Loopholes and the 'Anti-Realism' of the Quantum World

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Loopholes and the 'Anti-Realism' of the Quantum World

Postby seemslikeadream » Thu Aug 16, 2018 1:19 pm

Loopholes and the 'Anti-Realism' of the Quantum World

The theoretical physicist John Wheeler once used the phrase “great smoky dragon” to describe a particle of light going from a source to a photon counter. “The mouth of the dragon is sharp, where it bites the counter. The tail of the dragon is sharp, where the photon starts,” Wheeler wrote. The photon, in other words, has definite reality at the beginning and end. But its state in the middle—the dragon’s body—is nebulous. “What the dragon does or looks like in between we have no right to speak.”

Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.
Wheeler was espousing the view that elementary quantum phenomena are not real until observed, a philosophical position called anti-realism. He even designed an experiment to show that if you hold on to realism—in which quantum objects such as photons always have definite, intrinsic properties, a position that encapsulates a more classical view of reality—then one is forced to concede that the future can influence the past. Given the absurdity of backward time-travel, Wheeler’s experiment became an argument for anti-realism at the level of the quantum.

But in May, Rafael Chaves and colleagues at the International Institute of Physics in Natal, Brazil, found a loophole. They showed that Wheeler’s experiment, given certain assumptions, can be explained using a classical model that attributes to a photon an intrinsic nature. They gave the dragon a well-defined body, but one that is hidden from the mathematical formalism of standard quantum mechanics.

Rafael Chaves, a physicist at the International Institute of Physics, and his colleagues used the emerging field of causal modeling to find a loophole in Wheeler’s delayed-choice experiment.
International Institute of Physics
Chaves’s team then proposed a twist to Wheeler’s experiment to test the loophole. With unusual alacrity, three teams raced to do the modified experiment. Their results, reported in early June, have shown that a class of classical models that advocate realism cannot make sense of the results. Quantum mechanics may be weird, but it’s still, oddly, the simplest explanation around.

Dragon Trap

Wheeler devised his experiment in 1983 to highlight one of the dominant conceptual conundrums in quantum mechanics: wave-particle duality. Quantum objects seem to act either like particles or waves, but never both at the same time. This feature of quantum mechanics seems to imply that objects have no inherent reality until observed. “Physicists have had to grapple with wave-particle duality as an essential, strange feature of quantum theory for a century,” said David Kaiser, a physicist and historian of science at the Massachusetts Institute of Technology. “The idea pre-dates other quintessentially strange features of quantum theory, such as Heisenberg’s uncertainty principle and Schrödinger’s cat.”

The phenomenon is underscored by a special case of the famous double-slit experiment called the Mach-Zehnder interferometer.

In the experiment, a single photon is fired at a half-silvered mirror, or beam splitter. The photon is either reflected or transmitted with equal probability—and thus can take one of two paths. In this case, the photon will take either path 1 or path 2, and then go on to hit either detector D1 or D2 with equal probability. The photon acts like an indivisible whole, showing us its particle-like nature.

Lucy Reading-Ikkanda/Quanta Magazine
But there’s a twist. At the point where path 1 and path 2 cross, one can add a second beam splitter, which changes things. In this setup, quantum mechanics says that the photon seems to take both paths at once, as a wave would. The two waves come back together at the second beam splitter. The experiment can be set up so that the waves combine constructively—peak to peak, trough to trough—only when they move toward D1. The path toward D2, by contrast, represents destructive interference. In such a setup, the photon will always be found at D1 and never at D2. Here, the photon displays its wavelike nature.

Wheeler’s genius lay in asking: what if we delay the choice of whether to add the second beam splitter? Let’s assume the photon enters the interferometer without the second beam splitter in place. It should act like a particle. One can, however, add the second beam splitter at the very last nanosecond. Both theory and experiment show that the photon, which until then was presumably acting like a particle and would have gone to either D1 or D2, now acts like a wave and goes only to D1. To do so, it had to seemingly be in both paths simultaneously, not one path or the other. In the classical way of thinking, it’s as if the photon went back in time and changed its character from particle to wave.

One way to avoid such retro-causality is to deny the photon any intrinsic reality and argue that the photon becomes real only upon measurement. That way, there is nothing to undo.

Such anti-realism, which is often associated with the Copenhagen interpretation of quantum mechanics, took a theoretical knock with Chaves’s work, at least in the context of this experiment. His team wanted to explain counterintuitive aspects of quantum mechanics using a new set of ideas called causal modeling, which has grown in popularity in the past decade, advocated by computer scientist Judea Pearl and others. Causal modeling involves establishing cause-and-effect relationships between various elements of an experiment. Often when studying correlated events—call them A and B—if one cannot conclusively say that A causes B, or that B causes A, there exists a possibility that a previously unsuspected or “hidden” third event, C, causes both. In such cases, causal modeling can help uncover C.

Chaves and his colleagues Gabriela Lemos and Jacques Pienaar focused on Wheeler’s delayed choice experiment, fully expecting to fail at finding a model with a hidden process that both grants a photon intrinsic reality and also explains its behavior without having to invoke retro-causality. They thought they would prove that the delayed-choice experiment is “super counterintuitive, in the sense that there is no causal model that is able to explain it,” Chaves said.

But they were in for a surprise. The task proved relatively easy. They began by assuming that the photon, immediately after it has crossed the first beam splitter, has an intrinsic state denoted by a “hidden variable.” A hidden variable, in this context, is something that’s absent from standard quantum mechanics but that influences the photon’s behavior in some way. The experimenter then chooses to add or remove the second beam splitter. Causal modeling, which prohibits backward time travel, ensures that the experimenter’s choice cannot influence the past intrinsic state of the photon.

Gabriela Lemos, a physicist at the International Institute of Physics, showed how a “hidden variable” could be affecting the results of the experiment.
Courtesy of Gabriela Barreto Lemos
Given the hidden variable, which implies realism, the team then showed that it’s possible to write down rules that use the variable’s value and the presence or absence of the second beam splitter to guide the photon to D1 or D2 in a manner that mimics the predictions of quantum mechanics. Here was a classical, causal, realistic explanation. They had found a new loophole.

This surprised some physicists, said Tim Byrnes, a theoretical quantum physicist at New York University, Shanghai. “What people didn’t really appreciate is that this kind of experiment is susceptible to a classical version that perfectly mimics the experimental results,” Byrnes said. “You could construct a hidden variable theory that didn’t involve quantum mechanics.”

“This was the step zero,” Chaves said. The next step was to figure out how to modify Wheeler’s experiment in such a way that it could distinguish between this classical hidden variable theory and quantum mechanics.

In their modified thought experiment, the full Mach-Zehnder interferometer is intact; the second beam splitter is always present. Instead, two “phase shifts”—one near the beginning of the experiment, one toward the end—serve the role of experimental dials that the researcher can adjust at will.

The net effect of the two phase shifts is to change the relative lengths of the paths. This changes the interference pattern, and with it, the presumed “wavelike” or “particle-like” behavior of the photon. For example, the value of the first phase shift could be such that the photon acts like a particle inside the interferometer, but the second phase shift could force it to act like a wave. The researchers require that the second phase shift is set after the first.

With this setup in place, Chaves’s team came up with a way to distinguish between a classical causal model and quantum mechanics. Say the first phase shift can take one of three values, and the second one of two values. That makes six possible experimental settings in total. They calculated what they expected to see for each of these six settings. Here, the predictions of a classical hidden variable model and standard quantum mechanics differ. They then constructed a formula. The formula takes as its input probabilities calculated from the number of times that photons land on particular detectors (based on the setting of the two phase shifts). If the formula equals zero, the classical causal model can explain the statistics. But if the equation spits out a number greater than zero, then, subject to some constraints on the hidden variable, there’s no classical explanation for the experiment’s outcome.

Chaves teamed up with Fabio Sciarrino, a quantum physicist at the University of Rome La Sapienza, and his colleagues to test the inequality. Simultaneously, two teams in China—one led by Jian-Wei Pan, an experimental physicist at the University of Science and Technology of China (USTC) in Hefei, China, and another by Guang-Can Guo, also at USTC—carried out the experiment.

Each team implemented the scheme slightly differently. Guo’s group stuck to the basics, using an actual Mach-Zehnder interferometer. “It is the one that I would say is actually the closest to Wheeler’s original proposal,” said Howard Wiseman, a theoretical physicist at Griffith University in Brisbane, Australia, who was not part of any team.

But all three showed that the formula is greater than zero with irrefutable statistical significance. They ruled out the classical causal models of the kind that can explain Wheeler’s delayed-choice experiment. The loophole has been closed. “Our experiment has salvaged Wheeler’s famous thought experiment,” Pan said.

Hidden Variables That Remain

Kaiser is impressed by Chaves’s “elegant” theoretical work and the experiments that ensued. “The fact that each of the recent experiments has found clear violations of the new inequality … provides compelling evidence that ‘classical’ models of such systems really do not capture how the world works, even as quantum-mechanical predictions match the latest results beautifully,” he said.

The formula comes with certain assumptions. The biggest one is that the classical hidden variable used in the causal model can take one of two values, encoded in one bit of information. Chaves thinks this is reasonable, since the quantum system—the photon—can also only encode one bit of information. (It either goes in one arm of the interferometer or the other.) “It’s very natural to say that the hidden variable model should also have dimension two,” Chaves said.

David Kaiser, a physicist and historian at MIT, wants to eliminate the possibility of any unseen experimental correlations by employing a random-number generator based on distant astrophysical objects.
But a hidden variable with additional information-carrying capacity can restore the classical causal model’s ability to explain the statistics observed in the modified delayed-choice experiment.

In addition, the most popular hidden variable theory remains unaffected by these experiments. The de Broglie-Bohm theory, a deterministic and realistic alternative to standard quantum mechanics, is perfectly capable of explaining the delayed-choice experiment. In this theory, particles always have positions (which are the hidden variables), and hence have objective reality, but they are guided by a wave. So reality is both wave and particle. The wave goes through both paths, the particle through one or the other. The presence or absence of the second beam splitter affects the wave, which then guides the particle to the detectors—with exactly the same results as standard quantum mechanics.

For Wiseman, the debate over Copenhagen versus de Broglie-Bohm in the context of the delayed-choice experiment is far from settled. “So in Copenhagen, there is no strange inversion of time precisely because we have no right to say anything about the photon’s past,” he wrote in an email. “In de Broglie-Bohm there is a reality independent of our knowledge, but there is no problem as there is no inversion—there is a unique causal (forward in time) description of everything.”

Kaiser, even as he lauds the efforts so far, wants to take things further. In current experiments, the choice of whether or not to add the second phase shift or the second beam splitter in the classic delayed-choice experiment was being made by a quantum random-number generator. But what’s being tested in these experiments is quantum mechanics itself, so there’s a whiff of circularity. “It would be helpful to check whether the experimental results remain consistent, even under complementary experimental designs that relied on entirely different sources of randomness,” Kaiser said.

To this end, Kaiser and his colleagues have built such a source of randomness using photons coming from distant quasars, some from more than halfway across the universe. The photons were collected with a one-meter telescope at the Table Mountain Observatory in California. If a photon had a wavelength less than a certain threshold value, the random number generator spit out a 0, otherwise a 1. In principle, this bit can be used to randomly choose the experimental settings. If the results continue to support Wheeler’s original argument, then “it gives us yet another reason to say that wave-particle duality is not going to be explained away by some classical physics explanation,” Kaiser said. “The range of conceptual alternatives to quantum mechanics has again been shrunk, been pushed back into a corner. That’s really what we are after.”

For now, the dragon’s body, which for a brief few weeks had come into focus, has gone back to being smoky and indistinct.

Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.
https://www.wired.com/story/quantum-world-anti-realism/
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They could still get him out of office.
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Re: Loopholes and the 'Anti-Realism' of the Quantum World

Postby seemslikeadream » Thu Aug 16, 2018 1:26 pm

:P

hey I read about other stuff occasionally....less interested.......who else has posted a Quantum thread recently?

when I bumped the thread the date changed I posted on August 5th before my interests were all the rage ...ok odd that the date changed in the post but not in the OP ...this conundrum is one for Mac or any of the other SLaD conspirators.....fuel for the fire :P


Wombaticus Rex » Thu Aug 16, 2018 9:48 am wrote:
dada » Thu Aug 16, 2018 9:38 am wrote:Something Chomsky said, that if you can't explain a concept in a clear, concise way, like explaining it to a ten year old in five minutes, there may be something wrong with the concept. Something like that. I think you get the point.


I'm not autistic, just insufferable: that wasn't Chomsky, but you deserve serious props for not attributing it to Einstein, or, for that matter, Richard Feynman. "An alleged scientific discovery has no merit unless it can be explained to a barmaid," some Brit toffer said. He was quoted in a book on Einstein and now that quote has undergone some truly impressive mutations in the wild.

Either way, I don't think you'll find many people less interested in the implications of Quantum mechanics than SLAD.

As a side note, I'm not trying to flag this for the moderators or anything, but: we've had a ban on Star Wars metaphors in effect here since Bush was still President. Just tuck that away for future reference.


maybe you can chalk this up to I am interested in everything or just SLaD in Quantumland coincidence

if there is an OP posted and no one responds does that mean no one heard the noise or no one here is any more interested than SLaD?

was this a future post brought back to collide with RI GD reality...who can say?

sometimes she behaves as a sort of particle with varying electric charge



“Here it is,” he said triumphantly, “a marvel of modern technology. Just put this on, and you will see the world of virtual particles.”
Image


What’s particularly remarkable about this passage is that in addition to serving the allegorical purposes of Gilmore’s quantum story, it also presages with astounding prescience augmented reality tools like Google Glass nearly twenty years before their existence:

https://www.brainpickings.org/2014/01/3 ... t-gilmore/


don't know what a slide rule is for


https://www.youtube.com/watch?v=QfzpAv1hi2Y


Don't know much about history,
Don't know much biology
Don't know much about a science book,
Don't know much about the french I took ...(it would help with the French Mystery TV series I am watching)
But I do know that I love you,
And I know that if you love me, too,
What a wonderful world this would be
Mazars and Deutsche Bank could have ended this nightmare before it started.
They could still get him out of office.
But instead, they want mass death.
Don’t forget that.
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Re: Loopholes and the 'Anti-Realism' of the Quantum World

Postby seemslikeadream » Mon Sep 03, 2018 8:37 am

Quantum weirdness in 'chicken or egg' paradox

Quantum weirdness in 'chicken or egg' paradox
Credit: University of Queensland

The "chicken or egg" paradox was first proposed by philosophers in Ancient Greece to describe the problem of determining cause-and-effect.

Now, a team of physicists from The University of Queensland and the NÉEL Institute has shown that, as far as quantum physics is concerned, the chicken and the egg can both come first.

Dr Jacqui Romero from the ARC Centre of Excellence for Engineered Quantum Systems said that in quantum physics, cause-and-effect is not always as straightforward as one event causing another.

"The weirdness of quantum mechanics means that events can happen without a set order," she said.

"Take the example of your daily trip to work, where you travel partly by bus and partly by train.

"Normally, you would take the bus then the train, or the other way round.

"In our experiment, both of these events can happen first," Dr Romero said.

"This is called `indefinite causal order' and it isn't something that we can observe in our everyday life."

To observe this effect in the lab, the researchers used a setup called a photonic quantum switch.

UQ's Dr Fabio Costa said that with this device the order of events—transformations on the shape of light—depends on polarisation.

"By measuring the polarisation of the photons at the output of the quantum switch, we were able to show the order of transformations on the shape of light was not set."

"This is just a first proof of principle, but on a larger scale indefinite causal order can have real practical applications, like making computers more efficient or improving communication."

The research was published in Physical Reviews Letters by the American Physical Society.
https://phys.org/news/2018-09-quantum-w ... radox.html
Mazars and Deutsche Bank could have ended this nightmare before it started.
They could still get him out of office.
But instead, they want mass death.
Don’t forget that.
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Re: Loopholes and the 'Anti-Realism' of the Quantum World

Postby seemslikeadream » Mon Sep 10, 2018 12:38 pm

Antimatter seen in two places at once thanks to quantum experiment

Antimatter waves can interfere just like regular matter

wacomka/Getty
By Leah Crane

A particle can be in two places at once – even if it is made of antimatter. Researchers have just performed an antimatter twist on a classic experiment used to show one of the foundational tenets of quantum mechanics: that all particles are also waves.

In the most basic version of the double-slit experiment, first performed in 1801, a beam of light illuminates a plate with two parallel slits cut into it. The light that passes through the slits hits a screen, creating stripes of light and darkness …https://www.newscientist.com/article/2179061-antimatter-seen-in-two-places-at-once-thanks-to-quantum-experiment/


A topological source of quantum light

10 September 2018
Abstract

Quantum light is characterized by distinctive statistical distributions that are possible only because of quantum mechanical effects. For example, single photons and correlated photon pairs exhibit photon number distributions with variance lower than classically allowed limits. This enables high-fidelity transmission of quantum information and sensing with lower noise than possible with classical light sources1,2. Most quantum light sources rely on spontaneous parametric processes such as down-conversion and four-wave mixing2. These processes are mediated by vacuum fluctuations of the electromagnetic field. Therefore, by manipulating the electromagnetic mode structure, for example with dispersion-engineered nanophotonic systems, the spectrum of generated photons can be controlled3,4,5,6,7. However, disorder, which is ubiquitous in nanophotonic fabrication, causes device-to-device spectral variations8,9,10,11. Here we realize topologically robust electromagnetic modes and use their vacuum fluctuations to create a quantum light source in which the spectrum of generated photons is much less affected by fabrication-induced disorder. Specifically, we use the topological edge states realized in a two-dimensional array of ring resonators to generate correlated photon pairs by spontaneous four-wave mixing and show that they outperform their topologically trivial one-dimensional counterparts in terms of spectral robustness. We demonstrate the non-classical nature of the generated light and the realization of a robust source of heralded single photons by measuring the conditional antibunching of photons, that is, the reduced likelihood of photons arriving together compared to thermal or laser light. Such topological effects, which are unique to bosonic systems, could pave the way for the development of robust quantum photonic devices.
https://www.nature.com/articles/s41586-018-0478-3


The reality of quantum computing could be just three years away

Sep 7, 2018

Quantum computing has moved out of the realm of theoretical physics and into the real world, but its potential and promise are still years away.

Onstage at TechCrunch Disrupt SF, a powerhouse in the world of quantum research and a young upstart in the field presented visions for the future of the industry that illustrated both how far the industry has come and how far the technology has to go.

For both Dario Gil, the chief operating officer of IBM Research and the company’s vice president of artificial intelligence and quantum computing, and Chad Rigetti, a former IBM researcher who founded Rigetti Computing and serves as its chief executive, the moment that a quantum computer will be able to perform operations better than a classical computer is only three years away.

“[It’s] generating a solution that is better, faster or cheaper than you can do otherwise,” said Rigetti. “Quantum computing has moved out of a field of research into now an engineering discipline and an engineering enterprise.”

Considering the more than 30 years that IBM has been researching the technology and the millions (or billions) that have been poured into developing it, even seeing an end of the road is a victory for researchers and technologists.

Achieving this goal, for all of the brainpower and research hours that have gone into it, is hardly academic.

The Chinese government is building a $10 billion National Laboratory for Quantum Information in Anhui province, which borders Shanghai and is slated to open in 2020. Meanwhile, the U.S. public research into quantum computing is running at around $200 million per year.


Source: Patin Informatics via Bloomberg News.

One of the reasons why governments, especially, are so interested in the technology is its potential to completely remake the cybersecurity landscape. Some technologists argue that quantum computers will have the potential to crack any type of encryption technology, opening up all of the networks in the world to potential hacking.

Of course, quantum computing is so much more than security. It will enable new ways of doing things we can’t even imagine because we have never had this much pure compute power. Think about artificial and machine learning or drug development; any type of operation that is compute-intensive could benefit from the exponential increase in compute power that quantum computing will bring.

Security may be the Holy Grail for governments, but both Rigetti and Gil say that the industrial chemical business will be the first place where the potentially radical transformation of a market will appear first.

What is quantum computing anyway?

To understand quantum computing it helps to understand the principles of the physics behind it.

As Gil explained onstage (and on our site), quantum computing depends on the principles of superposition, entanglement and interference.

Superposition is the notion that physicists can observe multiple potential states of a particle. “If you a flip a coin it is one or two states,” said Gil. Meaning that there’s a single outcome that can be observed. But if someone were to spin a coin, they’d see a number of potential outcomes.

Once you’ve got one particle that’s being observed, you can add another and pair them thanks to a phenomenon called quantum entanglement. “If you have two coins where each one can be in superpositions and then you can have measurements can be taken” of the difference of both.

Finally, there’s interference, where the two particles can be manipulated by an outside force to change them and create different outcomes.

“In classical systems you have these bits of zeros and ones and the logical operations of the ands and the ors and the nots,” said Gil. “The classical computer is able to process the logical operations of bits expressed in zeros and ones.”

“In an algorithm you put the computer in a super positional state,” Gil continued. “You can take the amplitude and states and interfere them and the algorithm is the thing that interferes… I can have many, many states representing different pieces of information and then i can interfere with it to get these data.”

These operations are incredibly hard to sustain. In the early days of research into quantum computing the superconducting devices only had one nanosecond before a qubit transforms into a traditional bit of data. Those ranges have increased between 50 and 100 microseconds, which enabled IBM and Rigetti to open up their platforms to researchers and others to conduct experimentation (more on that later).

The physical quantum computer

As one can imagine, dealing with quantum particles is a delicate business. So the computing operations have to be carefully controlled. At the base of the machine is what basically amounts to a huge freezer that maintains a temperature in the device of 15 millikelvin — near absolute zero degrees and 180 times colder than the temperatures in interstellar space.

“These qubits are very delicate,” said Gil. “Anything from the outside world can couple to it and destroy its state and one way to protect it is to cool it.”

Wiring for the quantum computer is made of superconducting coaxial cables. The inputs to the computers are microwave pulses that manipulates the particles creating a signal that is then interpreted by the computers’ operators.

Those operators used to require a degree in quantum physics. But both IBM and Rigetti have been working on developing tools that can enable a relative newbie to use the tech.



Quantum computing in the “cloud”

Even as companies like IBM and Rigetti bring the cost of quantum computing down from tens of millions of dollars to roughly $1 million to $2 million, these tools likely will never become commodity hardware that a consumer buys to use as a personal computer.

Rather, as with most other computing these days, quantum computing power will be provided as a service to users.

Indeed, Rigetti announced onstage a new hybrid computing platform that can provide computing services to help the industry both reach quantum advantage — that tipping point at which quantum is commercially viable — and to enable industries to explore the technologies to acclimatize to the potential ways in which typical operations could be disrupted by it.

“A user logs on to their own device and use our software development kit to write a quantum application,” said Rigetti. “That program is sent to a compiler and kicks off an optimization kit that runs on a quantum and classical computer… This is the architecture that’s needed to achieve quantum advantage.”

Both IBM and Rigetti — and a slew of other competitors — are preparing users for accessing quantum computing opportunities on the cloud.

IBM has more than a million chips performing millions of quantum operations requested by users in over 100 countries around the world.

“In a cloud-first era I’m not sure the economic forces will be there that will drive us to develop the miniaturized environment in the laptop,” Rigetti said. But the ramifications of the technology’s commercialization will be felt by everyone, everywhere.

“Quantum computing is going to change the world and it’s all going to come in our lifetime, whether that’s two years or five years,” he said. “Quantum computing is going to redefine every industry and touch every market. Every major company will be involved in some capacity in that space.”
https://techcrunch.com/2018/09/07/the-r ... ears-away/
Mazars and Deutsche Bank could have ended this nightmare before it started.
They could still get him out of office.
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