Abstract

We report the first experimental investigation on the long standing, fundamental problem of QED intra-atom quantum correlations.^1-3 It is well known that the quantum analysis of the problem of the interacting atoms in free-space, based on a Dicke Hamiltonian, shows an unexpected noncausal intra-atom correlation transfer. We show theoretically that the problem is fully resolved by the use of the complete Hamiltonian, i.e., including the anti-resonant contribution.⁴ However, in spite of the existence of this satisfactory quantum-theoretical approach, the transfer of QED correlations within a single-mode plane, Casimir microcavity still shows a somewhat puzzling noncausal behavior when the atoms are correlated by transverse interactions, i.e., taking place in the direction parallel to the plane of the cavity and then orthogonal to the K-vector of the allowed mode. The "spatial" counterpart of this problem, the establishment of the transverse "size" of the mode, was already investigated by us in an early paper.⁵ By the present work, the investigation is carried out to the theoretical, as said, and the experimental analysis of the complete correlation problem in space-time, then fully implying the Einstein causality within the process. The experimental layout is shown in Fig. 1. Two equal, mutally delayed, 50 fs, 10 Mw laser pulses generated by an amplified CPM laser are injected in an active microcavity with a molecular solution of oxazine-725 as active medium: the two pulses are then the "pump" of two identical, virtually "thresholdless" microlasers placed at a transverse distance d in the cavity plane. The space-time properties of the quantum coupling taking place between the microlasers determines the shape and the mutual time-delay of the output microlaser sub-picosecond pulses. These shapes are analyzed by a standard mixing up-conversion technique by beating in a thin KDP slab the microlaser output with an ultrashort pulse supplied by the pumping laser: this is shown in Fig. 1. Figure 2 shows the time-delay measured between the output pulses vs the delay of the exciting ones. The linear dependence between the two delays, expressed by the straight line in Fig. 2, μm plies a full absence of space-time quantum correlations among the excited atoms. On the other hand, the "plateau" of the curve, implying the presence of transverse correlations, also gives the transverse "speed" of the intra-atom exchange of excitation.

© 1994 IEEE