Question on Entanglement

[emcelhannon]

Thanks for trying, but I'm feeling a little obtuse. Just tell me to kiss off when you start to feel like your talking to a wall.

 

It might help if I pull specific information from the article that confuses me. In our previous posts we discussed that measured light that has been collapsed and light that remains in a superposition are indistinguishable, because when it enters our measuring apparatus it always appears collapsed, (because we just measured it)

(This is where my misconception probably is. The slightly different language is intended to reveal possible errors in my understanding.)

 

The article reads, "By performing statistical tests on the light received at the other end of the cable, one can tell whether it remains in the superposition of states or has already been observed..." I'm assuming that "already been observed" means collapsed, and I should mention that I have no clue what statistical tests would be performed.

 

I'm also confounded by the final comment, " Such a test would only work if the collapse occurs after (as opposed to before) observation;"

I'm under the impression that observation equals collapse.

 

Gribbon published some theory about unbreakable codes using entanglement for communication in an afterword of one of his cat/kittens books. I thought I'd kept up with everything until that point. I couldn't make sense out of it, so I feel I missed something fundamental somewhere. It's been years since I read that book. Even after reading many books, and periodicals in the field, my comprehension of the standard model seems coherent....except for this shortcoming.

 

I'm beginning to think my neurons just aren't packed tight enough to make the connection.

I appreciate your patience, even if you're only able to be patient till now.

[swansont]

The difference is knowing how to set up your polarizer to make the measurement. If the superposition is between 0º and 90º states (Up and Right), you will get a different answer if you measure at any other angle: if you put your polarizer at 1º, some of the Up photons will be detected as being at 91º. If the eavesdropper then recreates a photon at 91º, most of the time that will be detected as a Right photon, rather than UP. That's how you can detect eavesdropping. "Already been observed" means the transmitted photon has been observed by an eavesdropper, rather than the partner photon having been observed.

[emcelhannon]

Got it. You hit on just what I was missing.

 

Thanks again

[5614]

The difference is knowing how to set up your polarizer to make the measurement. If the superposition is between 0º and 90º states (Up and Right), you will get a different answer if you measure at any other angle: if you put your polarizer at 1º, some of the Up photons will be detected as being at 91º. If the eavesdropper then recreates a photon at 91º, most of the time that will be detected as a Right photon, rather than UP. That's how you can detect eavesdropping. "Already been observed" means the transmitted photon has been observed by an eavesdropper, rather than the partner photon having been observed.

Am I correct in saying that the axis of polarisation must be agreed upon by the two parties prior to the quantum communication line being used? If so this initial communication must be by classical means, and could thus be intercepted. If some 3rd party did intercept this, and knew the setup, could they successfully wire tap the communication without leaving a trace?

 

And practically how does one define a 0º axis on an experiment such as this?