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I've been reading 'How the Hippies saved Physics', which describes a design for a superluminal communication device, of which the crucial part was a laser which duplicated an incoming photon many times. The reason this won't work is what is now known as the no-cloning theorem - a quantum state can't be duplicated in this way. It may appear that a laser can do this, but it can't.

The thing is that I have vague memories of reading that when the laser was first talked about, it was claimed that quantum theory forbade such a device. What I'd like to know is whether there were such claims, and if so were they based on the idea that a laser would duplicate a quantum state.

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up vote 5 down vote accepted

Yeah, Neils Bohr and John Von Neumann were skeptics:

Many prominent physicists thought it could not even work, based on their knowledge of physical principles. In quantum mechanics, the uncertainty principle developed by Einstein, says that the energy (and therefore the frequency, by E=hv) of a photon can't be known to great precision in a short time. In masers, photons last for a very short time. Therefore, no less than Neils Bohr and John von Neumann thought it couldn't work, even after it had been created. The solution to this apparent paradox is that, though the photons all have the same frequency and direction, which atoms do the emitting and when remains unknown. The emitting atoms maintain an anonymity that avoids uncertainty >violation.

Read more: Why Was the Laser Light Invented? |

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I would only add that the story is told in full in book "How the Laser Happened: Adventures of a Scientist" by Charles Hard Townes (who received the Nobel Prize in Physics in 1964 for "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle"). – Leos Ondra May 9 '12 at 19:42

As far as I know, initially, the main requirement for lasing was population inversion. It can easily be shown that this is not possible for a pure two (energy )level system. I suppose this is what you are referring to.

However, since then, using quantum interference in multi-level systems, one can have lasing without inversion. LWI in atomic vapor. A laser does not duplicate a quantum state nor am I aware of any such device that can do this.

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I wonder why was it downvoted. I can see a point in not voting it up (comparing with the accepted answer), but not in downvoting. – Yrogirg Aug 13 '12 at 10:19

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