The mirror image of teleportation: using one pre-shared entangled pair, you can send two classical bits by transmitting only one qubit.
Why it matters
Together with teleportation it pins down the exchange rate between classical bits, qubits and entanglement — the foundation of quantum information theory.
After this you will be able to
Hand-picked and ordered. If you only have time for one, take the first.
A rigorous but genuinely well-taught course by John Watrous covering single systems, multiple systems, quantum circuits and the core protocols. The cleanest bridge from intuition to real notation.
Free, self-contained and rigorous, with exercises. Widely used as the standard course text in Europe and the best single free replacement for a paid textbook.
Short, complete, copy-pasteable Qiskit tutorials for individual algorithms and protocols. Useful when you want one working example of a specific thing rather than a whole course.