Quantum teleportation

Quantum teleportation is a process by which the quantum state of a particle at one location can be inferred from another location without the particle being transmitted. The process utilizes the properties of quantum entanglement for two, distant, particles to determine the state of a third particle.

Quantum mechanics forbids the state of a particle from being copied exactly from one particle to another. However, through quantum teleportation, information about the state of a particle can be instantaneously transferred through the measurement of another particle even if separated by a long distance. While the process does not involve the physical transfer of matter, it does alter the state of the original particle measured and requires a second classical channel of communication to transport the result of the measurement.

Quantum teleportation was first theorized in a paper published in 1993 by an international team of six scientists Asher Peres, William Wootters, Richard Jozsa, Charles Bennett, Gille Brassard, and Claude Crépeau. It was first performed experimentally in 1997 by two separate teams led by Sandu Popescu and Anton Zeilinger. Quantum teleportation has now been demonstrated experimentally using various systems, including:

• single photons
• coherent light fields
• nuclear spins
• trapped ions

In quantum mechanics, entanglement is a phenomenon where two or more objects have to be described in reference to one another, even if the objects are spatially separated. Quantum entanglement leads to correlations between observable physical properties of the systems.

A basic understanding of the protocol behind quantum teleportation can be demonstrated with a pair of entangled particles, a sender (Alice) and a receiver (Bob). If Alice interacts with a third particle (of unknown state), the outcome of that interaction can be determined by measurements of Bob due to the correlations between the two. Therefore information about the third particles state has been transported across space.

However, to determine the state of the third particle, Bob must receive the outcome of the measurement from Alice via a classical channel. Since the process alters the state of the third particle quantum teleportation doe not break the no-cloning principle of quantum mechanics.

The measurement of the third particle by Alice is known as a Bell measurement. Bell states or EPR pairs are specific quantum states of a two qubit system (i.e. a quantum mechanical system composed of two interacting two-level subsystems) that represent the simplest and maximal examples of quantum entanglement.

Basic diagram of the quantum teleportation protocol