Can Entangled Particles Be Used for Quantum Communication?

[Lost in Space]

I've read that supposedly no coherent information can be instantaneously teleported across space between single pairs of entangled particles because of random fluctuations. But might a way of overcoming this be to take two separated sets of more than one pair of entangled particles and arrange and contain each set in an array or ring and send and read their states as a binary bitstream, i.e. affected/unaffected?

 

[SpectraCat]

I've read that supposedly no coherent information can be instantaneously teleported across space between single pairs of entangled particles because of random fluctuations. But might a way of overcoming this be to take two separated sets of more than one pair of entangled particles and arrange and contain each set in an array or ring and send and read their states as a binary bitstream, i.e. affected/unaffected?

I used to think that it might be possible, and I wasted a fair amount of time messing around with gedanken experiments to try to show that. The "randomness" you mention is not the most important issue (at least I don't think it is). Ultimately, what I realized is that, once the observers (Alice and Bob by convention) are separated by a space-like interval, then they cannot know whether it is their measurement that "collapsed" the entangled pair, determining both values, or whether they are reading the value created when their partner's measurement "collapsed" the entanglement. In fact, relativity tells us that there is no way to uniquely determine the order of the two measurement events when they are occurring in non-intersecting light-cones. So one observer might see that Alice's measurement came first, while the another would say that Bob's measurement came first ... however all observers agree that, whichever measurement they think came first, the entanglement was broken and the states of *both* particles were determined immediately after that first measurement.

The only way information can be extracted from such a channel is if Alice and Bob compare their results some how, but of course that can only happen if the exchange information over a normal (i.e. lightspeed or slower) channel. This is why people say that quantum entanglement does not violate relativity, even though the "collapse" of the entanglement seems to happen instantaneously, even for space-like separation of measurement events. Note that this has been tested experimentally .. check Dr. Chinese's website for summaries of and links to the relevant papers.

 

[Lost in Space]

I used to think that it might be possible, and I wasted a fair amount of time messing around with gedanken experiments to try to show that. The "randomness" you mention is not the most important issue (at least I don't think it is). Ultimately, what I realized is that, once the observers (Alice and Bob by convention) are separated by a space-like interval, then they cannot know whether it is their measurement that "collapsed" the entangled pair, determining both values, or whether they are reading the value created when their partner's measurement "collapsed" the entanglement. In fact, relativity tells us that there is no way to uniquely determine the order of the two measurement events when they are occurring in non-intersecting light-cones. So one observer might see that Alice's measurement came first, while the another would say that Bob's measurement came first ... however all observers agree that, whichever measurement they think came first, the entanglement was broken and the states of *both* particles were determined immediately after that first measurement.

The only way information can be extracted from such a channel is if Alice and Bob compare their results some how, but of course that can only happen if the exchange information over a normal (i.e. lightspeed or slower) channel. This is why people say that quantum entanglement does not violate relativity, even though the "collapse" of the entanglement seems to happen instantaneously, even for space-like separation of measurement events. Note that this has been tested experimentally .. check Dr. Chinese's website for summaries of and links to the relevant papers.

Thanks for your reply, Spectracat. I appreciate that there is no way to establish who is responsible for the collapse across space without confirmation via a lightspeed or lower channel, but what I was asking was if it's really necessary to know who's responsible, and is there a case that all one needs to know is whether there has been a collapse or not in a given particle? If a collapse can be identified, perhaps there's a way of reading the particles in a series, comparing each to its last known state and establishing whether there has since been a change of state and converting this to a binary value?
It's interesting what you say about QE not violating relativity. Do you think then that entangled particles could also be subject to relativistic time effects? If one of an entangled pair were to be accelerated to sublight speed or placed within the vicinity of a black hole would the resultant time dilation mean that any collapse would no longer be instantaneous? If this is true and there was indeed a time lag between the particles wouldn't that mean that any generation of a collapse in the unaccelerated or gravitationally unaffected particle would seem to be generated in the future of the other, whose time was slowed down relative to it? And if we were to bring the particles together once more in 'normal' spacetime after experiencing a relativistic time differential, would the time differential remain or could it be the two particles somehow come back to a shared 'present'? Also, could it be possible that a relativistic time differential may have a significance in determining who is actually collapsing the state?
Lots of questions I know, but as far as I'm aware no experiments have been carried out for temporal separation although reportedly information has been experimentally teleported across space.

 

[Lost in Space]

Just to add to my last post, I still think it may be possible to bypass the problem of being able to determine who collapses the state albeit in a restricted kind of way. If we know that a state of a particle in an array has been unaffected we can call that value 0. If we know that a state has been collapsed there are two possibilites of who collapsed it. Perhaps if we had specific numerical values attached to precoded messages, could we then with the aid of a computer, close in on the message by filtering the randomness? Something like a chess player who only fixates on sensible moves?

 

[Ryan_m_b]

Heres an analogy I often use to explain why entanglement can never be used for communication.

Alice and Bob both have black boxes. Inside each box is a spinning coin. To open a box a button must be pressed on the top of the box, the box lid will open 1 second later. In the second between the press of the button and the opening of the lid the coin will fall showing either heads or tails. Correspondingly the coin in the other box will also fall over (showing the opposite side of the coin).

Bob wants to know if Alice has sent him a message so he presses his box and sees the coin is heads. But Bob has no way of knowing if the coin is heads because he pressed the button or if Alice pressed her button at some earlier time. The only way to know is to contact her and ask by conventional means.

You see the problem? One can never know if the info one is measuring is from somebody else or because of the action of checking for a message itself.

 

[San K]

If we know that a state of a particle in an array has been unaffected we can call that value 0. P

how would we/alice/bob know that the state of the particle is unaffected (or affected)?

...without Alice and Bob communicating at speed of light or slower...

 

[Ryan_m_b]

how would we/alice/bob know that the state of the particle is unaffected (or affected)?

...without Alice and Bob communicating at speed of light or slower...

They couldn't, that's the problem with a QCD

 

[Khashishi]

Suppose you give up on being certain about the result of a quantum communication, and settle for probability greater than 50% of transmitting a bit of information correctly faster than light. Is this possible at all? I suspect the answer is still no.

 

[O.B.1]

Well I'm not as clued up on entanglement as you guys are so forgive me for being a layman, this is how I see it...

If it were possible to set a specific state for a particular particle, then perhaps the following method would work...

Referring to each particle as a bit, for ease of understanding.
According to what I've read about about entanglement, a bit can either be in one state or another, but we can't tell which state it will be in until we observe it.
What we can tell, is that the other bit will be in the opposite state than the bit we've just observed.

Hence, an array of bits will either be an image, or a negative image, at any given time of observation. The state of all bits in the array would have to be synced, and set at the same time to their appropriate values in order to get a complete image.

It's easy enough to determine if the image is negative and to correct it on one side or the other, so it doesn't really matter if the bits are constantly switching states, just as long as they're all switching at the same time.
The big question is, can a bit be set to a specific state in comparison to the other bits in the same array.
For instance, can I set all bits to State-A, and then the last 3 bits to State-B?
So that I have five particles, who's state represents X and three particles who's state represents Y?
This would mean that my entangled particles on the other side, at any given moment in time,
will read either 00000111, or 11111000.

If so, then the next thing is to have have two separate arrays, for sending and receiving, in each device.

We'll also set specific time constants for sending and receiving.
(Using minutes as an example time interval...)
Alice can only set her sender-bits on the first time constant, and Bob can
only read his receiver-bits on the second time constant.
Bob and Alice's devices must obviously be synced in time exactly.
It's also important that Bob reads his receiver bits continuously, whether Alice's bits have changed or not, (meaning he will get the same image until Alice changes her bits)

Another example. I send an image on my device.
At 1:00am my sender-bits are encoded with the image.
(I can encode my message at 1:00, 1:02, 1:04, etc)

At 1:01am the other device checks its receiver bits (as it would on 1:03, 1:05 etc) and finds and update has been made.
Whether the bits are in their first state or second state is irrelevant, as long as their states are consistent among each other.
This way we get a full image, either negative or normal.
If the image is negative after compiling all the bits, then we simple correct it programatically.

 

[DrChinese]

For instance, can I set all bits to State-A, and then the last 3 bits to State-B? So that I have five particles, who's state represents X and three particles who's state represents Y?
This would mean that my entangled particles on the other side, at any given moment in time,
will read either 00000111, or 11111000.

No. If you set a Alice's bit to a specific value, it will be set to a specific value for Bob. It will not be entangled. For your idea to work, we want to send a message from Alice to Bob. Alice needs to do something that leads to a specific result for Bob. There is no such action, other than collapsing the wave function in a specific basis, that Alice can do. On the other hand, there is no way to determine if Bob did that either. Time ordering does not matter for that function.