View Full Version : FTL Communication Channels and Other Issues Resulting from Eraser

2007-Nov-05, 02:49 AM
Hi all,

The link for "A Double-Slit Quantum Eraser Experiment (http://grad.physics.sunysb.edu/~amarch/)" was brought to our attention by papageno.

It is the best description of "entanglement" that I have seen. Of course given my none existent background in these subjects that doesn't mean much :).

Reading through the article has raised some points/questions for me which I would like to have cleared.

Let's start with:

Why can't this be used to transmit information faster than speed of light?

Take a look at the last diagram.
Imagine that the detector P and the "polarizer" are on spaceship P. Also imagine that the "quarter wave plates", the "double slit" and the detector S are on space ship S. Let's say the BBO and the laser assembly are on spaceship BBO.
Let's assume that spaceships S and P are 1 light years apart(for the sake of simplicity). If the operator on ship P starts moving the polarizer in and out of the path of the P photon stream, it should coincide with the formation and destruction of the fringe pattern on ship S. Why can't this be used for instantaneous communication?

I do recall reading about entanglement and being explained that the pair particles can simultaneously and apparently faster than the speed of light communication change paths. However because these changes are apparently random on both sides the phenomena can not be used for transmitting information (At least that's how I understood things).

But with Eraser it seems that the phenomena can be manipulated by an operator.

I do realize that I am overlooking something but can't see what that is.

Ken G
2007-Nov-05, 03:25 AM
Quantum entanglement, and erasure, are both fascinating to be sure, but they are way oversold as mysterious phenomenon and all kinds of hooey is written about them. First let's just start with the idea that the QWPs can alter the interference pattern, which is actually not mysterious at all. The website says:
"It is peculiar then, that the presence of the quarter wave plates causes the s photons to so drastically change their behavior. One can't help but ask, how do these photons know that we could know which slit they went through? "
But again, there's no mystery here. An interference pattern requires interference, and an R polarized wave will not interfere with an L wave when you look at the patterns of bright and dark on the wall. That's not a quantum phenomenon at all, the same holds for a purely classical treatment of light (or indeed of any wave that can be polarized).

On to the entanglement. Entanglement is indeed pretty bizarre, but the key issue that is often overlooked is that none of these experiments yield anything at all surprising if there is no coincidence counter present. If you have two separate experimenters taking data, with no coincidence counter, neither will ever have any idea what is happening on the other path of the experiment. That's why there's no communication between them. To get the surprising results, you always need the coincidence counter, so you always need communication. You can't get communication, you have to build in the communication.

Finally, we have the issue of erasure. The erasure can happen at any time, even at times that seem causally impossible, because there is no causality involved in the erasure. Erasure, or not, is simply about how the data is packaged. The pattern that shows no interference is actually two patterns at once, slightly displaced, and each showing interference fringes. Package them separately, and voila, you have fringes, package them together, and zoom, no fringes. That's all that is happening, it is information management. There really isn't very much happening there that physical, and what there is is all contained in the mechanics of waves. It's the combination of wave physics and the packaging in terms of quantum events that are coincident, but only the coincidence counter can establish that coincidence. Thus, no communication without communication.

2007-Nov-05, 03:36 AM
Thanks Ken G,

After posting I did start worrying that I had left out the coincidence counter and that might end up being the important factor.
I will require couple of days digesting this.

Thanks again for the quick reply.