Quantum Entanglement

[Ellen Stuttle]

I'm starting a new thread so that those of us who would like to talk about quantum entanglement might be able to find the posts instead of having them buried in the very long thread "The Logical Leap."

The first post, on Aspect's Experimental Set-Up, was moved from that thread. I left a link where the post originally appeared.

[Ellen Stuttle]

Aspect's Experimental Set-Up

Here's a section from The Age of Entanglement which describes Alain Aspect's becoming excited by Bell's 1964 paper and explaining to Bell an idea for an improved experimental design.

It also includes material on the status meanwhile of two other experimenters, Clauser and Fry.

There are morsels of nourishment for everyone's respective biases in the excerpt. I'll reserve comments for later.

The Age of Entanglement

Louisa Gilder

2008

Alfred A. Knopf

31: In Which the Settings Are Changed 1975-1982

pp. 282-85

[italics in original; other emphases added]

Hanging next to the door of Bell's office at CERN was a poster of a long-necked Modigliani lady in a hat; her eyes and the eyes of Bell himself watched the twenty-seven-year-old Alain Aspect, a genial, mustachioed graduate student talking eagerly about boxes of water.

It was early in 1975, and Aspect had just returned to Europe from a three-year stint of French "national service," teaching in Cameroon. Soon after his return, he had suffered what he described as a coup de foudre. "In October 1974," he remembered, "I read John Bell's famous paper 'On the Einstein-Podolsky-Rosen Paradox,' and it was love at first sight. This was the most exciting subject I could dream of." Immediately he decided to make Bell's theorem the subject of his Ph.D. thesis at his alma mater, the University of Paris-South, in Orsay.

Meanwhile, Clauser was trying to get a job. "I must have applied to at least a dozen different places, and at all of them I was totally rejected." Universities were uneasy about hiring a professor who would encourage the next generation to question the foundations of quantum theory. Finally Clauser found an opening at the Lawrence Livermore National Laboratory, in the hills east of Oakland, researching plasmas (David Bohm's first love).

"I don't know anything at all about plasma physics," Clauser announced at his job interview. "But I do know a lot about doing experimental physics. I'm a very talented experimental physicist."

"You can learn plasma physics," was the reply. He was hired in 1976 and stayed there for a decade. At Livermore, his self-proclaimed skills as an experimentalist were well used. But almost wasted was an equal, though unproclaimed, skill. Clauser had the gift--rare even in the teaching profession--of explaining complicated subjects to students clearly, vividly, and patiently. The university career where he could have combined these skills did not materialize in the three decades since he first applied for such a job.

"Back in the sixties and seventies, reputable physicists did not ask questions about quantum mechanics," explained Fry in 2000. "I think that Clauser took the brunt of this attitude--in part, I believe, because he was actually doing the experiment, not just talking about the theory."

Fry himself had better luck with academia. In the midst of performing his experiment, he was granted tenure. Thirty years later, by then the head of the physics department at Texas A&M, he learned that this open-minded institutional decision was thanks to an intervention from Frank Pipkin, Holt's adviser at Harvard. Realizing that the tenure committee was about to reject the Bell experimenter, one of Fry's friends asked Pipkin to come to College Station, Texas.

"If you had sent me just Ed's file to look at, I would have rejected him very quickly," Pipkin told the committee. "However, after spending a day in his lab I can tell you that this guy is a winner and I would bet on his success." Pipkin's renown in atomic physics won over the skeptical committee.

Bell himself was acutely aware of the stigma attached to the experiments his work had inspired, but thus far Aspect was not. Before heading to West Africa in 1972, Aspect remembered, "I had a quite good education in classical physics, and I knew my education in quantum physics was extremely bad." The classes he had taken on the subject comprised equation-solving with little discussion of physical meaning, let alone inculcation of any stigmas.

So for his three equatorial years in Cameroon, Aspect taught himself quantum mechanics, using a recent textbook by the great French physicist Claude Cohen-Tannoudji. This book had two strengths: "First, it is real physics," said Aspect. "Second, it is neutral with respect to foundation. No brainwashing, no 'Bohr solved all of that.'" As a result, "I was able to solve the equations but nobody had washed my brain.

"I was totally convinced by Einstein and Bell," he said. But what experiment to do? In rereading Bell's 1964 paper, Aspect realized that its last lines told him "there was still an important test to be done."

He raced to Geneva to tell Bell his idea.

Bell had ended his paper on a cautionary note. If there was enough time for a light-speed signal to correlate the particles, then entanglement would lose much of its mystery. Conceivably, quantum mechanics might work, wrote Bell, only when "the settings of the instruments are made sufficiently in advance to allow them to reach some mutual rapport" by exchange of signals at the speed of light. In that connection, experiments of the type proposed [in 1957] by Bohm and Aharonov, in which the settings are changed during the flight of the particles, are crucial.

The practical problem with this experiment was that the huge, fragile, piles-of-plates polarizers at either end of the Freedman-Clauser experiment could not move into their settings quickly. Aspect had come up with a beautiful (and, importantly, frugal) alternative idea. Its main ingredient was water.

"Each polarizer," Aspect explained to Bell, "would be replaced by a setup involving a switching device followed by two polarizers in two different orientations." At any given time, the switch would allow a path to only one of the two polarizers. "The switch would rapidly redirect the incident light from one polarizer to the other one," leaving no time for light-speed signals to facilitate any kind of "mutual rapport" between distant ends of the apparatus. He turned to the blackboard, where he wrote the appropriate inequality for the situation "if the two switches work at random and are uncorrelated."

Aspect's "switches" were two glass boxes full of water, over forty-two feet apart from each other, on either side of the beam of cascading calcium that produced the photons. Each box of water carried a sound wave, far higher than the human ear can hear. (Transducers on either side of the boxes converted electrical signals into this ultrasonic wave.)

Sound waves, unlike light waves, need a medium--hence the silence of outer space. They operate by repeatedly compressing and then relieving pressure in their medium, so that the air or water they move through becomes alternately denser and thinner, denser and thinner. When the water is thinned by the ultrasonic wave, photons can pass through to the polarizer beyond; when it is dense, it deflects the photons to the other polarizer, set at the alternate setting. The wave cycles far faster than a photon can travel the twenty-one feet separating the source from the switch. "The switching between the two channels would occur about every ten nanoseconds," Aspect explained. Meanwhile, it would take a light signal four times that long to travel between the two locations.

It would not, he admitted, be the ideal scheme, "since the change is not truly random, but rather quasiperiodic. Nevertheless, the two switches on the two sides would be driven by different generators at different frequencies," meaning that the two boxes would oscillate at different rates, and in practice the rates would drift. "It is then very natural," said Aspect, "to assume that they function in an uncorrelated way." *

When Aspect finished his eager presentation, he stood silently awaiting a reply. Bell asked his first question with a trace of irony: "Have you a permanent position?" Aspect was only a graduate student, but--because of the uniqueness of the French system, and in drastic contrast to his counterparts in America--his position at the École normale supérieure was actually permanent. Even with this advantage, it was not easy.

"There will be serious fights," Bell warned him. But the stigma was not the only thing he worried about. "One should not spend all his time on concepts. You are an experimentalist, which keeps your feet on the ground, so you are not in so much danger. For me, I am a theorist and this subject must remain my hobby.

"If you spend all your time thinking about it, you are in danger of becoming crazy."

*This experiment (published in

Physical Review Letters

for Christmas 1982) was so difficult to carry out that Aspect and his student, Jean Dalibard, listed the machinist, Gérard Roger, as an author.

[Xray]

Aspect's Experimental Set-Up

Here's a section from The Age of Entanglement which describes Alain Aspect's becoming excited by Bell's 1964 paper and explaining to Bell an idea for an improved experimental design.

It also includes material on the status meanwhile of two other experimenters, Clauser and Fry.

There are morsels of nourishment for everyone's respective biases in the excerpt. I'll reserve comments for later.

Ellen,

I'm a laypersom whom Dragonfly's many posts on the subject finally got to "take the plunge" and start reading about QM. I have to rely on books by scientists written for laypersons, which can give interested laypeople at least a little glimpse (but only a glimpse) into this fantastic world. It is crucial for the layperson to know his/her limits, and those who believe that reading a few books on QM for laypeople will suffice to "lecture" a great mind like Dragonfly on questions which present themselves to an expert who has delved as deeply into the subject as DF are in error (to put it mildly). I’m thinking of what just happened on the Logical Leap thread, where again a great opportunity for many posters and readers to learn more about a subject got ‘lost in altercation’ by unjustified polemic attacks against DF.

Layperson that I am, I therefore apologize beforehand if any of my questions on QM sounds foolish, and in case I talk nonsense, don't hesitate DF or others who have studied QM to point it out and correct the errors. I want to learn as much as I can (in the very limited scope that I have).

I'm currently reading a book on QM by Anton Zeilinger, its German title is Einsteins Schleier ('Einstein's Veil'). The book was published in 2003; on p. 92, Zeilinger mentions the Aspect experiments, and also the experiments John Clauser conducted with Steven Freedman in the USA

Zeilinger:

"Es [das von Steven Freedman und John Clauser durchgeführte Experiment] bestätigte bereits klar die Vorhersagen der Quantenphysik und zeigte eine Verletzung der Bellschen Ungleichung. Interessant war die Motivation von John Clauser. Er hatte es für unmöglich gehalten, dass die Welt tatsächlich so verrückt ist, wie dies die quantenmechanische Verschränkung impliziert. Umso glaubwürdiger ist daher sein experimentelles Ergebnis, das seiner Erwartung widersprach. Zeigt es ja, dass ein lokal realistisches Bild der Welt nicht haltbar ist. Die berühmtesten Experimente dieser Art sind die von Alain Aspect und seinen Mitarbeitern, die zu Beginn der Achtzigerjahre in Paris durchgeführt wurden. Diese Experimente waren lange Zeit die präzisesten Demonstrationen der Quantenphysik für verschränkte Paare.“

Attempted translation by me:

“It [the experiment conducted by Steven Freedman and John Clauser] already clearly confirmed the predictions by quantum physics and showed a violation of Bell’s inequality.

Interesting in that context was John Clauser’s motivation: for he had thought it impossible that the world actually is as crazy as the quantum entanglement implies. All the more credible is the result of the experiments. For it clearly shows that a local realistic concept of the world is untenable.

The most famous experiments of that kind were conducted by Alain Aspect and his co-workers at the beginning of the 1980s in Paris. These experiments have for a long time been the precisest demonstrations by QM for entangled pairs.”

Ellen, if memory serves (I can't find your post on that right now), you also mentioned Nicolas Gisin on the other thread.

Zeilinger wrote that Gisin could demonstrate quantum entanglement over a distance of 20 km.

Zeilinger wrote this in 2003, and you spoke about a more recent experiment by Gisin in 2008. What did it show?

Edited September 4, 2010 by Xray

[Ellen Stuttle]

Ellen, if memory serves (I can't find your post on that right now), you also mentioned Nicolas Gisin on the other thread.

Zeilinger wrote that Gisin could demonstrate quantum entanglement over a distance of 20 km.

Zeilinger wrote this in 2003, and you spoke about a more recent experiment by Gisin in 2008. What did it show?

Gisin's name was in one of the excerpts I posted from The Age of Entanglement, which I'm currently reading, but that book only tells the tale as far as 2005. The references to a more recent experiment followed from a link Shayne posted:

http://www.physorg.com/news132830327.html

I don't think the experimenters were named in the physorg news write-up. If they were, I've forgotten who they were.

Please keep in mind that I'm just learning the details of this subject myself -- though I've heard talk about it for years. I don't know the "curricula vitae" yet of most of the people in the field -- just the big names who are often referred to in physics circles (such as Einstein, Bohr, Schrödinger, Heisenberg, de Broglie, Bell, Bohm).

Also...please use paragraph breaks if you'd like replies from me. The first part of your post used them, but then you reverted to the no-space-between-paragraphs style. I have eye problems, and type done like that on a computer screen is extra hard for me to read.

Ellen

[Ellen Stuttle]

Attempted translation by me:

“It [the experiment conducted by Steven Freedman and John Clauser] already clearly confirmed the predictions by quantum physics and showed a violation of Bell’s inequality.

Interesting in that context was John Clauser’s motivation: for he had thought it impossible that the world actually is as crazy as the quantum entanglement implies. All the more credible is the result of the experiments. For it clearly shows that a local realistic concept of the world is untenable.

The most famous experiments of that kind were conducted by Alain Aspect and his co-workers at the beginning of the 1980s in Paris. These experiments have for a long time been the precisest demonstrations by QM for entangled pairs.”

Notice that Aspect also expected NOT to find the phenomenon. See his comment in the excerpt I posted (my underline):

"I was totally convinced by Einstein and Bell," he said. But what experiment to do? In rereading Bell's 1964 paper, Aspect realized that its last lines told him "there was still an important test to be done."

What his being "totally convinced by Einstein and Bell" means is that he thought that "a local realistic concept" would be the right one. ("Local" means communication at the speed of light or slower. "Realistic" means some cause which uniquely determines the result, instead of a probabilistic distribution. Einstein argued against Bohr. Bell sided with Einstein.)

Ellen

[Xray]

Attempted translation by me:

“It [the experiment conducted by Steven Freedman and John Clauser] already clearly confirmed the predictions by quantum physics and showed a violation of Bell’s inequality.

Interesting in that context was John Clauser’s motivation: for he had thought it impossible that the world actually is as crazy as the quantum entanglement implies. All the more credible is the result of the experiments. For it clearly shows that a local realistic concept of the world is untenable.

The most famous experiments of that kind were conducted by Alain Aspect and his co-workers at the beginning of the 1980s in Paris. These experiments have for a long time been the precisest demonstrations by QM for entangled pairs.”

Notice that Aspect also expected NOT to find the phenomenon. See his comment in the excerpt I posted (my underline):

"I was totally convinced by Einstein and Bell," he said. But what experiment to do? In rereading Bell's 1964 paper, Aspect realized that its last lines told him "there was still an important test to be done."

What his being "totally convinced by Einstein and Bell" means is that he thought that "a local realistic concept" would be the right one. ("Local" means communication at the speed of light or slower. "Realistic" means some cause which uniquely determines the result, instead of a probabilistic distribution. Einstein argued against Bohr. Bell sided with Einstein.)

Ellen

And what is called Bell's Theorem is actually the contradiction between Bell's original assumption and the results of his own QM experiments?

I also have a question regarding the "no conscious observer needed" issue.

First question: Am I correct in thinking that according to your understanding of current QM theory there's no longer considered to be a need for an "observer" in "collapsing" the wave equation?

Yes. That is: no conscious observer is needed. Sometimes the interaction with the environment is also called an "observation". See for example E. Joos et al. Decoherence and the Appearance of a Classical World in Quantum Theory, 2nd edition, Springer 2003, and Maximilian Schlosshauer, Decoherence and the Quantum-to-classical Transition, Springer 2007.

Does this mean that the mere possibility that the way the particles take through e. g. a double slit can be measured has as a result that no interference pattern can be seen?

As for Bohm's non-local hidden variable theory: it accepts the non-separability but would assume interaction betweeen the particles at superluminal speed?

Edited September 9, 2010 by Xray