Persistent coherence of quantum superpositions in an optimally doped cuprate revealed by 2D spectroscopy

Understanding of the precise mechanisms of high-temperature superconductivity is elusive. In particular, in order to solve the puzzle of the pairing mechanism, it is important to understand the detailed nature of the excitations at energies around the superconducting gap. While measurements of the dynamics of excited electronic populations have been able to give some insight, they have largely neglected the intricate dynamics of quantum coherence. Here, we apply multidimensional coherent spectroscopy for the first time to a prototypical cuprate and report unprecedented coherent dynamics persisting for ~500 fs, originating directly from the quantum superposition of optically excited states separated by 20 - 60 meV. These results reveal the correlation between high and low energy excitations, and indicate that the interplay between many-body states on different energy scales conserves phase coherence. In revealing these dynamics we demonstrate that multidimensional coherent spectroscopy can address electronic correlations and interrogate many-body quantum systems in unprecedented ways.