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jschiwal 01-20-2005 01:43 AM

Will the development of quantum computing lead to faster than light communication?
This is a link to the third page of a Scientific American Article on quantum computing and cryptography.


Ultimately cryptographers want some form of quantum repeater--in essence, an elementary form of quantum computer that would overcome distance limitations. A repeater would work through what Albert Einstein famously called "spukhafte Fernwirkungen," spooky action at a distance. Anton Zeilinger and his colleagues at the Institute of Experimental Physics in Vienna, Austria, took an early step toward a repeater when they reported in the August 19, 2004, issue of Nature that their group had strung an optical-fiber cable in a sewer tunnel under the Danube River and stationed an "entangled" photon at each end. The measurement of the state of polarization in one photon (horizontal, vertical, and so on) establishes immediately an identical polarization that can be measured in the other.
I think that this is the theoretical principle that prompted Einstein's famous quote 'God doesn't play dice with the Universe.' Allowing for the possibility of faster than light communication between two photon pairs may undermine the theory of relativity.

J.W. 01-20-2005 03:22 AM

Hard to say. I'm no physicist but in my opinion, any claims to measure the position of a single photon would violate Heisenberg's Uncertainty Principle At least from what I understand. -- J.W.

vharishankar 01-20-2005 03:29 AM

And from wikipedia -- article on the uncertainty principle.

jschiwal 01-20-2005 06:43 PM

If you read the full article, they are using Heisenberg's Uncertainty Principle. If someone tries to tap in, the properties of the photon will be changed.

I think that the Uncertainty principle states that you can't determine one property such as phase without changing another property such as momentum. They are only concerned with measuring a single property, so if the others are changed that's OK.

My main point is that this is a theoretical conflict between quantum theory and relativity. It would be interesting if this quantum effect can be realized and used.


Scanning through the wiki article (thanks for that link) there is this quote:
An example of a pair of conjugate variables is the x-component of angular momentum (spin) vs. the y-component of angular momentum.

Capt_Caveman 01-20-2005 11:06 PM

I read about an interesting experiment that a group from UMBC performed early on in the entanglement work. They basically made entangled photon pairs and then sent them through a beam splitter so that the members of each pair went in different directions. In one direction, the photons passed through a filter that absorbed photons, except that the letters UMBC were cut out of the filter and some of the photons would pass through the letters. The other member of each pair went of in a different direction to a detector. When they measured the non-filtered photons, they formed the image "UMBC". Pretty freaky.

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