Superdense Coding

Superdense coding is, in a sense, the mirror image of quantum teleportation: teleportation uses two classical bits and entanglement to transmit one qubit's worth of quantum state, while superdense coding uses one qubit and entanglement to transmit two classical bits. The sender and receiver start with a shared Bell pair. To send one of four possible two-bit messages, the sender applies one of four gates (identity, X, Z, or XZ) to their half of the entangled pair, then physically sends that single qubit to the receiver. The receiver, now holding both halves of the original entangled pair, performs a Bell-basis measurement that deterministically reveals which of the four gates was applied, and therefore which two-bit message was sent. Like teleportation, superdense coding does not exceed the classical channel capacity when the cost of originally distributing the entangled pair is counted, and it does not allow faster-than-light signaling. Its practical relevance today is mostly as a teaching example and as a building block in quantum network protocol design, rather than a deployed communication technique.