Category: Physics

Preparations for the construction of the first detector module of the Deep Underground Neutrino Experiment are rapidly progressing. Members of the international DUNE collaboration have begun the final tests of detector components that will be shipped to South Dakota. There they will become part of a one-of-a-kind experiment designed to study some of the most elusive particles in the universe: neutrinos.

DUNE is an experiment aimed at exploring the nature of neutrinos. Scientists hope that unlocking the secrets of these particles will shed light on some of the biggest mysteries in physics, such as why the universe is made of matter and how neutron stars and black holes are forged in the aftermath of exploding stars.

DUNE, hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory, will be housed at two locations: the Fermilab site in Illinois and the Sanford Underground Research Facility in South Dakota. The far detector, an enormous structure envisioned to ultimately comprise four modules, will be located 1.5 kilometers underground at SURF. Each module will be a liquid-argon time projection chamber, or LArTPC, designed to be filled with 17,000 tons of argon, an element commonly found in air that is ideal for studying neutrinos.

The first DUNE detector module to be built at SURF will employ horizontal drift technology. When neutrinos collide with the argon atoms inside the module, they will produce charged particles. These charged particles knock out electrons as they travel through the argon. The electrons are attracted by a strong electric field towards anode plane assemblies, or APAs, which record a projection of where the electrons were produced. By measuring the timing of when electrons hit the APAs, scientists are able to reconstruct three-dimensional particle tracks.

When complete, the first detector module will contain 150 APAs, each a large rectangular plane approximately 2.3 meters by 6 meters in size and composed of tightly wound copper-beryllium wires. “We are going to effectively plaster a wall with a grid of wires,” said Justin Evans, a professor of physics at the University of Manchester. He is leading the effort to build APAs in the U.K.

The technology for the first module was successfully tested in a scaled-down version called ProtoDUNE at the CERN Neutrino Platform in 2019. Although it was only one-twentieth of the size to the final DUNE detector module, it still was the largest LArTPC ever constructed and operated.

“That was the first time that we in the U.K. built any of these wire grids and took data from them,” said Evans. “And they worked wonderfully.”

In addition to proving that the detector technology would work, the horizontal drift prototype revealed where design improvements could be made. With this in mind, scientists designed ProtoDUNE II, an upgraded prototype that will undergo tests at CERN this year.

“With a prototype, there’s always lessons learned,” said Thomas Wieber, the installation team leader at CERN. “We want to prove that what we think is going to work better actually does work better.”

DUNE scientists are also working on developing the technology for a vertical drift detector, which is the planned technology for the second far detector module. Preparations for testing the new technology in a separate prototype detector, known as vertical drift module-0, are underway at CERN as well.

Testing the full assembly process

Testing all aspects of the horizontal drift module assembly also involves ensuring that all the detector components, which come from DUNE collaborators around the world, will arrive safely. To ensure this process goes smoothly, ProtoDUNE II also has served as a testbed for the installation process. The groups involved in manufacturing the detector components brought all their test parts together at CERN to assemble and test the prototype.

“We have had collaborators coming in from all over the world,” said Daniela Macina, the installation coordinator for the horizontal drift detector at CERN. “This is the first time we’ve integrated and installed the final DUNE horizontal drift detectors all together.”

ProtoDUNE II contains four APAs. All four were tested in a cold box filled with super-cold gaseous nitrogen in order to ensure that the electronics function properly at frigid temperatures. All four APAs passed the test. They then were assembled in the ProtoDUNE II cryostat, along with other parts of the prototype. These parts include the electronics and light sensors, which identify photons that are released when neutrinos interact with the liquid argon in the detector.

“We needed to assemble things according to a procedure that will be as close as possible to the one we’ll use in South Dakota,” Macina said.

Later this year, the team will fill ProtoDUNE II with liquid argon and shoot a beam of particles through the detector to test it. “We don’t expect big surprises, because there were only minor changes between ProtoDUNE and ProtoDUNE II,” said Macina.

As the final work on ProtoDUNE II takes place at CERN, scientists are also ramping up production of the APAs that will be installed at SURF. Of the 150 APAs that will be installed in the first module of the far detector, 136 APAs will be produced at the Daresbury Laboratory in the U.K., and another 14 will be produced at the University of Chicago.

To support the mass production of these parts, the Daresbury Laboratory in the U.K. has constructed an APA factory where the team has set up four production lines, each with six-meter-long machines for winding the wires around steel frames to create the APAs.

“We have four of those set up, all working in parallel,” Evans said. “We’re trying to get to the point where each production line can produce one APA every two months.”

Shipping detector components to South Dakota

Activities related to the detector installation have also started up in South Dakota, where the excavation of the caverns for the DUNE far detector are about 60% complete. In early November, the team shipped one of the APAs from the first prototype at CERN to SURF for a logistics test. The process of transporting the long, rectangular APA from Europe to the United States, offloading the structure, then lowering it a mile below the ground through a relatively narrow mineshaft was successfully completed.

“We’re using older APAs to validate those procedures so that we’re not going to damage anything when we start bringing the real APAs to SURF,” said Fermilab scientist Eric James, a technical coordinator who focuses on DUNE’s horizontal drift detector.

Despite all the details that need to be worked out to bring the first DUNE detector module to life, DUNE scientists are not losing sight of their ultimate goal: exploring a new frontier of neutrino science.

“I’m hugely excited about what we will see when we switch this thing on,” Evans said. “There is so much the neutrino can tell us about the universe.”

Provided by Fermi National Accelerator Laboratory 

In recent months, the neutrino research facility at the European laboratory CERN has been bustling with activity. Scientists, engineers and technicians from around the world have gathered there to assemble a large prototype of a new particle detector to study the neutrino, one of the most mysterious types of particles in the universe.

Neutrinos are everywhere, but they rarely interact with matter. Each second, trillions of these particles traverse our bodies and leave without a trace. By studying these ghost-like particles, physicists hope to answer questions, such as: Why is the universe made of matter? What is the relationship between the four forces of nature? How are black holes formed in the aftermath of an exploding star?

Researchers working on the international Deep Underground Neutrino Experiment, hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory, hope to solve these mysteries. Their work on the prototype detector at CERN brings them a step closer to achieving this goal.

The infrastructure needed for DUNE is expansive. It includes a new particle accelerator at Fermilab, which will produce a neutrino beam that will pass through 1,300 kilometers of earth before reaching the Sanford Underground Research Facility in South Dakota. At SURF, these particles will be greeted by the DUNE far detector, a gigantic subterranean detector housed 1.5 kilometers below the surface. The detector will comprise huge detector modules containing argon, an element whose highly stable nature makes it perfect for studying neutrinos. Excavation of the underground caverns for the DUNE far detector is about 60% complete.

Testing new technology

Members of the DUNE collaboration, which includes scientists and engineers from more than 35 countries, are busy at work designing, testing and building the components of the first two DUNE detector modules to be installed at SURF. Module one will be a horizontal drift detector, which is based on a tried-and-tested technique that will be scaled up for DUNE. The mass production of components for this first module already has begun. The second module, known as the vertical drift detector, will feature new technology. Testing has been ongoing for the last two years.

“I expect exciting physics out of both the horizontal and vertical drift detectors,” said Steve Kettell, the technical coordinator for the vertical drift detector, based at the DOE’s Brookhaven National Laboratory. “But the vertical drift technology opens up significant opportunities for building additional detectors that are lower in cost and easier to install.”

Horizontal vs. vertical

On a basic level, horizontal and vertical drift detectors work in the same way. When a neutrino interacts with an argon atom inside the detector’s liquid-argon-filled chamber, the particles produced in this interaction release electrons. A strong electric field between opposite sides of the detector chamber pushes these loose electrons to an anode, a large structure that detects the arrival of charged particles. In a horizontal drift detector, the electric field exists between two opposing walls, and the electrons drift horizontally; in a vertical drift detector, the electric field runs between the bottom and top of the detector, and the electrons drift vertically. The argon-neutrino interaction also produces a brief flash of light that both detectors capture with a separate photon detection system.

“Fundamentally, there’s nothing different about vertical drift and horizontal drift,” Kettell explained. “We are detecting neutrino events in essentially the same manner.”

The differences are in the details. The anode of the horizontal drift detector consists of large planes of tightly wound wires, known as anode plane assemblies, or APAs. They are 6 meters tall and 2.3 meters wide. The anode of the vertical drift detector, on the other hand, will be composed of charge readout planes, or CRPs. They are large, perforated-printed circuit boards that are 3 meters by 3.5 meters in size and have copper strips printed onto their surfaces. Like the wires in the APAs, the copper strips in the CRPs will collect the drifting electrons.

The DUNE vertical drift detector will feature multilayer CRPs at the top and at the bottom. “The CRPs have perforated 2.5-millimeter holes, so that electric charge can pass through and go to another layer to get collected,” said Dominique Duchesneau, leader of the CRP consortium and a physicist at the French National Centre for Scientific Research. Each CRP layer has differently orientated copper strips, he added, which “gives you the possibility to have multiple views of the electrons.”

A key advantage of CRPs is that because they are made of simple metal-plated circuit boards rather than a tight coil of wires, they are cheaper and easier to manufacture and install than APAs.

“With the vertical drift detector, we’re trying to demonstrate that we can build a less expensive detector that works equally well,” Kettell said.

Because the vertical drift detector technology requires fewer elements than the horizontal drift, it provides a larger active volume. A larger active volume means that there will be more space in which particle interactions can be collected, said Inés Gil-Botella, a DUNE physics coordinator based at the Centre for Energy, Environmental and Technological Research in Spain. “You’re maximizing the possibility of seeing neutrino interactions in this liquid argon.”

Another innovation is the photon detection system DUNE scientists plan to build for the vertical drift detector, an upgrade of the ARAPUCA technology developed for the first DUNE far detector module. This new system will cover all four cryostat walls as well as the cathode with photon detection modules. (In contrast, in the horizontal drift detector, the photon detectors only are embedded in the APA planes, behind the wires.) To power and read out the photo sensors on the high-voltage cathode, which is set to 300 kilovolts, the vertical drift team uses a powerful laser that provides power via optical fibers.

In addition, the argon within the vertical drift detector will be doped with xenon to enhance the number of photons that get detected when particles interact with atoms in the liquid —and to enhance the uniformity of light detection throughout the chamber. Together, these features will make this photon detection system more capable of detecting low-energy physics events, such as those triggered by supernovae or solar neutrino events, Gil-Botella said.

A bustle of activity

The team working on the DUNE vertical drift detector comes from around the world. Major contributions are being made by CERN, France, Italy, Spain and the U.S. But members also come from several other countries in Europe, Asia and Latin America. “There’s been tremendous progress on many fronts,” said Kettell.

This group has been busy. To date, they have successfully tested small-scale, 32-centimeter-by-32-centimeter CRPs in a 50-liter liquid-argon-filled chamber fitted with a cathode, electronics and a photon detection system. This early prototype was able to collect data from cosmic-ray tracks with “good signal-to-noise performance,” said Kettell. They have also tested full-size, 3-meter by 3.5-meter CRPs with the cathode, electronics, and the photon detection system in a large coldbox at CERN.

The team has demonstrated that the components of the vertical drift detector could read out signals at 300 kilovolts—the high voltage that will be needed for creating the electric field in the full-sized DUNE detector. They have also shown that electrons can drift six meters—the maximum distance electrons will travel in the final-size module—and use the CRPs to receive these tracks. “The next big milestone we’ll face is the installation of all of the systems together at a larger scale,” Gil-Botella said.

The team is now assembling parts into a larger vertical drift prototype, dubbed “vertical drift module-0,” in a large cryogenic vessel at CERN, about the size of a small house. This prototype will contain two full-sized CRPs on both the top and bottom of the detector, with the cathode installed in the middle, as well as an advanced photon detection system. Electrons knocked loose in the upper half of the detector will drift upward toward the CRP set at the top, and electrons produced in the lower half will drift in the down direction, until they reach the CRP layers at the bottom. CRP development has been led by France, with the construction of the top CRPs in France and the bottom CRPs in the U.S.

The DUNE researchers aim to complete the installation of the vertical drift prototype detector in spring 2023. Once complete, the team will fill the detector with liquid argon and turn it on, so that scientists can observe the tracks left by particle beams and cosmic rays that pass through it.

Ultimately, the goal is to have the components of the vertical drift detector ready to be installed in one of the large caverns in South Dakota in 2027.

“What I really would like to see is the installation of the first CRPs in the big cryostat at SURF, which will come in several years,” Duchesneau said. “In the meantime, I think module-0 running and taking data in the real configuration of the vertical drift is a very exciting step.”

Provided by Fermi National Accelerator Laboratory 

A research team, led by Professor Yuji Nakamura of the Department of Mechanical Engineering at Toyohashi University of Technology, discovered that the flickering of flames can be freely controlled by moving two flames closer together or further apart. Until now, it had been known that interference between flames separated by a certain distance causes the flames to flicker during in-phase or anti-phase. However, it was not possible to stably express the state of “stopping the flickering of flames” that should occur under critical conditions where the phase changes.

The research team has succeeded in stably expressing the state of “stopping the flickering of flames” by periodically adjusting the distance between flames. This makes it possible to freely control the flickering of the flames, and to elucidate the essence of flickering flames. The research is published in the journal Physical Review Applied.

The flickering of flames is a familiar phenomenon that is easy to observe. At the same time, it is also a mysterious and interesting phenomenon with vast complexity. For example, once flickering flames have interfered with each other, only the stable flickering mode is selectively expressed. Depending on the distance between the flames, the “in-phase mode” that fluctuates in the same phase and the “anti-phase mode” that fluctuates in the opposite phase are selectively expressed. There are also mysterious phenomena such as different fluctuation frequencies in those modes. Using these guidelines, it is possible to achieve various fluctuating states.

And yet, there is no example that shows “stopping the flickering by interfering with the fluctuating flames.” In the past, it was shown that this state can be achieved by arranging three flames (known as “death mode” in reference to the complete absence of movement). However, researchers have yet to understand the reason why death mode cannot be achieved with two flames.

When examining this phenomenon, the research team found that the death mode is expressed by adjusting the distance between the two flames closer or further apart in a certain cycle.

“When conducting experiments involving flame-to-flame interference, flickering will temporarily stop if the flames are gradually brought closer or further apart,” explains Dr. Ju Xiaoyu, lead author and researcher at the time of the project. “However, if the flames are kept in that position, they will eventually start flickering again.

“Since the flames eventually flicker, we know that flickering is a stable state. The fact that there is a delay period until the flames settle into a stable state means that if we can create a situation where flickering can be stopped within that time scale, the flickering should be stopped permanently. We were able to prove that this prediction is correct by periodically adjusting the distance between flames closer and further apart. We also demonstrated that the reason for this phenomenon can be explained by hydrodynamic properties. Moving forward, we will proceed with research aimed at constructing a theory.”Play

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“It has been known that the flame flickering mode is determined by interference between flames,” says Professor Yuji Nakamura, leader of the research team. “Researchers in applied physics have attempted to explain this phenomenon as nonlinear physics instead of combustion engineering. Nevertheless, their explanation felt inadequate to me due to its failure to consider hydrodynamics. In response, I began to earnestly research this theme.

“I was amazed to witness a phenomenon in which flickering temporarily stopped in an intermediate state between in-phase and anti-phase flickering. I felt a strong desire to elucidate this mysterious transition state, a theme which has not been addressed by previous research. From the beginning, I had the idea of constantly adjusting the distance between flames to take advantage of the time delay until they settled into a stable state. Ultimately, I was able to organize this method with the help of Dr. Ju.”

Professor Nakamura concludes, “Introducing this phenomenon at events such as academic conference is sure to capture the interest of the audience without exception. However, the audience will, without exception, raise questions regarding examples of practical application. For example, ‘How can these findings be used?’ I repeatedly answered such questions by posing my own question—’Actually, I only began researching this phenomenon out of personal curiosity, so I’d like to ask how you think my findings can be used?’

“This experience has led me to start my research presentations by asking the audience to refrain from questions on practical application. I believe that one appeal of conducting basic research at a university is being able to purely immerse yourself in curiosity, without the need to consider practical application.”

Future outlook

Although the research team is not considering practical application of their research at the present time, they plan to delve deeper into the theme not only through experiments, but also through numerical and theoretical analyses. This will be done in the name of basic research that is unique to universities; that is, through the elucidation of mysterious phenomenon.

The team plans to proceed as international joint research in collaboration with Dr. Ju and many international researchers who have expressed interest in their research. Through the international dissemination of research seeds originating in Japan, the team would like to convey to the world that this kind of (currently impractical) basic research can be pursued vigorously in Japan.

More information: Xiaoyu Ju et al, Flame Flickering can Cease Under Normal Gravity and Atmospheric Pressure in a Horizontally Moving Dual Burner System, Physical Review Applied (2023). DOI: 10.1103/PhysRevApplied.19.014060

Provided by Toyohashi University of Technology 

Scientists have used a common weather forecasting technique for insights into how powerful lasers turn hunks of solid material into soups of electrically charged particles known as plasmas.

Using this time-tested technique in a new context could help researchers make important measurements in inertial confinement fusion devices, a concept being explored as a way to harness fusion energy. This process, which powers the sun and stars, could be a green way to create electricity on Earth without producing greenhouse gases or long-lived radioactive waste.

The scientists from PPPL, a U.S. Department of Energy (DOE) national laboratory managed by Princeton University, have measured the dense cloud of plasma produced by a strong laser striking a solid target. The intense heat caused atoms from the surface to evaporate and emit energetic X-ray light.

The measurement gauged the speed of the light using the Doppler effect—the same phenomenon that causes ambulance sirens to rise in pitch as they approach and then fall as they move away. Meteorologists rely on this effect to measure the speed of thunderstorms.

Physicists want to gain a better understanding of dense plasma in part because it is produced by inertial confinement fusion devices—facilities like the DOE’s National Ignition Facility at Lawrence Livermore National Laboratory that last year produced more fusion power than the energy it took to heat the plasma. The more physicists can understand the behavior of the resulting dense plasma, which is ten billion times denser than the plasma inside doughnut-shaped magnetic tokamaks, the more likely they could create fusion more efficiently.

The scientists found evidence for the existence of a barrier, or sheath, between the outer and inner layers of the dense plasma cloud. This observation suggests that laser-produced dense plasma behaves similarly to less-dense plasma. The finding marks the first time scientists have used this Doppler technique to measure very dense plasma. The experiment was performed using Colorado State University’s Advanced Laser for Extreme Photonics (ALEPH) facility.

The results show that dense plasma behaves much like other types of plasmas do, behavior that scientists have not been able to directly observe until now, reported Frances Kraus, lead author of a paper reporting the results in Physical Review Letters.

Prior to this finding, researchers did not know whether they could measure X-rays with precision in dense plasma, which can obscure such observations. “Scientists did not believe that you could pick out the X-ray behavior within all of the other signals,” Kraus said. “But our diagnostic shows that you can.”

Invented in 1960, lasers are used for a wide range of tasks, including surgery, welding, and printing. A laser—the term stands for “light amplification by stimulated emission of radiation”—consists of rays of light that all have the same consists of rays of light that all have the same frequency. In addition, the photon particles that make up laser light are all traveling in the same direction and orientation. As lasers have become more powerful, there has grown a need to understand their fundamental characteristics.

“You have to understand what these lasers create before you can find uses for them,” Kraus said. “This is fundamental stuff. These super-powerful lasers will have lots of applications in the future. We just don’t know yet what they will be.”

More information: B. F. Kraus et al, Ablating Ion Velocity Distributions in Short-Pulse-Heated Solids via X-Ray Doppler Shifts, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.235001

Journal information: Physical Review Letters 

Provided by Princeton Plasma Physics Laboratory

Materials typically conduct electricity or insulate against it—so experimental and theoretical physicists have been captivated by a compound called samarium hexaboride (SmB₆) that appears to do both. Numerous studies over the course of 50 years have revealed that SmB₆ acts like an insulator as well as an electricity-conducting metal.

Now, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) say it’s possible to image the exact position of electrons along the surface of SmB₆ with single-atom precision, enabling a breakthrough in understanding the compound’s properties and why it can both insulate and conduct. The findings, published in Science, build upon SEAS research reported in 2019 that determined that SmB₆ is a topological insulator—meaning it conducts electricity along its surfaces but not its insides.

Despite the 2019 discovery about SmB₆‘s surface metallicity, many questions remained about its overall metallicity and why different measurements didn’t generate consistent results.

“Anyone with a voltmeter should be able to tell you whether a material is conductive or insulating,” says Harris Pirie, first author of the paper and a former Ph.D. student in the lab of Clowes Professor of Science Jenny Hoffman at SEAS. But that was never the case with SmB₆. “That intrigued us and thrust SmB₆ even further into the spotlight within the physics community.

To understand the peculiar behavior of how electrical charges move within SmB₆, Pirie and collaborators from University of Oxford, University of Illinois at Chicago (UIC), and other institutions needed to develop a new imaging approach to detect the distribution of electrons on SmB₆.

In conversations with Dirk Morr of UIC, the team homed in on the idea of adapting a scanning tunneling microscope, which uses an unimaginably small needle tip to measure atomic structures on the surface of a material. Instead of measuring atomic structure, they used it to detect a magnetic resonance in SmB₆ at a cold temperature, a signature of the magnetic interactions that turn the would-be metal into a low-temperature insulator. “That magnetic interaction generates a clear resonance we can measure, and we predicted that if we could measure its excitation energy at different surface points, it would reveal the electronic charge at that position,” Pirie says.

Scanning the needle across the surface of SmB₆ to map out the electronic charges across all surface points, the team created a readout that “looks like a topographic map you would have of a mountain range,” Pirie says. Except those mountain peaks and valleys are the size of atoms.

Using this method, the team has captured the first picture of electrons accumulating around atomic defects on SmB₆‘s surface—even surfaces created by cross-sectioning a sample of SmB₆ into fragments. “It was clear right from the moment we took that measurement, we had found the electrons we were looking for,” Pirie says. “We saw these amazing wave patterns that the electrons formed around the defects, indicating a high signal-to-noise ratio. It was a very cool moment.”

“The work gives us new understanding of the importance of single atom defects in topological materials,” says Hoffman, the paper’s senior author.

Pirie is now looking to modify the scanning tunneling microscope method to build new quantum devices. “The atomic defects we’ve identified could be useful in building quantum circuits. The needle of the scanning tunneling microscope can come so ridiculously close to the sample material that it’s no longer passively imaging it—it can touch and change the sample,” he says. “I’m interested in seeing if we can move atoms around on SmB₆, pushing all the electrons into specific, controlled channels or puddles. The hope is that by strategically constructing atomic defects on SmB₆, electrons could be precisely trapped to form qubits—the basic units necessary for quantum computing.”

That might help solve a major barrier to a workable quantum computer: to operate stably, the quantum state of a a quantum computer’s qubits must be entirely prohibited from entangling with electrons in the surrounding environment.

The imaging method could give scientists a powerful high-resolution tool to see what electrons are doing on various materials and compounds, Pirie says. “This tool can look at what the electric charge around just one atom is doing—allowing us to see the world at a smaller scale. There are so many fundamental questions this could help us answer about the world around us.”

More information: Harris Pirie et al, Visualizing the atomic-scale origin of metallic behavior in Kondo insulators, Science (2023). DOI: 10.1126/science.abq5375

Journal information: Science 

Provided by Harvard John A. Paulson School of Engineering and Applied Sciences 

In a demonstration that promises to help scale up quantum computers based on tiny dots of silicon, RIKEN physicists have succeeded in connecting two qubits—the basic unit for quantum information—that are physically distant from one another.

Many big IT players—including the likes of IBM, Google and Microsoft—are racing to develop quantum computers, some of which have already demonstrated the ability to greatly outperform conventional computers for certain types of calculations. But one of the greatest challenges to developing commercially viable quantum computers is the ability to scale them up from a hundred or so qubits to millions of qubits.

In terms of technologies, one of the front-runners to achieve large-scale quantum computing is silicon quantum dots that are a few tens of nanometers in diameter. A key advantage is that they can be fabricated using existing silicon fabrication technology. But one hurdle is that, while it is straightforward to connect two quantum dots that are next to each other, it has proved difficult to link quantum dots that are far from each other.

“In order to connect many qubits, we have to densely cram many of them into a very small area,” says Akito Noiri of the RIKEN Center for Emergent Matter Science. “And it’s very hard to use wires to connect such very densely packed qubits.”

Now, Noiri and co-workers have realized a two-qubit logic gate between physically distant silicon spin qubits.

“While there has been a lot of work in this area using various approaches, this is the first time that anyone has succeeded in demonstrating a reliable logic gate formed by two distant qubits,” says Noiri. “The demonstration opens up the possibility of scaling up quantum computing based on silicon quantum dots.”

To connect the two qubits, the team used a method known as coherent spin shuttling, which allows single spin qubits to be moved across an array of quantum dots without affecting their phase coherence—an important property for quantum computers since it carries information. This method involves pushing electrons through an array of qubits by applying a voltage.

Although the physical separation between the two qubits was relatively short, Noiri is confident that it can be extended in future studies. “We want to increase the separation to about a micrometer or so,” he says. “That will make the method more practical for future use.”

More information: Akito Noiri et al, A shuttling-based two-qubit logic gate for linking distant silicon quantum processors, Nature Communications (2022). DOI: 10.1038/s41467-022-33453-z

Journal information: Nature Communications 

Provided by RIKEN 

Over last few decades, study of topological phases of matter in solid-state electron systems has been extended to other research fields, including synthetic dimensions, and motivates extensive research on topological materials in many systems. Synthetic dimensions in photonics constructed by coupling internal degrees of freedom of light has manifested as a powerful platform for creating synthetic lattices with artificial connectivities.

Synthetic dimensions offer exciting new ways to explore higher-dimensional physics in lower geometrical dimensionality, simplify the setup for achieving unusual functionalities that are hard to be achieved in real space, and manipulate light in multiple ways.

Among them, dynamically modulated ring resonator systems, where resonant modes with equally spaced frequencies are coupled by external modulation to construct a synthetic frequency dimension, can provide great experimental flexibility and reconfigurability to construct more complex lattice structures.

Topological phases of matter play an essential role in many branches of photonics for providing exotic properties, which can be classified by their topological invariants.

For a one-dimensional (1D) system, such as the well-known SSH model, the topology is characterized by the Zak phase, which has been demonstrated within several experimental schemes in photonics. In spite of these progresses, there has been a remarkable challenge that the topological information of conventional 1D SSH model cannot be directly distinguished from the measured band structure in the current photonic or condensed matter platforms due to the identical band shapes for both topological trivial and non-trivial cases.

Study of 1D synthetic SSH model constructed along the frequency dimension of dynamically modulated ring resonators would provide richer physics and new routes to extract topological phase information.

In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Xianfeng Chen and Professor Luqi Yuan from State Key Laboratory of Advanced Optical Communication Systems and Networks, School of Physics and Astronomy, Shanghai Jiao Tong University, China and co-workers for the first time demonstrated a direct experimental measurement of Zak phase from the bulk band structure for a synthetic SSH model by utilizing the frequency axis of light, constructed in two coupled ring resonators by the bichromatic modulations at different amplitudes.

This novel study is enabled by merging topological photonics and band structure analysis in the synthetic frequency dimension and can further promote both fields by introducing a new way for exploring topological phases of matter with experimental feasibility and reconfigurability.

The 1D SSH model along the frequency dimension is constructed by two coupled ring resonators of 10.2 m long. The symmetric and antisymmetric supermodes in the ring resonator system have frequency splitting of 2κ=Ω/3=2π•6.67 MHz (Ω is the free spectral range) and are connected by the electro-optic phase modulator (EOM), which provides bichromatic sinusoidal modulations at different amplitudes. The supermodes act as synthetic lattice sites, with alternating hopping amplitudes g₁ and g₂.

The topologically non-trivial and trivial transmission cases can be converted flexibly by the external modulation. The Zak phase to be measured can be obtained theoretically by integrating the Berry curvature over the first Brillouin zone and take two values, which are 0 for the topologically trivial case and π for the topologically non-trivial case, respectively.

To implement the proposal in the experiment, the projected band structure of the synthetic lattice is obtained through the time-resolved band structure spectroscopy by collecting the drop-port transmission spectrum through the output fiber coupler via linearly scanning the frequency of the input laser source.

A data-analysis scheme called resonant method is applied to extract the geometric phases encoded in the bulk band due to projections of the band structures onto superpositions of the two supermodes with the frequency difference as the lattice sites in the frequency dimension.

The proposal is validated in experiments performed at the telecom wavelength, where distinguishable time-resolved transmission spectra in the non-trivial and trivial phases are obtained. Zak phase values (∼ 0 and 0.98π) are then extracted in different topological phases with a good fit between the simulation results and the experimental measurements.

This work for the first time experimentally provides the evidence of directly reading the topological Zak phase from the distinguishable time-resolved transmission spectra in a synthetic SSH model, constructed with the frequency dimension in modulated ring resonators.

The proposed method for characterizing the topological invariant is universal in the synthetic frequency dimension and points out a new route in exploring topological phases of matter with experimental feasibility and reconfigurability. The ability of providing alternating modulations between synthetic lattice sites in experiment therefore offers new possibility to construct more complex lattice structures with nonuniform connectivities in the frequency dimension.

The topologically non-trivial and trivial transmission spectra bring possible ingredient towards exploring spectral non-reciprocity and potential applications in optical communications.

More information: Guangzhen Li et al, Direct extraction of topological Zak phase with the synthetic dimension, Light: Science & Applications (2023). DOI: 10.1038/s41377-023-01126-1

Journal information: Light: Science & Applications 

Provided by Chinese Academy of Sciences 

Benefiting from superior capability in manipulating wavefront of electromagnetic waves, metasurfaces have provided a flexible platform for designing ultracompact and high-performance devices with unusual functionalities. Despite various advances in this field, the unique functionalities achieved by metasurfaces have come at the cost of the structural complexity, resulting in a time-consuming parameter sweep for the conventional metasurface design.

Although artificial neural networks provide a flexible platform for significantly improving the design process, the current metasurface designs are restricted to generating qualitative field distributions. Therefore, new artificial neural networks should be proposed to inversely predict metasurface-based devices with quantitative functionalities.

In a new paper published in Light: Advanced Manufacturing, a team of scientists, led by Professor Yiming Zhu from Terahertz Technology Innovation Research Institute, and Shanghai Key Lab of Modern Optical System, University of Shanghai for Science and Technology, China, and co-workers have developed a network architecture consisting of a tandem neural network and an iterative algorithm to predict metasurfaces with high-accuracy functionalities.

As a proof-of-principle example, they design multifoci metalens (predict by the network architecture) that possess multiple focal points with identical polarization states, as well as accurate intensity ratios. In addition, metalens for generating two focal points with orthogonal polarization states and accurate intensity ratios, and vortex generators for generating position- and polarization-dependent converged vortices were predicted/designed and experimentally demonstrated.

In comparison with the traditional forward-design, the inverse designed methods in this work can automatically and intelligently optimize the ratio of field intensities between two focal points. The tandem neural network enables unprecedented capability in globally optimizing the whole (target) amplitudes and phase. Therefore, one can use the optimized amplitudes and phase to easily design the metasurface to generate quantitative functionalities.

The presented approach will promote machine learning in further designing ultracompact devices with high-accuracy and quantitative functionalities in imaging, detecting, and sensing

More information: Yang Zhu et al, Metasurfaces designed by a bidirectional deep neural network and iterative algorithm for generating quantitative field distributions, Light: Advanced Manufacturing (2023). DOI: 10.37188/lam.2023.009

Provided by Chinese Academy of Sciences 

Holography, invented by Gabor, provides an approach for recording and reconstructing the complete information (i.e. intensity and phase) of the light from an object. Since its invention, holographic-related technologies have been widely applied in numerous areas, such as optical display, imaging, data storage, encryption and metrology.

In a new paper published in Light: Science & Applications, a team of scientists led by Professors Yueqiang Hu and Huigao from Hunan University and Zhenwei Xie from Shenzhen University developed a unique method for applying angular momentum (AM) holography for information multiplexing.

Holography based on conventional optical devices such as spatial light modulators (SLM) suffers from the disadvantages of low resolution and small field-of-view, hindering its practical applications. On the other hand, scientists can access the high resolution, ultra-thin thickness, and high-performance counterpart by replacing the conventional components with metasurfaces.

Metasurfaces, composed of two-dimensional subwavelength arrays of nanoscale scatters, allow for manipulating lights with multiple degrees of freedom (DoFs). It provides a new generation of versatile platforms for optical multiplexing holography.

In this context, different physical dimensions of light, such as wavelength, incidence angle, state of polarization (SoP) and time, have been exploited as independent information channels for holographic systems. Having almost exhausted the existing physical dimensions for multiplexing holography, the AM dimension of light has emerged as a new opportunity.

The optical AM, a quantum mechanical description of the photon, has boosted numerous applications in both classical and quantum optical fields, including optical tweezers, the spin-Hall effect, and quantum microscopy. The AM is categorized as orbital angular momentum (OAM) and spin angular momentum (SAM).

Using appropriate spatial frequency sampling of a hologram in momentum space, OAM has been implemented as an independent information carrier for optical holography. OAM-multiplexed holography shows unprecedented capacity for optical information processing due to the unbounded helical mode and intrinsic orthogonality. The linear polarization channels are added to the OAM-dependent holography, further enhancing the information capacity. The SAM has also been explored for multiplexing holography, ranging from spin-dependent to spin-decoupled.

Numerous efforts have been devoted to generating full-polarization vectorial holography. It can offer unlimited multiplexing channels in principle due to the capacity to control the arbitrary polarization vectors on the Poincare sphere (PS), showing great potential for high-capacity optical encryption. Adding the SAM dimension to the existing OAM-multiplexed holography and spatially manipulating the full-polarization vectors simultaneously has not yet been investigated.

The most intuitive design approach is segmenting or interleaving several kinds of meta-atoms, each corresponding to a specific functionality. Such methods will restrict its efficiency and introduce undesirable cross-talks. To overcome these drawbacks, a metasurface consisting of non-interleaved meta-atoms would be an excellent choice. However, the realization of AM holography using either the interleaved or the non-interleaved strategies has remained elusive.

The research team have theoretically and experimentally proven the AM-holography paradigm based on full synergy of the SAM and OAM via a minimalist metasurface. The design methodology of AM-holography depends on independently controlling the two spin eigenstates and arbitrarily overlaying them in each operation channel. This has thereby spatially modulated the resulting waveform at will.

The research team demonstrates an AM meta-hologram that can reconstruct two sets of distinct holographic images to prove the concept. These is the spin-orbital locked (SOL) and spin-superimposed (SS) ones, resulting in the multi-dimensional and multi-channel holography determined by the incident AM. The multi-channel AM meta-holograms have offered additional security locks, allowing us to construct the advanced optical nested encryption platform to revolutionize the existing optical encryption schemes that suffered from limited data capacity or low security.

In the optical nested encryption strategy, the SOL and SS holographic images are employed to encrypt and decrypt the optical information in a specific sequence, making the encoded information invulnerable to certain brute-force attacks.

The nested encryption scheme theoretically owns unbounded information channels from the AM holography, catering to the ever-growing requirement of parallel high-security information transmission. It is worth noting that the design strategy is generalized and can be extended to achieve other waveform shaping functionalities such as spatial structured-light generation and polarization knots.

More information: Hui Yang et al, Angular momentum holography via a minimalist metasurface for optical nested encryption, Light: Science & Applications (2023). DOI: 10.1038/s41377-023-01125-2

Journal information: Light: Science & Applications 

Provided by Chinese Academy of Sciences 

In the early 2000s, scientists observed lightning discharge producing X-rays comprising high energy photons—the same type used for medical imaging. Researchers could recreate this phenomenon in the lab, but they could not fully explain how and why lightning produced X-rays. Now, two decades later, a Penn State-led team has discovered a new physical mechanism explaining naturally occurring X-rays associated with lightning activity in the Earth’s atmosphere.

They published their results on March 30 in Geophysical Research Letters.

The team’s finding could also shed light on another phenomenon: the small shock sometimes felt when touching a metal doorknob. Called spark discharge, it occurs when a voltage difference is created between a body and a conductor. In a series of lab experiments in the 1960s, scientists discovered that spark discharges produce X-rays—just as lightning does. More than 60 years later, scientists are still conducting lab experiments to better understand the mechanism underpinning this process.

Lightning consists in part of relativistic electrons, which emit spectacular high-energy bursts of X-rays with tens of mega electron-volt energies called terrestrial gamma-ray flashes (TGFs). Researchers have created simulations and models to explain the TGF observations, but there is a mismatch between simulated and actual sizes, according to lead author Victor Pasko, Penn State professor of electrical engineering. Pasko and his team mathematically modeled the TGF phenomenon to better understand how it can occur in observed compact space.

“The simulations are all very big—usually several kilometers across—and the community has difficulty reconciling this right now with actual observations, because when lightning propagates, it’s very compact,” Pasko said, explaining that lightning’s space channel is typically several centimeters in scale, with electric discharge activity producing X-rays expanding around tips of these channels up to 100 meters in extreme cases. “Why is that source so compact? It’s been a puzzle until now. Since we’re working with very small volumes, it may also have implications for the lab experiments with spark discharges underway since the 1960s.”

Pasko said that they developed the explanation for how an electric field amplifies the number of electrons, driving the phenomenon. The electrons scatter on individual atoms, which constitute the air, as they experience acceleration. As the electrons move, most of them go forward as they gain energy and multiply, similar to a snow avalanche, allowing them to produce more electrons. As the electrons avalanche, they produce X-rays, which launch the photons backward and produce new electrons.

“From there, the question we wanted to answer mathematically was, “What is the electric field you need to apply in order to just replicate this, to launch just enough X-rays backwards to allow amplification of these select electrons?'” Pasko said.

The mathematical modeling established a threshold for the electric field, according to Pasko, which confirmed the feedback mechanism that amplifies the electron avalanches when X-rays emitted by the electrons travel backward and generate new electrons.

“The model results agree with the observational and experimental evidence indicating that TGFs originate from relatively compact regions of space with spatial extent on the order of 10 to 100 meters,” Pasko said.

In addition to describing high-energy phenomena related to lightning, Pasko said the work may eventually help to design new X-ray sources. The researchers said they plan to examine the mechanism using different materials and gases, as well as different applications of their findings.

More information: Victor P. Pasko et al, Conditions for Inception of Relativistic Runaway Discharges in Air, Geophysical Research Letters (2023). DOI: 10.1029/2022GL102710

Journal information: Geophysical Research Letters 

Provided by Pennsylvania State University