[evanb]

In superdense coding you send one quantum bit ahead of time and hang onto it’s entangled partner. Then later you send one classical but which, when combined with the quantum bit you shared earlier, conveys two classical bits of your choice.

[attilakun]

You don't send any classical bits in superdense coding. You start out with a pair of entangled qubits in the state |00>+|11>. Then, if Alice has the first qubit and Bob has the second, by manipulating the first qubit Alice can perform 4 actions:

1. Nothing, which leaves the combined state unchanged: |00>+|11>.

2. Flip the sign of the base state |11>, thereby changing the combined state to |00>-|11>.

3. Flip the first bit which changes the combined state to |10>+|01>.

4. Perform choice 3 and flip the sign of base state |10>, which changes the combined state to -|10>+|01>.

Now notice that all 4 possible combined states are orthogonal to each other. But we reached each orthogonal state by manipulating only Alice's qubit. When Bob receives Alice's qubit he can put the 2 qubits together and see which state the combined system is in. You wouldn't be able to do this by sending a classical bit as that can't participate in entanglement. And entanglement is needed to access 4 different orthogonal states via manipulating just one qubit.

[evanb]

You're right---I forgot the protocol. Thanks for the correction!