  In the past decades, quantum plasmonics has become an active area due to its
potential applications in on-chip plasmonic devices for quantum information
processing. However, the fundamental physical process, i.e., how a quantum
state of light evolves in the photon-plasmon conversion process, has not been
clearly understood. Here, we report a complete characterization of the
plasmon-assisted extraordinary optical transmission process through quantum
process tomography. By inputting various coherent states to interact with the
plasmonic structure and detecting the output states with a homodyne detector,
we reconstruct the process tensor of the photon-plasmon conversion process.
Both the amplitude and phase information of the process are extracted, which
explains the evolution of the quantum-optical state after the coupling with
plasmons. Our experimental demonstration constitutes a fundamental block for
future on-chip applications of quantum plasmonic circuits.
