Interesting this story
appears after the one about Lord Hagues warning on encryption.
It may be possible to send quantum-encrypted messages through space, after physicists showed a beam of light sent to a satellite could return to Earth with its quantum properties intact, according to new research published in Physical Review Letters. Quantum cryptography relies on the properties of quantum mechanics to encode …
"Well if that's the case, then you've found a way to communicate faster than the speed of light."
IANAQP but I think ... No because although the entanglement can occur at infinite distance, you can't use it to send information. You can query the particle, and be sure that your distant correspondent will see the same state that you do: the state, if you like, "has been communicated FTL" (in fact, it is the same state). But what you can't do is any meaningful information encoding.
Alice can't say: "I'll measure this and get a 1, and therefore send a 1 to Bob" but she can say "I measured this and got a 1, therefore Bob got a 1"
'Alice can't say: "I'll measure this and get a 1, and therefore send a 1 to Bob" but she can say "I measured this and got a 1, therefore Bob got a 1'
That's more or less my understanding of it too.
You can have a pair of entangled particles but their state is unresolved and therefore unknown until you examine one of them, at which point you know what the state of the other particle will be when it is examined, even though it may be arbitrarily far away.
An analogy would be to have two people, separated by distance, flipping coins; neither person knows whether they'll get a head or a tail until they flip their coin but once they have they'll know how the other coin will land when the other person flips it.
It would be possible to devise a protocol to make use of this, but not to convey information directly. For example, let's say the two coin-tossers, Alice & Bob, have agreed to send an encrypted message between them and have a choice of two encryption keys, 1 & 2. Alice & Bob agree that if Alice tosses her coin and gets a 'Head' then they'll use encryption key 1 but if her coin lands as a 'Tail' they'll use encryption key 2. When Alice tosses her coin and gets a 'Head' she encrypts the message using encryption key 1, and once Bob has tossed his coin he'll know he has to use encryption key 1 to decrypt the message; they haven't needed to transmit the information about which key to use. However, the message itself, once Alice has encrypted it, would still need to be sent via classical non-instantaneous methods.
"Then the next question... if you change the state of one... is the state of the other changed simultaneously too?"
Not quite, if my understanding is correct (who am I kidding, no-one understands this stuff.) If you have a pair of quantum entangled photons which have opposite polarisation, the state of the remote photon only becomes concrete once you observe the state of the near photon.
And no, they're not concrete before you make the observation. This was the subject of heavy debate between supporters of Albert Einstein and supporters of Niels Bohr throughout the first half of the 20th century, before an experiment using polarised lasers devised by John Bell in the 60's demonstrated that the 'spooky action at a distance' so despised by Einstein was actually happening.
It doesn't mean information can travel faster than light though, changing the state of the near photon doesn't instantly change the state of the remote photon, so it doesn't break the theory of General Relativity.
" I don't know why this is the case, though."
It is true because any attempt to manipulate the a quantum state destroys entanglement. All that can be done is to measure the state at which point we also know what state will be measured at the remote location.
So entanglement allows 2 parties to both observe the same random number but not to manipulate it. We can use the observed random number as a virtually unbreakable encryption key - which is why so much research is being done. Any actual communication is still limited to the speed of light.