Free-space quantum communication achieved over 144 km

A free-space quantum-communication experiment carried out in the Canary Islands by researchers from eight universities and research institutes in Austria, Germany, The Netherlands, the United Kingdom, and Italy has achieved quantum-key distribution over a 144 km distance-an order of magnitude longer than the previous record, and at last approaching distance scales large enough for ground-to-satellite communication.

A free-space link set up between the islands of La Palma (the quantum sender “Alice”) and Tenerife (receiver “Bob”) contained a high-power UV-laser-pumped downconverting beta-barium-borate-crystal source (808 nm) and optics to produce polarization-entangled photon pairs; one photon was measured on La Palma and the other captured by the 1 m telescope at the Tenerife Optical Ground Station and measured there. Two polarizing beamsplitters analyzed either the 0°/90° or ±45° directions, randomly split by a nonpolarizing beamsplitter. Time-tagging of photons, aided by GPS (global-positioning-system) data, a conventional Internet connection, and fast time-correlation software at Alice’s end, found the coincident photon pairs and determined a coincidence count rate of about 20 to 40 per second, resulting in the generation of a quantum-cryptographic key. Contact Rupert Ursin at [email protected].