Quantum dense coding
Bob can send two bits of information to Alice with a single photon if they share a pair of entangled photons. To begin, one photon each is sent to Alice and Bob. The photons are in one of the four Bell states. Bob then performs either one of the three unitary transformations on his photon, transferring the pair into another Bell state, or simply does nothing the fourth option. He then sends the photon to Alice who measures the state of the pair. Since there are four possible outcomes of this measurement, Bob has sent twice as much information as can be sent classically with a two-state particle.