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  1. To protect the confidentiality of a message during its transmission, people encrypt it. However, noise in the transmission channels can be a source of concern regarding how faithful the message transmission may be after it has been decrypted. This is particularly important for secrets shared using quantum scale messengers. For example, a classical secret takes the shape of a string of zeros and ones, whereas a quantum secret is akin to an unknown quantum state of two entangled particles carrying the secret. This is because no two quantum particles can be in the same state at any given time. In a new study published in EPJ D, Chen-Ming Bai from Shaanxi Normal University, Xi'an, China, and colleagues calculate the degree of fidelity of the quantum secret once transmitted and explore how to avoid eavesdropping. What is exciting about quantum secrets is that they make it possible to share a secret among a number of participants. Yet, only certain participants can reconstruct the secret by collaborating. Creating a permission system to decide who can access the secret requires the development of a specific procedure. In this study, the authors provide a concrete example of how such an approach could work with three participants. Since noise in the transmission channel has a great influence on the quantum secret shared, the authors analyse the impacts of two kinds of noisy channels on sharing quantum secrets. Indeed, the quantum system inevitably interacts with the external environment to produce quantum noise, which leads to entanglement between the quantum state and the environment. In particular, the authors evaluate the consequences of quantum noise and the resulting degree of damping of the encrypted signal by examining its physical characteristics, like its amplitude. This helps determine how faithfully the secret has been transmitted to the receiver. Bai and colleagues find that the fidelity of the encrypted message's transmission improves depending on the quantum state in which the particles carrying the secret find themselves. The authors subsequently provide an optimised strategy to enhance fidelity in secret transmission. < Here >
  2. Experiments to confirm we can see single photons offer new ways to probe our understanding of quantum reality Paul Kwiat asks his volunteers to sit inside a small, dark room. As their eyes adjust to the lack of light, each volunteer props his or her head on a chin rest—as you would at an optometrist’s—and gazes with one eye at a dim red cross. On either side of the cross is an optical fiber, positioned to pipe a single photon of light at either the left or the right side of a volunteer’s eye. Even as he verifies the human eye’s ability to detect single photons, Kwiat, an experimental quantum physicist at the University of Illinois at Urbana–Champaign, and his colleagues are setting their sights higher: to use human vision to probe the very foundations of quantum mechanics, according to a paper they submitted to the preprint server arXiv on June 21. Rather than simply sending single photons toward a volunteer’s eye through either the left or the right fiber, the idea is to send photons in a quantum superposition of effectively traversing both fibers at once. Will humans see any difference? According to standard quantum mechanics, they will not—but such a test has never been done. If Kwiat’s team produces conclusive results showing otherwise, it would question our current understanding of the quantum world, opening the door to alternative theories that argue for a dramatically different view of nature in which reality exists regardless of observations or observers, cutting against the grain of how quantum mechanics is interpreted today. “It could possibly be evidence that something’s going on beyond standard quantum mechanics,” says Rebecca Holmes, Kwiat’s former student who designed the equipment, and who is now a researcher at the Los Alamos National Laboratory. The effort to determine whether humans can directly detect single photons has a storied history. In 1941 researchers from Columbia University reported in Science the human eye can see a flash from as few as five photons landing on the retina. More than three decades later Barbara Sakitt, a biophysicist then at the University of California, Berkeley, performed experiments suggesting that the eye could see a single photon. But these experiments were far from conclusive. “The problem with all these experiments is that they were just trying to use ‘classical’ light sources” that do not reliably emit single photons, Holmes says. That is, there was no guarantee each of these early trials involved just one photon. Then, in 2012, came firm evidence that individual photoreceptors, or rod cells, can detect single photons—at least in the eyes of a frog. Leonid Krivitsky of the Agency for Science, Technology and Research in Singapore and his colleagues extracted rod cells from adult frogs’ eyes and performed laboratory tests showing the cells reacted to single photons. Now, “there’s absolutely no doubt that individual photoreceptors respond to single photons,” Kwiat says. That is not the same as saying those rod cells do the same in a living frog—or, for that matter, a human being. So Kwiat, along with Illinois colleague physicist Anthony Leggett and others, began envisioning tests of human vision using single-photon sources. Soon Kwiat’s group, which now included Holmes, was actually experimenting. But “we got beat on that,” Holmes says. In 2016 a team led by biophysicist Alipasha Vaziri, then at the University of Vienna, reported using single-photon sources to show “humans can detect a single-photon incident on their eye with a probability significantly above chance.” Kwiat’s team, somewhat skeptical of the result, wants to improve the statistics by doing a much larger number of trials with many more subjects. Their key concern is the low efficiency of the eye as a photon detector. Any incident photon has to get past the cornea, the clear outer layer of the eye, which reflects some of the light. The photon then enters a lens that, together with the cornea, focuses the light on the retina at the back of the eye. But between the lens and the retina is a clear, gel-like substance that gives the eye its shape—and this too can absorb or scatter the photon. Effectively, less than 10 percent of the photons that hit the cornea make it to the rod cells in the retina, which result in nerve signals that travel into the brain, causing perception. So getting statistically significant results that rise above chance is a daunting challenge. “We are hoping in the next six months to have a definitive answer,” Kwiat says. That has not stopped them from dreaming up new experiments. In the standard setup a half-silvered mirror steers a photon to either the left or the right fiber. The photon then lands on one side or the other of a volunteer’s retina, and the subject has to indicate which by using a keyboard. But it is trivial (using quantum optics) to put the photon in a superposition of going through both fibers, and onto both sides of the eye, at once. What occurs next depends on what one believes happens to the photon. Physicists describe a photon’s quantum state using a mathematical abstraction called the wave function. Before the superposed photon hits the eye its wave function is spread out, and the photon has an equal probability of being seen on the left or the right. The photon’s interaction with the visual system acts as a measurement that is thought to “collapse” the wave function, and the photon randomly ends up on one side or the other, like a tossed coin coming up “tails” or “heads.” Would humans see a difference in the photon counts on the left versus the right when perceiving superposed photons as compared with photons in classical states? “If you trust quantum mechanics, then there should be no difference,” Kwiat says. But if their experiment finds an irrefutable, statistically significant difference, it would signal something amiss with quantum physics. “That would be a big. That would be a quite earth-shattering result,” he adds. Such a result would point toward a possible resolution of the central concern of quantum mechanics: the so-called measurement problem. There’s nothing in the theory that specifies how measurements can collapse the wave function, if indeed wave functions do collapse. How big should the measuring apparatus be? In the case of the eye, would an individual rod cell do? Or does one need the entire retina? What about the cornea? Might a conscious observer need to be in the mix? Some alternative theories solve this potential problem by invoking collapse independently of observers and measurement devices. Consider, for instance, the “GRW” collapse model (named after theorists Giancarlo Ghirardi, Alberto Rimini and Tullio Weber). The GRW model and its many variants posit wave functions collapse spontaneously; the more massive the object in superposition, the faster its collapse. One consequence of this would be that individual particles could remain in superposition for interminably long times whereas macroscopic objects could not. So, the infamous Schrödinger’s cat, in GRW, can never be in a superposition of being dead and alive. Rather it is always either dead or alive, and we only discover its state when we look. Such theories are said to be “observer-independent” models of reality[.] If a collapse theory such as GRW is the correct description of nature, it would upend almost a century of thought that has tried to argue observation and measurement are central to the making of reality. Crucially, when the superposed photon lands on an eye, GRW would predict ever-so-slightly different photon counts for the left and the right sides of the eye than does standard quantum mechanics. This is because differently sized systems in the various stages of the photon’s processing—such as two light-sensitive proteins in two rod cells versus two assemblies of rod cells and associated nerves in the retina—would exhibit different spontaneous collapse rates after interacting with a photon. Although both Kwiat and Holmes stress it is highly unlikely they will see a difference in their experiments, they acknowledge that any observed deviation would hint at GRW-like theories. Michael Hall, a theoretical quantum physicist at the Australian National University who was not part of the study, agrees GRW would predict a very small deviation in the photon counts, but says such deviations would be too tiny to be detected by the proposed experiment. Nevertheless, he thinks any aberration in the photon counts would deserve attention. “It would be quite serious. I find that unlikely but possible,” he says. “That would be amazingly interesting.” Kwiat also wonders about the subjective perception of quantum states versus classical states. “Is there any perceptual difference on the part of the person when they directly observe a quantum event?” he asks. “The answer is ‘probably not,’ but we really don’t know. You can’t know the answer to that unless either you have a complete physical model down to the quantum mechanical level of what’s going on in the human visual system—which we don’t have—or you do the experiment.” Robert Prevedel, a member of Vaziri’s 2016 team who is now at the European Molecular Biology Laboratory in Germany, is more interested in teasing out exactly where collapse actually occurs in the chain of events. Does it happen at the beginning, when a photon strikes a rod cell? Or in the middle, with generation and transmission of neural signals? Or does it happen at the end, when the signals register in conscious perception? He suggests firing superposed photons at extracted retinas and recording from different levels of visual processing (say, from rod cells or from the different types of photo cells that make up the retina) to see how long the superposition lasts. Prevedel thinks first absorption by a rod should destroy the photon’s superposition. But “if we can see quantum [superposition] in any of the subsequent levels inside the different cell layers in the retina, or any downstream neuronal circuits even, that would be really a breakthrough,” he says. “This would be an amazing finding.” There is, of course, an elephant in the room: human consciousness. Could conscious perception ultimately cause the collapse of the quantum state, making the photon show up on one or the other side? Prevedel doubts consciousness has anything whatsoever to do with measurement and collapse. “Consciousness…arises in our brain as the combined effect of millions, if not billions, of cells and neurons. If there is a role of consciousness in the detection of quantum superposition, it’d involve a really macroscopic object on the level of the entire brain, i.e. a huge ensemble of atoms and electrons that make up the biological cells,” Prevedel says. “From all that we know, this kind of macroscopic object would not be able to sustain quantum [superposition].” < Here >
  3. "Excitingly, this research direction will also enable us to test the fundamental limits of quantum mechanics," said researcher Michael Vanner. Scientists have built a tiny drum that both vibrates and remains still when struck by a drumstick made of light. The feat blurs the lines between the quantum world and the visible world defined by classical physics. In the world of quantum physics, an object can exist as both a particle and wave and can be in two places at once. It is a world filled with "spooky action." But quantum phenomena mostly go unseen -- the invisible tricks have to be measured by sensors. The latest research offers a magnifying glass with which scientists can watch and study quantum physics. "Such systems offer significant potential for the development of powerful new quantum-enhanced technologies, such as ultra-precise sensors, and new types of transducers," lead researcher Michael Vanner, who studies quantum physics at Imperial College London, said in a news release. "Excitingly, this research direction will also enable us to test the fundamental limits of quantum mechanics by observing how quantum superpositions behave at a large scale." For several years, scientists have been trying to generate quantum action on a tiny drum using a drumstick made of laser light. They made progress, but kept falling short of their goal. Using an unconventional approach, scientists in England and Australia finally made a breakthrough. They described their feat in the New Journal of Physics. "We adapted a trick from optical quantum computing to help us play the quantum drum. We used a measurement with single particles of light -- photons -- to tailor the properties of the drumstick," said Martin Ringbauer from the University of Queensland. "This provides a promising route to making a mechanical version of Schrodinger's cat, where the drum vibrates and stands still at the same time." The experiments allowed scientists to observe mechanical interferences fringes for the first time, and they could pave the way for a theory that bridges the gap between quantum world and the mechanical world. < Here >
  4. Surprisingly, the Schrödinger Equation — the fundamental equation of quantum mechanics — emerges while studying massive astronomical structures. Propagation of waves through an astrophysical disk can be understood using the Schrödinger Equation. Image credit: James Tuttle Keane, California Institute of Technology. Massive astronomical objects are frequently encircled by groups of smaller objects that revolve around them, like the planets around the Sun. For example, supermassive black holes are orbited by swarms of stars, which are themselves orbited by enormous amounts of rock, ice, and other space debris. Due to gravitational forces, these huge volumes of material form into flat, round disks. These disks, made up of countless individual particles orbiting en masse, can range from the size of our Solar System to many light-years across. Astrophysical disks of material generally do not retain simple circular shapes throughout their lifetimes. Instead, over millions of years, they slowly evolve to exhibit large-scale distortions, bending and warping like ripples on a pond. Exactly how these warps emerge and propagate has long puzzled astronomers, and even computer simulations have not offered a definitive answer, as the process is both complex and prohibitively expensive to model directly. Caltech planetary scientist Konstantin Batygin turned to an approximation scheme called perturbation theory to formulate a simple mathematical representation of disk evolution. This approximation, often used by astronomers, is based upon equations developed by mathematicians Joseph-Louis Lagrange and Pierre-Simon Laplace. Within the framework of these equations, the individual particles and pebbles on each particular orbital trajectory are mathematically smeared together. In this way, a disk can be modeled as a series of concentric wires that slowly exchange orbital angular momentum among one another. As an analogy, in the Solar System one can imagine breaking each planet into pieces and spreading those pieces around the orbit the planet takes around the Sun, such that the Sun is encircled by a collection of massive rings that interact gravitationally. The vibrations of these rings mirror the actual planetary orbital evolution that unfolds over millions of years, making the approximation quite accurate. Using this approximation to model disk evolution, however, had unexpected results. “When we do this with all the material in a disk, we can get more and more meticulous, representing the disk as an ever-larger number of ever-thinner wires,” Dr. Batygin said. “Eventually, you can approximate the number of wires in the disk to be infinite, which allows you to mathematically blur them together into a continuum. When I did this, astonishingly, the Schrödinger Equation emerged in my calculations.” The Schrödinger Equation is the foundation of quantum mechanics. It describes the non-intuitive behavior of systems at atomic and subatomic scales. One of these non-intuitive behaviors is that subatomic particles actually behave more like waves than like discrete particles — a phenomenon called wave-particle duality. This new work suggests that large-scale warps in astrophysical disks behave similarly to particles, and the propagation of warps within the disk material can be described by the same mathematics used to describe the behavior of a single quantum particle if it were bouncing back and forth between the inner and outer edges of the disk. The Schrödinger Equation is well studied, and finding that such a quintessential equation is able to describe the long-term evolution of astrophysical disks should be useful for scientists who model such large-scale phenomena. “Additionally, it is intriguing that two seemingly unrelated branches of physics can be governed by similar mathematics,” Dr. Batygin said. “This discovery is surprising because the Schrödinger Equation is an unlikely formula to arise when looking at distances on the order of light-years.” “The equations that are relevant to subatomic physics are generally not relevant to massive, astronomical phenomena. Thus, I was fascinated to find a situation in which an equation that is typically used only for very small systems also works in describing very large systems.” A paper reporting this work is published in the Monthly Notices of the Royal Astronomical Society. < Here >
  5. A team of researchers with Università degli Studi di Padova and the Matera Laser Ranging Observatory in Italy has conducted experiments that add credence to John Wheeler's quantum theory thought experiment. In their paper published on the open access site Science Advances, the group describes their experiment and what they believe it showed. The nature of light has proven to be one of the more difficult problems facing physicists. Nearly a century ago, experiments showed that light behaved like both a particle and a wave, but subsequent experiments seemed to show that light behaved differently depending on how it was tested, and weirdly, seemed to know how the researchers were testing it, changing its behavior as a result. Back in the late 1970s, physicist Johan Wheeler tossed around a thought experiment in which he asked what would happen if tests allowed researchers to change parameters after a photon was fired, but before it had reached a sensor for testing—would it somehow alter its behavior mid-course? He also considered the possibilities as light from a distant quasar made its way through space, being lensed by gravity. Was it possible that the light could somehow choose to behave as a wave or a particle depending on what scientists here on Earth did in trying to measure it? In this new effort, the team in Italy set out to demonstrate the ideas that Wheeler had proposed—but instead of measuring light from a quasar, they measured light bounced from a satellite back to Earth. The experiment consisted of shooting a laser beam at a beam splitter, which aimed the beam at a satellite traveling in low Earth orbit, which reflected it back to Earth. But as the light traveled back to Earth, the researchers had time to make a choice whether or not to activate a second beam splitter as the light was en route. Thus, they could test whether the light was able to sense what they were doing and respond accordingly. The team reports that the light behaved just as Wheeler had predicted—demonstrating either particle-like or wave-like behavior, depending on the behavior of those studying it. < Here >
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