  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.
