Cooperative Phenomena in Disordered Quantum Systems (CODEQ)
A central challenge for advancing quantum technologies is to understand how interactions and decoherence emerge in many-body quantum systems. While these effects are well characterized in ultracold, highly controlled systems, they remain largely concealed at elevated temperatures, where thermal noise and structural disorder induce strong inhomogeneous broadening that masks the underlying microscopic dynamics. In this project, we develop advanced nonlinear spectroscopy techniques based on femtosecond laser technology and interferometric detection schemes to access these hidden interaction mechanisms in disordered quantum systems. Using coherent two-dimensional spectroscopy and multiple-quantum coherence detection, we can probe inter-particle interactions and related collective phenomena with exceptional sensitivity. These ultrafast methods allow us to capture the quantum systems in quasi-static snapshots, while at the same time revealing subtle interaction processes and collective decoherence mechanisms with high resolution.
Figure 1: a) Level scheme describing a single particle and two interacting particles. In the two-particle system, either one particle can be excited (states 01,10) or both (state 11), giving rise to two-photon absorption. The two-particle-state is only visible in the experiment, if an interaction between the particles (interaction strength ∆) introduces an asymmetry between the two-photon excitation pathways. b) The experiment is based on 2 to 4 femtosecond laser pulses interacting with the system inducing a sequence of population and coherence states. We probe the two-photon pathways by inducing a so called 2-quantum coherence (2QC), or in general we probe nQC pathways. c) Experimental 2QC signal, revealing interactions in an extremely dilute rubidium atom ensemble. The atom density is only 8×106 cm-3, corresponding to a mean inter-atomic distance of 27 µm. This highlights the extreme sensitivity of the method to probe interactions.
