# Entanglement com

1. Jun 21, 2013

### jaramsli

I am writing a sci-fi story that contains an 'entanglement com', a point-to-point connection between Earth and some distant planet, allowing for real-time communication.

I know absolutely nothing about physics, but I know there is a problem with observing a qbit, the other bits will realign themselves to become "consistent with the observation" - don't remember where I read the quoted text,
probably some computer magazine back in the 20th century.

So, my question is: How would the entanglement com work, if at all?

2. Jun 21, 2013

### Staff: Mentor

It does not work at all. According to the current laws of physics, it is impossible to transmit any information faster than the speed of light. Entanglement does not help in any way, you would still need a classical channel to transmit information.

3. Jun 21, 2013

### Tenshou

What are quantum computers about? I thought they were about getting information to move through entanglement?

4. Jun 21, 2013

### Staff: Mentor

They still have to move physical objects, or send electromagnetic signals from A to B, or use some similar method to transmit information.

5. Jun 21, 2013

### Tenshou

So, what exactly is quantum computing, if it isn't trying to get entanglement to work

6. Jun 22, 2013

### phinds

7. Jun 22, 2013

### jaramsli

Unfortunately, one fool can ask more questions than ten wise men can answer:

"According to the current laws of physics, it is impossible to transmit any information faster than the speed of light."
Of course. I may be a fool, but not that much of a fool :)

"Entanglement does not help in any way, you would still need a classical channel to transmit information."

From this I can conclude that if two particles are entangled, and the state of one is changed, either

a) The state of the other will not change instantaneously, but rather, the state change will propagate through space at the speed of light

or

b) The state change may be instantaneous, but will not be observable

or

c) There is a limit to the distance between the two particles

or

d) Something else that I didn't think about

I am afraid I have no idea what a 'classical channel' is.

8. Jun 22, 2013

### Staff: Mentor

The state change is not observable (as in "see, now it changed!"). You can observe the state afterwards, but you cannot know the state it had before, otherwise there is no entanglement.
"instantaneous" is not the right expression - it depends on the reference frame, and the time-order of the observations in your reference frame does not matter.

The current internet, a phone line, mail, avian carriers, ... everything you can use to transfer regular bits.

9. Jun 22, 2013

### jaramsli

Thanks again.

Would I be right to assume that if two particles were entangled and the state of one changed, and although the state change is not observable, I could read the changed state from the other particle, say, one nanosecond later?

10. Jun 23, 2013

### Staff: Mentor

Yes.
You can even read it one nanosecond earlier ;).

11. Jun 23, 2013

### jaramsli

Then, if our particle, lets name it p1 for short had its state changed again, the other particle, let's call it p2, would reflect the change?

The state of p1 could be derived from the state of p2?

If it works once, it works twice, right?

If the state, let's call it S, changed every nanosecond to S(i), then the state of the other, let's call it T would change into a derived state T(i) every nanosecond and every T(i) could be recorded locally every nanosecond?

12. Jun 23, 2013

### Staff: Mentor

That is the key point, and it can be done only once. As soon as this can be done, the entanglement is lost, and further modifications will not influence the measurements on the other particle.

13. Jun 23, 2013

### jaramsli

Thanks! I get the impression that I understand it now, I know it will pass!

In any case, I ran out of questions :)