Recipe

Step-by-step: how teleportation actually works

Mission 6: Quantum Teleportation. Part of the Quantum Courier Service, a free course from the Oxford University Quantum Information Society.

To pull off this magic trick without violating no-cloning, Alice and Bob need a very specific three-ingredient recipe: The VIP Qubit, a completely normal telephone, and a Bell Pair (entangled twins acting as telepathic fuel).

Step 1: The Quantum Collision

Alice mathematically smashes the VIP Qubit and her half of the entangled Bell Pair together. They are now completely tangled up.

Step 2: The Sacrifice

This is the point of no return. Alice slams her hand down and measures both of her qubits. CRACK. The delicate superpositions collapse violently into boring, classical dust (like 0s and 1s). The VIP Qubit is gone from her lab forever.

Step 3: The Hand-Off

Here is the part everyone gets wrong. Nothing shoots down a bridge to Bob. The instant Alice measures, the entanglement does its quiet work: Bob's blank qubit is already carrying the VIP state, hidden under one of exactly four possible disguises. Which disguise is fixed by Alice's two measurement outcomes. But Bob can't tell yet, staring at his own qubit, it still looks like meaningless static, exactly as it did a moment ago. Nothing raced across town, and no usable information has travelled.

Step 4: The Phone Call

Alice picks up a normal telephone and calls Bob. "Hey Bob! The results of my sacrifice were a 0 and a 1!" (Crucially, because Alice has to make a slow classical phone call to give Bob the password, no usable information travels faster than light. Einstein's ghost breathes a sigh of relief).

Step 5: The Unscrambling

The two bits Alice phones over tell Bob exactly which of the four disguises his qubit is wearing, and therefore which move peels it off. There are only four possibilities: do nothing at all, a bit-flip, a phase-twist, or both together. Bob applies the single move the two bits point to, the disguise drops, and he is holding the perfect VIP Qubit!