[General Battuta]

Basically the whole uncertainty thing is a theoretical construct and you could just as accurately say that no data transmission ever took place and the spin of each particle was predetermined. I know, sometimes interpretations of quantum physics are unnecessarily confusing.

This is a problematic statement. One argument (the hidden variables interpretation) claims this, but there's some experimental evidence that there are actually no hidden variables and that the spins really are in a state of quantum superposition until the moment of measurement.

You're trying to simplify quantum mechanics to a classical approximation - believe me, we'd all like to. But unfortunately that hasn't been viable for several decades.

[Ghostavo]

How would quantum entanglement help with communication, even if you had another communication channel then?

[General Battuta]

How would quantum entanglement help with communication, even if you had another communication channel then?

Here's some reading related to cryptography:
http://en.wikipedia.org/wiki/Quantum_cryptography
http://en.wikipedia.org/wiki/Quantum_key_distribution

These rely on the combination of quantum entanglement with a classical channel.

I don't feel confident enough about my quantum kung fu to speak to more elaborate applications.

[samiam]

Quantum entangle-o-gragy communication works all the time in Mass Effect 2, I don't see the problem.

This is a problematic statement. One argument (the hidden variables interpretation) claims this, but there's some experimental evidence that there are actually no hidden variables and that the spins really are in a state of quantum superposition until the moment of measurement.

That's not an argument, it's an interpretation. It's a simpler and more GD-worthy interpretation than the cat state.

[Nuke]

you should probibly look at the way traditional wireless communication works. anything in the radio spectrum needs a carrier wave which is modulated with the information you want to send. at the other end you look for a frequency that is close to to the desired carrier wave, and when you have locked on to that wave form you can then subtract the carrier from it and retrieve the information (there are different modulations you can use for analog or digital data). in addition, when dealing with digital information you also need to have a protocol for identifying and retrieving frames. when the vaue can either be 0 or 1, you dont have a unique value for when a frame starts. usually a transition from the idle state followed by a bit pattern denote the start of a frame, then comes data, and some form of error checking (to both detect errors and to confirm that what was received was in fact a frame). optical systems probably work slightly different, but im not going into that, but you still need a protocol to make sure that what you're getting is data.

for entanglement to work as a means of communication you would need to be able to predictably control a particle's state. which according to the posts in this thread is not how entanglement works. for it to work at the other end you would need to see the state of the particle at all times, which again is impossible. you need to see everything, even the noise, to lock onto a data frame.

[Ghostavo]

But if the entanglement "snaps" when you measure it, won't that make it useless except as a random number generator that two parties can secretly share?

[samiam]

I guess it would, and I have no idea where Bioware came up with the idea of quantum communication.

[Bobboau]

You can change a property of an entangled particle and the change manifests in the other

ok, I must have misunderstood you right here, what I originally meant (and you were responding to here) was that you can change the property of a particle, after both sides had measured it, and then when measured again both particles would have maintained their relationship

[General Battuta]

Quantum entangle-o-gragy communication works all the time in Mass Effect 2, I don't see the problem.

This is a problematic statement. One argument (the hidden variables interpretation) claims this, but there's some experimental evidence that there are actually no hidden variables and that the spins really are in a state of quantum superposition until the moment of measurement.

That's not an argument, it's an interpretation. It's a simpler and more GD-worthy interpretation than the cat state.

It's an interpretation that has at least some experimental evidence stacked against it, and it may (I'm not up on my quantum) have been ruled out entirely by the Bell experiments.

[samiam]

Extensions of QEM both with and without hidden variables provide the same accuracy of prediction, although I'm not quite sure what that means.

In any case a Bell test involves using polarizers to force a certain state, not merely observing a quibit, so it's not quite the same process. Observing a quibit is still going to produce results consistent with nonlocal hidden variables.

[General Battuta]

The Bell theorem rules out all local hidden variable theories, though not nonlocal ones.

[samiam]

Whoop, I just edited my post.

Bohm theory agrees perfectly with nondeterministic quantum mechanics. It's a more parsimonious explanation for superposition, unless the discussion keeps going on and gets into many worlds or pilot waves.

[Mikes]

Enjoy being unable to secure your passwords by this time next year! Also Skynet.

No matter how often I read it Samiam,... that one liner you wrote there still cracks me up! Also Skynet.

lol.

[BengalTiger]

I guess it would, and I have no idea where Bioware came up with the idea of quantum communication.

Probably in the same place George Lucas came up with light sabers.

Thought experiment:

Let's say there are 16 coins, all entangled in pairs. The sender has 8 and the receiver has the rest. They are on the opposite sides of the known universe.

The sender tosses 7 of them, while the number 8 one is measured by the receiver.
The sender checks whether he has heads or tails on the 7 coins he flipped. He then tosses the coins that have a tail but need to have a heads and vice versa, until he gets the right combination.
The receiver keeps on measuring the 8th coin, only to find out it's still laying on the same side.

When the sender's coins are flipped so that they represent the information to be sent, he measures the 8th coin and flips it until he gets the opposite side.
The receiver then finds the 8th coin changed sides and reads the other 7, does a NOT operation to each one and gets the message.

Now since quantum comms are impossible, where is the flaw in this thought experiment?

[samiam]

The very act of measuring a coin flips it. So the chance that you're receiving a message or a random observation is 50/50.

[BengalTiger]

Well then, the receiver measures the number 8 coin and gets:

H->T->H->T->T; and the double tails means it was flipped on the other side, time to receive the message.

[Ghostavo]

You can't measure a particle OR impose a state without severing the link.

Otherwise it wouldn't be any more difficult than transmitting information over the (STL) wire.

[General Battuta]

Thought experiment:

Let's say there are 16 coins, all entangled in pairs. The sender has 8 and the receiver has the rest. They are on the opposite sides of the known universe.

The sender tosses 7 of them, while the number 8 one is measured by the receiver.
The sender checks whether he has heads or tails on the 7 coins he flipped. He then tosses the coins that have a tail but need to have a heads and vice versa, until he gets the right combination.
The receiver keeps on measuring the 8th coin, only to find out it's still laying on the same side.

When the sender's coins are flipped so that they represent the information to be sent, he measures the 8th coin and flips it until he gets the opposite side.
The receiver then finds the 8th coin changed sides and reads the other 7, does a NOT operation to each one and gets the message.

Now since quantum comms are impossible, where is the flaw in this thought experiment?[/color]

This doesn't work on two levels.

1) You cannot assign a coin a value. It comes up randomly heads or tails when you look at it. You cannot flip the coin more than once; once you look at it, it's either heads or tails, now and forever.

2) As stated quite clearly earlier in the thread, it is impossible to tell whether the other side has already examined the coin when you look at it. There's no way to distinguish 'this coin has never been examined, and I've just examined it' from 'the other side has examined this coin, and I'm seeing the opposite of what they're seeing'.

Think of it this way: you're given 8 coins in a box with a closed lid. You're told that either someone flipped all the coins and then put them in the box as they fell, or the coins have all been flipped randomly inside the box after it was closed. You're told that you have to determine which, or they'll kick your dog.

How do you stop your dog from being kicked? You can't. Either way the coins were flipped randomly.

[watsisname]

H->T->H->T->T; and the double tails means it was flipped on the other side

No it doesn't.  Each measurement has a 50/50 chance of being spin UP or DOWN.  Observing the particle doesn't guarantee the spin will flip anymore than flipping a coin guarantees you'll go from H->T or vice versa.

To reiterate Battuta, this means there is no way of knowing if the observed spin of your particle is a result of your measurement, or due to the entanglement effects from the other side making their measurement, without a carrier signal, which is not FTL.

Also, what Ghostavo said.

edit:  And what Battuta ninja'd with. D:

[samiam]

Assuming that it was possible to violate the Bell Inequality and create particles with identical spin, I wonder how badly that would screw physics up. Would you get a perpetual magnet motion milkshake or something?