Information Transmission with Entangled Photons

[DanDaManJC]

Published in nature...

Abstract:

Quantum teleportation1 is central to the practical realization of quantum communication2, 3. Although the first proof-of-principle demonstration was reported in 1997 by the Innsbruck4 and Rome groups5, long-distance teleportation has so far only been realized in fibre with lengths of hundreds of metres6, 7. An optical free-space link is highly desirable for extending the transfer distance, because of its low atmospheric absorption for certain ranges of wavelength. By following the Rome scheme5, which allows a full Bell-state measurement, we report free-space implementation of quantum teleportation over 16 km. An active feed-forward technique has been developed to enable real-time information transfer. An average fidelity of 89%, well beyond the classical limit of 2/3, is achieved. Our experiment has realized all of the non-local aspects of the original teleportation scheme and is equivalent to it up to a local unitary operation5. Our result confirms the feasibility of space-based experiments, and is an important step towards quantum-communication applications on a global scale.

http://www.nature.com/nphoton/journal/v4/n6/full/nphoton.2010.87.html

I don't have a subscription to nature though, so no details :-/. That said... how does this work? I know how quantum entanglement works through the preservation of angular momentum when described in terms of spin. But I thought it was fundamentally impossible to instantaneously transmit information?

IIRC the basic issue was that even if the state on the receiving end changes instantaneously, the receiver wouldn't know which basis to use and when the message started. Although those issues could be fixed with a defined protocol -- there was something more fundamental blocking this kind of information transmission.

Edit:
well the wiki page has good info
http://en.wikipedia.org/wiki/Quantum_teleportation

 

[William Gaatjes]

I am interested in the process as well.
I would like to know how 2 photons are entangled, then separated 16 km apart from each other while maintaining same state. After that, how is exactly measured that both photons work in a synchro like principle ?
How is all this measured ? How do they measure the spin from both photons at the same time ?



In the best sense this reminds me of synchro's (i know it is not the same principle, do not bother to point that out please ):

When the transmitter’s shaft is turned, the synchro receiver’s shaft turns such that its "electrical position" is the same as the transmitter’s. What this means is that when the transmitter is turned to electrical zero, the synchro receiver also turns to zero. If the transmitter is disconnected from the synchro receiver and then reconnected, its shaft will turn to correspond to the position of the transmitter shaft.

http://www.engineersedge.com/instrumentation/self_synchronizing_motors_2.htm

 

[DanDaManJC]

Hmm I see. The synchro analogy seems to make more sense

maybe I'll email one of my profs for access

 

[Matthiasa]

If you can fully access it, it does state how all of it was done. (a requirement for being published )
But like everything in Nature it goes way over my head. (I only have access because of the university I attend)
So no explanations from me.

Still interesting stuff though.

OH and DanDaManJC if you are still attending check your university's library website, as it probably has access.

 

[silverpig]

I'll take a look a bit later. Nature articles are usually quite simple and elegant. Should be a nice paper.

 

[William Gaatjes]

This website has a good explanation about entanglement for those without a subscription to Nature :

The process of using certain crystals to split incoming photons into pairs of photons is called parametric down-conversion.

Normally the photons exit the crystal such that one is aligned in a horizontally polarized light cone, the other aligned vertically. By adjusting the experiment, the horizontal and vertical light cones can be made to overlap. Even though the polarization of the individual photons is unknown, the nature of quantum mechanics predicts they differ.

To illustrate, if an entangled photon meets a vertical polarizing filter (analagous to the fence in Figure 4.4), the photon may or may not pass through. If it does, then its entangled partner will not because the instant that the first photon's polarization is known, the second photon's polarization will be the exact opposite.

It is this instant communication between the entangled photons to indicate each other's polarization that lies at the heart of quantum entanglement. This is the "spooky action at a distance" that Einstein believed was theoretically implausible.

Straight to the point :
http://www.davidjarvis.ca/entanglement/quantum-entanglement.shtml

Some information first :
http://www.davidjarvis.ca/dave/entanglement/



And some music :

http://www.youtube.com/watch?v=mUSxwFOM60g

http://www.youtube.com/watch?v=eo4u4JJAPGk

http://www.youtube.com/watch?v=qz-nhcfTfWQ&feature=channel

http://www.youtube.com/watch?v=sPG6OQy2iwI&feature=channel

http://www.youtube.com/watch?v=DEZNDO4pdys&feature=related

 

[William Gaatjes]

In figure 6.4 of the website i posted above, the interference pattern of polarized light through a slit reminds me of a 2D version of the the 2p electron cloud.

Why is this ? Is this coincidence ?


http://www.search.com/reference/Atomic_orbital

http://mwolff.tripod.com/spin.html


Some more music :

http://www.youtube.com/watch?v=4rGvq3cdJkk&feature=related

http://www.youtube.com/watch?v=3vfLvZCdT9g&feature=related