  Superdense coding is a paradigmatic protocol in quantum information science,
employing a quantum communication channel to send classical information more
efficiently. As we show here, it can be understood as a particular case of a
prepare and measure experiment, a scenario that has attracted growing attention
for its fundamental and practical applications. Formulating superdense coding
as a prepare and measure scenario allows us to provide a
semi-device-independent witness of entanglement that significantly improves
over previous tests. Furthermore, we also show how to adapt our results into
self-testing of maximally entangled states and also provide a semidefinite
program formulation allowing to efficiently optimize, for any shared quantum
state, the probability of success in the superdense coding protocol.
