Teleportation and Superdense Coding

Quantum teleportation moves the exact state of one qubit onto a distant qubit, consuming one shared entangled pair and two classical bits; superdense coding is the reverse trade, delivering two classical bits with one transmitted qubit. Both are proved theory, both are demonstrated on real hardware, and neither sends any information faster than light.

What problem does teleportation solve?

Say Alice holds a qubit — a quantum bit whose state is a weighted combination of |0⟩ and |1⟩, the weights being amplitudes (numbers whose squares give measurement probabilities). She wants Bob, far away, to end up holding a qubit in exactly that state. She cannot look at it and phone him the description: measuring collapses the state to a plain 0 or 1, destroying the amplitudes. She cannot photocopy it either — the no-cloning theorem, a proved consequence of quantum mechanics' linearity, says no device can duplicate an unknown quantum state.

Quantum teleportation, published by Bennett and five coauthors in 1993, is the escape hatch: the state's information is transferred without ever being read. The name oversells nothing physical — no matter moves, and the original qubit's state is necessarily destroyed in the process (no-cloning again: if Bob gains the state, Alice must lose it). What travels is two ordinary classical bits, plus one prepaid resource: an entangled pair of qubits — two qubits prepared so their measurement outcomes are perfectly linked, no matter the distance (see the entanglement chapter) — with one half held by Alice and the other by Bob. Full protocol background lives on our teleportation page.