Physicists Create Quantum Link Between Photons That Don't Exist At the Same Time 364
sciencehabit writes "Physicists have long known that quantum mechanics allows for a subtle connection between quantum particles called entanglement, in which measuring one particle can instantly set the otherwise uncertain condition, or 'state,' of another particle—even if it's light years away. Now, experimenters in Israel have shown that they can entangle two photons that don't even exist at the same time. Anton Zeilinger, a physicist at the University of Vienna, says that the experiment demonstrates just how slippery the concepts of quantum mechanics are. 'It's really neat because it shows more or less that quantum events are outside our everyday notions of space and time.'"
Re:Wait for the retraction (Score:5, Informative)
We already knew that.
Whatever "we" you mean count me out.
According to GR gravity is facilitated via a retarded potential [wikipedia.org], and of course GR survived so far every conceivable test and has been shown to make correct predictions were Newtonian gravity failed.
So no, gravity does not operate faster than light.
The summary makes a bigger deal of this than it is (Score:5, Informative)
When you read the article, this isn't actually too controversial. All that's being done is changing the timing of of when the measurements are taken and when the intermediate photons become entangled. It's really just using the entanglement process to spread out the time over which the quantum state data is transmitted. You basically have a quantum data historical record.
I can certainly see this opening up useful new capabilities in quantum computing and measurement of quantum phenomena, but it doesn't change our understanding of quantum events and how they interact with our "everyday notions of space and time.".
Re:Wait for the retraction (Score:5, Informative)
Yeah, this. What the AC may be confused about is that faster than light travel is (as far as we know) not possible in space, but the distance between two points can increase faster than light could travel because there's nothing stopping space itself from expanding that fast.
Re:Photon model broken (Score:5, Informative)
Today, we call them "solid", "gas", "plasma", and "liquid" respectively.
Re:Wait for the retraction (Score:3, Informative)
Dark energy - a term coined to hide the fact that "we don't know". Dark energy seems to be accelerating the expansion after a period of deceleration, this is baffling but fits observational results. The theory is that gravity used to slow the expansion down, but apparently we passed a cut-off point where space has become stretched enough so that gravity is too weak - another force is taking over and stretching space again. A force with no obvious cause, not to us at least.
Re:Wait for the retraction (Score:5, Informative)
Entanglement can be used to exchange keys for secret communication. It allows two parties to create a shared key without anyone being able to intercept it. In principle, this key can be as long as the message itself and perfectly random, so a simple 'xor' operation is all it takes to make the message completely undecryptable. In more detail:
Alice wants to send a secret message to Bob.
They (or anybody else, really) create a bunch of entangled photons, half going to Alice and the entangled counterparts going to Bob. This all happens at normal speeds (not faster than light), but can be prepared in advance.
If anyone tries to eavesdrop during transmission of the entangled photons, Alice and Bob are able to detect the fact that the photons are no longer in a superimposed state and start over with a new bunch.
Now Alice and Bob measure the photons. They have no control over the outcome of the measurements, which will be completely random, but they do know that they will both get the same result (or rather, exactly the opposite result). This becomes their cryptographic key.
Now Alice encrypts her message with this key and sends it to Bob using traditional communication channels, for example a carrier pidgeon.
Bob uses his identical key to decrypt the message.
The only faster-than-light part of the story is that the entangled photons "chose" their state at the time of the measurement. Before the measurement, they were in a superimposed state. This means the information for the key didn't even exist yet in any way and can therefore never be intercepted by anyone. It only came into existence at the time the photons were measured, simultaneously for Alice and Bob. (Take the word "simultaneously" with a grain of salt, because as the article shows, they can even be separated in time). And the encrypted message without the key is just a series of random bits.
Re:Before and after (Score:2, Informative)
AFAIK, it's simplest to describe things with the time-independent Schrödinger equation [wikipedia.org]: H Psi = E Psi. This is close enough for most stable molecular states in chemistry. However, if you're talking about state transitions or spectroscopy or (as in this case) entanglement, you have no choice but to use the time-dependent Schrödinger equation: i h-bar d Psi / d t = H Psi, which is MUCH more difficult.
That H is not a variable but any suitable Hamiltonian operator [wikipedia.org], which usually has second-derivative goodness AND a complicated Psi-dependent potential energy term. BTW whoever made that Wikipedia page on the Hamiltonian: thank you, it's very clearly written!
AFAIK, entropy is not described at the quantum level, so Ilya Prigogine's "arrow of time" doesn't really exist, and you can just reverse the behaviour of time by putting a minus sign in front of it and see what happens. In Physics, I read this can be done by drawing a Feynman diagram and turning it upside-down.
I hope this helped answer your question...
N.B. if any "real" quantum chemist or quantum physicist reads this and cringes, please mod this down; it's better to give no information than false information!