Entanglement Experiment Points Toward Quantum Repeaters

Reporting in the Aug. 19 issue of Nature, researchers at Universität Wien in Vienna, Austria, describe a setup that simulates one aspect of the action of a quantum repeater in a long-distance quantum key distribution system. In the work, the researchers teleported a polarization state from one entangled photon to another 600 m across the Danube River through an 800-m optical fiber.

Entangled states naturally decohere over distance in a communications channel. Performing teleportation, purification and quantum memory functions, quantum repeaters would "entanglement swap" quantum states along shorter segments of the optical path, thereby extending the distance over which communication fidelity could be preserved.

The researchers generated an entangled pair of 788-nm photons by spontaneous parametric down-conversion in a BBO crystal and directed the photons to measurement stations on either side of the river. The result of a Bell-state experiment that they performed at one end of the setup ("Alice") on one of the photons was transmitted via a classical free-space microwave channel to the other end ("Bob") 1.5 µs before the receipt of the other photon through the fiber.

The result signaled the application of a voltage to an acousto-optic modulator, which transformed the received photon into the proper quantum state.

The teleportation fidelity was 84, 86 and 90 percent for the three polarization states used. An examination of the polarization stability over the 28-hour measurement runs confirmed that the setup displays high tolerance to real-world factors such as changing temperature.