Talk:No cloning theorem

From Wikipedia, the free encyclopedia

	Physics Portal This article is within the scope of WikiProject Physics, which collaborates on articles related to physics.
???	This article has not yet received a rating on the assessment scale. [FAQ]
???	This article has not yet received an importance rating within physics.
Help with this template Please rate this article, and then leave comments here to explain the ratings and/or to identify the strengths and weaknesses of the article.

[edit] Clarification of implications?

This is generally a very clear article, but the principle raises some questions for newcomers. The second paragraph deals with quantum entanglement, but what about Bose–Einstein condensates and lasers? In the case of a condensate, I guess the same quantum state is reached because it is a known and definite state forced by cooling, rather then a superposition. But does a photon in a laser not stimulate emission of another identically-polarised photon, even if that polarisation is not known? --Cedders^tk 08:25, 16 May 2006 (UTC)

I think that would be an example of entanglement. E.g., if |0> and |1> are the two possible polarization states of the inital photon, then the final state can be |0>|0> or |1>|1>. So, if we start with a superposition of |0> and |1> (with coefficients a and b), we have:

a|0> + b|1> --> a|0>|0> + b|1>|1>

Whereas, for cloning we would need:

a|0> + b|1> --> (a|0> + b|1>)x(a|0> + b|1>) = aa|0>|0> + ab|0>|1> + ba|1>|0> + bb|0>|0>

In other words, cloning refers to the creation of two separable states, and that's what the theorem forbids. -- Tim314 00:37, 19 February 2007 (UTC)