Bessel beams are studied within the general framework of quantum optics. The two modes of the electromagnetic field are quantized and the basic dynamical operators are identified. The algebra of these operators is analyzed in detail; it is shown that the operators that are usually associated to linear momentum, orbital angular momentum and spin do not satisfy the algebra of the translation and rotation group, and that this algebra is not closed. Some physical consequences of these results are examined.

The transition probability for the emission of a Bessel photon by an atomic system is calculated within first order perturbation theory. We derive a closed expression for the electromagnetic potentials beyond the paraxial approximation that permits a systematic multipole approximation . The matrix elements between center of mass and internal states are evaluated for some specially relevant cases. This permits to clarify the feasibility of observing the rotational effects of twisted light on atoms predicted by the calculations. It is shown that the probability that the internal state of an atom acquires orbital angular momentum from light is, in general, maximum for an atom located at the axis of a Bessel mode. For a Gaussian packet, the relevant parameter is the ratio of the spread of the atomic center of mass wave packet to the transversal wavelength of the photon.

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In this comment we show that the ambiguity of entropic quantities calculated in [1] for fermionic fields in the context of Unruh effect is not related to the properties of anticommuting fields, as claimed in [1], but rather to wrong mathematical manipulations with them and not taking into account a fundamental superselection rule of quantum field theory. * Electronic address: kbradler@cs.mcgill.ca

Doppler-free optical double-resonance spectroscopy is used to study the   s p p 5 5 6 1 2 3 2 3 2 excitation sequence in room-temperature rubidium atoms. This involves a  s p 5 5 F or ±2 electric quadrupole transitions.