Research

The interplay of the concepts of complementarity and interference in the time-energy domain is 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. The level scheme is depicted in the small inset of the figure. We find that the spectrum of resonance fluorescence exhibits a signature of vacuum-mediated interference effects. In the figure, the black curve shows the spectrum as monitored by a detector with perfect frequency resolution, whereas the blue line shows the corresponding spectrum without the interference contribution. By contrast, the total intensity of the fluorescence light 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 Mercury ions, our model system turns out to be an ideal candidate to provide evidence for vacuum-induced atomic coherences as yet unconfirmed in atomic systems.

The dynamics of an atomic few-level system can depend on the phase of driving fields coupled to the atom if certain conditions are satisfied. This is of particular interest to control interference effects, which can alter the system properties considerably. Here, we discuss the mechanisms of such phase control and interference effects in an atomic three-level system in lambda-configuration, where the upper state spontaneously decays into the two lower states. The lower states are coupled by a driving field, which we treat as quantized. This allows for an interpretation on the single photon level for both the vacuum and the driving field. By analyzing the system behavior for a driving field initially in non-classical states with only few Fock number states populated, we find that even though the driving field is coupled to the lower states only, it induces a multiplet of upper states. Then interference occurs independently in three-level subsystems in V-configuration, each formed by two adjacent upper states and a single dressed lower state.

In many current schemes of theoretical and experimental interest, the control of the quantum dynamics is limited by the influence of incoherent processes such as spontaneous emission. The spontaneous emission acts as a source of decoherence, e.g. prohibiting the storage of atomic population in an excited state. The inherent statistical nature of these incoherent processes further does not not allow for a deterministic control of the quantum dynamics. Thus here the aim is to suppress the spontaneous emission. For this, we use an intense external driving field of low frequency, which induces additional multiphoton pathways between the various bound states of the atom. The interference between all possible one- and multiphoton pathways from a given initial state to a final state then gives rise to a modified system dynamics. In particular, we use this technique to suppress the usual spontaneous decay of excited atomic states.

The scheme described here is similar to our other projects in that it also discusses interference effects. The realization of the interference effects, however, is different: While usually the interference is induced by a coherent driving field, here the interfering pathways are induced by incoherent pump fields. We study the inelastic spectral intensity emitted in the spontaneous decay of two near-degenerate atomic states to a common ground state, where the external incoherent pump fields couple the two levels to an upper state. The analysis focuses on the interplay of the interference induced by the incoherent pump fields with the interference between the two spontaneous decay channels, and we find that the incoherent relaxation processes are altered by the external incoherent pump fields. In a more recent study, we have shown that interferences induced by incoherent driving fields can also be used in collective systems.