Quantum entangled-photon Chinese satellite.

[Mirimir]

Here, from <http://www.scottaaronson.com/blog/?p=2464>:

> A few weeks ago, Hensen et al., of the Delft University of Technology
> and Barcelona, Spain, put out a paper reporting the first experiment
> that violates the Bell inequality in a way that closes off the two
> main loopholes simultaneously: the locality and detection loopholes.
> Well, at least with ~96% confidence. This is big news, not only
> because of the result itself, but because of the advances in
> experimental technique needed to achieve it. Last Friday, two
> renowned experimentalists—Chris Monroe of U. of Maryland and Jungsang
> Kim of Duke—visited MIT, and in addition to talking about their own
> exciting ion-trap work, they did a huge amount to help me understand
> the new Bell test experiment. So OK, let me try to explain this.
> 
> While some people like to make it more complicated, the Bell
> inequality is the following statement. Alice and Bob are cooperating
> with each other to win a certain game (the “CHSH game“) with the
> highest possible probability. They can agree on a strategy and share
> information and particles in advance, but then they can’t communicate
> once the game starts. Alice gets a uniform random bit x, and Bob gets
> a uniform random bit y (independent of x). Their goal is to output
> bits, a and b respectively, such that a XOR b = x AND y: in other
> words, such that a and b are different if and only if x and y are
> both 1. The Bell inequality says that, in any universe that satisfies
> the property of local realism, no matter which strategy they use,
> Alice and Bob can win the game at most 75% of the time (for example,
> by always outputting a=b=0).
> 
> What does local realism mean? It means that, after she receives her
> input x, any experiment Alice can perform in her lab has a definite
> result that might depend on x, on the state of her lab, and on
> whatever information she pre-shared with Bob, but at any rate, not on
> Bob’s input y. If you like: a=a(x,w) is a function of x and of the
> information w available before the game started, but is not a
> function of y. Likewise, b=b(y,w) is a function of y and w, but not
> of x. 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. Clearly, at the moment two “entangled” particles are
> created, but before they separate, one of them flips a tiny coin and
> then says to the other, “listen, if anyone asks, I’ll be spinning up
> and you’ll be spinning down.” Then the naïve, doofus physicists
> measure one particle, find it spinning down, and wonder how the other
> particle instantly “knows” to be spinning up—oooh, spooky!
> mysterious! Anyway, if that’s how you think it has to work, then you
> believe in local realism, and you must predict that Alice and Bob can
> win the CHSH game with probability at most 3/4.
> 
> What Bell observed in 1964 is that, even though quantum mechanics
> doesn’t let Alice send a signal to Bob (or vice versa) faster than
> the speed of light, it still makes a prediction about the CHSH game
> that conflicts with local realism. (And thus, quantum mechanics
> exhibits what one might not have realized beforehand was even a
> logical possibility: it doesn’t allow communication faster than
> light, but simulating the predictions of quantum mechanics in a
> classical universe would require faster-than-light communication.) In
> particular, if Alice and Bob share entangled qubits, say
> $$\frac{\left| 00 \right\rangle + \left| 11
> \right\rangle}{\sqrt{2}},$$ then there’s a simple protocol that lets
> them violate the Bell inequality, winning the CHSH game ~85% of the
> time (with probability (1+1/√2)/2 > 3/4). Starting in the 1970s,
> people did experiments that vindicated the prediction of quantum
> mechanics, and falsified local realism—or so the story goes.

Discussion in <https://news.ycombinator.com/item?id=10269297>

OK, so local realism is dead. But what is it? An excerpt from above:

> 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.

And here's the tl;dr:

> (And thus, quantum mechanics exhibits what one might not have
> realized beforehand was even a logical possibility: it doesn’t allow
> communication faster than light, but simulating the predictions
> of quantum mechanics in a classical universe would require
> faster-than-light communication.)

Another chunk:

> The violation of the Bell inequality has a schizophrenic status in
> physics.  To many of the physicists I know, Nature’s violating the
> Bell inequality is so trivial and obvious that it’s barely even
> worth doing the experiment: if people had just understood and
> believed Bohr and Heisenberg back in 1925, there would’ve been no
> need for this whole tiresome discussion.

Me, I like the many worlds interpretation. But it's just an
interpretation. What matters is the math.