quantum mechanics (15)

Deux Ex Machina...

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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

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Double Slit...

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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

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Flatland...

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Image Source: Link below

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

Flatland: “The book used the fictional two-dimensional world of Flatland to comment on the hierarchy of Victorian culture, but the novella’s more enduring contribution is its examination of dimensions.” Source: Wikipedia

After decades of exploration in nature’s smallest domains, physicists have finally found evidence that anyons exist. First predicted by theorists in the early 1980s, these particle-like objects only arise in realms confined to two dimensions, and then only under certain circumstances — like at temperatures near absolute zero and in the presence of a strong magnetic field.

Physicists are excited about anyons not only because their discovery confirms decades of theoretical work, but also for practical reasons. For example, Anyons are at the heart of an effort by Microsoft to build a working quantum computer.

This year brought two solid confirmations of the quasiparticles. The first arrived in April, in a paper featured on the cover of Science, from a group of researchers at the École Normale Supérieure in Paris. Using an approach proposed four years ago, physicists sent an electron gas through a teeny-tiny particle collider to tease out weird behaviors — especially fractional electric charges — that only arise if anyons are around. The second confirmation came in July when a group at Purdue University in Indiana used an experimental setup on an etched chip that screened out interactions that might obscure anyon behavior.

MIT physicist Frank Wilczek, who predicted and named anyons in the early 1980s, credits the first paper as the discovery but says the second lets the quasiparticles shine. “It’s gorgeous work that makes the field blossom,” he says. Anyons aren’t like ordinary elementary particles; scientists will never be able to isolate one from the system where it forms. They’re quasiparticles, which means they have measurable properties like a particle — such as a location, maybe even a mass — but they’re only observable as a result of the collective behavior of other, conventional particles. (Think of the intricate geometric shapes made by group behavior in nature, such as flocks of birds flying in formation or schools of fish swimming as one.)

The known universe contains only two varieties of elementary particles. One is the family of fermions, which includes electrons, as well as protons, neutrons, and the quarks that form them. Fermions keep to themselves: No two can exist in the same quantum state at the same time. If these particles didn’t have this property, all matter could simply collapse to a single point. It’s because of fermions that solid matter exists.

The rest of the particles in the universe are bosons, a group that includes particles like photons (the messengers of light and radiation) and gluons (which “glue” quarks together). Unlike fermions, two or more bosons can exist in the same state at the same time. They tend to clump together. It’s because of this clumping that we have lasers, which are streams of photons all occupying the same quantum state.

Physicists prove the existence of two-dimensional particles called 'anyons', Stephen Omes, Astronomy (December 2020)

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Breaking Physics...

 

Topics: Quantum Computer, Quantum Mechanics, Thermodynamics

In what could prove to be a momentous accomplishment for fundamental physics and quantum physics, scientists say they’ve finally figured out how to manufacture a scientific oddity called a time crystal.

Time crystals harness a quirk of physics in which they remain ever-changing yet dynamically stable. In other words, they don’t give off energy as they change conformation, making them an apparent violation of the natural law that all things gradually turn towards entropy and disorder.

Now, it seems like it’s possible for these things to exist, after all, Quanta Magazine reports. At least, that’s according to what a massive team of researchers from Stanford, Princeton, and elsewhere working with Google’s quantum computing labs claimed in preprint research shared online last week. Aside from being an incredible scientific discovery in abstract — time crystals represent a new, bizarre phase of matter — the discovery could have profound implications for the finicky world of quantum computing.

“The consequence is amazing: You evade the second law of thermodynamics,” study coauthor and Max Planck Institute for the Physics of Complex Systems director Roderich Moessner told Quanta.

Google Claims To Create Time Crystals Inside Quantum Computer, Dan Robitzski, Futurism

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The Weirdest Matter...

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This simulation shows how a fracton-filled material would be expected to scatter a beam of neutrons.
H. Yan et al., Physical Review Letters

 

Topics: Condensed Matter Physics, Quantum Mechanics, Theoretical Physics

Your desk is made up of individual, distinct atoms, but from far away its surface appears smooth. This simple idea is at the core of all our models of the physical world. We can describe what’s happening overall without getting bogged down in the complicated interactions between every atom and electron.

So when a new theoretical state of matter was discovered whose microscopic features stubbornly persist at all scales, many physicists refused to believe in its existence.

“When I first heard about fractons, I said there’s no way this could be true because it completely defies my prejudice of how systems behave,” said Nathan Seiberg, a theoretical physicist at the Institute for Advanced Study in Princeton, New Jersey. “But I was wrong. I realized I had been living in denial.”

The theoretical possibility of fractons surprised physicists in 2011. Recently, these strange states of matter have been leading physicists toward new theoretical frameworks that could help them tackle some of the grittiest problems in fundamental physics.

Fractons are quasiparticles — particle-like entities that emerge out of complicated interactions between many elementary particles inside a material. But fractons are bizarre even compared to other exotic quasiparticles because they are totally immobile or able to move only in a limited way. There’s nothing in their environment that stops fractons from moving; rather it’s an inherent property of theirs. It means fractons’ microscopic structure influences their behavior over long distances.

“That’s totally shocking. For me it is the weirdest phase of matter,” said Xie Chen, a condensed matter theorist at the California Institute of Technology.

The ‘Weirdest’ Matter, Made of Partial Particles, Defies Description, Thomas Lewton, Quanta Magazine

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Space-Based Quantum Technology...

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(Credit: Yurchanka Siarhei/Shutterstock)

Topics: Computer Science, Quantum Computer, Quantum Mechanics

Quantum technologies are already revolutionizing life on Earth. But they also have the potential to change the way we operate in space. With the U.S., China, and Europe all investing heavily in this area, these changes are likely to be with us sooner rather than later.

So how will space-based quantum technologies make a difference?

Now, we get an overview thanks to the work of Rainer Kaltenbaek at the Institute for Quantum Optics and Quantum Information, in Austria, and colleagues throughout Europe, who have mapped out the future in this area and set out the advances that space-based quantum technologies will make possible.

While quantum computing and quantum communication grab most of the headlines, Kaltenbaek and colleagues point out that other quantum technologies are set to have equally impressive impacts. Take, for example, atom interferometry with quantum sensors.

These devices can measure with unprecedented accuracy any change in motion of a satellite in orbit as it is buffeted by tiny variations in the Earth’s gravitational field. These changes are caused by factors such as the movement of cooler, higher-density water flows in the deep ocean, flooding, the movement of the continents, and ice flows.

The Future of Space-Based Quantum Technology, Discover/Physics arXiv

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Quantum Microscope...

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Artist’s impression of UQ’s new quantum microscope in action. Credit: The University of Queensland

Topics: Biology, Biotechnology, Instrumentation, Quantum Mechanics, Quantum Optics

In a major scientific leap, University of Queensland researchers have created a quantum microscope that can reveal biological structures that would otherwise be impossible to see.

This paves the way for applications in biotechnology, and could extend far beyond this into areas ranging from navigation to medical imaging.

The microscope is powered by the science of quantum entanglement, an effect Einstein described as “spooky interactions at a distance.”

Professor Warwick Bowen, from UQ’s Quantum Optics Lab and the ARC Centre of Excellence for Engineered Quantum Systems (EQUS), said it was the first entanglement-based sensor with performance beyond the best possible existing technology.

“This breakthrough will spark all sorts of new technologies — from better navigation systems to better MRI machines, you name it,” Professor Bowen said.

“Entanglement is thought to lie at the heart of a quantum revolution. We’ve finally demonstrated that sensors that use it can supersede existing, non-quantum technology.

“This is exciting — it’s the first proof of the paradigm-changing potential of entanglement for sensing.”

Major Scientific Leap: Quantum Microscope Created That Can See the Impossible, University of Queensland

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Muon g-2...

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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

 

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Colloidal Quantum Dots...

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FIG. 1. (a) Schematic of La Mer and Dinegar's model for the synthesis of monodispersed CQDs. (b) Representation of the apparatus employed for CQD synthesis. Reproduced with permission from Murray et al., Annu. Rev. Mater Res. 30(1), 545–610 (2000). Copyright 2000 Annual Reviews.

Topics: Energy, Materials Science, Nanotechnology, Quantum Mechanics, Solar Power

ABSTRACT
Solution-processed colloidal quantum dot (CQD) solar cells are lightweight, flexible, inexpensive, and can be spray-coated on various substrates. However, their power conversion efficiency is still insufficient for commercial applications. To further boost CQD solar cell efficiency, researchers need to better understand and control how charge carriers and excitons transport in CQD thin films, i.e., the CQD solar cell electrical parameters including carrier lifetime, diffusion length, diffusivity, mobility, drift length, trap state density, and doping density. These parameters play key roles in determining CQD thin film thickness and surface passivation ligands in CQD solar cell fabrication processes. To characterize these CQD solar cell parameters, researchers have mostly used transient techniques, such as short-circuit current/open-circuit voltage decay, photoconductance decay, and time-resolved photoluminescence. These transient techniques based on the time-dependent excess carrier density decay generally exhibit an exponential profile, but they differ in the signal collection physics and can only be used in some particular scenarios. Furthermore, photovoltaic characterization techniques are moving from contact to non-contact, from steady-state to dynamic, and from small-spot testing to large-area imaging; what are the challenges, limitations, and prospects? To answer these questions, this Tutorial, in the context of CQD thin film and solar cell characterization, looks at trends in characterization technique development by comparing various conventional techniques in meeting research and/or industrial demands. For a good physical understanding of material properties, the basic physics of CQD materials and devices are reviewed first, followed by a detailed discussion of various characterization techniques and their suitability for CQD photovoltaic devices.

Advanced characterization methods of carrier transport in quantum dot photovoltaic solar cells, Lilei Hu, Andreas Mandelis, Journal of Applied Physics

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No Strings Attached...

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Image Source: Physicist finds loose thread of string theory puzzle, Cay Leytham-Powell, University of Colorado at Boulder, Phys.org

Topics: Einstein, General Relativity, Quantum Mechanics, String Theory

For decades, most physicists have agreed that string theory is the missing link between Einstein's theory of general relativity, describing the laws of nature at the largest scale, and quantum mechanics, describing them at the smallest scale. However, an international collaboration headed by Radboud physicists has now provided compelling evidence that string theory is not the only theory that could form the link. They demonstrated that it is possible to construct a theory of quantum gravity that obeys all fundamental laws of physics, without strings. They described their findings in Physical Review Letters last week.

When we observe gravity at work in our universe, such as the motion of planets or light passing close to a black hole, everything seems to follow the laws written down by Einstein in his theory of general relativity. On the other hand, quantum mechanics is a theory that describes the physical properties of nature at the smallest scale of atoms and subatomic particles. Though these two theories have allowed us to explain every fundamental physical phenomenon observed, they also contradict each other. As of today, physicists have severe difficulties to reconcile the two theories to explain gravity on both the largest and smallest scale.

Explaining gravity without string theory, Radboud University, Phys.org

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Quasiparticles, and Graphene...

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Telltale traces In this doping vs magnetic field conductance map, the magnetic field is varied along the vertical axis. Horizontal yellow streaks show Brown-Zak fermions propagating along straight trajectories with high mobility (low resistance), whereas slanted indigo lines show the cyclotron motion around Brown-Zak fermions. The slope of these lines enabled the researchers to obtain the degeneracy (and find an additional quantum number) of these new quasiparticles. (Courtesy: J Barrier)

Topics: Fermions, Graphene, Nanotechnology, Quantum Mechanics

Researchers at the University of Manchester in the UK have identified a new family of quasiparticles in superlattices made from graphene sandwiched between two slabs of boron nitride. The work is important for fundamental studies of condensed-matter physics and could also lead to the development of improved transistors capable of operating at higher frequencies.

In recent years, physicists and materials scientists have been studying ways to use the weak (van der Waals) coupling between atomically thin layers of different crystals to create new materials in which electronic properties can be manipulated without chemical doping. The most famous example is graphene (a sheet of carbon just one atom thick) encapsulated between another 2D material, hexagonal boron nitride (hBN), which has a similar lattice constant. Since both materials also have similar hexagonal structures, regular moiré patterns (or “superlattices”) form when the two lattices are overlaid.

If the stacked layers of graphene-hBN are then twisted, and the angle between the two materials’ lattices decreases, the size of the superlattice increases. This causes electronic band gaps to develop through the formation of additional Bloch bands in the superlattice’s Brillouin zone (a mathematical construct that describes the fundamental ideas of electronic energy bands). In these Bloch bands, electrons move in a periodic electric potential that matches the lattice and does not interact with one another.

New family of quasiparticles appears in graphene, Isabelle Dumé, Physics World

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Integrated Nanodiamonds...

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Nanophotonic integration for simultaneously controlling a large number of quantum mechanical spins in nanodiamonds. (Image: P. Schrinner/AG Schuck)

Topics: Nanotechnology, Quantum Computer, Quantum Mechanics, Semiconductor Technology

(Nanowerk News) Physicists at Münster University have succeeded in fully integrating nanodiamonds into nanophotonic circuits and at the same time addressing several of these nanodiamonds optically. The study creates the basis for future applications in the field of quantum sensing schemes or quantum information processors.

The results have been published in the journal Nano Letters ("Integration of Diamond-Based Quantum Emitters with Nanophotonic Circuits").

Using modern nanotechnology, it is possible nowadays to produce structures that have feature sizes of just a few nanometers.

This world of the most minute particles – also known as quantum systems – makes possible a wide range of technological applications, in fields which include magnetic field sensing, information processing, secure communication, or ultra-precise timekeeping. The production of these microscopically small structures has progressed so far that they reach dimensions below the wavelength of light.

In this way, it is possible to break down hitherto existent boundaries in optics and utilize the quantum properties of light. In other words, nanophotonics represents a novel approach to quantum technologies.

Controlling fully integrated nanodiamonds, Westfälische Wilhelms-Universität Münster

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Clocking Dark Matter...

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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

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Right-Handed Photons...

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

Note: A primer on quarks at Hyperphysics</a>

On 17 January 1957, a few months after Chien-Shiung Wu’s discovery of parity violation, Wolfgang Pauli wrote to Victor Weisskopf: “Ich glaube aber nicht, daß der Herrgott ein schwacher Linkshänder ist” (I cannot believe that God is a weak left-hander). But maximal parity violation is now well established within the Standard Model (SM). The weak interaction only couples to left-handed particles, as dramatically seen in the continuing absence of experimental evidence for right-handed neutrinos. In the same way, the polarisation of photons originating from transitions that involve weak interaction is expected to be completely left-handed.

The LHCb collaboration recently tested the handedness of photons emitted in rare flavor-changing transitions from a b-quark to an s-quark. These are mediated by the bosons of the weak interaction according to the SM – but what if new virtual particles contribute too? Their presence could be clearly signaled by a right-handed contribution to the photon polarization.

In pursuit of right-handed photons, A report from the LHCb experiment, CERN Courier

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Schrödinger’s Clock...

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Credit: Getty Images

Topics: Modern Physics, Quantum Mechanics, Theoretical Physics

Albert Einstein’s twin paradox is one of the most famous thought experiments in physics. It postulates that if you send one of two twins on a return trip to a star at near light speed, they will be younger than their identical sibling when they return home. The age difference is a consequence of something called time dilation, which is described by Einstein’s special theory of relativity: the faster you travel, the slower time appears to pass.

But what if we introduce quantum theory into the problem? Physicists Alexander Smith of Saint Anselm College and Dartmouth College and Mehdi Ahmadi of Santa Clara University tackle this idea in a study published today in the journal Nature Communications. The scientists imagine measuring a quantum atomic clock experiencing two different times while it is placed in superposition—a quirk of quantum mechanics in which something appears to exist in two places at once. “We know from Einstein’s special theory of relativity that when a clock moves relative to another clock, the time shown on it slows down,” Smith says. “But quantum mechanics allows you to start thinking about what happens if this clock were to move in a superposition of two different speeds.”

Superposition is a strange aspect of quantum physics where an object can initially be in multiple locations simultaneously, yet when it is observed, only one of those states becomes true. Particles can be placed in superposition in certain experiments, such as those using a beam splitter to divide photons of light, to show the phenomenon in action. Both of the particles in superposition appear to share information until they are observed, making the phenomenon useful for applications such as encryption and quantum communications.

Quantum Time Twist Offers a Way to Create Schrödinger’s Clock, Jonathan O'Callaghan, Scientific American

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