Quantum teleportation
Quantum teleportation moves an unknown qubit state from one qubit to another by consuming one pre-shared entangled pair and sending two ordinary classical bits. The original is destroyed in the process, nothing travels faster than light, and no matter is "beamed" — it is plumbing, not magic, and real hardware uses it to move quantum states between chips and across networks.
| Task | Best classical | Quantum |
|---|---|---|
| Transmit one unknown qubit state | impossible — measuring destroys it, and no-cloning forbids copying | 1 Bell pair + 2 classical bits |
What problem does it solve?
You hold a qubit in some state you do not know, and you need that exact state on a different qubit somewhere else — another chip, another lab. The obvious moves fail. You cannot measure and re-transmit: a measurement returns one bit and destroys the amplitudes. You cannot copy it first: the no-cloning theorem forbids duplicating an unknown quantum state.
Teleportation solves it with two resources: an entangled pair shared in advance, and two classical bits sent when you are ready.
Why is there no classical equivalent?
A qubit state is a pair of complex amplitudes — continuous values. Describing them precisely takes unboundedly many classical bits, and you cannot learn them from one copy anyway. Teleportation sidesteps the problem entirely: the state is transferred exactly, and neither sender nor receiver learns what it was. The two classical bits that cross the wire carry no information about the state — they are uniformly random whatever is being sent, which is also why the protocol cannot signal faster than light.
How does the protocol work?
Three qubits: q0 holds the state to send (here prepared with RY(0.9)), q1 is the sender's half of a Bell pair, q2 is the receiver's half.
- Share entanglement: H on q1, CX from q1 to q2 — a Bell pair. This can happen long before there is anything to send.
- Bell measurement: the sender applies CX from q0 to q1, then H on q0, and measures both. Two classical bits come out; each of the four outcomes is equally likely.
- Correct: the receiver applies X to q2 if the q1 bit was 1, then Z if the q0 bit was 1. After that, q2 holds the original state exactly — and q0 no longer does.
Until the two bits arrive, the receiver's qubit is statistically pure noise. The classical channel is not bookkeeping; it is the step that makes the state usable and the reason causality survives.
One honest caveat about the demo below: our circuit IR has no classical control, so the corrections cannot be applied conditionally. The demo stops just before them — you see the pre-correction state, grouped by measurement outcome.
What it is not
Worth stating plainly, because this protocol attracts more mythology than any other.
- Not faster-than-light communication. Nothing usable arrives until two classical bits travel by ordinary means.
- Not copying. The sender's qubit ends up as a measurement record; only one instance of the state ever exists.
- Not moving matter. Only the state — the information — moves between qubits that already exist at both ends.
- Not a speedup. There is no algorithmic advantage here. Its importance is architectural: teleportation is the mechanism behind linking QPU modules, quantum network repeaters, and gate teleportation in fault-tolerant designs.
What does this look like on real hardware?
Teleportation is one of the most thoroughly demonstrated protocols in the field: first shown with photons in 1997, and demonstrated ground-to-satellite over more than 1,000 km by the Micius mission in 2017. Deterministic on-chip teleportation is routine on superconducting and trapped-ion systems, and trapped-ion architectures use closely related state transfer as everyday machinery for moving information between zones.
Running the full protocol on a gate-based QPU requires mid-circuit measurement with real-time feed-forward, which several current platforms now support — check capabilities on the QPU index. Our in-browser simulator does not model feed-forward, which is why the Lab demo shows the pre-correction state honestly rather than pretending.