Stimulated emission and no-cloning theorem

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Arminius
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Stimulated emission and no-cloning theorem

Postby Arminius » Sun Aug 17, 2014 8:57 am UTC

One of the major aspects of quantum cryptography is the non-cloning theorem which states that a quantum state cannot be copied without destroying it.

Stimulated emission is when an incident photon triggers the release of another one from an atom in an excited state with the same information.

Isn't there a contradiction? Isn't stimulated emission "copying" (ou cloning) the quantum state (for example polarisation)?

(Yes, I know there are some answers out there but some are actually contradictory)

stianhat
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Re: Stimulated emission and no-cloning theorem

Postby stianhat » Sun Aug 17, 2014 7:21 pm UTC

I thought that theorem only applied copy as in "reproduce". If so, there is no contradiction.

lgw
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Re: Stimulated emission and no-cloning theorem

Postby lgw » Mon Aug 18, 2014 2:37 am UTC

You can't clone all of the quantum information, but you can clone some of it. Stimulated emission was actually the first way this way this was done.

Partial cloning - what could possibly go wrong?

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quantropy
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Re: Stimulated emission and no-cloning theorem

Postby quantropy » Mon Aug 18, 2014 1:49 pm UTC

Arminius wrote:Isn't there a contradiction? Isn't stimulated emission "copying" (ou cloning) the quantum state (for example polarisation)?


From my post on physics stack exchange

A short history of the laser:
1917: Einstein describes theory of stimulated emission
Following decades: Lasers (and masers) are thought to be impossible due to quantum theory (Everyone knows Einstein was wrong about quantum theory :)
1951 Charles Townes builds a maser - some people are just awkward (John von Neumann: 'That can't be right', Niels Bohr: 'But that is not possible' - see How the Laser Happened: Adventures of a Scientist)
1982 Nick Herbert invents FLASH - A superluminal communicator based upon a new kind of quantum measurement (see How the hippies saved physics : science, counterculture, and the quantum revival )
Later in 1982: No-cloning theorem is devised, explaining why FLASH won't work.

This shows that even some of the best minds have been tripped up by the distinction between cloning a state and whatever it is that lasers do.

Frenetic Pony
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Re: Stimulated emission and no-cloning theorem

Postby Frenetic Pony » Tue Aug 19, 2014 1:13 am UTC

This thread does give me motivation to try and find a remembered paper on nondestructive quantum teleportation, and see if that was just my memory or was something real. That being said

quantropy wrote:This shows that even some of the best minds have been tripped up by the distinction between cloning a state and whatever it is that lasers do.


Lasers man

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How do they work?

Arminius
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Re: Stimulated emission and no-cloning theorem

Postby Arminius » Tue Aug 19, 2014 8:15 pm UTC

Yes, between the stackexchange posts and wikipedia I got completely confused.

I get partial copying possible (up to 5/6th, why?), no copying possible, full copying possible... so what is it in the end?

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LaserGuy
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Re: Stimulated emission and no-cloning theorem

Postby LaserGuy » Wed Aug 20, 2014 6:20 pm UTC

I think it depends on the state that you're trying to copy. It is possible to perfectly clone certain, well-defined states. It is not possible to clone general states.

Arminius
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Re: Stimulated emission and no-cloning theorem

Postby Arminius » Mon Aug 25, 2014 5:36 pm UTC

Could you elaborate on that please?

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LaserGuy
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Re: Stimulated emission and no-cloning theorem

Postby LaserGuy » Tue Aug 26, 2014 2:04 am UTC

You can clone an eigenstate, and you can clone states that satisfy the relation <A|B> = |<A|B>|^2. A general state is neither an eigenstate, nor does it satisfy this relation, so it can't be cloned perfectly.


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