Quantum interference enforced by time-energy complementarity.

The interplay of the concepts of complementarity and interference in the time-energy domain are studied. In particular, we theoretically investigate the fluorescence light from a J = 1/2 to J= 1/2 transition that is driven by a monochromatic laser field. We find that the spectrum of resonance fluorescence exhibits a signature of vacuum-mediated interference effects, whereas the total intensity is not affected by interference. We demonstrate that this result is a consequence of the principle of complementarity, applied to time and energy. Since the considered level scheme can be found, e.g., in (198)Hg(+) ions, our model system turns out to be an ideal candidate to provide evidence for as yet experimentally unconfirmed vacuum-induced atomic coherences

Quantum control of interacting multiatom systems

Control schemes in dipole-dipole interacting two-particle systems are discussed. In contrast to single-particle systems, in such collective systems vacuum-induced dipole-dipole interactions can couple transitions regardless of the orientation of the two involved dipole moments. We show that these couplings gives rise to a significant modification of the system properties, and demonstrate how they can be exploited to alter the system dynamics. In particular, we discuss a setup where the long-time dynamics crucially depends on the relative position of the two atoms, and demonstrate that such systems are a promising candidate for the realization of a multidimensional decoherence free subspace. © 2007 American Institute of Physics

Breakdown of the few-level approximation in collective systems

In contrast to single atoms, in collective systems, the vacuum couples transitions with orthogonal dipole moments. This leads to a geometry-dependent dynamics and to a breakdown of the few-level approximation in collective systems. © 2007 Optical Society of America

Breakdown of the few-level approximation in dipole-dipole interacting systems

The validity of the few-level approximation is investigated in a system of two dipole-dipole interacting four-level atoms. Each atom is modelled by two complete sets of angular momentum multiplets. We provide two independent arguments demonstrating that the few-level approximation in general leads to incorrect predictions if it is applied to the Zeeman subleveis of the atomic level scheme. First, we show that the artificial omission of subleveis generally leads to incorrect eigenenergies of the system. The second counterexample involves an external laser field and illustrates that the relevant states in each atom are not only determined by the laser field polarization, but also by the orientation of the atomic separation vector. As the physical origin of this outcome, we identify the dipole-dipole interaction between orthogonal dipole transitions of different atoms. Our interpretation enables us to identify conditions on the atomic level structure as well as special geometries in which (partial) few-level approximations are valid

Probing quantum superposition states with few-cycle laser pulses

The quantum dynamics of a two-level system illuminated by a few-cycle pulse with an adjustable carrierenvelope (C-E) phase is investigated theoretically. We consider the weak-field regime where tunneling processes and multiphoton ionization are negligible. It is shown that the upper state population exhibits a strong dependence on the C-E phase and on the time of arrival of the few-cycle pulse if the system is initially prepared in a coherent superposition state. We demonstrate that this effect can be employed to probe the coherence properties of the superposition state and allows one to determine the phase of the laser that prepares this state. © 2009 Optical Society of America