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Comment by chulan | Hacker News Reader

Quantum entanglement seems to allow instantaneous info transfer over any distance. Any Quantum Physicists on here?

Any Quantum Physicists on here?

j0.

Indeed it does seem to allow that, and this is right at the heart of the quantum measurement problem. The three popular ways out of this are:

1. To ignore it entirely, stating that we don't really care about the implications of quantum mechanics and are just happy that it's useful for calculating things. LA LA LA!

2. To declare that yes, well, maybe it sort-of does, but it doesn't matter because there's no actual information being transmitted. I can't use quantum entanglement to send a message to Alpha Centauri, since I have no control over the sequence of bits which get transmitted; the most I can do is ensure that both they and I wind up with the same string of completely random bits.

3. Go to a no-collapse ("many-worlds") theory of quantum mechanics, in which there isn't any superluminal communication required (why? It's nonobvious and hard to explain but if you're willing to trust me then you can just go along with it for now...)

My personal preference is for (3) but the implications of many-worlds theories are so horrifying that it remains quite controversial.

Not _info_ transfer but it allows instantaneous collapse of the entangled state into known states after the measurement is done on a entangled system.

Here is how I remember it (and I am just a comp sci nerd, not a physicist so please correct me):

* you have 2 qubits Q1 and Q2. You entangle them. Now you have an entangled system Q1Q2.

* You separate Q1 and Q2 in space. Let's say you put Q2 on a spaceship and blast it into space, but leave Q1 in the lab on earth.

* After some time you measure Q1 to get its value. At that point the entanglement of Q1Q2 collapses. You now know the value of Q1 that you just measured and at the same time Q2 is forced to a known value too.

* At _appears_ as if you could send information this way but you cannot. Think about it. You don't know what value you'll measure in the lab for Q1. So you can't force Q2 to be a certain value either.

* Let's think about it another way -- suppose you tell the spaceship operator that if Q2 collapses to |0> then they should turn the spaceship immediately around and head back home and if it collapses to |1> they should arm their weapons and prepare for an alien attack. Now you are on earth in control of Q1, and you want to force Q2 to be measured to |1> because you know the aliens are coming. There is nothing you can do to Q1 to force Q2 to be measured as |1>.

OK, but can you detect in the spaceship when Q2 collapses? (I know no quantum physics beyond a few articles0

If so why can't you simply modulate the signal on top of a series of collapsing entangled pairs. Basically morse code with the timing of the collapses, you don't care what value they collapse too.

Nice try, but no - basically, to use your phrasing, there's nothing to detect when Q2 collapses.

No. You can ask the spaceship to measure Q2 after a certain period of time. So "fly away and at a specific time T measure Q2" would be the command. Sorry I should have made that more clear.

Some people like to point out that technically the information isn't traveling through space, but to me this seems like a rather academic distinction. This is the relevant wikipedia article:

http://en.wikipedia.org/wiki/Bells_theorem