Quantum teleportation

Quantum teleportation is a protocol that transfers the unknown quantum state of one Qubit to another distant qubit using a shared pair of entangled qubits plus two bits of ordinary classical communication. It moves information about a state, not any physical particle, and despite the name it is neither faster-than-light communication nor the transport of matter.

#The protocol

Suppose Alice holds a qubit in an unknown state she wants to send to Bob. Beforehand, they share a pair of qubits in a maximally entangled Bell state, one held by each. The protocol proceeds in three steps (Bennett et al. 1993):

1. Alice performs a joint measurement (a Bell-state measurement) on her unknown qubit and her half of the entangled pair. This yields one of four outcomes, encoded in two classical bits, and destroys the original state on her side.
2. Alice sends those two classical bits to Bob over a normal channel such as fiber or radio.
3. Bob applies one of four fixed quantum gate corrections to his half of the entangled pair, chosen by the two bits, and his qubit is left in the exact state Alice started with.

The shared Quantum entanglement is the resource that makes this possible, and the two classical bits are indispensable.

#Why it is not faster-than-light

The correction step is essential and depends on classical bits that travel no faster than light. Until Bob receives them, his qubit is in a state that carries no usable information about Alice's input; measuring it early yields random results. So teleportation cannot transmit information faster than the classical channel allows, preserving causality and Einstein's speed limit. This is a common and important misconception to correct.

#Why it works without violating no-cloning

Teleportation may seem to conflict with the no-cloning theorem, which forbids making an identical copy of an unknown quantum state. It does not, because the joint Bell-state measurement in the first step irreversibly destroys the original on Alice's side before Bob reconstructs it. At no moment do two copies of the state coexist. This is also why teleportation cannot be used to broadcast a state to many receivers from a single unknown input: each protocol run consumes both the input and one entangled pair to produce exactly one output.

#What is and is not transferred

• Transferred: the complete quantum state, including its Superposition amplitudes and phase, even though that state was unknown and never measured directly.
• Not transferred: any physical particle or matter. Bob's qubit was already present; it is merely reconfigured.
• Not duplicated: the original is necessarily destroyed, consistent with the no-cloning theorem, so teleportation relocates a state rather than copying it.

#Experiments and uses

The first experimental demonstrations came in 1997 using entangled photons (Bouwmeester et al. 1997). Since then teleportation has been shown over long optical fibers, across free space to satellites, and between different physical systems, and it has been extended to more complex states (Pirandola et al. 2015). It is not a way to move objects. Its practical roles are as a building block: it underpins quantum repeaters for long-distance quantum networks, enables gate teleportation as a primitive inside Quantum error correction and fault-tolerant circuits, and lets modular quantum computers link separate processors. In these roles it is a routine and essential tool rather than science fiction.