Superdense Coding can be viewed as teleportation in reverse. The idea is to transmit two classical bits of information by sending a single qubit through the quantum channel. Suppose Alice wants to send a two-bit message to Bob [1, 4, 6]. She could send two qubits with the message encoded in them.
A single qubit on its own can’t transmit two classical bits of information. However, superdense coding allows a single qubit to perform this. This option requires that the two parties initially share a pair of entangled qubits. Assume initially Alice and Bob share a Bell state:
The first qubit in each term is Alice’s half of the state and the second qubit is used by Bob. Note that the Bell state is a fixed state. Alice and Bob don’t need to prepare the state. It is assumed that some third party prepared the Bell state beforehand and sent one of the entangled qubits to Alice and the other one to Bob. Hence, Alice and Bob each possess half of the Bell state that is they share one unit of the entangled pair.