  Entanglement distribution based on time-bin qubits is an attractive option
for emerging quantum networks. We demonstrate a 4.09 GHz repetition rate source
of photon pairs entangled across early and late time bins separated by 80 ps.
Simultaneous high rates and high visibilities are achieved through frequency
multiplexing the spontaneous parametric down conversion output into 8 time-bin
entangled pairs. We demonstrate entanglement visibilities as high as 99.4%,
total entanglement rates up to 3.55e6 coincidences/s, and predict a
straightforward path towards achieving up to an order of magnitude improvement
in rates without compromising visibility. Finally, we resolve the density
matrices of the entangled states for each multiplexed channel and express
distillable entanglement rates in ebit/s, thereby quantifying the tradeoff
between visibility and coincidence rates that contributes to useful
entanglement distribution. This source is a fundamental building block for
high-rate entanglement-based quantum key distribution systems or advanced
quantum networks.
