Quantum Theory Experiment Said to Prove ‘Spooky’ Interactions
nytimes.com
nytimes.com
"There are two particles entangled together such that when one becomes up or down, the other becomes the opposite, and measuring either will determine both, instantaneously, regardless of distance" is a much harder thing to swallow.
Occam's Razor says that there are hidden variables that are pre-selecting state, and we just don't know about them until we measure. ESPECIALLY when one of the requirements for the system is that we can only measure, not influence the outcome of the measurement.
So please, someone who knows more than I do - why can't there be hidden variables being set at entanglement that pre-determine the measurement outcome?
The real problem is that when you measure A in the "up" direction, and then B in the "10 degrees east of up" direction, then B seems to know that you measured A in the "up" direction.
That is to say: B's probability distribution as a function of the direction its being measured is dependent on the direction that A is measured. There's no way to construct an "A-independent" probability distribution of B's results for arbitrary directions. The probabilities won't sum to 1 and still match experimental results.
It's unfortunate that "A up" therefore "B up" is a degenerate case of this reality where classicality actually works, because it leads to confusion.
Also, follow the links others have provided for a more formal explanation :)
exactly. The probabilities are 1 only in theory. For example theoretical Malus law over all angles would give 1. On practice - i couldn't find single photon Malus law confirmation. A slight deviation from Malus law like cutting the tail at high angles off and thus decreasing the total to below 1 (ie. when/if photons at high angles being lost disproportionally much more intensively than Malus law states) would bring the experimentally measured S to values higher than 2 while still being in the realm of local realism.
"Lecture 20: Bell inequalities and nonlocality" from John Watrous of the University of Calgary [2] is a short and accessible introduction to the CHSH game. fizx mentioned Scott Aaronson's "Bell inequality violation finally done right" [3] which is about the same recent experiment as the NYT article but much better. It contains a concise description of the CHSH game.
[1] https://en.wikipedia.org/wiki/Bell's_theorem
[2] https://cs.uwaterloo.ca/~watrous/CPSC519/LectureNotes/20.pdf
It's not strictly correct to say that Occam's Razor says we should have hidden variables. Occam's Razor is the idea that in deciding between two or more similar theories (which both have to be correct), we should prefer the simpler of the theories. Bell's theorem tells us that we should reject the notion of local hidden variables.
Importantly, this does allow the notion of nonlocal hidden variables. One such theory is the de Broglie-Bohm theory, which claims that the wavefunction is an actual physical entity, referred to as the pilot wave or guiding wave. It would be nonlocal because the wavefunction collapse of one particle to measure its properties would necessarily influence the other particle at superluminal speeds. Another nonlocal hidden variable theory is superdeterminism, which claims that everything that ever happened and ever will happen has already been determined, so the correlations are not a result of some spooky interaction, they were just "predestined" to be correlated. This would again be nonlocal because the universe as a whole would have knowledge of the particle properties a priori.
Also, it should be noted that quantum entanglement does not allow information to travel faster than light.
The Delft researchers were able to entangle two electrons separated by a distance of 1.3 kilometers, slightly less than a mile, and then share information between them.
...
Researchers like Dr. Hanson envision a quantum communications network formed from a chain of entangled particles girdling the entire globe. Such a network would make it possible to securely share encryption keys, and know of eavesdropping attempts with absolute certainty.
This implies that entanglement allows faster-than-light communication (transmitting arbitrary information across the entangled link). I thought that was a no-no, and that entanglement still didn't allow FTL communication despite FTL-like properties.
Can somebody help out here?
A better discussion can be had at http://www.scottaaronson.com/blog/?cat=33
Perhaps the best way to explain local realism is that it’s the thing
you believe in, if you believe all the physicists babbling about
“quantum entanglement” just missed something completely obvious.
The finding is another blow to one of the bedrock principles of
standard physics known as “locality,” which states that an object is
directly influenced only by its immediate surroundings.
- no realism
- no locality
- neither realism nor locality
Any theory, such as quantum mechanics, that violates Bell's
inequalities must abandon either locality or realism;
There is also a third option that almost no-one mentions, and that's superdeterminism; i.e. that free will doesn't exist.
http://plato.stanford.edu/entries/qm-relational/
I'm not super knowledgeable about this (and whether it even meets the criteria), but perhaps you will find it interesting.
Basically exactly the same as this http://www.flownet.com/ron/QM.pdf (5.3), but in a well-known textbook or some review paper in a journal with good reputation. I run from time-to-time into a discussion which just needs such pointer...
http://arxiv.org/pdf/quant-ph/9605002v2.pdf
I don't know if that exact paper was ever published, but an earlier version was formally published here:
C. H. Adami and N. J. Cerf, “Information Theory of Quantum Entanglement and Measurement,” Physica D 120 (1998) 62-81.
But this paper turns out to be a rehash of an older result (Zurek, 1991)
http://arxiv.org/abs/quant-ph/0306072
which in turn is more or less a rehash of an even older result (1955) by von Neuman:
http://arxiv.org/pdf/1311.7649v1.pdf (See the references for the original source.)
And there is some evidence that even Bohr and Heisenberg actually understood this.
Basically, the physics community bifurcates among those who deny this is true (e.g. Lubos Motl) and those among whom it is considered common knowledge. Indeed, when I submitted my paper to Physics Today back in 2000 it was rejected on the grounds that it was nothing new.
The hard part of QM is not understanding it. The hard part is accepting that it is true. Because people really want to believe that the reason that we can make reproducible measurements is that these measurements are a faithful reflection of a underlying metaphysical reality that is actually "out there" in some sense. But that turns out not to be the case. As David Mermin so eloquently puts it (http://arxiv.org/abs/quant-ph/9609013) we are in fact made of "correlations without correlata."
I wish some student somewhere would publish a review paper in some reputable place, clearly describing this cascade of entanglements from start to the end using an example of photomultiplier. Cascade starting from a photon and ending with a 'classical bit' sitting in the memory cell...
https://en.wikipedia.org/wiki/Measurement_in_quantum_mechani...
and
https://en.wikipedia.org/wiki/Quantum_decoherence
?
If you don't like Wikipedia, the references section are chock-full of "real" papers, including this one:
A more parsimonious reading of the article would assume that the author knows, and assumes you know, that FTL communication is impossible, and that the interesting aspect of the communication possible through an entangled quantum network is, as stated, its security and resilience to eavesdropping.
But since the 1970s, a series of precise experiments by physicists are increasingly erasing doubt — alternative explanations that are referred to as loopholes — that two previously entangled particles, even if separated by the width of the universe, could instantly communicate.
So I don't think my reading was insufficiently parsimonious.
Edit:
Entanglement is akin to the following... Assume Alice and Bob are distantly separated. Alice has an arbitrary float variable called x. Alice and Bob both have a function rand() generating floats between zero and one, with a shared seed so that they can both generate the same random numbers.
Classical correlation is that they both can call rand() with their identical seeds to get the same random numbers.
Entanglement is that Bob's rand() is actually ((x + rand()) % 1), where Bob instantly and magically has access to Alice's variable x. No information can be transmitted, because the distribution of random numbers hasn't changed, and Bob doesn't have direct access to x. But something is different, and if Bob is clever, he might be able to do some fun stuff.
This isn't a perfect analogy, but perhaps it helps.
Really? That's not random, that's pseudo-random, so the secret is still just the seed distributed before hand. True randomness would be entirely uninvolved from other state, including location or time. This is really just combining the sharing and the generating into one process so you CAN share true randomness without sharing other state.
In other words, let's not blame the Times for something they did not actually say.
Instead what I inferred from this is that since entanglement can only be resolved once per pair of particles, a communications system based on entanglement might provide perfect forward secrecy even in an environment in which very large numbers are able to be factored by quantum computers.
(copied from my response to a similar comment above):
But since the 1970s, a series of precise experiments by physicists are increasingly erasing doubt — alternative explanations that are referred to as loopholes — that two previously entangled particles, even if separated by the width of the universe, could instantly communicate.
I don't think there's a good English word for the concept of an influence that isn't causal and doesn't transmit information. That's why it's often called "spooky action at a distance" and why attempts to describe it with simple English sentences are likely to lead to confusion.
The quotes you posted above refer to human communications, and don't mention FTL. That's what I responded to.
The quote you posted now, is clearly referring to the fact that the particles both react to one measurement instantly, regardless of difference. It makes no mention of people communicating with one another FTL.
Edit: I agree with tgflynn that the Times just overloaded the word "communicate."
I believe quantum key sharing requires a classical channel in addition to the quantum channel, hence no information is transmitted faster than light.
This sounds like lazy evaluation.
From the first sentence:
"[Scientists] conducted an experiment they say proves one of the most fundamental claims of quantum theory — that objects separated by great distance can instantaneously affect each other’s behavior."
Emphasis mine. This is totally wrong.
The second sentence:
"The finding is another blow to one of the bedrock principles of standard physics known as “locality,” which states that an object is directly influenced only by its immediate surroundings."
Again, this is wrong. Quantum physics is entirely consistent with locality. It contradicts "local hidden variable models." Despite the similar name, this is different from locality. (Local hidden variable models are local, classical models.)