quantum mechanics (41)

Fusion's Holy Grail...

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A view of the assembled experimental JT-60SA Tokamak nuclear fusion facility outside Tokyo, Japan. JT-60SA.ORG

Topics: Applied Physics, Economics, Energy, Heliophysics, Nuclear Fusion, Quantum Mechanics

Japan and the European Union have officially inaugurated testing at the world’s largest experimental nuclear fusion plant. Located roughly 85 miles north of Tokyo, the six-story JT-60SA “tokamak” facility heats plasma to 200 million degrees Celsius (around 360 million Fahrenheit) within its circular, magnetically insulated reactor. Although JT-60SA first powered up during a test run back in October, the partner governments’ December 1 announcement marks the official start of operations at the world’s biggest fusion center, reaffirming a “long-standing cooperation in the field of fusion energy.”

The tokamak—an acronym of the Russian-language designation of “toroidal chamber with magnetic coils”—has led researchers’ push towards achieving the “Holy Grail” of sustainable green energy production for decades. Often described as a large hollow donut, a tokamak is filled with gaseous hydrogen fuel that is then spun at immense high speeds using powerful magnetic coil encasements. When all goes as planned, intense force ionizes atoms to form helium plasma, much like how the sun produces its energy.

[Related: How a US lab created energy with fusion—again.]

Speaking at the inauguration event, EU energy commissioner Kadri Simson referred to the JT-60SA as “the most advanced tokamak in the world,” representing “a milestone for fusion history.”

“Fusion has the potential to become a key component for energy mix in the second half of this century,” she continued.

The world’s largest experimental tokamak nuclear fusion reactor is up and running, Andrew Paul, Popular Science.

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'Teleporting' Images...

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High-dimensional quantum transport enabled by nonlinear detection. In our concept, information is encoded on a coherent source and overlapped with a single photon from an entangled pair in a nonlinear crystal for up-conversion by sum frequency generation, the latter acting as a nonlinear spatial mode detector. The bright source is necessary to achieve the efficiency required for nonlinear detection. Information and photons flow in opposite directions: one of [the] Bob’s entangled photons is sent to Alice and has no information, while a measurement on the other in coincidence with the upconverted photon establishes the transport of information across the quantum link. Alice need not know this information for the process to work, while the nonlinearity allows the state to be arbitrary and unknown dimension and basis. Credit: Nature Communications (2023). DOI: 10.1038/s41467-023-43949-x

Topics: Applied Physics, Computer Science, Cryptography, Cybersecurity, Quantum Computers, Quantum Mechanics, Quantum Optics

Nature Communications published research by an international team from Wits and ICFO- The Institute of Photonic Sciences, which demonstrates the teleportation-like transport of "patterns" of light—this is the first approach that can transport images across a network without physically sending the image and a crucial step towards realizing a quantum network for high-dimensional entangled states.

Quantum communication over long distances is integral to information security and has been demonstrated with two-dimensional states (qubits) over very long distances between satellites. This may seem enough if we compare it with its classical counterpart, i.e., sending bits that can be encoded in 1s (signal) and 0s (no signal), one at a time.

However, quantum optics allow us to increase the alphabet and to securely describe more complex systems in a single shot, such as a unique fingerprint or a face.

"Traditionally, two communicating parties physically send the information from one to the other, even in the quantum realm," says Prof. Andrew Forbes, the lead PI from Wits University.

"Now, it is possible to teleport information so that it never physically travels across the connection—a 'Star Trek' technology made real." Unfortunately, teleportation has so far only been demonstrated with three-dimensional states (imagine a three-pixel image); therefore, additional entangled photons are needed to reach higher dimensions.

'Teleporting' images across a network securely using only light, Wits University, Phys.org.

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The "Tiny Ten"...

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Researchers are working to overcome challenges related to nanoscale optoelectronic interconnects, which use light to transmit signals around an integrated circuit. IMAGE: PROVIDED BY NCNST

Topics: Biology, Materials Science, Nanoengineering, Nanomaterials, Nanotechnology, Quantum Mechanics

The promise of nanotechnology, the engineering of machines and systems at the nanoscale, is anything but tiny. Over the past decade alone, there has been an explosion in research on how to design and build components that solve problems across almost every sector, and nanotechnology innovations have led to huge advancements in our quest to address humanity’s grand challenges, from healthcare to water to food security.

Like any area of scholarship, there are still so many unknowns. And yet, there are more talented scientists and engineers endeavoring to better comprehend and harness the power of nanotechnology than ever before. The future is bright for nanotechnology and its applications.

In celebration of its 20th anniversary, the National Center for Nanoscience and Technology, China (NCNST), a subsidiary of the prestigious Chinese Academy of Sciences, partnered with Science Custom Publishing to survey nanoscience experts from the journal and across the globe about the most knotty and fascinating questions that still need to be answered if we are to advance nanotechnology in society.

The Tiny Ten: Experts weigh in on the top 10 challenges remaining for nanoscience & nanotechnology, Science Magazine

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Quantum Switch...

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Credit: CC0 Public Domain

Topics: Condensed Matter Physics, Materials Science, Quantum Computer, Quantum Mechanics

Quantum scientists have discovered a rare phenomenon that could hold the key to creating a 'perfect switch' in quantum devices, which flips between being an insulator and a superconductor.

The research, led by the University of Bristol and published in Science, found these two opposing electronic states exist within purple bronze, a unique one-dimensional metal composed of individual conducting chains of atoms.

Tiny changes in the material, for instance, prompted by a small stimulus like heat or light, may trigger an instant transition from an insulating state with zero conductivity to a superconductor with unlimited conductivity and vice versa. This polarized versatility, known as "emergent symmetry," has the potential to offer an ideal On/Off switch in future quantum technology developments.

Lead author Nigel Hussey, Professor of Physics at the University of Bristol, said, "It's a really exciting discovery that could provide a perfect switch for quantum devices of tomorrow.

"The remarkable journey started 13 years ago in my lab when two Ph.D. students, Xiaofeng Xu, and Nick Wakeham, measured the magnetoresistance—the change in resistance caused by a magnetic field—of purple bronze."

In the absence of a magnetic field, the resistance of purple bronze was highly dependent on the direction in which the electrical current was introduced. Its temperature dependence was also rather complicated. Around room temperature, the resistance is metallic, but as the temperature is lowered, this reverses and the material appears to be turning into an insulator. Then, at the lowest temperatures, the resistance plummets again as it transitions into a superconductor.

Despite this complexity, surprisingly, the magnetoresistance was found to be extremely simple. It was essentially the same irrespective of the direction in which the current or field was aligned and followed a perfect linear temperature dependence all the way from room temperature down to the superconducting transition temperature.

Research reveals rare metal could offer revolutionary switch for future quantum devices, Queen's University Belfast, Phys.org.

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Pines' Demon...

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Lurking for decades: researchers have discovered Pines' demon, a collection of electrons in a metal that behaves like a massless wave. It is illustrated here as an artist’s impression. (Courtesy: The Grainger College of Engineering/University of Illinois Urbana-Champaign)

Topics: Particle Physics, Quantum Mechanics, Research, Solid-State Physics, Theoretical Physics

For nearly seven decades, a plasmon known as Pines’ demon has remained a purely hypothetical feature of solid-state systems. Massless, neutral, and unable to interact with light, this unusual quasiparticle is reckoned to play a key role in certain superconductors and semimetals. Now, scientists in the US and Japan say they have finally detected it while using specialized electron spectroscopy to study the material strontium ruthenate.

Plasmons were proposed by the physicists David Pines and David Bohm in 1952 as quanta of collective electron density fluctuations in a plasma. They are analogous to phonons, which are quanta of sound, but unlike phonons, their frequency does not tend to zero when they have no momentum. That’s because finite energy is needed to overcome the Coulomb attraction between electrons and ions in a plasma in order to get oscillations going, which entails a finite oscillation frequency (at zero momentum).

Today, plasmons are routinely studied in metals and semiconductors, which have conduction electrons that behave like a plasma. Plasmons, phonons, and other quantized fluctuations are called quasiparticles because they share properties with fundamental particles such as photons.

In 1956, Pines hypothesized the existence of a plasmon which, like sound, would require no initial burst of energy. He dubbed the new quasiparticle a demon in honor of James Clerk Maxwell’s famous thermodynamic demon. Pines’ demon forms when electrons in different bands of metal move out of phase with one another such that they keep the overall charge static. In effect, a demon is the collective motion of neutral quasiparticles whose charge is screened by electrons from another band.

Demon quasiparticle is detected 67 years after it was first proposed. Edwin Cartlidg, Physics World.

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Quantum Slow Down...

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Topics: Chemistry, Computer Science, Quantum Computer, Quantum Mechanics

Scientists at the University of Sydney have, for the first time, used a quantum computer to engineer and directly observe a process critical in chemical reactions by slowing it down by a factor of 100 billion times.

Joint lead researcher and Ph.D. student Vanessa Olaya Agudelo said, "It is by understanding these basic processes inside and between molecules that we can open up a new world of possibilities in materials science, drug design, or solar energy harvesting.

"It could also help improve other processes that rely on molecules interacting with light, such as how smog is created or how the ozone layer is damaged."

Specifically, the research team witnessed the interference pattern of a single atom caused by a common geometric structure in chemistry called a "conical intersection."

Conical intersections are known throughout chemistry and are vital to rapid photochemical processes such as light harvesting in human vision or photosynthesis.

Chemists have tried to directly observe such geometric processes in chemical dynamics since the 1950s, but it is not feasible to observe them directly, given the extremely rapid timescales involved.

To get around this problem, quantum researchers in the School of Physics and the School of Chemistry created an experiment using a trapped-ion quantum computer in a completely new way. This allowed them to design and map this very complicated problem onto a relatively small quantum device—and then slow the process down by a factor of 100 billion. Their research findings are published August 28 in Nature Chemistry.

"In nature, the whole process is over within femtoseconds," said Olaya Agudelo from the School of Chemistry. "That's a billionth of a millionth—or one quadrillionth—of a second.

"Using our quantum computer, we built a system that allowed us to slow down the chemical dynamics from femtoseconds to milliseconds. This allowed us to make meaningful observations and measurements.

"This has never been done before."

Joint lead author Dr. Christophe Valahu from the School of Physics said, "Until now, we have been unable to directly observe the dynamics of 'geometric phase'; it happens too fast to probe experimentally.

"Using quantum technologies, we have addressed this problem."

Scientists use a quantum device to slow down simulated chemical reactions 100 billion times. University of Sydney, Phys.org.

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Tunnel Falls...

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Chip off the old block: Intel’s Tunnel Falls chip is based on silicon spin qubits, which are about a million times smaller than other qubit types. (Courtesy: Intel Corporation)

Topics: Applied Physics, Chemistry, Electrical Engineering, Quantum Computer, Quantum Mechanics

Intel – the world’s biggest computer-chip maker – has released its newest quantum chip and has begun shipping it to quantum scientists and engineers to use in their research. Dubbed Tunnel Falls, the chip contains a 12-qubit array and is based on silicon spin-qubit technology.

The distribution of the quantum chip to the quantum community is part of Intel’s plan to let researchers gain hands-on experience with the technology while at the same time enabling new quantum research.

The first quantum labs to get access to the chip include the University of Maryland, Sandia National Laboratories, the University of Rochester, and the University of Wisconsin-Madison.

The Tunnel Falls chip was fabricated on 300 mm silicon wafers in Intel’s “D1” transistor fabrication facility in Oregon, which can carry out extreme ultraviolet lithography (EUV) and gate and contact processing techniques.

Intel releases 12-qubit silicon quantum chip to the quantum community, Martijn Boerkamp, Physics World.

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Beyond Attogram Imaging...

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When X-rays (blue color) illuminate an iron atom (red ball at the center of the molecule), core-level electrons are excited. X-ray excited electrons are then tunneled to the detector tip (gray) via overlapping atomic/molecular orbitals, which provide elemental and chemical information about the iron atom. Credit: Saw-Wai Hla

Topics: Applied Physics, Instrumentation, Materials Science, Nanomaterials, Quantum Mechanics

A team of scientists from Ohio University, Argonne National Laboratory, the University of Illinois-Chicago, and others, led by Ohio University Professor of Physics, and Argonne National Laboratory scientist, Saw Wai Hla, have taken the world's first X-ray SIGNAL (or SIGNATURE) of just one atom. This groundbreaking achievement could revolutionize the way scientists detect materials.

Since its discovery by Roentgen in 1895, X-rays have been used everywhere, from medical examinations to security screenings in airports. Even Curiosity, NASA's Mars rover, is equipped with an X-ray device to examine the material composition of the rocks on Mars. An important usage of X-rays in science is to identify the type of materials in a sample. Over the years, the quantity of materials in a sample required for X-ray detection has been greatly reduced thanks to the development of synchrotron X-rays sources and new instruments. To date, the smallest amount one can X-ray a sample is in an attogram, which is about 10,000 atoms or more. This is due to the X-ray signal produced by an atom being extremely weak, so conventional X-ray detectors cannot be used to detect it. According to Hla, it is a long-standing dream of scientists to X-ray just one atom, which is now being realized by the research team led by him.

"Atoms can be routinely imaged with scanning probe microscopes, but without X-rays, one cannot tell what they are made of. We can now detect exactly the type of a particular atom, one atom-at-a-time, and can simultaneously measure its chemical state," explained Hla, who is also the director of the Nanoscale and Quantum Phenomena Institute at Ohio University. "Once we are able to do that, we can trace the materials down to the ultimate limit of just one atom. This will have a great impact on environmental and medical sciences and maybe even find a cure that can have a huge impact on humankind. This discovery will transform the world."

Their paper, published in the scientific journal Nature on May 31, 2023, and gracing the cover of the print version of the scientific journal on June 1, 2023, details how Hla and several other physicists and chemists, including Ph.D. students at OHIO, used a purpose-built synchrotron X-ray instrument at the XTIP beamline of Advanced Photon Source and the Center for Nanoscale Materials at Argonne National Laboratory.

Scientists report the world's first X-ray of a single atom, Ohio University, Phys.org.

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Straining Moore...

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Topics: Applied Physics, Chemistry, Computer Science, Electrical Engineering, Materials Science, Nanotechnology, Quantum Mechanics, Semiconductor Technology

Gordon Moore, the co-founder of Intel who died earlier this year, is famous for forecasting a continuous rise in the density of transistors that we can pack onto semiconductor chips. James McKenzie looks at how “Moore’s law” is still going strong after almost six decades but warns that further progress is becoming harder and ever more expensive to sustain.

When the Taiwan Semiconductor Manufacturing Company (TSMC) announced last year that it was planning to build a new factory to produce integrated circuits, it wasn’t just the eye-watering $33bn price tag that caught my eye. What also struck me is that the plant, set to open in 2025 in the city of Hsinchu, will make the world’s first “2-nanometer” chips. Smaller, faster, and up to 30% more efficient than any microchip that has come before, TSMC’s chips will be sold to the likes of Apple – the company’s biggest customer – powering everything from smartphones to laptops.

But our ability to build such tiny, powerful chips shouldn’t surprise us. After all, the engineer Gordon Moore – who died on 24 March this year, aged 94 – famously predicted in 1965 that the number of transistors we can squeeze onto an integrated circuit ought to double yearly. Writing for the magazine Electronics (38 114), Moore reckoned that by 1975 it should be possible to fit a quarter of a million components onto a single silicon chip with an area of one square inch (6.25 cm2).

Moore’s prediction, which he later said was simply a “wild extrapolation”, held true, although, in 1975, he revised his forecast, predicting that chip densities would double every two years rather than every year. What thereafter became known as “Moore’s law” proved amazingly accurate, as the ability to pack ever more transistors into a tiny space underpinned the almost non-stop growth of the consumer electronics industry. In truth, it was never an established scientific “law” but more a description of how things had developed in the past as well as a roadmap that the semiconductor industry imposed on itself, driving future development.

Moore's law: further progress will push hard on the boundaries of physics and economics, James McKenzie, Physics World

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Challenging the Standard Model...

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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

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Quantum Vortexes...

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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.

 

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ALPS and Dark Matter...

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Magnet row of the ALPS experiment in the HERA tunnel: In this part of the magnets, intense laser light is reflected back and forth, from which axions are supposed to form. Credit: DESY, Marta Maye

Topics: Dark Matter, Materials Science, Particle Physics, Quantum Mechanics

The ALPS (Any Light Particle Search) experiment, which stretches a total length of 250 meters, is looking for a particularly light type of new elementary particle. The international research team wants to search for these so-called axions or axion-like particles using twenty-four recycled superconducting magnets from the HERA accelerator, an intense laser beam, precision interferometry, and highly sensitive detectors.

Such particles are believed to react only extremely weakly with known kinds of matter, which means they cannot be detected in experiments using accelerators. ALPS is therefore resorting to an entirely different principle to detect them: in a strong magnetic field, photons—i.e., particles of light—could be transformed into these mysterious elementary particles and back into [light] again.

"The idea for an experiment like ALPS has been around for over 30 years. By using components and the infrastructure of the former HERA accelerator, together with state-of-the-art technologies, we are now able to realize ALPS II in an international collaboration for the first time," says Beate Heinemann, Director of Particle Physics at DESY.

Helmut Dosch, Chairman of DESY's Board of Directors, adds, "DESY has set itself the task of decoding matter in all its different forms. So ALPS II fits our research strategy perfectly, and perhaps it will push open the door to dark matter."

The ALPS team sends a high-intensity laser beam along a device called an optical resonator in a vacuum tube, approximately 120 meters in length, in which the beam is reflected backward and forwards and is enclosed by twelve HERA magnets arranged in a straight line. If a photon were to turn into an axion in the strong magnetic field, that axion could pass through the opaque wall at the end of the line of magnets.

Once through the wall, it would enter another magnetic track almost identical to the first. Here, the [axion] could then change back into a photon, which would be captured by the detector at the end. A second optical resonator is set up here to increase the probability of an [axion[ turning back into a photon by a factor of 10,000.

This means if [light] does arrive behind the wall, it must have been an axion in between. "However, despite all our technical tricks, the probability of a photon turning into an axion and back again is very small," says DESY's Axel Lindner, project leader and spokesperson of the ALPS collaboration, "like throwing 33 dice and them all coming up the same."

In order for the experiment to actually work, the researchers had to tweak all the different components of the apparatus to maximum performance. The light detector is so sensitive that it can detect a single photon per day. The precision of the system of mirrors for the light is also record-breaking: the distance between the mirrors must remain constant to within a fraction of an atomic diameter relative to the wavelength of the laser.

World's most sensitive model-independent experiment starts searching for dark matter, Deutsches Elektronen-Synchrotron, Phys.org.

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Removing the Spookiness...

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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 articleexplaineropinion 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

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Strange Metals II...

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Credit: CC0 Public Domain

Topics: Applied Physics, Chemistry, Materials Science, Metamaterials, Quantum Mechanics

The behavior of so-called "strange metals" has long puzzled scientists—but a group of researchers at the University of Toronto may be one step closer to understanding these materials.

Electrons are discrete, subatomic particles that flow through wires like molecules of water flowing through a pipe. The flow is known as electricity, and it is harnessed to power and control everything from lightbulbs to the Large Hadron Collider.

In quantum matter, by contrast, electrons don't behave as they do in normal materials. They are much stronger, and the four fundamental properties of electrons—charge, spin, orbit, and lattice—become intertwined, resulting in complex states of matter.

"In quantum matter, electrons shed their particle-like character and exhibit strange collective behavior," says condensed matter physicist Arun Paramekanti, a professor in the U of T's Department of Physics in the Faculty of Arts & Science. "These materials are known as non-Fermi liquids, in which the simple rules break down."

Now, three researchers from the university's Department of Physics and Centre for Quantum Information & Quantum Control (CQIQC) have developed a theoretical model describing the interactions between subatomic particles in non-Fermi liquids. The framework expands on existing models and will help researchers understand the behavior of these "strange metals."

Their research was published in the journal Proceedings of the National Academy of Sciences (PNAS). The lead author is physics Ph.D. student Andrew Hardy, with co-authors Paramekanti and post-doctoral researcher Arijit Haldar.

"We know that the flow of a complex fluid like blood through arteries is much harder to understand than water through pipes," says Paramekanti. "Similarly, the flow of electrons in non-Fermi liquids is much harder to study than that in simple metals."

Hardy adds, "What we've done is construct a model, a tool, to study non-Fermi liquid behavior. And specifically, to deal with what happens when there is symmetry breaking, when there is a phase transition into a new type of system."

"Symmetry breaking" is the term used to describe a fundamental process found in all of nature. Symmetry breaks when a system—whether a droplet of water or the entire universe—loses its symmetry and homogeneity and becomes more complex.

Researchers develop new insight into the enigmatic realm of 'strange metals', Chris Sasaki, University of Toronto, Phys.org

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Slits in Time...

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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

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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.

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Scanning With a Twist...

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How it works: illustration of the quantum twisting microscope in action. Electrons tunnel from the probe (inverted pyramid at the top) to the sample (bottom) in several places at once (green vertical lines) in a quantum-coherent manner. (Courtesy: Weizmann Institute of Science)

Topics: Chemistry, Entanglement, Materials Science, Nanotechnology, Quantum Mechanics

When the scanning tunneling microscope debuted in the 1980s, the result was an explosion in nanotechnology and quantum-device research. Since then, other types of scanning probe microscopes have been developed, and together they have helped researchers flesh out theories of electron transport. But these techniques probe electrons at a single point, thereby observing them as particles and only seeing their wave nature indirectly. Now, researchers at the Weizmann Institute of Science in Israel have built a new scanning probe – the quantum twisting microscope – that detects the quantum wave characteristics of electrons directly.

“It’s effectively a scanning probe tip with an interferometer at its apex,” says Shahal Ilani, the team leader. The researchers overlay a scanning probe tip with ultrathin graphite, hexagonal boron nitride, and a van der Waals crystal such as graphene, which conveniently flopped over the tip like a tent with a flat top about 200 nm across. The flat end is key to the device’s interferometer function.  Instead of an electron tunneling between one point in the sample and the tip, the electron wave function can tunnel across multiple points simultaneously.

“Quite surprisingly, we found that the flat end naturally pivots so that it is always parallel with the sample,” says John Birkbeck, the corresponding author of a paper describing this work. This is fortunate because any tilt would alter the tunneling distance and hence strength from one side of the plateau to the other. “It is the interference of these tunneling paths, as identified in the measured current, that gives the device its unique quantum-wave probing function,” says Birkbeck.

Scanning probe with a twist observes the electron’s wavelike behavior, Anna Demming, Physics World

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Spooky Action Between Friends...

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Credit: Petrovich9/Getty Images

Topics: Entanglement, Particle Physics, Quantum Mechanics, Research, Theoretical Physics

Reference: Albert Einstein colorfully dismissed quantum entanglement—the ability of separated objects to share a condition or state—as “spooky action at a distance.” Science.org

For the first time, scientists have observed quantum interference—a wavelike interaction between particles related to the weird quantum phenomenon of entanglement—occurring between two different kinds of particles. The discovery could help physicists understand what goes on inside an atomic nucleus.

Particles act as both particles and waves. And interference is the ability of one particle’s wavelike action to diminish or amplify the action of other quantum particles like two boat wakes crossing in a lake. Sometimes the overlapping waves add up to a bigger wave, and sometimes they cancel out, erasing it. This interference occurs because of entanglement, one of the weirder aspects of quantum physics, which was predicted in the 1930s and has been experimentally observed since the 1970s. When entangled, the quantum states of multiple particles are linked so that measurements of one will correlate with measurements of the others, even if one is on Jupiter and another is on your front lawn.

Dissimilar particles can sometimes become entangled, but until now, these [mismatched] entangled particles weren’t known to interfere with one another. That’s because part of measuring interference relies on two wavelike particles being indistinguishable from each other. Imagine two photons, or particles of light, from two separate sources. If you were to detect these photons, there would be no way to determine which source each came from because there is no way to tell which photon is which. Thanks to the quantum laws governing these very small particles, this ambiguity is actually measurable: all the possible histories of the two identical photons interfere with one another, creating new patterns in particles’ final wavelike actions.

Scientists See Quantum Interference between Different Kinds of Particles for the First Time, Stephanie Pappas, Scientific American

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Rule Breakers...

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Credit: Matt Harrison Clough (original image at link)

Topics: Entanglement, High Energy Physics, Particle Physics, Quantum Mechanics

Breaking the rules is exciting, especially if they have been held for a long time. This is true not just in life but also in particle physics. Here the rule I’m thinking of is called “lepton flavor universality,” and it is one of the predictions of our Standard Model of particle physics, which describes all the known fundamental particles and their interactions (except for gravity). For several decades after the invention of the Standard Model, particles seemed to obey this rule.

Things started to change in 2004 when the E821 experiment at Brookhaven National Laboratory on Long Island announced its measurement of a property of the muon—a heavy version of the electron—known as its g-factor. The measurement wasn’t what the Standard Model predicted. Muons and electrons are both parts of a class of particles called leptons (along with a third particle, the tau, as well as the three generations of neutrinos). The rule of lepton flavor universality says that because electrons and muons are charged leptons, they should all interact with other particles in the same way (barring small differences related to the Higgs particle). If they don’t, then they violate lepton flavor universality—and the unexpected g-factor measurement suggested that’s just what was happening.

If particles really were breaking this rule, that would be exciting in its own right and also because physicists believe that the Standard Model can’t be the ultimate theory of nature. The theory doesn’t explain why neutrinos have mass, what makes up the invisible dark matter that seems to dominate the cosmos, or why matter won out over antimatter in the early universe. Therefore, the Standard Model must be merely an approximate description that we will need to supplement by adding new particles and interactions. Physicists have proposed a huge number of such extensions, but at most one of these theories can be correct, and so far none of them has received any direct confirmation. A measured violation of the Standard Model would be a flashlight pointing the way toward this higher theory we seek.

Rule-Breaking Particles Pop Up in Experiments around the World, Andreas Crivellin, Scientific American

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Quipu...

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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

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