The quantum teleportation was verified experimentally

Quantum teleportation: Transfer flying quantum bits at the push of a button

Hybrid technology enables the transmission of photonic qubits with great reliability

15.08.2013

With the help of quantum mechanical entanglement of spatially separated light fields, scientists from Tokyo and Mainz have succeeded in teleporting photonic quantum bits extremely reliably. Around 15 years after the first attempts in the field of optical teleportation, a decisive breakthrough has been achieved. The success of the experiment carried out in Tokyo is based on a hybrid technology in which two conceptually different, previously incompatible approaches are linked. "Discrete, digital optical quantum information can be transmitted continuously and thus, so to speak, at the push of a button," explains Prof. Dr. Peter van Loock from Johannes Gutenberg University Mainz (JGU). As a physics theorist, Van Loock advised the experimental physicists led by Akira Furusawa from the University of Tokyo on how they can carry out the teleportation experiment most efficiently and ultimately also verify a successful quantum teleportation. The research work has now been published in the renowned specialist magazine Nature released.

Quantum teleportation enables the transfer of arbitrary quantum states from a sender, called Alice, to a spatially distant receiver, called Bob. The prerequisite is that Alice and Bob first share an entangled quantum state, e.g. in the form of entangled photons, over the distance. Quantum teleportation is of fundamental importance for the processing of quantum information (quantum computing) and quantum communication. For quantum communication in particular, photons are considered to be the ideal information carrier, as they enable signals to be transmitted at the speed of light. A photon can be used to represent a quantum bit or qubit - analogous to a bit in classic information processing. One then speaks of "flying quantum bits".

The first attempts to teleport individual photons, also known as light particles, go back to the Viennese physicist Anton Zeilinger. In the meantime, various experiments have been carried out, but the teleportation of a photonic quantum bit using conventional methods has reached its limits due to experimental inadequacies and fundamental principles.

The key to the experiment in Tokyo is a hybrid technology. With their help, it has been possible to experimentally achieve a completely deterministic quantum teleportation of photonic qubits, in which the teleportation takes place with extremely high reliability. The accuracy of the transmission is 79-82 percent for four different qubits. In addition, even with a low degree of entanglement, the qubits could be teleported much more efficiently than in previous experiments.

The concept of entanglement goes back to Erwin Schrödinger and describes the finding that two quantum systems, for example two light particles, share a common state and their behavior is more dependent on one another than is classically possible. In the Tokyo experiment, the entanglement of many photons with many photons created a continuous entanglement in which not just a few individual light particles, but the complete amplitudes and phases of two light fields are quantum-correlated with one another. Previous experiments, on the other hand, had only entangled a single photon with another single photon - a less efficient solution. "The entanglement of photons worked very well in the Tokyo experiment - practically at the push of a button as soon as the laser was switched on," says van Loock, Professor of Theory of Quantum Optics and Quantum Information, describing the experiment. This continuous entanglement was achieved by so-called squeezed light, which takes on the shape of an ellipse in the phase space of the light field. Once the entanglement has been created, a third light field can be attached to the transmitter. From there, in principle, any and any number of states can be transmitted to the receiver. "In our experiment there were exactly four sufficiently representative test states that were transmitted by Alice using the entanglement and generated corresponding states for Bob. Thanks to the continuous entanglement, it is possible that the photonic qubits are deterministic, i.e. with every attempt to Bob be transferred, "adds van Loock.

Earlier experiments on optical teleportation were designed differently and are incompatible to this day. From the point of view of physics theory it was assumed that the two different approaches, the discrete and the continuous world, are to be combined. The fact that the experiment with hybrid technology has actually succeeded represents a technological breakthrough. "Now the two worlds are converging," says van Loock.