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September 19, 2023• Physics 16, 160

The Linac Coherent Light Source, an x-ray free-electron laser at SLAC National Accelerator Laboratory, lights up for the first time after an upgrade that should allow it to deliver up to one million x-ray pulses per second.

G. Stewart/SLAC National Accelerator Laboratory

G. Stewart/SLAC National Accelerator Laboratory

X-ray free-electron lasers (XFELs) first came into existence two decades ago. They have since enabled pioneering experiments that see both the ultrafast and the ultrasmall. Existing devices generate short and intense x-ray pulses at a rate of around 100 x-ray pulses per second. But one of these facilities, the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory in California, is set to eclipse this pulse rate. The LCLS Collaboration has now announced first light for its upgraded machine, LCLS-II. When it is fully up and running, LCLS-II is expected to fire one million pulses per second, making it the worlds most powerful x-ray laser.

The LCLS-II upgrade signifies a quantum leap in the machines potential for discovery, says Robert Schoenlein, the LCLSs deputy director for science. Now, rather than demonstration experiments on simple, model systems, scientists will be able to explore complex, real-world systems, he adds. For example, experimenters could peer into biological systems at ambient temperatures and physiological conditions, study photochemical systems and catalysts under the conditions in which they operate, and monitor nanoscale fluctuations of the electronic and magnetic correlations thought to govern the behavior of quantum materials.

The XFEL was first proposed in 1992 to tackle the challenge of building an x-ray laser. Conventional laser schemes excite large numbers of atoms into states from which they emit light. But excited states with energies corresponding to x-ray wavelengths are too short-lived to build up a sizeable excited-state population. XFELs instead rely on electrons traveling at relativistic speed through a periodic magnetic array called an undulator. Moving in bunches, the electrons wiggle through the undulator, emitting x-ray radiation that interacts with other bunches and becomes amplified. The result is a bright x-ray beam with laser coherence.

The first XFEL was built in Hamburg, Germany, in 2005. Today that XFEL emits soft x-ray radiation, which has wavelengths as short as a few nanometers. LCLS switched on four years later and expanded XFELs reach to the much shorter wavelengths of hard x rays, which are essential to atomic-resolution imaging and diffraction experiments. These and other facilities that later appeared in Japan, Italy, South Korea, Germany, and Switzerland have enabled scientists to probe catalytic reactions in real time, solve the structures of hard-to-crystallize proteins, and shed light on the role of electronphoton coupling in high-temperature superconductors. The ability to record movies of the dynamics of molecules, atoms, and even electrons also became possible because x-ray pulses can be as short as a couple of hundred attoseconds.

The upgrades to LCLS offer a new mode of XFEL operation, in which the facility delivers an almost continuous x-ray beam in the form of a megahertz pulse train. For the original LCLS, the pulse rate, which maxed out at 120 Hz, was set by the properties of the linear accelerator that produced the relativistic electrons. Built out of copper, a conventional metal, and operated at room temperatures, the accelerator had to be switched on an off 120 times per second to avoid heat-induced damage. In LCLS-II some of the copper has been replaced with niobium, which is superconducting at the operating temperature of 2 K. Bypassing the damage limitations of copper, the dissipationless superconducting elements allow an 8000-fold gain in the maximum repetition rate. The new superconducting technology is also expected to reduced jitter in the beam, says LCLS director Michael Dunne. Greater stability and reproducibility, higher repetition rate, and increased average power will transform our ability to look at a whole range of systems, he adds.

LCLS-II is a boon for time-resolved chemistry-focused experiments, says Munira Khalil, a physical chemist at the University of Washington in Seattle. Khalil, a user of LCLS, plans to take advantage of the photon bounty of the dynamical experiments. She hopes such experiments may fulfill a chemists dream: real-time observations of the coupled motion of atoms and electrons. With extra photons, scientists could also probe dilute samples, potentially shedding light on how metals bind to specific sites in proteinsa process relevant to the function of half of all of natures proteins.

The megahertz pulse rate also means that experiments that previously took days to perform could now be completed in hours or minutes, says Henry Chapman of the Center for Free Electron Laser Science at DESY, Germany. At LCLS and later at Hamburgs XFEL, Chapman ran pioneering experiments to determine the structures of proteins. The method he used, called serial crystallography, involves merging the diffraction patterns of multiple samples sequentially injected into the XFELs beam. Serial crystallography has allowed scientists to determine the structures of biologically relevant proteins that form crystals too small to study with conventional crystallography techniques. Chapman says that the increased throughput enabled by LCLS-II will permit much more ambitious experiments, such as measurements of biomolecular reactions on timescales from femtoseconds to microseconds. One could also think of an on the fly analysis that feeds back into the experiment to discover optimal conditions for drug binding or catalysis, he says.

For Khalil, the dramatic speedup of the experiments is a key advance of LCLS-II, as she thinks it will make these kinds of experiments accessible to a wider group of people. Until now, she says, XFEL facilities were mostly used by people who had the opportunity to work extensively at XFELs as postdocs or graduate students. Many more experimenters should now be able to enter the XFEL arena, she says. Its an exciting time for the field.

Matteo Rini

Matteo Rini is the Editor of Physics Magazine.

The slow drift of microscale features on a surface reveals the force driving atoms from the hot to the cold side of the material. Read More

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First Light for a Next-Generation Light Source - Physics

Photo by Andrew George on Unsplash

The intellectual exchange between Einstein and Bohr on quantum theory remains one of the most momentous and well-documented dialogues in the history of physics.

They were two towering figures in 20th-century physics, and their debates primarily revolved around the interpretation of quantum mechanics. At the core of their disagreement were differing views on determinism, realism, and the nature of measurement in quantum systems.

Einstein was a proponent of local realism, advocating for a deterministic universe where physical properties have well-defined values independent of observation. He famously said, God does not play dice with the universe, indicating his discomfort with the inherent randomness that quantum mechanics seemed to introduce.

Bohr, on the other hand, embraced the probabilistic nature of quantum mechanics and defended the Copenhagen interpretation. According to this view, quantum systems dont have definite properties until they are measured, and the act of measurement itself plays a crucial role in determining the state of the system.

One of the most famous instances of their debate was Einsteins formulation of the EPR paradox (Einstein-Podolsky-Rosen paradox) in 1935. This thought experiment aimed to show that quantum mechanics was incomplete because it allowed for spooky action at a distance, where entangled particles could instantaneously affect each other regardless of the distance separating them. Einstein viewed this as a violation of the principle of locality.

Bohr countered by arguing that Einsteins paradox was based on classical intuitions that didnt fully apply in the quantum realm. According to Bohr, the seeming non-locality is a result of our classical expectations, and theres no violation of physical principles in the quantum description.

Given the extensive developments in quantum mechanics since their era, I cannot help but wonder how these formidable minds would converse in the context of 21st-century advances!

This article is an attempt to hypothesize their perspectives on five topics and providing an analytical account of their likely viewpoints.

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If Einstein and Bohr met for a beer today, what would they talk about? - Medium

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Our research focuses on the foundations of quantum mechanics, quantum information theory, and practical applications of quantum information. Examples include operational reconstruction of quantum theory and quantum particle statistics, exploring quantum phenomena at macroscopic scales, and advancing quantum verification theory.

Our Team is part of the Quantum Optics, Quantum Nanophysics and Quantum Information group of the Faculty of Physics.We are member of the Vienna Center for Quantum Science and Technology (VCQ), one of the largest quantum hubs in Europe, and of the Austrian Cluster of Excellence (quantA), advancing basic research in quantum sciences, aiming to expand the frontiers of knowledge and thus being the engine for future innovations.

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Internal further training & Coaching:The Vienna Doctoral School as well as the department of human resources offer plenty of opportunities to grow your skills in over 600 courses to choose from free of charge.

Fair salary:The basic salary of EUR 2.457,00 (30h, 14x p.a.) increases if we can credit professional experience.The employment duration is 4 years. Initially limited to 1.5 years, the employment relationship is automatically extended to4 yearsif the employer does not terminate it within the first 12 months by submitting a non-extension declaration.

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We look forward to new personalities in our team!The University of Vienna has an anti-discriminatory employment policy and attaches great importance to equal opportunities, theadvancement of womenanddiversity. We lay special emphasis on increasing the number of women in senior and in academic positions among the academic and general university staff and therefore expressly encourage qualified women to apply. Given equal qualifications, preference will be given to female candidates.

University of Vienna. Space for personalities. Since 1365.

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Application deadline:10/06/2023

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University Assistant Predoctoral, Physics job with UNIVERSITY OF ... - Times Higher Education

Two scientists have won a $100,000 prize for describing both the structure and a way to detect mysterious black hole photon spheres. These enigmatic structures form at the edges of black holes, and could reveal the underlying physics that govern the most extreme objects in the cosmos.

Alexandru Lupsasca, of Vanderbilt University, and Michael Johnson, of Harvard University, won the New Horizon Prize in Physics "for elucidating the sub-structure and universal characteristics of black hole photon rings, and their proposed detection by next-generation interferometric experiments."

The New Horizons award is given each year to early career researchers by the Breakthrough Prize Foundation, and the prize money is donated by tech billionaires Sergey Brin, Priscilla Chan and Mark Zuckerberg, Yuri and Julia Milner, and Anne Wojcicki. A second prize was also awarded this year to Mikhail Ivanov, of MIT, Oliver Philcox, of Columbia University and the Simons Foundation, and Marko Simonovi, of the University of Florence for their work on the universe's 'cosmological collider'.

When photons stream toward a black hole, most are either bent away or (if they cross its event horizon) engulfed permanently in the dark abyss. Yet some rare light particles avoid this fate instead they surf the cosmic monster's gaping mouth in a series of tight orbits and, if the black hole is spinning, steal some of its rotational energy to miraculously spring free.

Related: The closest black holes to Earth may be 10 times closer than we thought

Detecting these photons for the first time would give physicists unprecedented insight into the most extreme objects in our universe, as well as how the known laws of physics break down in the presence of their infinite gravitational pulls.

"Gravity is the big mystery. To date we don't know how to combine Einstein's theory of general relativity, which is the relativistic picture of gravity as the bending of space-time, with quantum mechanics," the theory of the very small, Alexandru Lupsasca, who used relativity to devise what the rings should look like and find the parameters that describe them, told Live Science.

"The problem is that gravity is very weak it's the weakest of all forces," Lupsasca said. "So to have a chance of understanding quantum gravity, we have to look where gravity is strongest. And nowhere is gravity stronger than around a black hole."

Supermassive black holes are enormous, measuring roughly the width of the solar system, so it can take a photon around six days travelling at the speed of light to make an orbit. At the end of these six days, photons can either perform a U-turn to make another orbit, or fly into or away from the black hole. The photons that slip a black hole's gravity emerge in the form of an ultrathin halo around the pure black chasm: a photon sphere.

Photon spheres can be broken down into even smaller rings, with the light that went in last nesting in near-infinite bands inside the light that went in first. Peeling back these layers would reveal a string of snapshots displaying every angle of the surrounding universe, beginning with the recent past and running all the way back to the scant remaining glimmers of light captured eons ago by the black hole.

"It's like a laundromat, it takes light from every angle, lets it tumble and shoots it off in every direction," Lupsasca said. At any given time, only some photons can fit: "there's always more photons coming in, but there's always some leaking out."

After making theoretical predictions of what the rings should look like, the pair and their colleagues set about devising ways to measure the halos. Johnson realized that the Event Horizon Telescope (the EHT, which he and other researchers had used to capture the first-ever image of a black hole) was perfect for this task, if only the photon sphere could be distinguished from the fuzzy band of other light streaming from the black hole.

To achieve this, Johnson reasoned, researchers would only need to place the EHT into an array with one more telescope to distinguish the first band of the photon sphere.

"The miracle is that and this is unlike anything we've ever studied in astronomy that you can add one orbiter that's enough to study the photon ring," Johnson told Live Science. "That was just a complete shock."

Lupsasca and Johnson are working on a pitch to have NASA launch a satellite carrying the extra telescope. If successful, they could obtain the very first image of the outer band of a photon sphere within 10 to 15 years. Doing so would enable them not only to judge the size of a black hole's event horizon and its spin but also, once they have measured a second band, to probe some of the most radical theories in physics.

"This photon ring comes from as close as you can see to the visible edge of the observable universe," Lupsasca said. "If that's not enough to get you excited, I don't know what gets you out of bed."

Read the rest here:

New Horizon Prize in Physics awarded to scientists chasing ... - Livescience.com