable.”

“We are confident that the nonlinear effects could have been further enhanced by getting closer to the quantum ground state, but we were limited by the temperature of our current cryostat. Our work provides a roadmap for achieving nonlinear vibrations in the quantum regime.”

Contrary to what has been observed so far in other mechanical resonators, the team of researchers found a method to boost the anharmonicity of a mechanical oscillator near its quantum regime. The results of this study set the first stepping stones for the future development of mechanical qubits or even quantum simulators.

As Adrian Bachtold remarks, “It is remarkable that we entered into ultra-strong coupling regime and observed strong anharmonicity in the resonator. But the damping rate becomes large at low temperature due to the coupling of the resonator to one quantum dot.”

“In future experiments that target cat states and mechanical qubits, it will be advantageous to couple nanotube vibrations to a double-quantum dot, since it enables strong nonlinearities together with long-lived mechanical states. The damping arising from the electron in the double-quantum dot is exponentially suppressed at low temperature, so that it should be possible to achieve damping rate of 10 Hz measured in nanotubes at low temperature.”

More information: C. Samanta et al, Nonlinear nanomechanical resonators approaching the quantum ground state, Nature Physics (2023). DOI: 10.1038/s41567-023-02065-9

Journal information: Nature Physics 

Provided by ICFO 

One of the most high-profile mysteries in physics today is what scientists refer to as the “Strong CP Problem.” Stemming from the puzzling phenomenon that neutrons do not interact with electric fields despite being made up of quarks—smaller, fundamental particles that carry electric charges—the Strong CP Problem puts into question the Standard Model of physics, or the set of theories scientists have been using to explain the laws of nature for years.

A team led by University of Minnesota Twin Cities theoretical physicists has discovered a new way to search for axions, hypothetical particles that could help solve this mystery. Working in collaboration with experimental researchers at the Fermilab National Accelerator Laboratory, the physicists’ new strategy opens up previously unexplored opportunities to detect axions in particle collider experiments.

The researchers’ paper is published and featured as the Editor’s suggestion in Physical Review Letters.

“As particle physicists, we’re trying to develop our best understanding of nature,” said Zhen Liu, co-author of the paper and an assistant professor in the University of Minnesota School of Physics and Astronomy. “Scientists have been tremendously successful in the past century in finding elementary particles through established theoretical frameworks. So, it’s extremely puzzling why neutrons do not couple to electric fields because in our known theory, we would expect them to. If we do discover the axion, it will be a great advance in our fundamental understanding of the structure of nature.”

One of the primary means for studying subatomic particles, and potentially discovering new ones, is collider experiments. Essentially, scientists force beams of particles to collide—and when they hit each other, the energy they produce creates other particles that pass through a detector, allowing researchers to analyze their properties.

Liu and his team’s proposed method involves measuring the “decay” product—or what happens when an unstable heavy particle transforms into multiple lighter particles—of the hypothetical axion into two muons—known particles that are essentially the heavier version of the electron. By working backward from the muon tracks in the detector to reconstruct such decays, the researchers believe they have a chance to locate the axion and prove its existence.

“With this research, we’re expanding ways we can search for the axion particle,” said Raymond Co, co-author of the paper and a postdoctoral researcher in the University of Minnesota School of Physics and Astronomy and William Fine Theoretical Physics Institute. “People have never used axion decay into muons as a way to search for the axion particle in neutrino or collider experiments before. This research opens up new possibilities to pave the way for future endeavors in our field.”

Liu and Co, along with University of Minnesota physics and astronomy postdoctoral researcher Kun-Feng Lyu and University of California, Berkeley postdoctoral researcher Soubhik Kumar, are behind the theoretical part of the research. They’re a part of the ArgoNeuT collaboration, which brings together theorists and experimentalists from across the country to study particles through experiments at Fermilab.

In this paper, the University of Minnesota-led theoretical team worked with the experimental researchers to perform a search for axions using their new method and existing data from the ArgoNeuT experiment. The researchers plan to use the experimental results to further refine their theoretical calculations of the axion production rate in the future.

More information: R. Acciarri et al, First Constraints on Heavy QCD Axions with a Liquid Argon Time Projection Chamber Using the ArgoNeuT Experiment, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.221802

Journal information: Physical Review Letters 

Provided by University of Minnesota 

An international research team has succeeded for the first time in measuring the electron spin in matter—i.e., the curvature of space in which electrons live and move—within “kagome materials,” a new class of quantum materials.

The results obtained—published in Nature Physics—could revolutionize the way quantum materials are studied in the future, opening the door to new developments in quantum technologies, with possible applications in a variety of technological fields, from renewable energy to biomedicine, from electronics to quantum computers.

Success was achieved by an international collaboration of scientists, in which Domenico Di Sante, professor at the Department of Physics and Astronomy “Augusto Righi,” participated for the University of Bologna as part of his Marie Curie BITMAP research project. He was joined by colleagues from CNR-IOM Trieste, Ca’ Foscari University of Venice, University of Milan, University of Würzburg (Germany), University of St. Andrews (UK), Boston College and University of Santa Barbara (U.S.).

Through advanced experimental techniques, using light generated by a particle accelerator, the Synchrotron, and thanks to modern techniques for modeling the behavior of matter, the scholars were able to measure electron spin for the first time, related to the concept of topology.

“If we take two objects such as a football and a doughnut, we notice that their specific shapes determine different topological properties, for example because the doughnut has a hole, while the football does not,” Domenico Di Sante explains. “Similarly, the behavior of electrons in materials is influenced by certain quantum properties that determine their spinning in the matter in which they are found, similar to how the trajectory of light in the universe is modified by the presence of stars, black holes, dark matter, and dark energy, which bend time and space.”

Although this characteristic of electrons has been known for many years, no one had until now been able to measure this “topological spin” directly. To achieve this, the researchers exploited a particular effect known as “circular dichroism“: a special experimental technique that can only be used with a synchrotron source, which exploits the ability of materials to absorb light differently depending on their polarization.

Scholars have especially focused on “kagome materials,” a class of quantum materials that owe their name to their resemblance to the weave of interwoven bamboo threads that make up a traditional Japanese basket (called, indeed, “kagome”). These materials are revolutionizing quantum physics, and the results obtained could help us learn more about their special magnetic, topological, and superconducting properties.

“These important results were possible thanks to a strong synergy between experimental practice and theoretical analysis,” adds Di Sante. “The team’s theoretical researchers employed sophisticated quantum simulations, only possible with the use of powerful supercomputers, and in this way guided their experimental colleagues to the specific area of the material where the circular dichroism effect could be measured.”

More information: Domenico Di Sante et al, Flat band separation and robust spin Berry curvature in bilayer kagome metals, Nature Physics (2023). DOI: 10.1038/s41567-023-02053-z

Journal information: Nature Physics 

Provided by Università di Bologna

High power/energy ultrafast fiber lasers have broadband applications in material processing, medicine, advanced manufacturing and other fields. Compared with solid-state lasers, fiber lasers have the advantages of compact systems, flexibility, good heat dissipation and high beam quality.

However, due to the serious nonlinear effect inside the fiber, the single-pulse energy and average power of the ultrafast fiber lasers, especially those with all-fiber structures, still lag behind that of the solid-state lasers. In recent years, the Mamyshev oscillator has attracted much research attention thanks to its potential in producing high energy ultrafast pulses.

Recently, a research group from National University of Defense Technology reported a Mamyshev oscillator based on all-polarization-maintaining fiber with core/cladding diameter of 10/125 μm, and realized a 153 nJ single pulse energy with a compressed pulse width of 73 fs. Furthermore, through adjusting the cavity parameters, a maximum 5th order harmonic mode-locking operation was obtained with an output average power of 3.4 W and a compressed pulse width of 100 fs.

Recently their work entitled “All-PM Fiber Mamyshev Oscillator Delivers Hundred-Nanojoule and Multi-Watt Sub-100 fs Pulses” was published on Ultrafast Science.

The pulse energy and average power of ultrafast fiber lasers have been greatly improved in recent years based on the Mamyshev oscillator with cascade spectral broadening and offset spectral filtering effect. The continuous light and weak pulses will be blocked in the Mamyshev oscillator. Due to the sufficient spectral broadening, the strong pulse can be survived in the cavity after two filters with different central wavelengths, and the ultrafast laser with large pulse energy can be obtained.

However, most of the previous results adopted spatial structures and introduced a large number of free-space elements for signal collimation and coupling, which render the system cumbersome and susceptible to interference.

“Our aim is to realize high power/energy ultrafast fiber lasers with all-fiber structure,” explained A/Prof. Can Li. In the Ultrafast Science paper, a Mamyshev oscillator based on all-polarization-maintaining fiber with core/cladding diameter of 10/125 μm was reported.

Compared with the conventional Mamyshev oscillators, which generally consist of two stages of gain fiber amplification, there is only a single amplification arm in the proposed laser. A piece of passive fiber was used to broaden the optical spectrum in the other arm, alleviating the nonlinear phase accumulation inside the cavity and making the system more compact in the meantime.

This research group respectively realized the high performance ultrafast fiber lasers with single pulse energy of 153 nJ and average power of 3.4 W, which represent the highest records in pulse energy and average power obtained from an all-fiber ultrafast laser oscillator with picosecond/femtosecond pulse duration.

“By adopting the fiber with larger core diameter and new pulse evolution mechanism that has a higher tolerance of nonlinearity phase accumulation, all-fiber ultrafast lasers with higher power/pulse energy are expectable,” says Professor Pu Zhou.

More information: Tao Wang et al, All-PM Fiber Mamyshev Oscillator Delivers Hundred-Nanojoule and Multi-Watt Sub-100 fs Pulses, Ultrafast Science (2023). DOI: 10.34133/ultrafastscience.0016

Provided by Ultrafast Science

After years of pioneering work, researchers at the Department of Energy’s SLAC National Accelerator Laboratory have completed the detector towers that will soon sit at the heart of the SuperCDMS SNOLAB dark matter detection experiment.

The team finished building the towers this past September, and SLAC, which leads the SuperCDMS project, sent the first two towers to SNOLAB in Ontario, Canada earlier this month.

SuperCDMS SNOLAB will look for relatively light dark matter particles, between about half the mass of the proton to roughly 10 proton masses, and in that range it will be the world’s most sensitive direct-detection experiment, said Richard Partridge, a senior staff scientist at SLAC and a long-time SuperCDMS researcher. That accomplishment comes down to two things: improvements in the detector design and the location of the experiment itself, Partridge said.

“It’s been a lot of fun,” Partridge said. “We’ve learned a lot of new things, and we’ve built some really interesting technology,” including flexible superconducting cables, electronics systems that function in extreme cold, and improved cryogenics systems—along with advances in shielding the detectors—that have made the detectors and surrounding systems better able to sense passing dark matter than ever before.

The experiment will also benefit from its new location 1.25 miles underground at SNOLAB, where the background of cosmic rays will interfere less with efforts to find a dark matter signal.

“SNOLAB and SuperCDMS are made for each other,” said SNOLAB Executive Director Jodi Cooley. “We are extremely excited about the potential for SuperCDMS to detect dark matter directly and advance our insight into the nature of the universe.”

Vitaly Yakimenko, SLAC’s deputy director for projects and infrastructure and SuperCDMS project director, said researchers look forward to bringing the experiment online. “It’s been 10 years of technological development to build these state-of-the-art detectors,” he said. The team hopes the experiment will capture signs of illusive dark matter particles, but regardless of the outcome, it will establish a path forward for even more sensitive experiments, Yakimenko said. “It’s a major accomplishment.”

JoAnne Hewett, head of SLAC’s Fundamental Physics Directorate and the lab’s chief research officer, said she was pleased by the experiment’s progress. “Understanding dark matter is one of the most important areas of research, both around the world and here at SLAC,” Hewett said. “We’re excited to reach this milestone and to work with our partners to build this cutting-edge experiment.”

Searching deep underground

Scientists know that all the visible matter in the universe—all the dust and planets and stars that we can already detect through telescopes—makes up only about 15% of what’s actually out there. The rest is dark matter, but no one knows exactly what that is. Physicists can tell it’s there through its gravitational pull on ordinary matter, but it is otherwise very hard to detect.

That’s where experiments like SuperCDMS SNOLAB come in. The project is the latest iteration of a series of experiments that use silicon and germanium crystals to try to find dark matter particles. These crystals are cooled to a fraction of a degree above absolute zero—hence the experiments’ name: Cryogenic Dark Matter Search, or CDMS. The hope is that at such low temperatures, researchers could detect passing dark matter particles by the tiny vibrations they create when colliding with the crystals.

Those collisions would also produce pairs of electrons and electron deficiencies, or holes, that move through the crystals, triggering more vibrations and amplifying the dark matter signal. Sophisticated superconducting electronics help detect these signals.

To make that job possible, the experiment will be built and operated at SNOLAB, 6,800 feet underground inside a nickel mine near Sudbury, Ontario. There, the SLAC-built detectors will be shielded from high-energy particles, called cosmic radiation, which can create unwanted background signals.

A long journey

Once the detectors were done, the next step for the SLAC team was getting them to SNOLAB—and here there were tradeoffs. To protect them from cosmic rays, the team wanted to get them to their new underground home as quickly as possible, which might suggest a direct route over the Rocky Mountains or even a flight to Ontario. However, the thinner atmosphere at higher altitudes affords less protection from cosmic rays.

“In principle, we want to keep things as low as possible, but there’s also a cost to the total number of days you’re on the surface,” said Tarek Saab, a University of Florida physicist and SuperCDMS spokesperson. “So, we want a route that gives you the least overall exposure to cosmic rays.”

In the end, the team decided to take a route east to Texas and then north to SNOLAB.

Meanwhile, SNOLAB has been busy getting the SuperCDMS facility ready for the detectors, the first two of which arrived on May 12 and made the trip 6,800 feet underground the following day. The remaining towers will arrive later this year, and initial preparations for the experiment are expected to be complete sometime in 2024, at which point the experimental team can begin taking initial data and working out any kinks that remain in the system. Researchers expect to run the experiment for three to four years before they have enough data to push the limits of what we know about dark matter.

But for now, everyone is looking forward to delivering the detectors, Saab said. “It will be a significant milestone to have the detectors at SNOLAB.”

Provided by SLAC National Accelerator Laboratory 

Quantum computers promise to solve challenging tasks in material science and cryptography that will remain out of reach even for the most powerful conventional supercomputers in the future. Yet, this will likely require millions of high-quality qubits due to the required error correction.

Progress in superconducting processors advances quickly with a current qubit count in the few hundreds. The advantages of this technology are the fast computing speed and its compatibility with microchip fabrication, but the need for ultra-cold temperatures ultimately confines the processor in size and prevents any physical access once it is cooled down.

A modular quantum computer with multiple separately cooled processor nodes could solve this. However, single microwave photons—the particles of light that are the native information carriers between superconducting qubits within the processors—are not suitable to be sent through a room temperature environment between the processors. The world at room temperature is bustling with heat, which easily disturbs the microwave photons and their fragile quantum properties like entanglement.

Researchers from the Fink group at the Institute of Science and Technology Austria (ISTA), together with collaborators from TU Wien and the Technical University of Munich, demonstrated an important technological step to overcome these challenges. They entangled low-energy microwave with high-energy optical photons for the very first time.

Such an entangled quantum state of two photons is the foundation to wire up superconducting quantum computers via room temperature links. This has implications not only for scaling up existing quantum hardware but it is also needed to realize interconnects to other quantum computing platforms as well as for novel quantum-enhanced remote sensing applications. Their results have been published in the journal Science.

Cooling away the noise

Rishabh Sahu, a postdoc in the Fink group and one of the first authors of the new study, explains, “One major problem for any qubit is noise. Noise can be thought of as any disturbance to the qubit. One major source of noise is the heat of the material the qubit is based on.”

Heat causes atoms in a material to jostle around rapidly. This is disruptive to quantum properties like entanglement, and as a result, it would make qubits unsuitable for computation. Therefore, to remain functional, a quantum computer must have its qubits isolated from the environment, cooled to extremely low temperatures, and kept within a vacuum to preserve their quantum properties.

For superconducting qubits, this happens in a special cylindrical device that hangs from the ceiling, called a “dilution refrigerator” in which the “quantum” part of the computation takes place. The qubits at its very bottom are cooled down to only a few thousandths of a degree above absolute zero temperature—at about -273 degrees Celsius. Sahu excitedly adds, “This makes these fridges in our labs the coldest locations in the whole universe, even colder than space itself.”

The refrigerator has to continuously cool the qubits but the more qubits and associated control wiring are added, the more heat is generated and the harder it is to keep the quantum computer cool. “The scientific community predicts that at around 1,000 superconducting qubits in a single quantum computer, we reach the limits of cooling,” Sahu cautions. “Just scaling up is not a sustainable solution to construct more powerful quantum computers.”

Fink adds, “Larger machines are in development but each assembly and cooldown then becomes comparable to a rocket launch, where you only find out about problems once the processor is cold and without the ability to intervene and correct such problems.”

Quantum waves

“If a dilution fridge cannot sufficiently cool more than a thousand superconducting qubits at once, we need to link several smaller quantum computers to work together,” Liu Qiu, postdoc in the Fink group and another first author of the new study, explains. “We would need a quantum network.”

Linking together two superconducting quantum computers, each with its own dilution refrigerator, is not as straightforward as connecting them with an electrical cable. The connection needs special consideration to preserve the quantum nature of the qubits.

Superconducting qubits work with tiny electrical currents that move back and forth in a circuit at frequencies about ten billion times per second. They interact using microwave photons—particles of light. Their frequencies are similar to the ones used by cellphones.

The problem is that even a small amount of heat would easily disturb single microwave photons and their quantum properties needed to connect the qubits in two separate quantum computers. When passing through a cable outside the refrigerator, the heat of the environment would render them useless.

“Instead of the noise-prone microwave photons that we need to do the computations within the quantum computer, we want to use optical photons with much higher frequencies similar to visible light to network quantum computers together,” Qiu explains. These optical photons are the same kind sent through optical fibers that deliver high-speed internet to our homes. This technology is well understood and much less susceptible to noise from heat. Qiu adds, “The challenge was how to have the microwave photons interact with the optical photons and how to entangle them.”

Splitting light

In their new study, the researchers used a special electro-optic device: an optical resonator made from a nonlinear crystal, which changes its optical properties in the presence of an electric field. A superconducting cavity houses this crystal and enhances this interaction.

Sahu and Qiu used a laser to send billions of optical photons into the electro-optic crystal for a fraction of a microsecond. In this way, one optical photon splits into a pair of new entangled photons: an optical one with only slightly less energy than the original one and a microwave photon with much lower energy.

“The challenge of this experiment was that the optical photons have about 20,000 times more energy than the microwave photons,” Sahu explains, “and they bring a lot of energy and therefore heat into the device that can then destroy the quantum properties of the microwave photons. We have worked for months tweaking the experiment and getting the right measurements.” To solve this problem, the researchers built a bulkier superconducting device compared to previous attempts. This not only avoids a breakdown of superconductivity, but it also helps to cool the device more effectively and to keep it cold during the short timescales of the optical laser pulses.

“The breakthrough is that the two photons leaving the device—the optical and the microwave photon—are entangled,” Qiu explains. “This has been verified by measuring correlations between the quantum fluctuations of the electromagnetic fields of the two photons that are stronger than can be explained by classical physics.”

“We are now the first to entangle photons of such vastly different energy scales.” Fink says, “This is a key step to creating a quantum network and also useful for other quantum technologies, such as quantum-enhanced sensing.”

More information: R. Sahu et al, Entangling microwaves with light, Science (2023). DOI: 10.1126/science.adg3812. www.science.org/doi/10.1126/science.adg3812

Journal information: Science 

Provided by Institute of Science and Technology Austria 

It may be that the famous Higgs boson, co-responsible for the existence of masses of elementary particles, also interacts with the world of the new physics that has been sought for decades. If this were indeed to be the case, the Higgs should decay in a characteristic way, involving exotic particles. At the Institute of Nuclear Physics of the Polish Academy of Sciences in Cracow, it has been shown that if such decays do indeed occur, they will be observable in successors to the LHC currently being designed.

When talking about the “hidden valley,” our first thoughts are of dragons rather than sound science. However, in high-energy physics, this picturesque name is given to certain models that extend the set of currently known elementary particles. In these so-called Hidden Valley models, the particles of our world as described by the Standard Model belong to the low-energy group, while exotic particles are hidden in the high-energy region.

Theoretical considerations suggest then the exotic decay of the famous Higgs boson, something that has not been observed at the LHC accelerator despite many years of searching. However, scientists at the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow argue that Higgs decays into exotic particles should already be perfectly observable in accelerators that are successors to the Large Hadron Collider—if the Hidden Valley models turn out to be consistent with reality.

“In Hidden Valley models we have two groups of particles separated by an energy barrier. The theory is that there could then be exotic massive particles which could cross this barrier under specific circumstances. The particles like Higgs boson or hypothetic Z’ boson would act as communicators between the particles of both worlds. The Higgs boson, one of the most massive particle of the Standard Model, is a very good candidate for such a communicator,” explains Prof. Marcin Kucharczyk (IFJ PAN), lead author of an article in the Journal of High Energy Physics, which presents the latest analyses and simulations concerning the possibility of detecting Higgs boson decays in the future lepton accelerators.

The communicator, after passing into the low energy region, would decay into two rather massive exotic particles. Each of these would, in picoseconds—that is, trillionths of a second—decay into another two particles, with even smaller masses, which would then be within the Standard Model.

So what signs would be expected in the detectors of future accelerators? The Higgs itself would remain unnoticed, as would the two Hidden Valley particles. However, the exotic particles would gradually diverge and eventually decay, generally into quark-antiquark beauty pairs visible in modern detectors as jets of particles shifted from the axis of the lepton beam.

“Observations of Higgs boson decays would therefore consist of searching for the jets of particles produced by quark-antiquark pairs. Their tracks would then have to be retrospectively reconstructed to find the places where exotic particles are likely to have decayed. These places, professionally called decay vertices, should appear in pairs and be characteristically shifted with respect to the axis of the colliding beams in the accelerator. The size of these shifts depends, among other things, on masses and average lifetime of exotic particles appearing during the Higgs decay,” says Mateusz Goncerz, M.Sc. (IFJ PAN), co-author of the paper in question.

The collision energy of protons at the LHC, currently the world’s largest particle accelerator, is up to several teraelectronvolts and is theoretically sufficient to produce Higgs capable of crossing the energy barrier that separates our world from the Hidden Valley. Unfortunately, protons are not elementary particles—they are composed of three valence quarks bound by strong interactions, capable of generating huge numbers of constantly appearing and disappearing virtual particles, including quark-antiquark pairs.

Such a dynamic and complex internal structure produces huge numbers of secondary particles in proton collisions, including many quarks and antiquarks with large masses. They form a background in which it becomes practically impossible to find the particles from the exotic Higgs boson decays that are being sought.

The detection of possible Higgs decays to these states should be radically improved by accelerators being designed as successors to the LHC: the CLIC (Compact Linear Collider) and the FCC (Future Circular Collider). In both devices it will be possible to collide electrons with their anti-material partners, the positrons (with CLIC dedicated to this type of collision, while FCC will also allow collisions of protons and heavy ions).

Electrons and positrons are devoid of internal structure, so the background for exotic Higgs boson decays should be weaker than at the LHC. Only will it be sufficiently so to discern the valuable signal?

In their research, physicists from the IFJ PAN took into account the most important parameters of the CLIC and FCC accelerators and determined the probability of exotic Higgs decays with final states in the form of four beauty quarks and antiquarks. To ensure that the predictions cover a wider group of models, the masses and mean lifetimes of the exotic particles were considered over suitably wide ranges of values.

The conclusions are surprisingly positive: all indications are that, in future electron-positron colliders, the background of exotic Higgs decays could be reduced even radically, by several orders of magnitude, and in some cases could even be considered negligible.

The existence of particle-communicators is not only possible in Hidden Valley models, but also in other extensions of the Standard Model. So if the detectors of future accelerators register a signature corresponding to the Higgs decays analyzed by the Cracow researchers, this will only be the first step on the road to understanding new physics. The next will be to collect a sufficiently large number of events and determine the main decay parameters that can be compared with the predictions of theoretical models of the new physics.

“The main conclusion of our work is therefore purely practical. We are not sure whether the new physics particles involved in Higgs boson decays will belong to the Hidden Valley model we used. However, we have treated this model as representative of many other proposals for new physics and have shown that if, as predicted by the model, the Higgs bosons decay into exotic particles, this phenomenon should be perfectly visible in those electron and positron colliders which are planned to be launched in the near future,” concludes Prof. Kucharczyk.

More information: Marcin Kucharczyk et al, Search for exotic decays of the Higgs boson into long-lived particles with jet pairs in the final state at CLIC, Journal of High Energy Physics (2023). DOI: 10.1007/JHEP03(2023)131

Provided by Polish Academy of Sciences