  Photonic implementations of reservoir computing (RC) promise to reach
ultra-high bandwidth of operation with moderate training efforts. Several
optoelectronic demonstrations reported state of the art performances for hard
tasks as speech recognition, object classification and time series prediction.
Scaling these systems in space and time faces challenges in control complexity,
size and power demand, which can be relieved by integrated optical solutions.
Silicon photonics can be the disruptive technology to achieve this goal.
However, the experimental demonstrations have been so far focused on spatially
distributed reservoirs, where the massive use of splitters/combiners and the
interconnection loss limits the number of nodes. Here, we propose and validate
an all optical RC scheme based on a silicon microring (MR) and time
multiplexing. The input layer is encoded in the intensity of a pump beam, which
is nonlinearly transferred to the free carrier concentration in the MR and
imprinted on a secondary probe. We harness the free carrier dynamics to create
a chain-like reservoir topology with 50 virtual nodes. We give proof of concept
demonstrations of RC by solving two nontrivial tasks: the delayed XOR and the
classification of Iris flowers. This forms the basic building block from which
larger hybrid spatio-temporal reservoirs with thousands of nodes can be
realized with a limited set of resources.
