BLOGS

Excited helium nuclei inflate like balloons, offering physicists a chance to study the strong nuclear force which binds the nucleus’s protons and neutrons. Kristina Armitage/Quanta Magazine

Topics: Modern Physics, Nobel Prize, Particle Physics, Quantum Mechanics, Steven Weinberg, Theoretical Physics

A new measurement of the strong nuclear force, which binds protons and neutrons together, confirms previous hints of an uncomfortable truth: We still don’t have a solid theoretical grasp of even the simplest nuclear systems.

To test the strong nuclear force, physicists turned to the helium-4 nucleus, which has two protons and two neutrons. When helium nuclei are excited, they grow like an inflating balloon until one of the protons pops off. Surprisingly, in a recent experiment, helium nuclei didn’t swell according to plan: They ballooned more than expected before they burst. A measurement describing that expansion, called the form factor, is twice as large as theoretical predictions.

“The theory should work,” said Sonia Bacca, a theoretical physicist at the Johannes Gutenberg University of Mainz and an author of the paper describing the discrepancy, which was published in Physical Review Letters. “We’re puzzled.”

For many years, physicists didn’t understand how to use the strong force to understand the stickiness of protons and neutrons. One problem was the bizarre nature of the strong force — it grows stronger with increasing distance rather than slowly dying off. This feature prevented them from using their usual calculation tricks. When particle physicists want to understand a particular system, they typically parcel out a force into more manageable approximate contributions, order those contributions from most important to least important, then simply ignore the less important contributions. With the strong force, they couldn’t do that.

Then in 1990, Steven Weinberg found a way to connect the world of quarks and gluons to sticky nuclei. The trick was to use an effective field theory — a theory that is only as detailed as it needs to be to describe nature at a particular size (or energy) scale. To describe the behavior of a nucleus, you don’t need to know about quarks and gluons. Instead, at these scales, a new effective force emerges — the strong nuclear force transmitted between nucleons by the exchange of pions.

A New Experiment Casts Doubt on the Leading Theory of the Nucleus, Katie McCormick, Quanta Magazine

A new study by KTH Royal Institute of Technology and Stanford University revises of our understanding of quantum vortices in superconductors. Pictured an artist’s depiction of quantum vortices. Credit: Greg Stewart, SLAC National Accelerator Laboratory

Topics: Modern Physics, Quantum Mechanics, Research, Superconductors

Within superconductors, little tornadoes of electrons, known as quantum vortices, can occur, which have important implications in superconducting applications such as quantum sensors. Now a new kind of superconducting vortex has been found, an international team of researchers reports.

Egor Babaev, professor at KTH Royal Institute of Technology in Stockholm, says the study revises the prevailing understanding of how electronic flow can occur in superconductors, based on work about quantum vortices that was recognized in the 2003 Nobel Prize award. The researchers at KTH, together with researchers from Stanford University, TD Lee Institute in Shanghai, and AIST in Tsukuba, discovered that the magnetic flux produced by vortices in a superconductor can be divided up into a wider range of values than thought.

That represents a new insight into the fundamentals of superconductivity and also potentially can be applied in superconducting electronics.

A vortex of magnetic flux happens when an external magnetic field is applied to a superconductor. The magnetic field penetrates the superconductor in the form of quantized magnetic flux tubes, which form vortices. Babaev says that originally research held that quantum vortices pass through superconductors each carrying one quantum of magnetic flux. But arbitrary fractions of quantum flux were not a possibility entertained in earlier theories of superconductivity.

Using the Superconducting Quantum Interference Device (SQUID) at Stanford University Babaev's co-authors, research scientist Yusuke Iguchi and Professor Kathryn A. Moler, showed at a microscopic level that quantum vortices can exist in a single electronic band. The team was able to create and move around these fractional quantum vortices, Moler says.

"Professor Babaev has been telling me for years that we could see something like this, but I didn't believe it until Dr. Iguchi actually saw it and conducted a number of detailed checks," she says.

Tiny quantum electronic vortexes can circulate in superconductors in ways not seen before, KTH Royal Institute of Technology, Phys.org.

Conceptual artwork of a pair of entangled quantum particles. Credit: Science Photo Library/Alamy Stock Photo

Topics: Modern Physics, Particle Physics, Quantum Mechanics, Theoretical Physics

Quantum entanglement is a complex phenomenon in physics that is usually poorly described as an invisible link between distant quantum objects that allows one to affect the other instantly. Albert Einstein famously dismissed this idea of entanglement as “spooky action at a distance.” Entanglement is better understood as information, but that’s admittedly bland. So nowadays, every news article, explainer, opinion piece, and artistic interpretation of quantum entanglement equates the phenomenon with Einstein’s spookiness. The situation has only worsened with the 2022 Nobel Prize in Physics going to Alain Aspect, John F. Clauser, and Anton Zeilinger for quantum entanglement experiments. But it’s time to cut this adjective loose. Calling entanglement spooky completely misrepresents how it actually works and hinders our ability to make sense of it.

In 1935, physicist Erwin Schrödinger coined the term entanglement, emphasizing that it was “not one but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought.” He was writing in response to a famous paper (known simply to physicists as the EPR argument) by Einstein, Boris Podolsky, and Nathan Rosen that claimed quantum physics was incomplete. The New York Times headline read, “Einstein attacks quantum theory,” which solidified the widespread perception that Einstein hated quantum physics.

The EPR argument concerns the everyday notion of reality as a collection of things in the world with physical properties waiting to be revealed through measurement. This is how most of us intuitively understand reality. Einstein’s theory of relativity fits into this understanding and says reality must be local, meaning nothing can influence anything else faster than the speed of light. But EPR showed that quantum physics isn’t compatible with these ideas—that it can’t account for a theory of local reality. In other words, quantum physics was missing something. To complete quantum physics, Einstein suggested scientists should look for a “deeper” theory of local reality. Many physicists responded in defense of quantum theory, but the matter remained unresolved until 1964 when physicist John S. Bell proposed an experiment that could rule out the existence of local reality. Clauser was the first to perform the test, which was later improved and perfected by Aspect and Zeilinger.

Quantum Entanglement Isn’t All That Spooky After All, Chris Ferrie, Scientific American

The classic double-slit experiment leads to characteristic interference patterns. Credit: Russell Knightly/SPL

Topics: Modern Physics, Optics, Quantum Mechanics

A celebrated experiment in 1801 showed that light passing through two thin slits interferes with itself, forming a characteristic striped pattern on the wall behind. Now, physicists have shown that a similar effect can arise with two slits in time rather than space: a single mirror that rapidly turns on and off causes interference in a laser pulse, making it change color.

The result is reported on 3 April in Nature Physics¹. It adds a new twist to the classic double-slit experiment performed by physicist Thomas Young, which demonstrated the wavelike aspect of light, but also — in its many later reincarnations — that quantum objects ranging from photons to molecules have a dual nature of both particle and wave.

The rapid switching of the mirror — possibly taking just one femtosecond (one-quadrillionth of a second) — shows that certain materials can change their optical properties much faster than previously thought possible, says Andrea Alù, a physicist at the City University of New York. This could open new paths for building devices that handle information using light rather than electronic impulses.

Romain Tirole, a quantum physicist at Imperial College London, and his collaborators shot an infrared laser at a surface made of layers of gold and glass with a thin coating of indium tin oxide (ITO), a material common in smartphone screens.

Under normal conditions, ITO is transparent to infrared light. But the researchers were able to make the material reflective using a second laser, which excited electrons in the material, affecting its optical properties. This could be done with pulses from the second laser that lasted for around 200 femtoseconds.

The researchers positioned a light sensor along the reflected beam. When they shot two ultrashort pulses separated by a few tens of femtoseconds — therefore turning the ITO mirror on twice in rapid succession — they saw that the waveform of the twice-reflected light changed in response. It went from a simple, monochromatic wave to a more complex one.

The results also showed that the ITO took less than ten femtoseconds to get excited — much faster than expected theoretically or from previous measurements. “The reason why everybody else thought it would be slower is that they used a different technique to measure the response time, which was limited to 50–100 fs,” says co-author Riccardo Sapienza, a physicist at Imperial College.

Light waves squeezed through ‘slits in time,’ Davide Castelvecchi, Nature

Atomic analog: when a beam of light is shone into a water droplet, the light is trapped inside. (Courtesy: Javier Tello Marmolejo)

Topics: Modern Physics, Optics, Quantum Mechanics, Quantum Optics, Research

Light waves confined in an evaporating water droplet provide a useful model of the quantum behavior of atoms, researchers in Sweden and Mexico have discovered. Through a simple experiment, a team led by Javier Marmolejo at the University of Gothenburg has shown how the resonance of light inside droplets of specific sizes can provide robust analogies to atomic energy levels and quantum tunneling.

When light is scattered by a liquid droplet many times larger than its wavelength, some of the light may reflect around the droplet’s internal edge. If the droplet’s circumference is a perfect multiple of the light’s wavelength inside the liquid, the resulting resonance will cause the droplet to flash brightly. This is an optical example of a whispering gallery mode, whereby sound can reflect around a circular room.

This effect was first described mathematically by the German physicist Gustav Mie in 1908 – yet despite the simplicity of the scenario, the rich array of overlapping resonances it produces can create some incredibly complex patterns, some of which have yet to be studied in detail.

Optical Tweezers

To explore the effect in more detail, Marmolejo and the team devised an experiment where they confined water droplets using optical tweezers. They evaporated the liquid by heating it with a fixed-frequency laser. As the droplets shrank, their circumferences will sometimes equal a multiple of the laser’s wavelength. At these “Mie resonances,” the droplets flashed brightly.

As they studied this effect, the researchers realized that the flashing droplets are analogous to the quantum behaviors of atoms. In these “optical atoms,” orbiting electrons are replaced with resonating photons. The electrostatic potential that binds electrons to the nucleus is replaced by the droplet’s refractive index, which tends to trap light in the droplet by internal reflection. The quantized energy levels of an atom are represented by the droplet sizes where Mie resonances occur.

Flashing droplets could shed light on atomic physics and quantum tunneling, Sam Jarman, Physics World.

A laser fusion power plant proposed by Longview Fusion Energy Systems would generate 1000 MWh or more of electricity. The plant would compress fusion fuel by using an indirect drive, the same approach used at the National Ignition Facility, which in December announced that it had produced ignition and gain, the first time that fusion researchers have attained those milestones.

Topics: Futurism, Lasers, Modern Physics, Nuclear Fusion

The attainment of fusion ignition and energy gain on the world’s most energetic laser late last year was indisputably a major scientific accomplishment. But the road to fusion as a viable source of energy will be a long one, if not a dead end. And if it does ultimately become a reality, most experts say that it is unlikely that a laser-driven fusion power plant will be based on the approach taken by the National Ignition Facility (NIF), where the fusion milestone occurred.

The December shot, which produced 1.5 times the 2 MJ of energy that was fired on the fusion fuel, has silenced skeptics who said that ignition could never be created by bombarding tiny capsules of deuterium–tritium fuel with lasers. (See “National Ignition Facility surpasses long-awaited fusion milestone,” Physics Today online, 13 December 2022.) “They have done something very important: demonstrating ignition and burn,” says Stephen Bodner, a retired head of the laser fusion branch at the US Naval Research Laboratory who once was a persistent critic of NIF’s approach.

And the milestone is likely to open the floodgates to new investments in the handful of startups that are pursuing inertial fusion energy (IFE). “I think you will see a proliferation of companies devoted to IFE or aspects of IFE because of this and because of investor interest,” says Todd Ditmire, a University of Texas at Austin physicist who is chief technology officer of Focused Energy, an IFE startup.

Yet despite the fanfare greeting the announcement, the fact is that the fusion energy yield from the successful shot amounted to less than 1% of the 300 MJ taken from the electricity grid to power NIF’s 192 beams. And the energy released was enough to boil about 10 tea kettles. Many experts say that economically viable fusion will require fusion reactions yielding energy gains of at least 100 times the energy deposited on the fuel capsule—two orders of magnitude greater than the NIF shot.

NIF success gives laser fusion energy a shot in the arm, David Kramer, Physics Today

Credit: Nicoletta Barolini

Topics: Chemistry, Graphene, Materials Science, Modern Physics, Nanotechnology

Graphullerene, an atom-thin material made of linked fullerene subunits, gives scientists a new form of modular carbon to play with.

Carbon, in its myriad forms, has long captivated the scientific community. Besides being the primary component of all organic life on earth, material forms of carbon have earned their fair share of breakthroughs. In 1996, the Nobel Prize in Chemistry went to the discoverers of fullerene, a superatomic symmetrical structure of 60 carbon atoms shaped like a soccer ball; in 2010, researchers working with an ultra-strong, atom-thin version of carbon, known as graphene, won the Nobel Prize in Physics.

Today in work published in Nature, researchers led by Columbia chemists Xavier Roy, Colin Nuckolls, and Michael Steigerwald, with postdoc and first author Elena Meirzadeh have discovered a new version of carbon that sits somewhere in between fullerene and graphene: graphullerene. It’s a new two-dimensional form of carbon made up of layers of linked fullerenes peeled into ultrathin flakes from a larger graphullerite crystal—just like how graphene is peeled from graphite crystals (the same material found in pencils).

“It is amazing to find a new form of carbon,” said Nuckolls. “It also makes you realize that there is a whole family of materials that can be made in a similar way that will have new and unusual properties as a consequence of the information written into the superatomic building blocks.”

Columbia Chemists Discover a New Form of Carbon: Graphene’s “Superatomic” Cousin, Ellen Neff, Quantum.Columbia.edu

Credit: sakkmesterke/ Getty Images

Topics: Lasers, Modern Physics, Quantum Computer, Quantum Mechanics

Physicists have devised a mind-bending error-correction technique that could dramatically boost the performance of quantum computers.

When the ancient Incas wanted to archive tax and census records, they used a device made up of a number of strings called a quipu, which encoded the data in knots. Fast-forward several hundred years, and physicists are on their way to developing a far more sophisticated modern equivalent. Their “quipu” is a new phase of matter created within a quantum computer, their strings are atoms, and the knots are generated by patterns of laser pulses that effectively open up [a] second dimension of time.

This isn’t quite as incomprehensible as it first appears. The new phase is one of many within a family of so-called topological phases, which were first identified in the 1980s. These materials display order not on the basis of how their constituents are arranged—like the regular spacing of atoms in a crystal—but on their dynamic motions and interactions. Creating a new topological phase—that is, a new “phase of matter”—is as simple as applying novel combinations of electromagnetic fields and laser pulses to bring order or “symmetry” to the motions and states of a substance’s atoms. Such symmetries can exist in time rather than space, for example in induced repetitive motions. Time symmetries can be difficult to see directly but can be revealed mathematically by imagining the real-world material as a lower-dimensional projection from a hypothetical higher-dimensional space, similar to how a two-dimensional hologram is a lower-dimensional projection of a three-dimensional object. In the case of this newly created phase, which manifests in a strand of ions (electrically charged atoms), its symmetries can be discerned by considering it as a material that exists in higher-dimensional reality with two-time dimensions.

“It is very exciting to see this unusual phase of matter realized in an actual experiment, especially because the mathematical description is based on a theoretical ‘extra’ time dimension,” says team member Philipp Dumitrescu, who was at the Flatiron Institute in New York City when the experiments were carried out. A paper describing the work was published in Nature on July 20.

Opening a portal to an extra time dimension—even just a theoretical one—sounds thrilling, but it was not the physicists’ original plan. “We were very much motivated to see what new types of phases could be created,” says study co-author Andrew Potter, a quantum physicist at the University of British Columbia. Only after envisioning their proposed new phase did the team members realize it could help protect data being processed in quantum computers from errors.

New Phase of Matter Opens Portal to Extra Time Dimension, Zeeya Merali, Scientific American

The ore pass at the Sanford Underground Research Facility in South Dakota. (Courtesy of Sanford Underground Research Facility, CC BY-NC-ND 4.0.)

Topics: Applied Physics, Modern Physics, Particle Physics, Theoretical Physics

The Deep Underground Neutrino Experiment (DUNE) will be the world’s largest cryogenic particle detector. Its aim is to study the most elusive of particles: neutrinos. Teams from around the world are developing and constructing detector components that they will ship to the Sanford Underground Research Facility, commonly called Sanford Lab, in the Black Hills of South Dakota. There the detector components will be lowered more than a kilometer underground through a narrow shaft to the caverns, where they will be assembled and operated while being sheltered from the cosmic rays that constantly rain down on Earth’s surface.

For at least two decades, the detector will be exposed to the highest-intensity neutrino beam on the planet. The beam will be generated 1300 km away by a megawatt-class proton accelerator and beamline under development at Fermilab in Batavia, Illinois. A smaller detector just downstream of the beamline will measure the neutrinos at the start of their journey, thereby enabling the experiment’s precision and scientific reach.

Building a ship in a bottle for neutrino science, Anne Heavey, FERMILAB, Physics Today

Living on: data taken by the now-defunct CDF experiment has revealed a surprising mass for the W boson. (Courtesy: Fermilab)

Topics: Fermilab, High Energy Physics, Modern Physics, Particle Physics, Steven Weinberg

The most precise measurement to date of the mass of the W boson has yielded a result seven standard deviations away from that predicted by the Standard Model of particle physics. The stunning result was obtained by a painstaking analysis of data taken at the Fermilab Tevatron collider in the US before it closed in 2011. The particle physics community must now study the results carefully to work out whether it is an incredible statistical fluke, an unknown experimental error, a flaw in the Standard Model, or a genuine indication of physics beyond the Standard Model.

The W boson is one of the most intriguing particles described by the Standard Model. Together with the neutral Z boson, the charged W boson mediates the weak interaction, which causes beta decay and several other important processes in particle physics. The weak interaction has long intrigued scientists searching for physics beyond the Standard Model, partly because it is the only force known to violate charge-parity symmetry. If particles in a process are exchanged for their antiparticles and the spatial coordinates are inverted, the weak interaction in this mirror image process is not always identical. This puzzle is not explained in the Standard Model.

W boson mass measurement surprises physicists, Tim Wogan, Physics World

A quantum probe for gravity: Physicists have detected a tiny phase shift in atomic wave packets due to gravity-induced relativistic time dilation – an example of the Aharonov-Bohm effect in action. (Courtesy: Shutterstock/Evgenia Fux)

Topics: General Relativity, Gravity, Modern Physics, Quantum Mechanics

The idea that particles can feel the influence of potentials even without being exposed to a force field may seem counterintuitive, but it has long been accepted in physics thanks to experimental demonstrations involving electromagnetic interactions. Now physicists in the US have shown that this so-called Aharonov-Bohm effect also holds true for a much weaker force: gravity. The physicists based their conclusion on the behavior of freefalling atomic wave packets, and they say the result suggests a new way of measuring Newton’s gravitational constant with far greater precision than was previously possible.

Yakir Aharonov and David Bohm proposed the effect that now bears their name in 1959, arguing that while classical potentials have no physical reality apart from the fields they represent, the same is not true in the quantum world. To make their case, the pair proposed a thought experiment in which an electron beam in a superposition of two wave packets is exposed to a time-varying electrical potential (but no field) when passing through a pair of metal tubes. They argued that the potential would introduce a phase difference between the wave packets and therefore lead to a measurable physical effect – a set of interference fringes – when the wave packets are recombined.

Seeking a gravitational counterpart

In the latest research, Mark Kasevich and colleagues at Stanford University show that the same effect also holds true for gravity. The platform for their experiment is an atom interferometer, which uses a series of laser pulses to split, guide and recombine atomic wave packets. The interference from these wave packets then reveals any change in the relative phase experienced along the two arms.

Physicists detect an Aharonov-Bohm effect for gravity, Edwin Cartlidge, Physics World

Image Source: Fermilab, and link below

Topics: Fermilab, High Energy Physics, Modern Physics, Neutrinos, Particle Physics

Solving big mysteries

The Deep Underground Neutrino Experiment is an international flagship experiment to unlock the mysteries of neutrinos. DUNE will be installed in the Long-Baseline Neutrino Facility, under construction in the United States. DUNE scientists will paint a clearer picture of the universe and how it works. Their research may even give us the key to understanding why we live in a matter-dominated universe — in other words, why we are here at all.

DUNE will pursue three major science goals: find out whether neutrinos could be the reason the universe is made of matter; look for subatomic phenomena that could help realize Einstein’s dream of the unification of forces; and watch for neutrinos emerging from an exploding star, perhaps witnessing the birth of a neutron star or a black hole.

DUNE at LBNF, Fermilab

(Credit: Giovanni Cancemi/Shutterstock) 

Topics: Biology, Condensed Matter Physics, Modern Physics, Quantum Mechanics

Note: After presenting my research proposal and acceptance by my committee, I've been taking a well-needed break from blogging. I'll post on and off until the New Year, which isn't too far off. Happy holidays!

In recent years, evidence has emerged that quantum physics seems to play a role in some of life’s fundamental processes. But just how it might do this is something of a mystery.

On the one hand, quantum phenomena are generally so delicate that they can only be observed when all other influences are damped – in other words in carefully controlled systems at temperatures close to absolute zero. By contrast, the conditions for life are generally complex, warm, and damp. Understanding this seemingly contradictory state of affairs is an important goal.

So physicists and biologists are keen to explore the boundaries of these very different regimes—life and quantum mechanics—to better understand where they might overlap.

Now Rainer Dumke at the Nanyang Technological University in Singapore and colleagues have created an exotic quantum state called entanglement using a superconducting qubit and a microscopic animal called a tardigrade. Along the way, the team has created the most extreme form of suspended animation ever recorded. “The tardigrade itself is shown to be entangled with the remaining subsystems,” they say.

To perform their entanglement experiment, Dumke and co cooled their tardigrade to below 10 millikelvins, almost to absolute zero, while reducing the pressure to a millionth of that in the atmosphere. In these conditions, no chemical reaction can occur so the tardigrade’s metabolism must have entirely halted stopped and the processes of life halted.

“This is to date the most extreme exposure to low temperatures and pressures that a tardigrade has been recorded to survive, clearly demonstrating that the state of cryptobiosis ultimately involves a suspension of all metabolic processes given that all chemical reactions would be prohibited with all its constituent molecules cooled to their ground states,” say the researchers.

In this condition, the tardigrade can be thought of as a purely dielectric element. Indeed, the researchers simulated their experiment by treating the tardigrade as a dielectric cube.

The experimental setup consisted of two superconducting capacitors, which when cooled can exist in a superposition of states called a qubit. They placed the tardigrade between the capacitor plates of one qubit so that it became an integral part of the capacitor. The team was then able to measure the effect of the tardigrade on the qubit’s properties.

How a Tardigrade "Micro Animal" Became Quantum Entangled with Superconducting Qubit, The Physics AriXiv Blog, Discovery Magazine

Quasiparticles in motion: illustration of ghost polaritons in a calcite crystal being “launched” to record distances by a gold microdisk. (Courtesy: HUST)

Topics: Condensed Matter Physics, Modern Physics, Quantum Mechanics

The existence of ghost hyperbolic surface polaritons has been demonstrated by an international collaboration including researchers in China and the US. Based at Huazhong University of Science and Technology (HUST), National University of Singapore (NUS), National Center for Nanoscience and Technology (NCNST), and the City University of New York (CUNY), the team showed that the polariton – a hybrid light-matter quasiparticle – has a record-breaking propagation distance of three times its photon wavelength. This ghost polariton is an exciting discovery that has applications in sub-wavelength, low-loss imaging, sensing, and information transfer. The full study is described in Nature.

Previously, hyperbolic polaritons, which arise from the strong coupling of electromagnetic radiation to lattice vibrations (phonons) in anisotropic crystals, had only been observed in two forms: bulk polaritons and surface polaritons. Bulk, volume-confined, hyperbolic polaritons (v-HPs) have a real out-of-plane wavevector and hence can propagate within the material supporting them. Surface-confined hyperbolic polaritons (s-HPs), however, have an entirely imaginary out-of-plane wavevector, and so decay exponentially away from the crystal surface, a property called evanescence. The hyperbolic dispersion of these polaritons is the result of the crystal’s dielectric anisotropy, which results in hyperbolic isofrequency contours in k-space (momentum space) and concave wavefronts in real space.

Most studies on v-HPs and s-HPs have been performed in thin layers of van der Waals crystals. These crystals comprise stacks of covalently bound 2D layers that are held together by weak van der Waals forces. However, in such crystal layers, there is no control over the optical axis. This is the direction in which propagating light experiences no birefringence and it is typically aligned with the layers.

Ghost surface polaritons seen for the first time, Kirsty McGhee, Physics World

Complementarity A new twist on the double-slit experiment. (Courtesy: Shutterstock/Andrey VP)

Topics: Modern Physics, Quantum Mechanics, Theoretical Physics

One of the most counterintuitive concepts in physics – the idea that quantum objects are complementary, behaving like waves in some situations and like particles in others – just got a new and more quantitative foundation. In a twist on the classic double-slit experiment, scientists at Korea’s Institute for Basic Sciences (IBS) used precisely controlled photon sources to measure a photon’s degree of wave-ness and particle-ness. Their results, published in Science Advances, show that the properties of the photon’s source influence its wave and particle character – a discovery that complicates and challenges the common understanding of complementarity.

The double-slit experiment is the archetypal example of complementarity at work. When a single photon encounters a barrier with two thin openings, it produces an interference pattern on a screen placed behind the openings – but only if the photon’s path is not observed. This interference pattern identifies the photon as a wave since a particle would create only one point of light on the screen. However, if detectors are placed at the openings to determine which slit the photon went through, the interference pattern disappears, and the photon behaves like a particle. The principle of complementarity states that both experimental outcomes are needed to fully understand the photon’s quantum nature.

Wave-particle duality quantified for the first time, Karmela Padavic-Callaghan, Physics World

Figure 1. The size contrast between conventional accelerator facilities and chip-based accelerators is dramatic. (a) The Next Linear Collider Test Accelerator facility at SLAC was used for early laser-acceleration experiments in 2012–15. (Image courtesy of the Archives and History Office/SLAC National Accelerator Laboratory.) (b) The first dielectric laser accelerator chips demonstrated at SLAC were made of fused silica and were each the size of a grain of rice. (Image courtesy of Christopher Smith/SLAC National Accelerator Laboratory.)

Topics: Applied Physics, Modern Physics, Particle Physics

Physics Today 74, 8, 42 (2021); https://doi.org/10.1063/PT.3.4815

Particle accelerators are among the most important scientific tools of the modern age. Major accelerator facilities, such as the 27-km-circumference Large Hadron Collider in Switzerland, where the Higgs boson was recently discovered, allow scientists to uncover fundamental properties of matter and energy. But the particle energies needed to explore new regimes of physics have increased to the TeV scale and beyond, and accelerator facilities based on conventional technologies are becoming prohibitively large and costly. Even lower-energy, smaller-scale accelerators used in medicine and industry are often cumbersome devices; they can weigh several tons and cost millions of dollars.

Efforts are consequently underway to develop more compact, less expensive accelerator technologies. One approach, a dielectric laser accelerator (DLA), uses an ultrafast IR laser to deliver energy to electrons inside a microchip-scale device. Efficient, ultrafast solid-state lasers and semiconductor fabrication methods developed over the past two decades have enabled a new breed of photonic devices that can sustain accelerating fields one to two orders of magnitude larger than conventional microwave-cavity accelerators.

The approach has the potential to dramatically shrink particle accelerators, thereby enabling ultrafast tabletop electron diffraction and microscopy experiments and tunable x-ray sources. An international effort is now underway to develop a laser-driven accelerator integrated on a silicon photonics platform: an “accelerator on a chip.”

Microchip accelerators, Joel England, Peter Hommelhoff, Robert L. Byer, Physics Today

LAWRENCE LIVERMORE NATIONAL LABORATORY

Topics: Energy, Environment, Modern Physics, Nuclear Fusion, Nuclear Power

More than a decade ago, the world’s most energetic laser started to unleash its blasts on tiny capsules of hydrogen isotopes, with managers promising it would soon demonstrate a route to limitless fusion energy. Now, the National Ignition Facility (NIF) has taken a major leap toward that goal. Last week, a single laser shot sparked a fusion explosion from a peppercorn-size fuel capsule that produced eight times more energy than the facility had ever achieved: 1.35 megajoules (MJ)—roughly the kinetic energy of a car traveling at 160 kilometers per hour. That was also 70% of the energy of the laser pulse that triggered it, making it tantalizingly close to “ignition”: a fusion shot producing an excess of energy.

 “After many years at 3% of ignition, this is super exciting,” says Mark Herrmann, head of the fusion program at Lawrence Livermore National Laboratory, which operates NIF.

NIF’s latest shot “proves that a small amount of energy, imploding a small amount of mass, can get fusion. It’s a wonderful result for the field,” says physicist Michael Campbell, director of the Laboratory for Laser Energetics (LLE) at the University of Rochester.

“It’s a remarkable achievement,” adds plasma physicist Steven Rose, co-director of the Centre for Inertial Fusion Studies at Imperial College London. “It’s made me feel very cheerful. … It feels like a breakthrough.”

And it is none too soon, as years of slow progress have raised questions about whether laser-powered fusion has a practical future. Now, according to LLE Chief Scientist Riccardo Betti, researchers need to ask: “What is the maximum fusion yield you can get out of NIF? That’s the real question.”

Fusion, which powers stars, forces small atomic nuclei to meld together into larger ones, releasing large amounts of energy. Extremely hard to achieve on Earth because of the heat and pressure required to join nuclei, fusion continues to attract scientific and commercial interest because it promises copious energy, with little environmental impact.

With explosive new result, laser-powered fusion effort nears ‘ignition’, Daniel Clery, Science Magazine

Feynman QED Diagram: Fermilab

Topics: Modern Physics, Particle Physics, Quantum Mechanics

Solving a mystery

More than 200 scientists from around the world are collaborating with Fermilab on the Muon g-2 physics experiment which probes fundamental properties of matter and space. Muon g-2 (pronounced gee minus two) allows researchers to peer into the subatomic world to search for undiscovered particles that may be hiding in the vacuum.

Residing at Fermilab's Muon Campus, the experiment uses the Fermilab accelerator complex to produce an intense beam of muons traveling at nearly the speed of light. Scientists will use the beam to precisely determine the value of a property known as the g-2 of the muon.

The muon, like its lighter sibling the electron, acts like a spinning magnet. The parameter known as "g" indicates how strong the magnet is and the rate of its gyration. The value of the muon's g is slightly larger than 2. This difference from 2 is caused by the presence of virtual particles that appear from the quantum vacuum and then quickly disappear into it again.

In measuring g-2 with high precision and comparing its value to the theoretical prediction, physicists aim to discover whether the experiment agrees with the theory. Any deviation would point to as yet undiscovered subatomic particles that exist in nature.

An experiment that concluded in 2001 at Brookhaven National Laboratory found a tantalizing 3.7 sigma (standard deviation) discrepancy between the theoretical calculation and the measurement of the muon g-2. With a four-fold increase in the measurement's precision, Muon g-2 will be more sensitive to virtual or hidden particles and forces than any previous experiment of its kind and can bring this discrepancy to the 5 sigma discovery level.

The centerpiece of the Muon g-2 experiment at Fermilab is a large, 50-foot-diameter superconducting muon storage ring. This one-of-a-kind ring, made of steel, aluminum, and superconducting wire, was built for the previous g-2 experiment at Brookhaven. The ring was moved from Brookhaven to Fermilab in 2013. Making use of Fermilab's intense particle beams, scientists will be able to significantly increase the science output of this unique instrument. The experiment started taking data in 2018.

U.S. Department of Energy - Fermilab: Muon g - 2

Clocking dark matter: optical clocks join the hunt for dark matter. (Courtesy: N Hanacek/NIST)

Topics: Dark Matter, Modern Physics, Quantum Mechanics

An optical clock has been used to set new constraints on a proposed theory of dark matter. Researchers including Jun Ye at JILA at the University of Colorado, Boulder, and Andrei Derevianko at the University of Nevada, Reno, explored how the coupling between regular matter and “ultralight” dark matter particles could be detected using the clock in conjunction with an ultra-stable optical cavity. With future upgrades to the performance of optical clocks, their approach could become an important tool in the search for dark matter.

Although it appears to account for about 85% of the matter in the universe, physicists know very little about dark matter. Most theoretical and experimental work so far has been focussed on hypothetical dark-matter particles, including WIMPS and axions, which have relatively large masses.  Alternatively, some physicists have proposed the existence of “ultralight” dark matter particles with extremely small masses that span many orders of magnitude (10⁻¹⁶–10⁻²¹ eV/c²).

According to the laws of quantum mechanics, the very smallest of these particles would have huge wavelengths, comparable to the sizes of entire dwarf galaxies – meaning they would behave like classical fields on scales we can easily measure.

Optical clock sets new constraints on dark matter, Sam Jarman, Physics World

Credit: Z. Shi et al., Proc. Natl. Acad. Sci. USA 117, 24634 (2020)

Topics: Materials Science, Modern Physics, Nanotechnology, Semiconductor Technology

If you ever manage to deform a diamond, you’re likely to break it. That’s because the hardest natural material on Earth is also inelastic and brittle. Two years ago, Ming Dao (MIT), Subra Suresh (Nanyang Technological University in Singapore), and their collaborators demonstrated that when bulk diamonds are etched into fine, 300-nm-wide needles, they become nearly defect-free. The transformation allows diamonds to elastically bend under the pressure of an indenter tip, as shown in the figure, and withstand extremely large tensile stresses without breaking.

The achievement prompted the researchers to investigate whether the simple process of bending could controllably and reversibly alter the electronic structure of nanocrystal diamond. Teaming up with Ju Li and graduate student Zhe Shi (both at MIT), Dao and Suresh have now followed their earlier study with numerical simulations of the reversible deformation. The team used advanced deep-learning algorithms that reveal the bandgap distributions in nanosized diamond across a range of loading conditions and crystal geometries. The new work confirms that the elastic strain can alter the material’s carbon-bonding configuration enough to close its bandgap from a normally 5.6 eV width as an electrical insulator to 0 eV as a conducting metal. That metallization occurred on the compression side of a bent diamond nanoneedle.

Diamond nanoneedles turn metallic, R. Mark Wilson, Physics Today