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The emission characteristics in the fluorescence of two laserdriven dipole-dipole-interacting three level atoms is investigated. When the light from both atoms is detected separately a correlation of the emission processes is observed in dependence of the dipole-dipole interaction. This opens the possibility to investigate the dipole-dipole interaction through the emission behavior. We present Monte-Carlo simulations which are in good agreement with the analytic solutions.PACS numbers: 42.50. Fx, 42.50.Lc, 42.50.Ar The problem of the influence of a neighboring atom on atomic emission behavior [1][2][3][4] has been investigated since a very long time. Especially the problem of the correlation of the emission of two neighboring atoms has found quite some interest [5][6][7]. This is a complex issue and the answer is very much dependent on various system parameters such as the strength of the pumping field, the life times and the wavelengths of the different transitions involved. An early theoretical calculation concluded that the dipole-dipole interaction between adjacent atoms is irrelevant for quantum jumps within a 3-level-system under the usual experimental conditions where the Rabi frequency of the pump field is large compared to all other rates in the problem [8,9]. Those results have been confirmed by quantum Monte-Carlo calculations [10][11][12], [13]. In this paper we reconsider the problem of dipole-dipole interaction and investigate especially the correlations in the emissions of two neighboring atoms which are observed individually. The arrangement is shown schematically in Fig. 1. We consider 2 identical nearby atoms in a trap, each with levels |1 , |2 and |3 . One of the atoms (say atom 1) is initially prepared, e.g. by pulsed excitation [14], in a metastable state |2 . A resonant cw pump interacts with both atoms. The initial state of atom 1 is such that it can start interacting with the pump either due to decay to the state |3 by emission of a photon or via excitation to the state |1 as a result of the dipole-dipole interaction. In the second case atom 2 goes to the state |2 . In our analysis we assume that the level spacing |1 ↔ |2 is close enough, so that the dipole-dipole interaction is effective only on this transition. The transitions |1 ↔ |3 and |2 ↔ |3 are supposed to lie in the optical domain, where the distance between two atoms is assumed to be much larger than the wavelengths of the corresponding transitions. For example in In + the wavelength on the |1 ↔ |2 corresponds to 9.3µm which could be two to three times the distance between two trapped ions [15]. We further assume that the fluorescence from each atom can be resolved individually by two distinct detectors 1 and 2. In what follow let us work in the limitIn our prepared system, detector 1 will not detect any fluorescence while atom 2 is fluorescing on the |1 ↔ |3 transition until the dipole-dipole interaction on the transition |1 ↔ |2 brings atom 1 towards the cycling transition |1 ↔ |3 and atom 2 towards the metastable state |2 ...

The different behaviour of first order interferences and second order correlations are investigated for the case of two coherently excited atoms. For intensity measurements this problem is equivalent to Young's double slit experiment and was investigated in an experiment by Eichmann et al. [Phys. Rev. Lett. 70, 2359(1993] and later analyzed in detail by Itano et al. [Phys. Rev. A 57, 4176 (1998)]. Our results show that in cases where the intensity interferences disappear the intensity-intensity correlations can display an interference pattern with a visibility of up to 100%. The contrast depends on the polarization selected for the detection and is independent of the strength of the driving field. The nonclassical nature of the calculated intensity-intensity correlations is also discussed. PACS number(s):