QUANTUM INFORMATION AND QUANTUM PHYSICS IN SPACE
Ursin, R.¹; Jennewein, Th.²; Schmitt-Manderbach, T.³; Weier, H.⁴; Perdigues, J.⁵; Rarity, J.⁶; Barbieri, C.⁷; Weinfurter, H.⁸; Zeilinger, A.^1
1Austrian Academy of Sciences; ²Ludwig-Maximilians-University; ³Institute for Experimental Physics; ⁴IQOQI; ⁵ESA/ESTEC; ⁶University of Bristol; ⁷University of Padova; ⁸University of Vienna, IQOQI

Quantum entanglement [1] is at the heart of quantum physics. At the same time it is the basis for novel quantum communication schemes, such as quantum cryptography [2] even over global distances[3, 4]. Bringing quantum entanglement to the space environment will open a new range of fundamental physics experiments, and will provide unique opportunities for quantum communication applications. We proposed tests of quantum communication in space, whereby an entangled photon Source is placed onboard the ISS, and two entangled photons are transmitted via a simultaneous down link and received at two distant ground stations. Furthermore, performing a series of consecutive single down links with separate ground stations will enable a test of establishing quantum cryptography even on a global scale [5]. This Space-QUEST proposal was submitted within ESA’s OA-2004 and was rated as ‘outstanding’ because of both, a novel and imaginative scientific content and for technological applications of quantum cryptography respectively. We report a proof-of-principle experiment where we were able to generate a quantum cryptographic key over a record-breaking distance 144 km over a free-space link between the Canary Islands La Palma and Tenerife [6, 7]. One photon from the entangled pair was measured locally. The second photon was sent via a transmitter telescope over the 144 km long free-space link to the Optical Ground Station (OGS) of the European Space Agency (ESA) on Tenerife [8]. The OGS, originally built to act as a transmitter and receiver for classical laser communication to and from satellites, a 1 m Richey-Chrétien/Coudé telescope was used to collect the single photons. The atmospheric turbulence caused significant beam wander in the focal plane. A suitable optical system was implemented to prevent the beam from wandering off the single photon detectors with a quantum efficiency of about 40%. We measured a loss of -30 dB on the entire quantum link. Each event in one of the detectors was locally labelled with a 64-bit computer generated tag, containing the detector channel and a time tag with a timing resolution of 156 ps synchronized with a 10 MHz oscillator directly disciplined by the Global Positioning System (GPS) with a relative drift of about 10-11 over 100 s. The identification of the coincident events where implemented by cross-correlating both sets of time tags using software which determined the offset (~487 µs) and unavoidable drift of the two timescales online. Future experiments with entangled photons will benefit from the increased precision of clock synchronization. Entangled photons shared between the two parties were used to establish 178 bits unconditional secure key [9] in total. Any attempt by an eavesdropper to intercept and copy the key is obvious to the receiving party, who notices errors in the transmission. This experiment demonstrated for the first time the use of an entangled photon source delivering the pair production rate required to realize an optical downlink from low-Earth satellites, such as the International Space Station (ISS) [10], to optical ground stations on Earth. This would allow a separation of the two entangled photons by more than 1400 km, clearly exceeding the possible distances for today¹s fiber technology [11]. References [1] E. Schrödinger. Die gegenwärtige Situation in der Quantenmechanik. Naturwissenschaften, 23:807–812; 823–828; 844–849, 1935. [2] C. H. Bennett and G. Brassard. Quantum cryptography. In Proceedings of IEEE International Conference on Computers, Systems, andSignal Processing, Bangalore, India, page 175, New York, 1984. IEEE. [3] J. G. Rarity, P. R. Tapster, P. M. Gorman, and P. Knight. 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