J. von Zanthier

Superradiance is one of the outstanding problems in quantum optics since Dicke introduced the concept of enhanced directional spontaneous emission by an ensemble of identical two-level atoms. The effect is based on correlated collective Dicke states which turn out to be highly entangled. Here we show that enhanced directional emission of spontaneous radiation can be produced also with statistically independent incoherent sources via the measurement of higher order correlation functions of the emitted radiation. Our analysis is applicable to a wide variety of quantum systems like trapped atoms, ions, quantum dots or NV-centers, and is also valid for statistically independent incoherent classical emitters. This is experimentally confirmed with up to eight independent thermal light sources.

A method for gaining information about the phonon-number moments and the generalized nonlinear and linear quadratures in the motion of trapped ions (in particular, position and momentum) is proposed, valid inside and outside the Lamb-Dicke regime. It is based on the measurement of first time derivatives of electronic populations, evaluated at the motion-probe interaction time t=0. In contrast to other state-reconstruction proposals, based on measuring Rabi oscillations or dispersive interactions, the present scheme can be performed resonantly at infinitesimal short motion-probe interaction times, remaining thus insensitive to decoherence processes.

We discuss a simple search problem which can be pursued with different methods, either on a classical or on a quantum basis. The system is represented by a chain of trapped ions. The ion to be searched is a member of that chain, consists, however, of an isotopic species different to the others. It is shown that the classical imaging may lead as fast to the final result as the quantum imaging. However, for the discussed case the quantum method gives more flexibility and higher precision when the number of ions considered in the chain is increasing. In addition, interferences are observable even when the distances between the ions is smaller than half a wavelength of the incident light.