Communication systems and entanglement

DrChinese

By the way, I fell victim to this little twist when I was first looking at this setup. I was sure I had found a way to signal FTL... but nature is tricky!!

UChr

Nice to hear. Yes very tricky.

Re 290, Figure 2: I have some questions - that might help me further:

To remove the suspicion of Which Path information I could try to modify my experimental set-up:
I could use optical fibers to transport p - photons from the source until the transmitter's PBS and also from source until the receiver’s PBS, and I could replace both the Double Slits with DS - substitutes:
1: a half-silvered mirror followed by optical fibers - adjusted in length equal to the difference between transmitted and reflected - and compiled to emulate a DS

2: an interferometer.
Arises then similar problems - or should this solve the WP – problem?

Re Walborn:
Figure 2 is without QWP and without polarizer - but shows interference. How does this work?

With the two QWP’s in place: Should then just a laserbeam with diagonal (positive) polarized photons cause interference - or is the presence of entanglement decisive?

DrChinese

To get interference for Alice, Bob must be detected as a wave and not a localized particle. Sending Bob through a fiber will turn Bob particle-like and that means no interference for Alice. And vice versa! Again, you must look at the entire context. It is not easy to detect both members of a pair as wave-like and even in principle this cannot be done on demand.

I didn't try to work through your examples all the way as these Quantum Eraser discussions get sidetracked almost immediately with the details of the setup and it gets impossible to move past that. The point is always the same as I mention: you don't get interference from photons in which it is possible, in principle, to know the which path information - regardless of whether you actually know it or not.

UChr

then no fiber - I thought maybe it was more smart - but it is a gedanken experiment, so no need to unnecessary practical difficulties.

So transmitter PBS(0) or PBS(45).
Receiver: a PBS(0) followed by two Mach-Zehnder interferometers - with for example a BS = half silvered mirror.

Maybe one of those who has studied the Walborn experiment, answer my two questions?

(Walborn figure 2 is without QWP and without polarizer - but shows interference. How does this work?

With the two QWP’s in place: Should then just a laserbeam with diagonal (positive) polarized photons cause interference - or is the presence of entanglement decisive?)

SpectraCat

Which Walborn experiment? Please link to the paper you are referencing.

DrChinese

Have you looked at this?

http://grad.physics.sunysb.edu/~amarch/

http://grad.physics.sunysb.edu/~amarch/Walborn.pdf

UChr

arXiv:quant-ph/0106078v1 13 juni 2001