1 August 1998 Ž . Optics Communications 153 1998 245–250 Interference pattern with a dark center in resonance fluorescence from two atoms driven by a squeezed vacuum field Z. Ficek a , R. Tanas b ´ a Department of Physics and Centre for Laser Science, The UniÕersity of Queensland, Brisbane 4072, Australia b Nonlinear Optics DiÕision, Institute of Physics, A. Mickiewicz UniÕersity, Poznan, Poland ´ Received 17 February 1998; revised 4 May 1998; accepted 4 May 1998 Abstract We study the resonance fluorescence from two interacting atoms driven by a squeezed vacuum field and show that this system produces an interference pattern with a dark center. We discuss the role of the interatomic interactions in this process and find that the interference pattern results from an unequal population of the symmetric and antisymmetric states of the two-atom system. We also identify intrinsically nonclassical effects versus classical squeezed field effects. q 1998 Elsevier Science B.V. All rights reserved. PACS: 42.50 1. Introduction It has been demonstrated in recent years that trapping of two atoms or ions at small interatomic distances enables one to observe collective phenomena and interference in wx the fluorescence field. For example, DeVoe and Brewer 1 have observed the phenomena of superradiance and subra- wx diance 2 in the fluorescence field emitted by two ions trapped at a distance comparable to the radiation wave- length and excited by weak laser pulses. At small inter- atomic distances the emission from each ion interferes with the others in a way that depending on the initial phase of the atomic dipoles the resulting radiation is either Ž . Ž . reduced subradiance or enhanced superradiance . wx Eichmann et al. 3 have observed the Young-type interference in the fluorescence field of two trapped ions. In this experiment, the ions were continuously driven by a weak coherent laser field and, depending on the polariza- tion, different degrees of interference were observed. The interference pattern in the Young-type experiment depends not only on the polarization but also on the intensity as well as on the direction of propagation of the driving field w x with respect to the interatomic axis 4–6 . For a weak driving field an interference pattern is observed due to the predominantly elastic scattering of the incident field. Un- der strong excitation, inelastic scattering dominates result- ing in a reduction of the fringe visibility. wx Kochan et al. 7 have shown that the fringe visibility of strongly driven atoms can be partially improved by cou- pling the atoms to a cavity mode. The cavity induces the interatomic correlations which alter the fringe visibility. wx Recently, Meyer and Yeoman 8 have reported an even stronger cavity-induced modification of the interference pattern that occurs when the coherent driving field is replaced by an incoherent field. They have shown that in contrast to the coherent excitation, the incoherent field produces an interference pattern with a dark center. They have interpreted this modification as an intrinsically non- classical effect arising from the destructive quantum inter- ference. In this paper, we report the existence of a similar contrast in behaviour of the interference pattern of two atoms driven in free space by a squeezed vacuum field. We discuss the dependence of the interference pattern on 0030-4018r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII S0030-4018 98 00249-1