Teleportation and Superdense Coding
Quantum teleportation moves the exact state of one qubit onto a faraway qubit. It uses up one shared entangled pair and two normal bits. Superdense coding is the reverse trade. It delivers two normal bits by sending one qubit. Both are proved theory. Both have been shown on real hardware. And neither one sends any information faster than light.
What problem does teleportation solve?
Why care? Future quantum computers and networks will need to move fragile quantum states from place to place. Teleportation is the standard way to do it. It is also a great test of whether you really understand entanglement, because it is easy to get wrong.
Say Alice holds a qubit. That is a quantum bit whose state is a weighted blend of |0⟩ and |1⟩. The weights are called amplitudes. Their squares give the measurement chances. Alice wants Bob, far away, to end up with a qubit in exactly that state.
She cannot look at it and phone him a description. Measuring it snaps the state to a plain 0 or 1, which wipes out the amplitudes. She cannot photocopy it either. The no-cloning theorem is a proved fact of quantum physics. It says no device can copy an unknown quantum state.
Quantum teleportation is the way out. Bennett and five coauthors published it in 1993. The state's information is moved without anyone ever reading it. The name is less magic than it sounds. No matter moves. And the original qubit's state is always destroyed in the process. (That is no-cloning again: if Bob gains the state, Alice must lose it.)
So what travels? Two ordinary bits. Plus one resource set up ahead of time: an entangled pair of qubits. Those are two qubits prepared so their measurement results are fully linked, no matter how far apart they are (see the entanglement chapter). Alice holds one half and Bob holds the other.
An everyday picture: two friends each keep one half of a torn movie ticket. The torn edges match only each other. That shared, prepared link is what makes the later message work. Where the picture breaks: the ticket halves already have fixed shapes, but an entangled pair has no fixed results until you measure. Full protocol background is on our teleportation page.
- How does the protocol work — and what is deferred measurement?
- What exactly should we see for a 0.8/0.6 message?
- Run deferred-measurement teleportationINTERACTIVE
- Teleport a definite |1⟩INTERACTIVE
- Why is this not faster-than-light communication?
- How do you send two classical bits with one qubit?
- Run superdense coding of the message 11INTERACTIVE
- What does this look like on real hardware today?
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