From Nature: “Cosmic test backs ‘quantum spookiness'”

Nature

02 February 2017
Elizabeth Gibney

The light from distant stars is used to fix settings in a new version of the iconic Bell test. Dr Fred Espenak/Science Photo Library.

A version of an iconic experiment to confirm quantum theory has for the first time used the light of distant stars to bolster the case for a phenomenon that Albert Einstein referred to as “spooky action at a distance”.

Einstein disliked the notion that objects can share a mysterious connection across any distance of space, and scientists have spent the past 50 years trying to make sure that their results showing this quantum effect could not have been caused by more intuitive explanations.

Quantum physics suggests that two so-called entangled particles can maintain a special connection — even at a large distance — such that if one is measured, that instantly tells an experimenter what measuring the other particle will show. This happens despite the fact neither particle has definite properties until it is measured. That unsettled some physicists, including Einstein, who favoured an alternative explanation: that quantum theory is incomplete, and that the outcomes instead depend on some predetermined, but hidden, variables.

The latest effort to explore the phenomenon, to be published in Physical Review Letters on 7 February, uses light emitted by stars around 600 years ago to select which measurements to make in a quantum experiment known as a Bell test. In doing so, they narrow down the point in history when, if they exist, hidden variables could have influenced the experiment.

“It’s a beautiful experiment,” says Krister Shalm, a quantum physicist at the US National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland. Although few expected it to disprove quantum mechanics, such experiments “keep pushing alternative theories to be more and more contrived and ridiculous”, he says. Similar techniques could, in the future, help to protect against hackers who try to crack quantum-cryptography systems, he adds.

Closing loopholes

Physicists at the University of Vienna, along with colleagues in China, Germany and the United States, developed a new version of the Bell test — a protocol devised by the physicist John Bell in the 1960s to distinguish between two possible explanations for the seemingly strange behaviour of the quantum world.

The test involves performing independent measurements on separated pairs of entangled quantum particles. Bell showed that, statistically, correlations between the results, once above a certain threshold limit, could not be explained by particles having hidden properties. Instead the coordinated outcomes seem to be the result of measurements on one particle mysteriously fixing the properties of the other.

Although Bell tests have supported quantum theory many times, they include assumptions that leave wiggle room for non-quantum explanations, and physicists have been trying to close these ‘loopholes’ ever since.

In 2015, they sealed a major victory when three separate teams, including Shalm’s, succeeded in simultaneously closing two major possible loopholes, by showing that entanglement could not be an illusion created by any speed-of-light communication between particles, or an artefact of only detecting certain photons.

See the following:
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.250402
http://www.nature.com/nature/journal/v526/n7575/abs/nature15759.html
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.250401

Freedom of choice

But they left open another loophole — one that is more subtle, and impossible to fully close, says Andrew Friedman, an astronomer at the Massachusetts Institute of Technology in Cambridge, and a co-author on the latest paper. Bell tests also assume that experimenters have free choice over which measurements they perform on each of the pair of photons. But some unknown effect could be influencing both the particles and what tests are performed (either by affecting choice of measurement directly, or more plausibly, by restricting the options that are available), to produce correlations that give the illusion of entanglement.

To narrow this freedom-of-choice loophole, researchers have previously put 144 kilometres between the source of entangled particles and the random-number generator that they use to pick experimental settings.

See
http://www.pnas.org/content/107/46/19708

The distance between them means that if any unknown process influenced both set-ups, it would have to have done so at a point in time before the experiment. But this only rules out any influences in the microseconds before: the latest paper sought to push this time back dramatically, by using light from two distant stars to determine the experimental settings for each photon. “We outsource the choice to the Universe itself,” says Friedman.

The team, led by physicist Anton Zeilinger at the University of Vienna, picked which properties of the entangled photons to observe depending on whether its two telescopes detected incoming light as blue or red. The colour is decided when the light is emitted, and does not change during travel. This means that if some unknown effect, rather than quantum entanglement, explains the correlation, it would have to have been set in motion at least around 600 years ago, because the closest star is 575 light-years (176 parsecs) away, says Friedman, who hopes to eventually push back this limit to billions of years ago by doing the experiment with light from more distant quasars. Their results found a level of correlation that supports ‘action at a distance’.

Protection against hackers

Technically, the experiment is impressive, say Ronald Hanson, a quantum physicist at the Delft University of Technology in the Netherlands. But, unlike the loopholes closed in 2015, this one can never be fully closed; confining it to further in the past is only possible by making new assumptions — in this case, for example, by assuming that no one messed with the photons immediately before they hit the telescopes, he says.

Others argue that although, fundamentally, the loophole is never closable, such experiments are valuable because new theories necessarily become more improbable and contrived, or eventually, end up assuming that everything in the Universe was determined at the time of the Big Bang — a philosophical view that most physicists reject. Reworking experiments to reduce and make better assumptions is therefore worthwhile, says Shalm.

Such experiments also have practical value, argues Friedman, because if quantum mechanics turns out to be explained by a different underlying theory, that discovery could impact the security of technologies that rely on quantum theory, such as quantum encryption. And trying to close such loopholes is useful because minimizing the assumptions in an experiment serves to also beef up protection against hackers who might otherwise exploit them, says Shalm, whose team at the NIST is exploring whether Bell tests could be used in quantum cryptography.

Harnessing cosmic phenomena is not the only way physicists are ensuring the independence of their measurement settings. In November, teams from around the world took part in the Big Bell Test, which tapped 100,000 game-playing volunteers worldwide to create random sequences of 0s and 1s, which physicists used to fix their measurement settings.

Preliminary analysis indicates that in this case, most — and possibly even all — of the experiments yet again supported quantum mechanics, says Morgan Mitchell at the Institute of Photonic Sciences (ICFO) in Barcelona, Spain, which coordinated the event. “Sorry, Einstein,” he says.

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Nature is a weekly international journal publishing the finest peer-reviewed research in all fields of science and technology on the basis of its originality, importance, interdisciplinary interest, timeliness, accessibility, elegance and surprising conclusions. Nature also provides rapid, authoritative, insightful and arresting news and interpretation of topical and coming trends affecting science, scientists and the wider public.

From Hopkins and Rutgers: “Between two worlds: Exotic insulator may hold clue to key mystery of modern physics”

Johns Hopkins University

Rutgers University

Dec 6, 2016
Arthur Hirsch

Scientists experiment with material that straddles world of classical physics and hidden quantum realm

Experiments using laser light and pieces of gray material the size of fingernail clippings may offer clues to a fundamental scientific riddle: what is the relationship between the everyday world of classical physics and the hidden quantum realm that obeys entirely different rules?

N. Peter Armitage

“We found a particular material that is straddling these two regimes,” said N. Peter Armitage, an associate professor of physics at Johns Hopkins University who led the research for the paper just published in the journal Science. Six scientists from Johns Hopkins and Rutgers University were involved in the work on materials called topological insulators, which can conduct electricity on their atoms-thin surface, but not in their insides.

Topological insulators were predicted in the 1980s, first observed in 2007, and have been studied intensively since. Made from any number of hundreds of elements, these materials have the capacity to show quantum properties that usually appear only at the microscopic level, but here appear in a material visible to the naked eye.

The experiments reported in Science establish these materials as a distinct state of matter “that exhibits macroscopic quantum mechanical effects,” Armitage said. “Usually we think of quantum mechanics as a theory of small things, but in this system quantum mechanics is appearing on macroscopic length scales. The experiments are made possible by unique instrumentation developed in my laboratory.”

In the experiments reported in Science, the elements bismuth and selenium make up dark gray material samples—each a few millimeters long and of different thicknesses—that were hit with “THz” light beams that are invisible to the unaided eye. Researchers measured the reflected light as it moved through the material samples and found indicators of a quantum state of matter.

Specifically, they found that as the light was transmitted through the material, the wave rotated a specific amount, which is related to physical constants that are usually only measurable in atomic scale experiments. The amount matched predictions of what would be possible in this quantum state.

The results add to scientists’ understanding of topological insulators, but also may contribute to the larger subject that Armitage says is the central question of modern physics: what is the relationship between the macroscopic classical world, and the microscopic quantum world from which it arises?

Scientists since the early 20th century have struggled with the question of how one set of physical laws governing objects above a certain size can co-exist alongside a different set of laws governing the atomic and subatomic scale. How does classical mechanics emerge from quantum mechanics, and where is the threshold that divides the realms?

Those questions remain to be answered, but topological insulators could be part of the solution.

“It’s a piece of the puzzle,” said Armitage, who worked on the experiments along with Liang Wu, who was a graduate student at Johns Hopkins when the work was done; Maryam Salehi of the Rutgers University Department of Material Science and Engineering; and Nikesh Koirala, Jisoo Moon, and Sean Oh of the Rutgers University Department of Physics and Astronomy.

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Rutgers, The State University of New Jersey, is a leading national research university and the state’s preeminent, comprehensive public institution of higher education. Rutgers is dedicated to teaching that meets the highest standards of excellence; to conducting research that breaks new ground; and to providing services, solutions, and clinical care that help individuals and the local, national, and global communities where they live.

The Johns Hopkins University opened in 1876, with the inauguration of its first president, Daniel Coit Gilman. “What are we aiming at?” Gilman asked in his installation address. “The encouragement of research … and the advancement of individual scholars, who by their excellence will advance the sciences they pursue, and the society where they dwell.”

The mission laid out by Gilman remains the university’s mission today, summed up in a simple but powerful restatement of Gilman’s own words: “Knowledge for the world.”

What Gilman created was a research university, dedicated to advancing both students’ knowledge and the state of human knowledge through research and scholarship. Gilman believed that teaching and research are interdependent, that success in one depends on success in the other. A modern university, he believed, must do both well. The realization of Gilman’s philosophy at Johns Hopkins, and at other institutions that later attracted Johns Hopkins-trained scholars, revolutionized higher education in America, leading to the research university system as it exists today.

From phys.org: “‘Quantum’ bounds not so quantum after all”

phys.org

July 1, 2016
Lisa Zyga

Quantum bounds are numbers (such as 4, 6, and 2√2) that naturally appear in quantum experiments, similar to how the number π emerges in circles. But just as how π pops up in a wide variety of areas beyond circles, in a new study physicists have found that quantum bounds are not exclusive to quantum theory but also emerge in purely classical experiments. The results suggest that attempts to define quantumness should not be concerned with quantum bounds, since there is nothing inherently quantum about them.

Components of the classical experiment that produces the same bounds that quantum experiments do. Credit: Frustaglia et al. ©2016 American Physical Society

The physicists, Diego Frustaglia et al., at the University of Sevilla in Spain, have published a paper on the emergence of quantum bounds in classical experiments in a recent issue of Physical Review Letters.

Different experiments, same bounds

In their study, the researchers performed three classical experiments that correspond to three famous quantum experiments involving quantum bounds. These quantum experiments are a sequential version of the Bell inequality and two other related quantum inequalities, all of which are used to distinguish between quantum and classical phenomena.

In order to show that a system exhibits quantum effects, these experiments traditionally attempt to show that a system can violate a quantum inequality. The greater the violation, the more quantum the system. The maximum violation of a quantum inequality is the quantum bound. The quantum bounds arise from probability distributions in the experiments and are specific numbers—for instance, the Bell inequality has a quantum bound of 2√2 (approximately 2.82), which is known as Tsirelson’s bound. The other two inequalities addressed here have quantum bounds of 4 and 6. Both theoretically and experimentally, no violation of a quantum inequality has ever surpassed these bounds.

In the new study, the researchers showed that these same quantum bounds emerge in experiments in which classical waves travel along an ordinary transmission line. The researchers found that the probabilities originating from the detection of wave intensities at the end of the transmission line follow the same distribution as the probabilities of detecting violations of the quantum inequalities. Specifically, the classical experiments yield bounds of 2.78, 3.93, and 5.93 for the three analogous experiments. In all three cases, these values are actually slightly closer to their theoretical values mentioned above than the values obtained in quantum experiments are, providing strong evidence that both quantum and classical experiments produce the same bounds.

Interpreting the results

One of the many implications of the study is that it offers new insight into what it means to be quantum. By showing that quantum bounds are not unique to quantum theory, but are universal bounds, the findings show that ongoing attempts to define quantum theory should not focus on these bounds.

Instead, the results provide a clue for finding a true quantum feature by revealing an important difference between the way in which the classical and quantum systems produce the same bounds. While the classical systems require some kind of extra resource, such as memory, the quantum systems do not. So a complete description of quantum theory should explain how quantum systems can violate the same bounds that classical systems do, but without using extra resources.

As the researchers explain, this approach of investigating classical systems to better understand quantum mechanics tends to be the opposite of most research.

“We somehow reverted the strategy followed by the founders of quantum theory,” Frustaglia told Phys.org. “In the early times of quantum mechanics, microscopic systems were subject to an intense questioning naturally biased towards classical physics. The result was a set of oddities interpreted as the paradigmatic features of the quantum realm: the particle-wave duality (is it a particle or a wave?), the Schrödinger’s cat (is it dead or alive?), and the Heisenberg’s uncertainty principle (where and how fast is it?).

“As a consequence, it was soon understood that quantum systems should be interrogated in their own specific language, eventually provided by modern quantum theory. It is then pertinent to address the possibility of interrogating classical systems with questions inspired by quantum physics. This is what we did, indeed, finding that classical systems with an underlying wave mechanism answer these questions in the same way truly quantum systems do. But one has to choose your system carefully: one would not be able to make it by using plain balls, for instance.”

In the future, the physicists plan to investigate how the universal bounds might emerge in the first place.

“Our results show that the ‘quantum’ bounds are common to many physical theories,” said coauthor Adán Cabello at the University of Sevilla. “This suggests that the reason for these bounds is something very simple and arguably inherent to the kind of theories we are interested in: theories in which ‘measurements’ produce repeatable results which are not affected by some other measurements.

“Surprisingly, this simple idea singles out many ‘quantum’ bounds. When we adopt this perspective, what is really significant is the fact that these bounds are actually reachable in nature. This shows that no hypothetical physical principle is acting and leads us to the conjecture that one of the physical principles that singles out quantum theory is precisely that one: There is no principle determining the probabilities of the outcomes of these ‘measurements.’

“One plan is to prove that this simple idea is responsible for all quantum bounds. Another plan is to test whether it is really true that these bounds can be reached with quantum systems. So far, and only very recently, H. S. Poh et al. have confirmed the so-called Tsirelson bound, 2√2, with four significant digits, but there is absolutely no experimental evidence of whether we can ‘touch’ these bounds in other scenarios. Also, it would be great to derive quantum theory from the assumption that there are no laws of nature determining or limiting the probabilities of measurement outcomes, and that the whole machinery of the theory follows from the aesthetic preference in the way we define ‘measurements.'”

Finally, the physicists also plan to investigate potential applications, such as building quantum technologies with the help of classical systems.

“Although inefficient in the sense that they require more memory or space, classical systems are sometimes better to produce ‘quantum’ numbers than quantum systems themselves,” Frustaglia said. “In contrast to quantum systems, which are very sensitive to the environment, the wires in our experiment can be bent, moved, heated, etc., and the results are the same. This suggests a future in which quantum technologies are actually built using quantum systems plus classical systems imitating quantum systems. It also raises the question as to whether similar ‘quantum’ features with potential functionalities can emerge in other supports as complex networks of artificial or biological nature. An appropriate answer to this questions requires multidisciplinary efforts that we are presently considering.”

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From Physics Today: “Questions and answers with Joseph Conlon” String Theory

Physics Today

17 June 2016
Jermey N. A. Matthews

Joseph Conlon. NO image credit.

The apple didn’t fall far from the tree,” says University of Oxford theoretical physicist Joseph Conlon. The author of Why String Theory?, reviewed in this month’s issue of Physics Today, says that from an early age he was good at math—a critical skill for a string theorist—thanks to the influence of his father and uncle, both PhD mathematicians, and his mother, a physics teacher.

By age 18 Conlon had earned a bachelor’s degree in mathematics from the local University of Reading in the UK; he did it part-time, while still in secondary school. Conlon followed that up by obtaining his bachelor’s and PhD degrees in physics at the University of Cambridge. At Oxford, he now focuses on phenomenological applications of string theory to particle physics and cosmology. “One thing I certainly benefited from is that if you [pursue] a physics undergraduate degree, having already done a math undergraduate degree, then you don’t need to concentrate on the math; you can just concentrate on understanding the physics concepts,” says Conlon.

For those who would question string theory’s validity because it can’t be experimentally tested, Conlon “presents a set of compelling arguments for the value of string theory while acknowledging its weaknesses and open challenges,” writes Gary Shiu in his Physics Today review. “Like courtroom juries, readers are encouraged to draw their own logical conclusions.” Conlon is also a cocreator of the public outreach website http://whystringtheory.com, which aims to be “a layman’s journey to the frontiers of physics.”

Physics Today books editor Jermey Matthews and senior editor Steven Blau, a theoretical physicist by training, recently caught up with Conlon to discuss the book.

PT: Why did you write the book?

CONLON: It’s to answer the question I think lots of people are asking: Why are so many people working on string theory if this is something you can’t directly say is the true theory of the universe at the smallest possible scales?

PT: So how would you answer the question “Why string theory?” for a nonexpert?

CONLON: String theory has brought ideas and insights and results to so many different areas beyond its supposedly core area of quantum gravity. The analogy I use in the book is it’s like in a gold rush, you get rich by selling spades, rather than by finding nuggets. String theory has … been able to provide spades to lots of people across mathematics and theoretical physics in so many different topics. And this is why so many people are interested in it.

PT: What inspired you to study string theory?

CONLON: I guess it was a fairly natural thing for me to do, given my interests and inclinations at the time. When I was in Cambridge, I was training in particle theory, and I was trying to learn as much particle theory as I could. You take courses on quantum field theory, you take courses on the standard model, you take a course in string theory.

The reason I wanted to carry on with the PhD in string theory was the feeling that lots of the standard model was carved out and understood in the 1970s and 1980s. String theory seemed more like something where I could get in and feel it wasn’t already done by the generation that came before.

The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth

PT: Were you ever tempted by any of the other alternative approaches to quantum gravity like loop quantum gravity or dynamic causal histories?

CONLON: Not really. I was never really exposed to them. As an undergraduate, it wasn’t something I learned or particularly had the option of learning then. And I haven’t been particularly tempted since then. From quite early on in my work on string theory I’ve been more interested in connecting it to experiments and observation. It’s great that people work on the formal problems of quantum gravity, but it’s not really my style of physics.

PT: As you were writing the book, was there something that you were hoping to be able to convey but said, “this is just too tough a nut to crack”? Did you have to leave anything on the table?

CONLON: Yes. There was a series of results around 1995 that were very important, involving D-branes. I ended up covering this less than I thought I would. And it partly was because I felt it was hard to try and convey to a general reader what was important about them without just dropping into buzz words.

PT: And, conversely, is there anything that you were particularly proud you were able to get across in simple language?

CONLON: I guess you have to ask the readers that. There are things I learned about—for example, the monstrous moonshine [a mathematical theory involving symmetries and related to conformal field theories] is a topic which I learned more about in the process of writing the book. I enjoyed writing about that because I learned about it at a slightly more technical level. It was a discovery process for me, too.

PT: According to the Physics Today review, your book also touches on “the sociology of string theory.” Was that your intention?

CONLON: Yes. Science is always more interesting when it’s done by humans, rather than [being] just abstract results. There’s also [a danger] you can get in if you look at someone very big [successful] and you say, “Gosh, they’ve gotten all these fantastic results. I can never possibly be like them. I’ll never be smart enough.”

But people are good at different things. Even though you might not be able to get the results that person did, you’ve got skills that they don’t have. I tried to convey that there are many, many different ways of being a good theoretical physicist. And part of that was by talking about the sociology, the different types of people who do the subject and do it successfully.

PT: Was explaining string theory to the general public a particular itch you wanted to scratch, or are you interested in writing other popular books?

CONLON: A bit of both. I thought string theory was being misrepresented, particularly in the general press, that there was this [notion] that string theory primarily was a theory of quantum gravity. And so string theory would then … compete with other theories of quantum gravity. And this is something I wanted to argue against because most people who work on string theory don’t focus on quantum gravity. That was the itch I wanted to scratch.

The book was also a chance to kind of let go the other side of my brain [used to write research papers] … and just write freely.

PT: What is your next project?

CONLON: In the process of finishing the book, basically I stopped doing research for six to nine months. So for the next two or three years I just want to do research because I enjoy doing research. And then I think I would like to write another book. I don’t know yet what it would be on.

PT: What books are you currently reading?

CONLON: I’ve got two on the go. The longer one, which I’m about halfway through, is [Winston] Churchill’s series The Second World War (Houghton Mifflin, ca. 1948–ca. 1953). And then the sort of more easy reading is one by Apollo astronaut (and physicist) Walter Cunningham, The All-American Boys: An Insider’s Look at the U.S. Space Program (revised edition, iPicturebooks, 2010).

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From PI: “New Experiment Clarifies How The Universe Is Not Classical”

Perimeter Institute

June 17, 2016
Erin Bow

“This is a great example of what’s possible when Perimeter and IQC work together. We can start with these exciting, abstract ideas and convert them to things we can actually do in our labs.”
– Kevin Resch, Faculty member, Institute for Quantum Computing

From left to right: Matthew Pusey (Perimeter postdoctoral researcher), Kevin Resch (IQC and University of Waterloo faculty member), Robert Spekkens (Perimeter faculty member), and Michael Mazurek (University of Waterloo and IQC PhD student) interact in a quantum optics lab at the Institute for Quantum Computing. No image credit.

Theorists from Perimeter and experimentalists from the Institute for Quantum Computing have found a new way to test whether the universe is quantum, a test that will have widespread applicability: they’ve proven the failure of noncontextuality in the lab.
_______________________________________________________________________________________________________________________________________

What does it mean to say the world is quantum? It’s a surprisingly difficult question to answer, and most casual discussions on the point are heavy on the hand-waving, with references to cats in boxes.

If we are going to turn the quantum-ness of the universe to our advantage through technologies like quantum computing, our definition of what it means to be quantum – or, more broadly, what it means to be non-classical – needs to be more rigorous. That’s one of the aims of the field of quantum foundations, and the point of new joint research carried out by theorists at Perimeter and experimentalists at the University of Waterloo’s Institute for Quantum Computing (IQC).

“We need to make precise the notion of non-classicality,” says Robert Spekkens, a faculty member at Perimeter, who led the work from the theoretical side. “We need to find phenomena that defy classical explanation, and then subject those phenomena to direct experimental tests.”

One candidate for something that defies classical explanation is the failure of noncontextuality.

“You can think of noncontextuality as the ‘if it walks like a duck’ principle,” says Matthew Pusey, a postdoctoral researcher at Perimeter who also worked on the project.

As the saying has it, if something walks like a duck and quacks like a duck, it’s probably a duck. The principle of noncontextuality pushes that further, and says that if something walks like a duck and quacks like a duck and you can’t tell it apart from a duck in any experiment, not even in principle, then it must be a duck.

Though noncontextuality is not something we often think about, it is a feature one would expect to hold in experiments. Indeed, it’s so intuitive that it seems silly to say it aloud: if you can’t tell two things apart, even in principle, then they’re the same. Makes sense, right?

But in the quantum universe, it’s not quite true.

Under quantum theory, two preparations of a system can return identical results in every conceivable test. But researchers run into trouble when they try to define exactly what those systems are doing. It turns out that in quantum mechanics, any model that assigns the systems well-defined properties requires them to be different. That’s a violation of the principle of noncontextuality.

To understand what’s happening, imagine a yellow box that spits out a mix of polarized photons – half polarized horizontally and half polarized vertically. A different box – imagine it to be orange – spits out a different mix of photons, half polarized diagonally and half polarized anti-diagonally.

Now measure the polarization of the photons from the yellow box and of the photons from the orange box. You can measure any polarization property you like, as much as you like. Because of the way the probabilities add up, the statistics of any measurement performed on photons from the yellow box are going to be identical to the statistics of the same measurement performed on photons from the orange box. In each case, the average polarization is always zero.

“Those two kinds of boxes, according to quantum theory, cannot be distinguished,” says Spekkens. “All the measurements are going to see exactly the same thing.”

You might think, following the principle of noncontextuality, that since the yellow and orange boxes produce indistinguishable mixes of photons, they can be described by the same probability distributions. They walk like ducks, so you can describe them both as ducks. But as it turns out, that doesn’t work.

In a noncontextual world, the fact that the yellow-box photons and orange-box photons are indistinguishable would be explained in the natural way: by the fact that the probability distribution over properties are the same. But the quantum universe resists such explanations – it can be proven mathematically that those two mixtures of photons cannot be described by the same distribution of properties.

“So that’s the theoretical result,” says Spekkens. “If quantum theory is right, then we can’t have a noncontextual model.”

But can such a theoretical result be tested? Theorists from Perimeter and experimentalists from IQC set out to discover that very thing.

Kevin Resch, a faculty member at IQC and the Department of Physics and Astronomy at the University of Waterloo, as well as a Perimeter Affiliate, worked on the project from the experimental end in his lab.

“The original method of testing noncontextuality required two or more preparation procedures that give exactly the same statistics,” he says. “I would argue that that’s basically not possible, because no experiments are perfect. The method described in our paper allows contextuality tests to deal with these imperfections.”

While previous attempts to test for the predicted failure of noncontextuality have had to resort to assuming things like noiseless measurements that are not achievable in practice, the Perimeter and IQC teams wanted to avoid such unrealistic assumptions. They knew they couldn’t eliminate all error, so they designed an experiment that could make meaningful tests of noncontextuality even in the presence of error.

Pusey hit on a clever idea to fight statistical error with statistical inference. Ravi Kunjwal, a doctoral student at the Institute for Mathematical Sciences in Chennai, India, who was visiting at the time, helped define what a test of noncontextuality should look like operationally. Michael Mazurek, a doctoral student with Waterloo’s Department of Physics and Astronomy and IQC, built the experimental apparatus – single photon emitters and detectors, just as in the yellow-and-orange box example above – and ran the tests.

“The interesting part of the experiment is that it looks really simple on paper,” says Mazurek. “But it wasn’t simple in practice. The analysis that we did and the standards that we held ourselves to required us to really get on top of the small systematic errors that are present in every experiment. Characterizing those errors and compensating for them was quite challenging.”

At one point, Mazurek used half a roll of masking tape to keep optical fibres from moving around in response to tiny shifts in temperature. Nothing about this experiment was easy, and much of it can only be described with statistics and diagrams. But in the end, the team made it work.

The result: an experiment that definitively shows the failure of noncontextuality. Like the pioneering work on Bell’s theorem, this research clarifies what it means for the world to be non-classical, and confirms that non-classicality experimentally.

Importantly, and in contrast to previous tests of contextuality, this experiment renders its verdict without assuming any idealizations, such as noiseless measurements or statistics being exactly the same. This opens a new range of possibilities.

Researchers in several fields are working to find “quantum advantages” – that is, things we can do if we harness the quantum-ness of the world that would not be possible in the classical world. Examples include quantum cryptography and quantum computation. Such advantages are the beams and girders of any future quantum technology we might be able to build. Noncontextuality can help researchers understand these quantum advantages.

“We now know, for example, that for certain kinds of cryptographic tasks and computational tasks, the failure of noncontextuality is the resource,” says Spekkens.

In other words, contextuality is the steel out of which the beams and girders are made.

“This is a great example of what’s possible when Perimeter and IQC work together,” says Resch, Canada Research Chair in Optical Quantum Technologies. “We can start with these exciting, abstract ideas and convert them to things we can actually do in our labs.”

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About Perimeter

Perimeter Institute is a leading centre for scientific research, training and educational outreach in foundational theoretical physics. Founded in 1999 in Waterloo, Ontario, Canada, its mission is to advance our understanding of the universe at the most fundamental level, stimulating the breakthroughs that could transform our future. Perimeter also trains the next generation of physicists through innovative programs, and shares the excitement and wonder of science with students, teachers and the general public.

From WIRED: “New Evidence Could Overthrow the Standard View of Quantum Mechanics”

WIRED

05.21.16
Dan Falk

Olena Shmahalo/Quanta Magazine

Of the many counterintuitive features of quantum mechanics, perhaps the most challenging to our notions of common sense is that particles do not have locations until they are observed. This is exactly what the standard view of quantum mechanics, often called the Copenhagen interpretation, asks us to believe. Instead of the clear-cut positions and movements of Newtonian physics, we have a cloud of probabilities described by a mathematical structure known as a wave function. The wave function, meanwhile, evolves over time, its evolution governed by precise rules codified in something called the Schrödinger equation. The mathematics are clear enough; the actual whereabouts of particles, less so. Until a particle is observed, an act that causes the wave function to “collapse,” we can say nothing about its location. Albert Einstein, among others, objected to this idea. As his biographer Abraham Pais wrote: “We often discussed his notions on objective reality. I recall that during one walk Einstein suddenly stopped, turned to me and asked whether I really believed that the moon exists only when I look at it.”

But there’s another view—one that’s been around for almost a century—in which particles really do have precise positions at all times. This alternative view, known as pilot-wave theory or Bohmian mechanics, never became as popular as the Copenhagen view, in part because Bohmian mechanics implies that the world must be strange in other ways. In particular, a 1992 study claimed to crystalize certain bizarre consequences of Bohmian mechanics and in doing so deal it a fatal conceptual blow. The authors of that paper concluded that a particle following the laws of Bohmian mechanics would end up taking a trajectory that was so unphysical—even by the warped standards of quantum theory—that they described it as “surreal.”

Nearly a quarter-century later, a group of scientists has carried out an experiment in a Toronto laboratory that aims to test this idea. And if their results, first reported* earlier this year, hold up to scrutiny, the Bohmian view of quantum mechanics—less fuzzy but in some ways more strange than the traditional view—may be poised for a comeback.

Saving Particle Positions

Bohmian mechanics was worked out by Louis de Broglie in 1927 and again, independently, by David Bohm in 1952, who developed it further until his death in 1992. (It’s also sometimes called the de Broglie–Bohm theory.) As with the Copenhagen view, there’s a wave function governed by the Schrödinger equation. In addition, every particle has an actual, definite location, even when it’s not being observed. Changes in the positions of the particles are given by another equation, known as the “pilot wave” equation (or “guiding equation”). The theory is fully deterministic; if you know the initial state of a system, and you’ve got the wave function, you can calculate where each particle will end up.

That may sound like a throwback to classical mechanics, but there’s a crucial difference. Classical mechanics is purely “local”—stuff can affect other stuff only if it is adjacent to it (or via the influence of some kind of field, like an electric field, which can send impulses no faster than the speed of light). Quantum mechanics, in contrast, is inherently nonlocal. The best-known example of a nonlocal effect—one that Einstein himself considered, back in the 1930s—is when a pair of particles are connected in such a way that a measurement of one particle appears to affect the state of another, distant particle. The idea was ridiculed by Einstein as “spooky action at a distance.” But hundreds of experiments, beginning in the 1980s, have confirmed that this spooky action is a very real characteristic of our universe.

In the Bohmian view, nonlocality is even more conspicuous. The trajectory of any one particle depends on what all the other particles described by the same wave function are doing. And, critically, the wave function has no geographic limits; it might, in principle, span the entire universe. Which means that the universe is weirdly interdependent, even across vast stretches of space. The wave function “combines—or binds—distant particles into a single irreducible reality,” as Sheldon Goldstein, a mathematician and physicist at Rutgers University, has written.

The differences between Bohm and Copenhagen become clear when we look at the classic “double slit” experiment, in which particles (let’s say electrons) pass through a pair of narrow slits, eventually reaching a screen where each particle can be recorded. When the experiment is carried out, the electrons behave like waves, creating on the screen a particular pattern called an “interference pattern.” Remarkably, this pattern gradually emerges even if the electrons are sent one at a time, suggesting that each electron passes through both slits simultaneously.

Those who embrace the Copenhagen view have come to live with this state of affairs—after all, it’s meaningless to speak of a particle’s position until we measure it. Some physicists are drawn instead to the Many Worlds interpretation of quantum mechanics, in which observers in some universes see the electron go through the left slit, while those in other universes see it go through the right slit—which is fine, if you’re comfortable with an infinite array of unseen universes.

By comparison, the Bohmian view sounds rather tame: The electrons act like actual particles, their velocities at any moment fully determined by the pilot wave, which in turn depends on the wave function. In this view, each electron is like a surfer: It occupies a particular place at every specific moment in time, yet its motion is dictated by the motion of a spread-out wave. Although each electron takes a fully determined path through just one slit, the pilot wave passes through both slits. The end result exactly matches the pattern one sees in standard quantum mechanics.

Lucy Reading-Ikkanda for Quanta Magazine

For some theorists, the Bohmian interpretation holds an irresistible appeal. “All you have to do to make sense of quantum mechanics is to say to yourself: When we talk about particles, we really mean particles. Then all the problems go away,” said Goldstein. “Things have positions. They are somewhere. If you take that idea seriously, you’re led almost immediately to Bohm. It’s a far simpler version of quantum mechanics than what you find in the textbooks.” Howard Wiseman, a physicist at Griffith University in Brisbane, Australia, said that the Bohmian view “gives you a pretty straightforward account of how the world is…. You don’t have to tie yourself into any sort of philosophical knots to say how things really are.”

But not everyone feels that way, and over the years the Bohm view has struggled to gain acceptance, trailing behind Copenhagen and, these days, behind Many Worlds as well. A significant blow came with the paper known as “ESSW,”** an acronym built from the names of its four authors. The ESSW paper claimed that particles can’t follow simple Bohmian trajectories as they traverse the double-slit experiment. Suppose that someone placed a detector next to each slit, argued ESSW, recording which particle passed through which slit. ESSW showed that a photon could pass through the left slit and yet, in the Bohmian view, still end up being recorded as having passed through the right slit. This seemed impossible; the photons were deemed to follow “surreal” trajectories, as the ESSW paper put it.

The ESSW argument “was a striking philosophical objection” to the Bohmian view, said Aephraim Steinberg, a physicist at the University of Toronto. “It damaged my love for Bohmian mechanics.”

But Steinberg has found a way to rekindle that love. In a paper published*** in Science Advances, Steinberg and his colleagues—the team includes Wiseman, in Australia, as well as five other Canadian researchers—describe what happened when they actually performed the ESSW experiment. They found that the photon trajectories aren’t surrealistic after all—or, more precisely, that the paths may seem surrealistic, but only if one fails to take into account the nonlocality inherent in Bohm’s theory.

The experiment that Steinberg and his team conducted was analogous to the standard two-slit experiment. They used photons rather than electrons, and instead of sending those photons through a pair of slits, they passed through a beam splitter, a device that directs a photon along one of two paths, depending on the photon’s polarization. The photons eventually reach a single-photon camera (equivalent to the screen in the traditional experiment) that records their final position. The question “Which of two slits did the particle pass through?” becomes “Which of two paths did the photon take?”

Importantly, the researchers used pairs of entangled photons rather than individual photons. As a result, they could interrogate one photon to gain information about the other. When the first photon passes through the beam splitter, the second photon “knows” which path the first one took. The team could then use information from the second photon to track the first photon’s path. Each indirect measurement yields only an approximate value, but the scientists could average large numbers of measurements to reconstruct the trajectory of the first photon.

The team found that the photon paths do indeed appear to be surreal, just as ESSW predicted: A photon would sometimes strike one side of the screen, even though the polarization of the entangled partner said that the photon took the other route.

But can the information from the second photon be trusted? Crucially, Steinberg and his colleagues found that the answer to the question “Which path did the first photon take?” depends on when it is asked.

At first—in the moments immediately after the first photon passes through the beam splitter—the second photon is very strongly correlated with the first photon’s path. “As one particle goes through the slit, the probe [the second photon] has a perfectly accurate memory of which slit it went through,” Steinberg explained.

But the farther the first photon travels, the less reliable the second photon’s report becomes. The reason is nonlocality. Because the two photons are entangled, the path that the first photon takes will affect the polarization of the second photon. By the time the first photon reaches the screen, the second photon’s polarization is equally likely to be oriented one way as the other—thus giving it “no opinion,” so to speak, as to whether the first photon took the first route or the second (the equivalent of knowing which of the two slits it went through).

The problem isn’t that Bohm trajectories are surreal, said Steinberg. The problem is that the second photon says that Bohm trajectories are surreal—and, thanks to nonlocality, its report is not to be trusted. “There’s no real contradiction in there,” said Steinberg. “You just have to always bear in mind the nonlocality, or you miss something very important.”

Faster Than Light

Some physicists, unperturbed by ESSW, have embraced the Bohmian view all along and aren’t particularly surprised by what Steinberg and his team found. There have been many attacks on the Bohmian view over the years, and “they all fizzled out because they had misunderstood what the Bohm approach was actually claiming,” said Basil Hiley, a physicist at Birkbeck, University of London (formerly Birkbeck College), who collaborated with Bohm on his last book, The Undivided Universe. Owen Maroney, a physicist at the University of Oxford who was a student of Hiley’s, described ESSW as “a terrible argument” that “did not present a novel challenge to de Broglie–Bohm.” Not surprisingly, Maroney is excited by Steinberg’s experimental results, which seem to support the view he’s held all along. “It’s a very interesting experiment,” he said. “It gives a motivation for taking de Broglie–Bohm seriously.”

On the other side of the Bohmian divide, Berthold-Georg Englert, one of the authors of ESSW (along with Marlan Scully, George Süssman and Herbert Walther), still describes their paper as a “fatal blow” to the Bohmian view. According to Englert, now at the National University of Singapore, the Bohm trajectories exist as mathematical objects but “lack physical meaning.”

On a historical note, Einstein lived just long enough to hear about Bohm’s revival of de Broglie’s proposal—and he wasn’t impressed, dismissing it as too simplistic to be correct. In a letter to physicist Max Born, in the spring of 1952, Einstein weighed in on Bohm’s work:

“Have you noticed that Bohm believes (as de Broglie did, by the way, 25 years ago) that he is able to interpret the quantum theory in deterministic terms? That way seems too cheap to me. But you, of course, can judge this better than I.”

But even for those who embrace the Bohmian view, with its clearly defined particles moving along precise paths, questions remain. Topping the list is an apparent tension with special relativity, which prohibits faster-than-light communication. Of course, as physicists have long noted, nonlocality of the sort associated with quantum entanglement does not allow for faster-than-light signaling (thus incurring no risk of the grandfather paradox or other violations of causality). Even so, many physicists feel that more clarification is needed, especially given the prominent role of nonlocality in the Bohmian view. The apparent dependence of what happens here on what may be happening there cries out for an explanation.

“The universe seems to like talking to itself faster than the speed of light,” said Steinberg. “I could understand a universe where nothing can go faster than light, but a universe where the internal workings operate faster than light, and yet we’re forbidden from ever making use of that at the macroscopic level—it’s very hard to understand.”

*Science paper:
Experimental nonlocal and surreal Bohmian trajectories

**Science paper:
Surrealistic Bohm Trajectories

See the full article here .

Please help promote STEM in your local schools.

Stem Education Coalition

From Nature: “Physics: Material to meaning” Book Review

Nature

Published online:
04 May 2016
Robert P. Crease

Robert P. Crease assesses Sean Carroll’s attempt to construct morality out of quantum field theory.

I don’t think I have ever read anything with a bigger ambition than The Big Picture, physicist Sean Carroll’s latest book.

The Big Picture: On the Origins of Life, Meaning, and the Universe Itself
Sean Carroll Dutton: 2016. ISBN: 9780525954828

Physics, Carroll writes, gives us a complete picture of the foundations of nature. Although that view has had an enormous impact on cosmology, materials science and other scientific fields, its implications for meaning and morality have yet to be determined. “Our values,” writes Carroll, “have not yet caught up to our best ontology.” In this book, he conducts a quest to catch up.

Carroll creates his big picture as follows. Quantum field theory provides a unified perspective on the subatomic realm. Carroll calls that the “Core Theory”, noting that its behaviour is fully captured by a formula called a Feynman path integral. Some features of the macro world can be directly tethered to it; others, including many concepts of thermodynamics, cannot. He calls these “emergent” features, ways of talking about the world that are not incompatible with Core Theory, yet cannot be grounded in it.

A bubble-chamber image showing the decay of a positive kaon particle. CERN.

In the fun parts of The Big Picture, Carroll demonstrates the absurdity of adding to the Core Theory to explain the possibility of things such as an afterlife or a transcendent underlying purpose. These are easy targets. The narrative begins to get awkward when it comes to, say, conscious experiences. These, Carroll writes, are “not part of the fundamental architecture of reality”; they are emergent, a handy way of talking about what brains do. Like entropy, he argues, consciousness is a concept that “we invent to give ourselves more useful and efficient descriptions of the world”. He calls his approach “poetic naturalism”. By using “poetic”, he means to give his blessing to ways of describing the world other than through fundamental physics — ways that, he says, can be meaningful if they are useful and don’t violate the Core Theory.

Carroll has a fluid, often engaging style, and the passages that explain science — including his appendix about the Feynman path integral — are excellent. The book brims, however, with avuncular clichés such as “Life is short, and certainty never happens”. Carroll confidently defines many concepts, including belief and consciousness, as if 2,500 years of philosophy have yielded little relevant to the subject; he dismisses the task of drawing careful distinctions and heeding subtleties as “ontologically fastidious”. All he finds in philosophical literature are a few interesting puzzles. It’s like getting a whirlwind tour of a city from a tour guide who doesn’t live there, but enthusiastically gives you capsule descriptions of favourite sites.

It is hardly surprising, therefore, that Carroll’s philosophical conclusions sound profound but leave us with disappointingly empty propositions, such as, “Morality exists only insofar as we make it so, and other people might not pass judgments in the same way that we do.” Outlining his own moral approach, Carroll offers a poetic naturalist’s version of the Ten Commandments, the “Ten Considerations”: greetings-card-like homilies such as “It Takes All Kinds”.

What’s fascinating about The Big Picture is that Carroll’s clarity and directness make its fundamental assumptions easy to spot, and whether you like this book will depend on whether you share them. Laboratories, as Carroll well knows, are workshops, controlled environments with unusual equipment, regulated conditions and specially trained workers. He writes from the perspective of such a worker who has come to believe that a mathematical physicist’s way of thinking is just how people think — or should think — about everything, even when they are not in a workshop or when they ponder values or the existence of God. Carroll describes deciding how to be morally good, for instance, as similar to a dinner-table conversation in which, like scientists collaborating, we “talk to others about their desires and how we can work together, and reason about how to make it happen”. Our group, he adds, “may include both vegetarians and omnivores, but with a good-faith effort”, universal satisfaction should result.

Reality, too, is just what things look like from a physicist’s perspective — and if it looks different to others, that is an illusion. When Carroll discusses time, he means the quantity that scientists measure. Everyday experience leads us to think that time flows in one direction, but he assures us that “in reality, both directions of time are created equal”. The ontologically fastidious would say, “Not so fast!” Time as lived by humans is something else again. Both outside and even inside workshops, to be bored or expectant, to hear a melody or to plan and execute an action is not to register one moment after another, but to retain previous ones and anticipate the next in an asymmetrical flow. Determining time in the workshop is an elaborate process, and assumes that you can mark it off as you can space, and then measure the spatial movement of something, whether it is the motions of heavenly bodies in ancient times or electronic transitions in caesium atoms in ours. Yet according to Carroll, this is real time.

If we accept the strict ontology of the workshop, as Carroll does, then we get his big picture and regard lived time, conscious experience and the rest of pre-workshop life as poetic and emergent. But there are broader ontologies in which the same things — which belong to the world described by the humanities and branches of biology, for instance — are regarded as fundamental, and as the driving force for workshop activity. Carroll’s is a naturalistic metaphysics.

Carroll brings tremendous passion to his writing. He is sure that honest human beings who care about the world make an effort to understand it as he does. He is right that science springs from certain basic human impulses to achieve goals and ward off threats. But where do his passion and certainty about this come from? They, too, are imported from and continue to be rooted in pre-workshop life. To find a way to talk about how scientific workshops emerge from life rather than the other way around — that would be a big picture indeed.

See the full article here .

Please help promote STEM in your local schools.

Stem Education Coalition

Nature is a weekly international journal publishing the finest peer-reviewed research in all fields of science and technology on the basis of its originality, importance, interdisciplinary interest, timeliness, accessibility, elegance and surprising conclusions. Nature also provides rapid, authoritative, insightful and arresting news and interpretation of topical and coming trends affecting science, scientists and the wider public.

From Ethan Siegel: “Ask Ethan: Can we use quantum entanglement to communicate faster-than-light?”

Starts with a Bang

5.7.16
Ethan Siegel

NanoSail-D poses after a successful laboratory deployment test NASA

Einstein called it spooky, but if we figure it out right, can we learn about distant star systems instantaneously?

“Trying to understand the way nature works involves a most terrible test of human reasoning ability. It involves subtle trickery, beautiful tightropes of logic on which one has to walk in order not to make a mistake in predicting what will happen.”

-Richard Feynman

Earlier this month, billionaire Yuri Milner and astrophysicist Stephen Hawking teamed up to announce the Breakthrough Starshot, an incredibly ambitious plan to send the first human-created spacecraft to other star systems within our galaxy. While a giant laser array could, feasibly, launch a low mass, microchip-sized spaceship towards another star at some ~20% the speed of light, it’s unclear how such an underpowered, small device like that would ever communicate across the vastness of interstellar space. But Olivier Manuel had an idea that he submitted for Ask Ethan:

It’s a long shot, but could quantum entanglement be used for communication?

Imagine you have two coins, where each one can turn up either heads or tails. You have one and I have one, and we’re located extremely far away from each other. We each toss them up in the air, catch them, and slap them down on the table. When we reveal the flip, we fully expect that there’s a 50/50 chance that each one of us will uncover a “heads” result and a 50/50 shot we’ll each get a “tails.” In the normal, unentangled Universe, your results and my results are completely independent of one another: if you get a “heads” result, there’s still a 50/50 shot for my coin to either display “heads” or “tails.” But under some circumstances, these results could be entangled, meaning that if we do this experiment and you get a “heads” result, you’ll know with 100% certainty that my coin is displaying “tails,” even before I told you. You’d know it instantaneously, even if we were separated by light years and not even a single second had passed.

The quantum mechanical Bell test for half-integer spin particles. Image credit: Wikimedia Commons user Maksim, under a c.c.a.-s.a.-3.0 license.

In quantum physics, we normally entangle not coins but individual particles like electrons or photons, where, for example, each photon can have a spin of either +1 or -1. If you measure the spin of one of them, you instantaneously know the spin of the other, even if it’s halfway across the Universe. Until you measure the spin of either one, they both exist in an indeterminate state; but once you measure even one, you immediately know both. We’ve done an experiment on Earth where we’ve separated two entangled photons by many miles, measuring their spins within nanoseconds of one another. What we find is that if we measure one of them to be +1, we know the other to be -1 at least 10,000 times faster than the speed of light would enable us to communicate.

A quantum optics setup. Image credit Matthew Broome

So now to Olivier’s question: could we use this property — quantum entanglement — to communicate from a distant star system to our own? The answer to that is yes, if you consider making a measurement at a distant location a form of communication. But when you say communicate, typically you want to know something about your destination. You could, for example, keep an entangled particle in an indeterminate state, send it aboard a spacecraft bound for the nearest star, and tell it to look for signs of a rocky planet in that star’s habitable zone. If you see one, make a measurement that forces the particle you have to be in the +1 state, and if you don’t see one, make a measurement that forces the particle you have to be in the -1 state.

Artist’s impression of a sunset from the world Gliese 667 Cc, in a trinary star system. Image credit: ESO/L. Calçada.

Therefore, you reason, the particle you have back on Earth will then either be in the -1 state when you measure it, telling you that your spacecraft found a rocky planet in the habitable zone, or it will be in the +1 state, telling you that it didn’t find one. If you know the measurement has been made, you should then be able to make your own measurement, and instantly know the state of the other particle, even if it’s many light years away.

The wave pattern for electrons passing through a double slit. If you measure “which slit” the electron goes through, you destroy the quantum interference pattern shown here. Image credit: Dr. Tonomura and Belsazar of Wikimedia Commons, under c.c.a.-s.a.-3.0.

It’s a brilliant plan, but there’s a problem: entanglement only works if you ask a particle, “what state are you in?” If you force an entangled particle into a particular state, you break the entanglement, and the measurement you make on Earth is completely independent of the measurement at the distant star. If you had simply measured the distant particle to be +1 or -1, then your measurement, here on Earth, of either -1 or +1 (respectively) would give you information about the particle located light years away. But by forcing that distant particle to be +1 or -1, that means, no matter the outcome, your particle here on Earth has a 50/50 shot of being +1 or -1, with no bearing on the particle so many light years distant.

A quantum eraser experiment setup, where two entangled particles are separated and measured. No alterations of one particle at its destination affect the outcome of the other. Image credit: Wikimedia Commons user Patrick Edwin Moran, under c.c.a.-s.a.-3.0.

This is one of the most confusing things about quantum physics: entanglement can be used to gain information about a component of a system when you know the full state and make a measurement of the other component(s), but not to create-and-send information from one part of an entangled system to the other. As clever of an idea as this is, Olivier, there’s still no faster-than-light communication.

Quantum teleportation, an effect (erroneously) touted as faster-than-light travel. In reality, no information is being exchanged faster than light. Image credit: American Physical Society, via http://www.csm.ornl.gov/SC99/Qwall.html.

Quantum entanglement is a wonderful property that we can exploit for any number of purposes, such as for the ultimate lock-and-key security system. But faster-than-light communication? Understanding why that’s not possible requires us to understand this key property of quantum physics: that forcing even part of an entangled system into one state or another doesn’t allow you to gain information about that forcing from measuring the remainder of the system. As Niels Bohr once famously put it:

If quantum mechanics hasn’t profoundly shocked you, you haven’t understood it yet.

The Universe plays dice with us all the time, much to Einstein’s chagrin. But even our best attempts to cheat at the game are thwarted by nature itself. If only all referees and umpires were as consistent as the laws of quantum physics!

See the full article here .

Please help promote STEM in your local schools.

Stem Education Coalition

“Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

From Science Alert: “Reality doesn’t exist until we measure it, quantum experiment confirms”

Science Alert

1 JUN 2015 [this cool article just appeared or re-appeared in social media.]
FIONA MACDONALD

Australian scientists have recreated a famous experiment and confirmed quantum physics’s bizarre predictions about the nature of reality, by proving that reality doesn’t actually exist until we measure it – at least, not on the very small scale.

That all sounds a little mind-meltingly complex, but the experiment poses a pretty simple question: if you have an object that can either act like a particle or a wave, at what point does that object ‘decide’?

Our general logic would assume that the object is either wave-like or particle-like by its very nature, and our measurements will have nothing to do with the answer. But quantum theory predicts that the result all depends on how the object is measured at the end of its journey. And that’s exactly what a team from the Australian National University has now found.

“It proves that measurement is everything. At the quantum level, reality does not exist if you are not looking at it,” lead researcher and physicist Andrew Truscott said in a press release.

Known as John Wheeler’s delayed-choice thought experiment, the experiment was first proposed back in 1978 using light beams bounced by mirrors, but back then, the technology needed was pretty much impossible. Now, almost 40 years later, the Australian team has managed to recreate the experiment using helium atoms scattered by laser light.

“Quantum physics predictions about interference seem odd enough when applied to light, which seems more like a wave, but to have done the experiment with atoms, which are complicated things that have mass and interact with electric fields and so on, adds to the weirdness,” said Roman Khakimov, a PhD student who worked on the experiment.

To successfully recreate the experiment, the team trapped a bunch of helium atoms in a suspended state known as a Bose-Einstein condensate, and then ejected them all until there was only a single atom left.

Bose-Einstein-condensates making waves a many-particle phenomenon

This chosen atom was then dropped through a pair of laser beams, which made a grating pattern that acted as a crossroads that would scatter the path of the atom, much like a solid grating would scatter light.

They then randomly added a second grating that recombined the paths, but only after the atom had already passed the first grating.

When this second grating was added, it led to constructive or destructive interference, which is what you’d expect if the atom had travelled both paths, like a wave would. But when the second grating was not added, no interference was observed, as if the atom chose only one path.

The fact that this second grating was only added after the atom passed through the first crossroads suggests that the atom hadn’t yet determined its nature before being measured a second time.

So if you believe that the atom did take a particular path or paths at the first crossroad, this means that a future measurement was affecting the atom’s path, explained Truscott. “The atoms did not travel from A to B. It was only when they were measured at the end of the journey that their wave-like or particle-like behaviour was brought into existence,” he said.

Although this all sounds incredibly weird, it’s actually just a validation for the quantum theory that already governs the world of the very small. Using this theory, we’ve managed to develop things like LEDs, lasers and computer chips, but up until now, it’s been hard to confirm that it actually works with a lovely, pure demonstration such as this one.

The full results* have been published in Nature Physics.

*Science paper:
Wheeler’s delayed-choice gedanken experiment with a single atom

See the full article here .

Please help promote STEM in your local schools.

Stem Education Coalition

From Ethan Siegel: “Can We Use Quantum Entanglement To Communicate Faster-Than-Light?”

Starts with a Bang

4.30.16
Ethan Siegel

The concept art of a solar sail (Japan’s IKAROS project) at a distant planet or star system. Image credit: Andrzej Mirecki of Wikimedia Commons, under a c.c.a.-s.a.-3.0 license.

Earlier this month, billionaire Yuri Milner and astrophysicist Stephen Hawking teamed up to announce the Breakthrough Starshot, an incredibly ambitious plan to send the first human-created spacecraft to other star systems within our galaxy. While a giant laser array could, feasibly, launch a low mass, microchip-sized spaceship towards another star at some ~20% the speed of light, it’s unclear how such an underpowered, small device like that would ever communicate across the vastness of interstellar space. But Olivier Manuel had an idea that he submitted for Ask Ethan:

It’s a long shot, but could quantum entanglement be used for communication?

It’s certainly worth considering. Let’s take a look at the idea.

Two coins: one showing heads and the other showing tails. Image credit: United States Mint, public domain.

Imagine you have two coins, where each one can turn up either heads or tails. You have one and I have one, and we’re located extremely far away from each other. We each toss them up in the air, catch them, and slap them down on the table. When we reveal the flip, we fully expect that there’s a 50/50 chance that each one of us will uncover a “heads” result and a 50/50 shot we’ll each get a “tails.” In the normal, unentangled Universe, your results and my results are completely independent of one another: if you get a “heads” result, there’s still a 50/50 shot for my coin to either display “heads” or “tails.” But under some circumstances, these results could be entangled, meaning that if we do this experiment and you get a “heads” result, you’ll know with 100% certainty that my coin is displaying “tails,” even before I told you. You’d know it instantaneously, even if we were separated by light years and not even a single second had passed.

The quantum mechanical Bell test for half-integer spin particles. Image credit: Wikimedia Commons user Maksim, under a c.c.a.-s.a.-3.0 license.

In quantum physics, we normally entangle not coins but individual particles like electrons or photons, where, for example, each photon can have a spin of either +1 or -1. If you measure the spin of one of them, you instantaneously know the spin of the other, even if it’s halfway across the Universe. Until you measure the spin of either one, they both exist in an indeterminate state; but once you measure even one, you immediately know both. We’ve done an experiment on Earth where we’ve separated two entangled photons by many miles, measuring their spins within nanoseconds of one another. What we find is that if we measure one of them to be +1, we know the other to be -1 at least 10,000 times faster than the speed of light would enable us to communicate.

By creating two entangled photons from a pre-existing system and separating them by great distances, we can know information about the state of one by measuring the state of the other. Image credit: Melissa Meister, of laser photons through a beam splitter, under c.c.-by-2.0 generic, from https://www.flickr.com/photos/mmeister/3794835939.

So now to Olivier’s question: could we use this property — quantum entanglement — to communicate from a distant star system to our own? The answer to that is yes, if you consider making a measurement at a distant location a form of communication. But when you say communicate, typically you want to know something about your destination. You could, for example, keep an entangled particle in an indeterminate state, send it aboard a spacecraft bound for the nearest star, and tell it to look for signs of a rocky planet in that star’s habitable zone. If you see one, make a measurement that forces the particle you have to be in the +1 state, and if you don’t see one, make a measurement that forces the particle you have to be in the -1 state.

Artist’s impression of a sunset from the world Gliese 667 Cc, in a trinary star system. Image credit: ESO/L. Calçada.

Therefore, you reason, the particle you have back on Earth will then either be in the -1 state when you measure it, telling you that your spacecraft found a rocky planet in the habitable zone, or it will be in the +1 state, telling you that it didn’t find one. If you know the measurement has been made, you should then be able to make your own measurement, and instantly know the state of the other particle, even if it’s many light years away.

The wave pattern for electrons passing through a double slit. If you measure “which slit” the electron goes through, you destroy the quantum interference pattern shown here. Image credit: Dr. Tonomura and Belsazar of Wikimedia Commons, under c.c.a.-s.a.-3.0.

It’s a brilliant plan, but there’s a problem: entanglement only works if you ask a particle, “what state are you in?” If you force an entangled particle into a particular state, you break the entanglement, and the measurement you make on Earth is completely independent of the measurement at the distant star. If you had simply measured the distant particle to be +1 or -1, then your measurement, here on Earth, of either -1 or +1 (respectively) would give you information about the particle located light years away. But by forcing that distant particle to be +1 or -1, that means, no matter the outcome, your particle here on Earth has a 50/50 shot of being +1 or -1, with no bearing on the particle so many light years distant.

A quantum eraser experiment setup, where two entangled particles are separated and measured. No alterations of one particle at its destination affect the outcome of the other. Image credit: Wikimedia Commons user Patrick Edwin Moran, under c.c.a.-s.a.-3.0.

This is one of the most confusing things about quantum physics: entanglement can be used to gain information about a component of a system when you know the full state and make a measurement of the other component(s), but not to create-and-send information from one part of an entangled system to the other. As clever of an idea as this is, Olivier, there’s still no faster-than-light communication.

Quantum teleportation, an effect (erroneously) touted as faster-than-light travel. In reality, no information is being exchanged faster than light. Image credit: American Physical Society, via http://www.csm.ornl.gov/SC99/Qwall.html.

Quantum entanglement is a wonderful property that we can exploit for any number of purposes, such as for the ultimate lock-and-key security system. But faster-than-light communication? Understanding why that’s not possible requires us to understand this key property of quantum physics: that forcing even part of an entangled system into one state or another doesn’t allow you to gain information about that forcing from measuring the remainder of the system. As Niels Bohr once famously put it:

If quantum mechanics hasn’t profoundly shocked you, you haven’t understood it yet.

The Universe plays dice with us all the time, much to Einstein’s chagrin. But even our best attempts to cheat at the game are thwarted by nature itself. If only all referees and umpires were as consistent as the laws of quantum physics!

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“Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan