  We experimentally demonstrate the active control of a plasmonic metamaterial
operating in the quantum regime. A two-dimensional metamaterial consisting of
unit cells made from gold nanorods is investigated. Using an external laser we
control the temperature of the metamaterial and carry out quantum process
tomography on single-photon polarization-encoded qubits sent through,
characterizing the metamaterial as a variable quantum channel. The overall
polarization response can be tuned by up to 33% for particular nanorod
dimensions. To explain the results, we develop a theoretical model and find
that the experimental results match the predicted behavior well. This work goes
beyond the use of simple passive quantum plasmonic systems and shows that
external control of plasmonic elements enables a flexible device that can be
used for quantum state engineering.
