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We study the time structure, the frequency composition, and the shot to shot fluctuations of the radiation emitted by a free-electron laser starting from shot noise in the electron beam longitudinal distribution, taking into account slippage and finite bunch length effects. We find a very difkrent behavior when the bunch length, Eg, is much longer than the cooperation length, E"or of the order of a few 8,. The field evolution is dominated by slippage eÃects in both cases, and shows the presence of superradiant spikes.

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Inconclusive photon subtraction (IPS) is a conditional measurement scheme to force nonlinear evolution of a given state. In IPS the input state is mixed with the vacuum in a beam splitter and then the reflected beam is revealed by ON/OFF photodetection. When the detector clicks we have the (inconclusive) photon subtracted state. We show that IPS on both channels of an entangled twin-beam of radiation improves the fidelity of coherent state teleportation if the energy of the incoming twin-beam is below a certain threshold, which depends on the beam splitter transmissivity and the quantum efficiency of photodetectors. We show that the energy threshold diverges when the transmissivity and the efficiency approach unit and compare our results with that of previous works on {\em conclusive} photon subtraction.Comment: slightly revised version, to appear in PR

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We discuss the cooperative decay of initial atomic excitation for a pencil-shaped active volume filled with two-level atoms. As long as the length of the sample is much smaller than a certain maximal cooperation length, the atom-field interaction producing the superradiant pulse can be treated in terms of the simplest possible laser model (single mode). The basic laser master equation turns out to be exactly solvable if specified for the superradiance limit which is characterized by two conditions: (i) The photons escape from the low-Q cavity so fast that they cannot feed themselves back into atomic excitation to any appreciable amount (this no-feedback condition is equivalent to the above-mentioned requirement for the length of the sample). (ii) The incoherent atomic decay due to natural relaxation is so slow that the individual atomic dipoles do not dephase before engaging themselves cooperatively in the interaction with the electromagnetic field. The present paper presents the derivation and general discussion of the equations describing the statistical properties of atoms and field in a superradiant pulse. Analytical and numerical solutions will be presented in a subsequent paper.

%'e show how a collection of noninverted two-level systems, under the action of a quasiresonant pump field, can produce an exponential amplification of a probe signal through the mechanism of a dynamical instability with spatial self-bunching in a way very similar to that experienced by a high-gain freeelectron laser. We offer a detailed description of this phenomenon, specify the conditions under which it takes place, and sketch possible experimental parameters for its observation in a gaseous medium. PACS number(s): 42.55. -f, 42.50. -p, 32.80.Pj

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