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Physical Review A

Reply to “Comment on ‘Optical coherence: A convenient fiction’ ”

Klaus Mølmer

Published Nov 1, 1998• 24 citations
Publisher

Abstract

The ambiguity of representing the density matrix of laser light by coherent states or by the number states is ``inherited'' from a similar ambiguity in the description of lasing mechanisms. Atomic excitation by electron impact is analyzed, and it is concluded that in normal discharge lasers, the lasing atoms become energetically entangled with the electrons and no mean dipoles are excited.

DOI
10.1103/physreva.58.4247

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The ambiguity of representing the density matrix of laser light by coherent states or by the number states is ``inherited'' from a similar ambiguity in the description of lasing mechanisms. Atomic excitation by electron impact is analyzed, and it is concluded that in normal discharge lasers, the lasing atoms become energetically entangled with the electrons and no mean dipoles are excited.
All compositions of a mixed-state density operator are equivalent for the prediction of the probabilities of future outcomes of measurements. For retrodiction, however, this is not the case. The retrodictive formalism of quantum mechanics provides a criterion for deciding that some compositions are fictional. Fictional compositions do not contain preparation device operators, that is operators corresponding to states that could have been prepared. We apply this to Mølmer's controversial conjecture that optical coherences in laser light are a fiction and find agreement with his conjecture. We generalize Mølmer's derivation of the interference between two lasers to avoid the use of any fictional states. We also examine another possible method for discriminating between coherent states and photon number states in laser light and find that it does not work, with the equivalence for prediction saved by entanglement.
The most commonly used lasers operate with population inversion. This requires an incoherent pumping power (from lamps, electrical discharges, etc.) that rapidly scales with the frequency of the generated laser radiation. In practice, this is the main obstacle for the operation in the continuous-wave regime of short-wavelength lasers in the UV, X-rays, etc. But lasers can also operate without population inversion using atomic coherences excited in the laser medium by coherent pumping with another laser. We present in this paper a short review of the field of lasing without inversion (LWI). In particular, we summarize the basic physics of LWI in the framework of both semiclassical theory and quantumjump formalism to describe light-matter interaction. We also review the experimental state-of-the-art in LWI. Most proof-of-principle demonstrations of LWI have used atomic vapours, so far. But recently gain without inversion (a pre-requisite for lasing) has been observed also in a technologically relevant medium: semiconductor nanostructures that can be tailored to have “artificial atom” electron states.