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pierredv : quantum-mechanics   34

Schrödinger’s kittens: New thought experiment breaks quantum theory | New Scientist, issue 3222, Mar 2019
"A twist on the famous Schrödinger’s cat thought experiment could undermine quantum physics – or provide a path to a deeper understanding of how the world works"
physics  quantum-mechanics  measurement  paradoxes  NewScientist 
8 days ago by pierredv
The Dirac Equation
Good explanation of derivation and consequence
Dirac  physics  quantum-mechanics 
5 weeks ago by pierredv
I'm building a machine that breaks the rules of reality | New Scientist, Apr 2018, Vlatko Vedral
"Tobias Schaetz ... he described an experiment looking at ions inside a crystal. He gave them some energy and watched how they cooled. Unlike a cup of coffee, which cools gradually, the ions seemed to lose energy for a while, but then the energy suddenly bounced back. It is proof of what we had suspected: the rules of classical thermodynamics don’t always apply in the quantum world."

"I thought I would make a quantum version of a heat engine ... The idea was to set up pairs of organic molecules and raise them to a high energy level by shining light on them. Left alone, the molecules will return to a slightly lower energy level, re-emitting light of a different frequency as they do so. ... If we set up the experiment just right, the emitted light won’t carry any information that could tell us which of the two molecules it came from. According to quantum theory, this forces them to become entangled, so that when one drops to the lower energy level, the other one automatically does too, with both emitting light in unison in a process called superradiance."

"... we were scooped ... Walmsley and his team saw that light was produced quicker than the classical rules of thermodynamics predict"

"Felix Binder, now at Nanyang Technological University in Singapore, has shown that quantum batteries can charge more quickly than normal ones"
NewScientist  thermodynamics  quantum-mechanics  Vlatko-Vedral 
11 weeks ago by pierredv
A quantum experiment suggests there’s no such thing as objective reality - MIT Technology Review apr 2019
Via John Helm

"Physicists have long suspected that quantum mechanics allows two observers to experience different, conflicting realities. Now they’ve performed the first experiment that proves it."

"last year Caslav Brukner, at the University of Vienna in Austria, came up with a way to re-create the Wigner’s Friend experiment in the lab by means of techniques involving the entanglement of many particles at the same time. The breakthrough that Proietti and co have made is to carry this out."

"The idea that observers can ultimately reconcile their measurements of some kind of fundamental reality is based on several assumptions."

<list follows>

"Proietti and co’s result suggests that objective reality does not exist. In other words, the experiment suggests that one or more of the assumptions—the idea that there is a reality we can agree on, the idea that we have freedom of choice, or the idea of locality—must be wrong."
MIT-Technology-Review  physics  quantum-mechanics  reality 
april 2019 by pierredv
Famous Experiment Dooms Pilot-Wave Alternative to Quantum Weirdness | Quanta Magazine
"There and in bouncing-droplet labs that soon sprang up at the Massachusetts Institute of Technology and elsewhere, droplets were seen to tunnel through barriers and perform other acts previously thought to be uniquely quantum. In reproducing quantum phenomena without any of the mystery, the bouncing-droplet experiments rekindled in some physicists de Broglie’s old dream of a reality at the quantum scale that consists of pilot waves and particles instead of probability waves and conundrums.

But a series of bouncing-droplet findings since 2015 has crushed this dream."
quantum-mechanics  physics 
october 2018 by pierredv
Reimagining of Schrödinger’s cat breaks quantum mechanics — and stumps physicists - Nature Sep 2018
In a multi-‘cat’ experiment, the textbook interpretation of quantum theory seems to lead to contradictory pictures of reality, physicists claim.
quantum-mechanics  NatureJournal 
september 2018 by pierredv
Quantum Amplitudes, Classical Ignorance & Quantum Information Processing | Saturday Mornings of Theoretical Physics, June 2018
Saturday, June 30, 2018 - 10:30
Venue:
Martin Wood Lecture Theatre, Clarendon Laboratory

A major theme in current theoretical physics is understanding the implications of quantum mechanics for the dynamics of composite systems. When sub-systems interact, they naturally become correlated. In the 1980s it was realised that by exploiting the correlations between quantum sub-systems it should be possible to (i) break current cryptographic systems by rapidly decomposing large numbers into their prime factors, and (ii) exchange confidential information in the secure knowledge that messages have not been intercepted. As we have become ever more dependent on the internet these explosive implications of quantum mechanics for cryptography have driven efforts to build quantum communication channels and quantum computers.

On June 30, to mark the completion of the Beecroft Building, the members of the Rudolf Peierls Centre for Theoretical Physics, who occupy the building above ground, and the members of the Quantum Information Technology hub (NQIT), who are installing kit in many of its subterranean laboratories, will join forces to explain the basic principles underlying the quantum dynamics of composite systems and to describe the challenge of implementing quantum computation and cryptography practically.

In the afternoon there will be three talks from NQIT, which is led by Oxford University. A general introduction to the programme will be followed by two talks about using ion traps and photonics to build a quantum computer. There will also have a chance to see the new labs via exclusive video and an opportunity to questions NQIT researchers.
Oxford  physics  quantum-mechanics  lectures 
july 2018 by pierredv
Quantum time: Is this where the flow of existence comes from? | New Scientist, April 2018
"Since all measurement outcomes are probabilistic, varying from case to case, the order in which we make measurements determines their outcome. “The true variability underlying physics is not the passing of time, but the ‘non-reproducibility’ of the outcome of quantum experiments,” says Connes.

That would mean that time isn’t fundamental. There is no order of time in the quantum world; temporal order appears only when processes such as measurement irreversibly turn quantum phenomena into observable classical phenomena. Applied to thermal systems, what emerges matches the second law of thermodynamics. “The flow has the same properties as what we call time,” says Rovelli."

"According to the most popular notion of how tunnelling occurs, derived from relativistic quantum field theory, it takes place in no time at all, with the electron travelling faster than light. That rings alarm bells. “Most of us are very careful about this – we shouldn’t really think about things travelling faster than light,” says Steinberg."

"The basic idea is to have ultracold atoms, cooled to a billionth of a degree above absolute zero, tunnel through a barrier formed by the tightly focused electromagnetic fields of a laser beam. “We’ve started to see the atoms tunnel through,” says Steinberg. “Now we have to add a measurement of how long they sit inside the barrier.”"

"Rovelli agrees that, in the end, there may not be one universally valid answer to the question of what time is. “When we think about time, we tend to think about it as a single package, and that’s definitely wrong,” he says. There’s the psychological time of our experience; the passing instants of time that clocks measure; relative time as explored by Einstein; time as entropy increase; and perhaps, now, time rooted in quantum ignorance. “It’s a beautiful problem because it brings together so many things,” says Rovelli. “I don’t think we’re anywhere near the end of the story, but there is real progress.”"
NewScientist  time  quantum-mechanics  relativity  Carlo-Rovelli 
july 2018 by pierredv
Richard Feynman at 100 - Nature May 2018
Paul Halpern celebrates the oeuvre of the brilliant, unconventional scientist.
physics  quantum-mechanics  Richard-Feynman  people  biography  NatureJournal 
may 2018 by pierredv
‘Quantum Radio’ May Aid Communications and Mapping Indoors, Underground and Underwater
via Dale hatfield

"Researchers at the National Institute of Standards and Technology (NIST) have demonstrated that quantum physics might enable communications and mapping in locations where GPS and ordinary cellphones and radios don’t work reliably or even at all, such as indoors, in urban canyons, underwater and underground."

"Traditionally, such atomic magnetometers are used to measure naturally occurring magnetic fields, but in this NIST project, they are being used to receive coded communications signals. "
quantum-mechanics  Communications  NIST 
january 2018 by pierredv
The geometry that could reveal the true nature of space-time | New Scientist issue 3136, 29 Jul 2017
"The discovery of an exquisite geometric structure is forcing a radical rethink of reality, and could clear the way to a quantum theory of gravity"

[Andrew Hodges, one of Penrose’s colleagues at Oxford] "showed that the various terms used in the BCFW method could be interpreted as the volumes of tetrahedrons in twistor space, and that summing them up led to the volume of a polyhedron."

"So why invoke virtual particles at all? ... The first is that dealing with them rather than with fields makes the maths more tractable. The other great advantage is that they help physicists visualise everything as the well-defined interactions between point-like particles, as opposed to the hazy goings-on between particles and fields. This fits nicely with the intuitive principle of locality, which holds that only things in the same spot in space and time can interact. Finally, the technique also helps enforce the principle of unitarity, which says that the probability of all outcomes should add up to 1."

Gluon interactions seemed to complex, but "In 1986, Stephen Parke and Tomasz Taylor from Fermilab near Batavia, Illinois, used Feynman diagrams and supercomputers to calculate the likelihoods of different outcomes for interactions involving a total of six gluons. A few months later, they made an educated guess at a one-line formula to calculate the same thing. It was spot on. More than 200 Feynman diagrams and many pages of algebra had been reduced to one equation, and the researchers had no idea why."

"In 2005, Ruth Britto, Freddy Cachazo, Bo Feng and Edward Witten [BCFW] were able to calculate scattering amplitudes without recourse to a single virtual particle and derived the equation Parke and Taylor had intuited for that six-gluon interaction"

[Nima Arkani-Hamed and his team at IAS] "arrived at a mind-boggling conclusion: the scattering amplitude calculated with the BCFW technique corresponds beautifully to the volume of a new mathematical object. They gave a name to this multi-dimensional concatenation of polyhedrons: the amplituhedron."

"It may transform physics, too ... because the amplituhedron does not embody unitarity and locality, those core principles baked into reality as described by Feynman diagrams. ... If so, locality is not a fundamental feature of space-time but an emergent one."
NewScientist  geometry  physics  gravity  field-theory  quantum-mechanics  twistors  Roger-Penrose  Richard-Feynman  Ed-Witten  maths 
december 2017 by pierredv
Reality check: The hidden connections behind quantum weirdness | New Scientist, April 2017
Love the physical analogy to a QM pilot wave in the bouncing oil droplet experiment

"Bohm proposed there was a hidden reality to quantum theory, meaning its crazy predictions of a world that doesn’t exist until you choose to look at it are just that: crazy."

As an alternative to the Copenhagen interpretation, "De Broglie suggested another: that particles are real and have equally real waves associated with them. In this picture, when a particle goes through one of the double slits, its “pilot” wave goes through both, interferes with itself, and then guides the particle to a location on the screen. . . . Bohm’s ideas made de Broglie revisit and revise his own pilot-wave theory. He developed a two-wave theory in which every particle rides a pilot wave, which in turn interacts with another wave that behaves like a wave function."

Also explains entanglement: "In the alternative picture, though, if particles are entangled, a common pilot wave guides them, and any change in the position or momentum of one particle instantly changes the pilot wave, thus influencing all the other particles."

" In 2005, Yves Couder and Emmanuel Fort at Denis Diderot University in Paris stumbled upon a physical analogue of pilot waves. . . . The interesting thing was what happened when this wave-particle system encountered a barrier, a fraction of a millimetre below the surface, with two gaps in it: a double slit. The walking droplet went over one or the other slit, while its pilot wave went over both, and the wave pattern that emerged on the other side guided the droplet on. "

Improved experiment: Pucci, ... and Bush, "Non-specular reflection of walking droplets" (Journal of Fluid Mechanics, DOI: 10.1017/jfm.2016.537, http://math.mit.edu/~bush/wordpress/wp-content/uploads/2016/09/Pucci-JFM-2016.pdf)

Problems:
1. "Bohmian mechanics is formulated to replicate the predictions of standard quantum mechanics: experimentally, it’s almost impossible to tell them apart"
2. not relativistic
NewScientist  quantum-mechanics  physics 
august 2017 by pierredv
First Object Teleported from Earth to Orbit - MIT Technology Review, Jul 2017
via John Helm
"Researchers in China have teleported a photon from the ground to a satellite orbiting more than 500 kilometers above."

"Finally, they measured the photons on the ground and in orbit to confirm that entanglement was taking place, and that they were able to teleport photons in this way. Over 32 days, they sent millions of photons and found positive results in 911 cases. "
MIT-Technology-Review  quantum-mechanics 
july 2017 by pierredv
Quantum Bayesianism Explained By Its Founder | Quanta Magazine
Interview with Christopher Fuchs
"Those interpretations [Copenhagen, many worlds, Bohmian] all have something in common: They treat the wave function as a description of an objective reality shared by multiple observers. QBism, on the other hand, treats the wave function as a description of a single observer’s subjective knowledge. It resolves all of the quantum paradoxes, but at the not insignificant cost of anything we might call “reality.” Then again, maybe that’s what quantum mechanics has been trying to tell us all along — that a single objective reality is an illusion."
Q. How does QBism get you around those limits?
A. "One way to look at it is that the laws of physics aren’t about the stuff “out there.” Rather, they are our best expressions, our most inclusive statements, of what our own limitations are. When we say the speed of light is the ultimate speed limit, we’re saying that we can’t go beyond the speed of light."
"I’ve become fascinated by these beautiful mathematical structures called SICs, symmetric informationally complete measurements — horrible name, almost as bad as bettabilitarianism. They can be used to rewrite the Born rule [the mathematical procedure that generates probabilities in quantum mechanics] in a different language, in which it appears that the Born rule is somehow deeply about analyzing the real in terms of hypotheticals."
QuantaMagazine  QBism  Bayesian  physics  quantum-mechanics  philosophy  interviews 
may 2016 by pierredv
Quantum Moves - on scienceathome.org
"Play Quantum Moves and contribute to cutting-edge physics research. Your task is to find clever ways of manipulating and moving atoms. By playing, you help physicists in the epic task of building a real quantum computer. This is your chance to push the boundaries of science!"
Via http://www.nature.com/news/human-mind-excels-at-quantum-physics-computer-game-1.19725, April 2016
quantum-mechanics  games 
april 2016 by pierredv
Human mind excels at quantum-physics computer game - Nature news Apr 2016
"With particles that can exist in two places at once, the quantum world is often considered to be inherently counterintuitive. Now, a group of scientists has created a video game that follows the laws of quantum mechanics, but at which non-physicist human players excel (J. J. W. H. Sørensen et al. Nature 532, 210–213; 2016)."

https://www.scienceathome.org/games/quantum-moves/game
quantum-mechanics  games  NatureJournal 
april 2016 by pierredv
A Fight for the Soul of Science - Quanta Magazine Dec 2015
"String theory, the multiverse and other ideas of modern physics are potentially untestable. At a historic meeting in Munich, scientists and philosophers asked: should we trust them anyway?"
QuantaMagazine  philosophy  science  scientific-method  string-theory  quantum-mechanics 
december 2015 by pierredv
Quantum purity: How the big picture banishes weirdness - New Scientist 11 April 2015
Based on work started by Lucien Hardy. “When I look at quantum theory, I see something that’s akin to the ad hoc nature of Kepler’s laws of planetary motion and of Lorentz transformations,” says Hardy. “What we need is some deeper set of principles.” Could it be that all the messy uncertainty of the quantum world simply stems from lack of information: making a measurement of a mixed state without having access to the larger pure state of which it is part? “The idea is not to get rid of the weirdness, but to explain it as a logical consequence of basic principles,” Chiribella says. "Fivefold way" per D’Ariano, Chiribella, Perinotti: five fundamental principles that must apply to any physical system to make a sensible measurement of it 1 Causality 2 Distinguishability 3 Composition 4 Compression 5 Tomography
NewScientist  explanation  quantum-mechanics  physics  *  information-theory 
august 2015 by pierredv
Is the Many Worlds hypothesis just a fantasy? - Aeon
"Nobody knows what happens inside quantum experiments. So why are some so keen to believe in parallel universes?" by Philip Ball -- article doesn't actually answer this question; it's a critique of many worlds interpretation -- "the Measurement Problem, which really comes down to this: between the rainbow-smear of probabilities in our equations and the matter-of-fact determinacy of everything we can actually measure, what on Earth is going on?" -- "the Measurement Problem, which really comes down to this: between the rainbow-smear of probabilities in our equations and the matter-of-fact determinacy of everything we can actually measure, what on Earth is going on?" -- closest to an answer of the Why question: "Most MWI popularizers think they are blowing our minds with this stuff, whereas in fact they are flattering them. ... The result sounds transgressively exciting while familiar enough to be persuasive."
philosophy  science  AeonMagazine  quantum-mechanics  many-worlds-interpretation  identity  selfhood  ontology  critique  modal-realism 
february 2015 by pierredv
25Sep14: Multiverse me: Should I care about my other selves? New Scientist #2988
Very well written discussion of moral-philosophical implications of multiple universes, quoting Tegmark and Deutsch
NewScientist  cosmology  quantum-mechanics  philosophy  physics  religion  morality  parallel.universes  multiverse  probability  risk  *  ethics 
november 2014 by pierredv
[1311.5253] An Introduction to QBism with an Application to the Locality of Quantum Mechanics
also via bit.ly/qbism Christopher A. Fuchs, N. David Mermin, Ruediger Schack (Submitted on 20 Nov 2013) "We give an introduction to the QBist interpretation of quantum mechanics. We note that it removes the paradoxes, conundra, and pseudo-problems that have plagued quantum foundations for the past nine decades. As an example, we show in detail how it eliminates quantum "nonlocality"."
quantum-mechanics  David  Mermin  QBism  arXiv 
july 2014 by pierredv
Physics: QBism puts the scientist back into science : Nature News & Comment
A participatory view of science resolves quantum paradoxes and finds room in classical physics for 'the Now', says N. David Mermin.
quantum-mechanics  David  Mermin  Bayesian  QBism  NatureJournal 
april 2014 by pierredv
Physics: Quantum quest : Nature News & Comment
The lesson, says Fuchs, isn't that Spekkens's model is realistic — it was never meant to be — but that entanglement and all the other strange phenomena of quantum theory are not a completely new form of physics. They could just as easily arise from a theory of knowledge and its limits. To get a better sense of how, Fuchs has rewritten standard quantum theory into a form that closely resembles a branch of classical probability theory known as Bayesian inference “It turns out that many principles lead to a whole class of probabilistic theories, and not specifically quantum theory,” says Schlosshauer.
Bayesian  physics  quantum-mechanics  NatureJournal 
september 2013 by pierredv
Physics goes social: How behaviour obeys quantum logic - opinion - 11 July 2013 - New Scientist
Andrei Khrennikov and Emmanuel Haven See their book "Quantum Social Science" "The idea of using quantum mechanics outside physics started more than a decade ago when we were trying to find new ways to model information in the social sciences – for example, information driving asset prices. We found that concepts from the quantum world can have insightful economic meaning. For instance, quantum potentials – a central concept in a particular interpretation of quantum mechanics – can play a role in the construction of pricing formulae." Other applications: Ellsberg paradox (behaviour in two stage gambles); quantum-like artificial intelligence; how voters are influenced by a stream of mass media information
Ellsberg  paradox  books  sociology  quantum-mechanics  NewScientist  AI  decision-making  Khrennikov  voting  Haven  gambling 
august 2013 by pierredv
Quantum weirdness: The battle for the basis of reality - physics-math - 05 August 2013 - New Scientist
Discussion of explanations of quantum entanglement, with the upshot shot that of the 4, reality, relativity, cause & effect and free will can't all be true at the same time.

"Reality, relativity, causality or free will? Take quantum theory at face value and at least one of them is an illusion – but which?"
free-will  quantum-mechanics  causality  NewScientist  reality  relativity  physics  * 
august 2013 by pierredv
Quantum minds: Why we think like quarks - life - 05 September 2011 - New Scientist
Strapline: "The fuzziness and weird logic of the way particles behave applies surprisingly well to how humans think" Application of Hilbert space theory ("quantum logic") to behavior "We make systematic errors when reasoning with probabilities, for example. Physicist Diederik Aerts of the Free University of Brussels, Belgium, has shown that these errors actually make sense within a wider logic based on quantum mathematics. The same logic also seems to fit naturally with how people link concepts together, often on the basis of loose associations and blurred boundaries. That means search algorithms based on quantum logic could uncover meanings in masses of text more efficiently than classical algorithms." Cites research by Aerts; Tversky & Shafir; Busemeyer & Pothos; Widdows, Cohen; etc.
physics  behavior  maths  NewScientist  quarks  quantum-mechanics 
november 2011 by pierredv

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