Quantum Teleportation

Quantum Teleportation

Quantum teleportation is one of the most counterintuitive protocols in quantum information. It transfers an unknown quantum state from one location to another using a pre-shared entangled pair and two classical bits: without the quantum state physically traveling through the channel.

You built this protocol in the Entanglement & Protocols track. This lesson revisits it as a network primitive: the operation that moves quantum states between nodes that already share entanglement, which is the job every layer of a quantum network ultimately exists to support.

The Protocol

Alice wants to send qubit |ψ⟩ = α|0⟩ + β|1⟩ to Bob. They share a Bell pair |Φ+⟩ = (|00⟩ + |11⟩)/√2.

Step 1: Alice's Bell measurement Alice performs a joint measurement on her qubit |ψ⟩ and her half of the Bell pair. This is a Bell state measurement (BSM): she applies a CNOT (from |ψ⟩ to her Bell pair qubit), then a Hadamard to |ψ⟩, then measures both qubits. Result: two classical bits (m1, m2).

Step 2: Classical communication Alice sends the two classical bits to Bob over a classical channel. This is why teleportation does not violate special relativity: the information must travel classically, limiting the protocol to light-speed.

After correction, Bob's qubit is in state |ψ⟩: identical to Alice's original.

What Gets Transferred?

Crucially: the state |ψ⟩ is destroyed at Alice's end (she measures it) and reconstructed at Bob's end. No physical qubit travels from Alice to Bob. The quantum information is "teleported" through the combination of entanglement and classical communication.

The no-cloning theorem is respected: Alice's qubit is in a known classical state (measured) after the protocol, so there is no duplicate.