EPN 40/3 27of quantum communication on a global scale, a task impossible on ground with current optical fiber and photon-detector technology. Currently, quantum communication on ground is limited to the order of 200 kilometers [4]. Bringing quantum communication into space is the only way to overcome this limit with state-of-the-art technology. Another area of applications is metrology, where quantum clock synchronization and quantum positioning [5] are studied. Furthermore, sources of quantum states in space may have applications in the new field of quantum astronomy. The proposed experimentsWe propose to ESA to perform these experiments in space by placing a quantum transceiver on the external pallet of the European Columbus module at the ISS (see Fig. 1). The entire terminal must not exceed the specifications given for pallet payloads as provided by ESA. The requirements are: size 1.39×1.17×0.86 m 3 , mass < 100 kg, and a peak power consumption of < 250 W, respectively. A preliminary design of a satellite-based quantum transceiver (including an entangled photon source, a weak pulse laser source, single photon detection modules together with two transceiver telescopes) based on state-of-the-art optical communication terminals and adapted to the needs of quantum communication has already been published in [6] (see Fig. 2). e entangled photons are transmitted to two distant ground stations via simultaneous down-links [7], allowing a test on entanglement and the generation of an unconditional secure quantum cryptographic key between stations separated by more than 1000 km.t is an open issue whether quantum laws, originally established to describe nature at the microscopic level of atoms, are also valid in the macroscopic domain such as long distances. Various proposals predict that quantum entanglement is limited to certain mass and length scales or is altered under specific gravitational circumstances. Testing the quantum correlations over distances achievable with systems placed in the Earth orbit or even beyond would allow verifying both the validity of quantum physics and the preservation of entanglement over distances impossible to achieve on ground. Using the large relative velocity of two orbiting satellites, one can perform experiments on entanglement where -due to special relativity -both observers can claim that they have performed the measurement on their system prior to the measurement of the other observer. In such an experiment it is no longer possible to think of any local realistic mechanisms that potentially influence one measurement outcome according to the other one. Moreover, quantum mechanics is also the basis for emerging technologies of quantum information science, presently one of the most active research fields in physics. Today's most prominent application is quantum key distribution (QKD) [3], i.e. the generation of a provably unconditionally secure key at distance, which is not possible with classical cryptography. The use of satellites allows for demonstrations SPAcE-quESt ExPERimE...
Models of quantum systems on curved space-times lack sufficient experimental verification. Some speculative theories suggest that quantum correlations, such as entanglement, may exhibit different behavior to purely classical correlations in curved space. By measuring this effect or lack thereof, we can test the hypotheses behind several such models. For instance, as predicted by Ralph et al [5] and Ralph and Pienaar [1], a bipartite entangled system could decohere if each particle traversed through a different gravitational field gradient. We propose to study this effect in a ground to space uplink scenario. We extend the above theoretical predictions of Ralph and coworkers and discuss the scientific consequences of detecting/failing to detect the predicted gravitational decoherence. We present a detailed mission design of the European Space Agency's Space QUEST (Space-Quantum Entanglement Space Test) mission, and study the feasibility of the mission scheme.
Link-budget calculations are a common way to assess system parameters, such as the required transmitter power and receiver sensitivity, in free-space optical ͑FSO͒ communication systems. One of the biggest challenges for long-range FSO deployment is its signal propagation under turbulent atmospheric conditions, which produce intensity fluctuations. Methods to estimate atmospheric-fading loss in radio-frequency systems cannot be adapted to the FSO channel. Until now no general closed-form methods have been developed to describe the fading loss in such a channel. A method to calculate the losses due to scintillation fading in the threshold approach, based on lognormal statistics of the received power, is presented.
Optical LEO downlinks from the Japanese OICETS to the optical ground station built by the German Aerospace Center (DLR) near Munich have been performed. This was the first optical LEO downlink on European grounds. The ground station received a 50-Mbit/s OOK signal at 847 nm on its 40-cm Cassegrain telescope and sent two spatially displaced beacon beams towards OICETS. Five out of eight trials could be performed successfully while the other three were hindered by cloud blockage. A BER of 10-6 has been reached. The elevation angle above the horizon ranged between 2° and 45°. The Fried parameter and the scintillation were measured with instruments inside the ground station. The beacon power received by the LUCE Terminal onboard OICETS has also been recorded. This paper describes the setup of the experiment and highlights the results of the measurement trials.
In this paper, a packet-level forward error correction (FEC) coding technique and pre-distortion adaptive optics (AO) technology are applied to a digital transmission scheme for optical feeder links in a geostationary Earth orbit (GEO) satellite communication system. The architectures of the gateway and the satellite are defined, including the building blocks of the interface between the radio frequency (RF) front-end and the optical front-end, as well as the digital signal processor. The system is designed to cater for Terabit/s highthroughput satellite (HTS) applications. The performance of the digital transmission scheme is evaluated in the forward and return links. The turbulent atmospheric optical channel is modeled for different optical ground station (OGS) altitudes. It is shown that fade mitigation techniques such as packet-level FEC coding and pre-distortion AO in the forward link, as well as large-aperture OGS telescope in the return link, are essential to close the link budget of a Terabit/s satellite communication system. Prepared using satauth.cls [Version: 2010/05/13 v2.00] 2 S. DIMITROV ET AL.Index-of-refraction turbulence (IRT) [3] is known to degrade the achievable data rates of FSO communication links [4]. For uplinks above 30 • elevation, the IRT falls within the weak turbulence regime [5], where the probability density function (PDF) of the irradiance at the receiver plane can be modeled by a log-normal distribution [6]. Moreover, the correlated fading events represent a lowpass process defined by the Greenwood frequency [7]. Regarding site selection and deployment, an OGS at sea level may suffer from strong influence of the turbulence, since the atmosphere is denser at lower altitudes. Astronomical sites are generally located on mountain tops, but these locations are usually isolated, and Terabit/s infrastructure needs to be developed. In this paper, 300 m, 900 m and 1500 m are chosen as examples of possible OGS altitudes.Adaptive optics (AO) is a technology used for correction of the incoming optical wavefront phase which is distorted by the turbulent atmosphere [8]. In ground-to-GEO FSO links, the scale size of the phase distortions is much larger than the feasible satellite telescope aperture size [3], and therefore, the spatial phase at the satellite receiving aperture is rather flat. Nevertheless, in an uplink scenario the traveling wavefront is initially perturbed by the atmosphere, and then it propagates through free space up to the GEO satellite. Thus, at the satellite plane the received wavefront profile differs from that of a pure diffracted laser beam. These distortions can be mitigated by applying pre-distortion AO (PAO) techniques to the outgoing optical wave at the ground station, using a beacon reference that senses the distortions introduced by the turbulent atmosphere.The simplest PAO correction method is tilt compensation of the first three spatial modes of the wavefront phase information [9]. It uses pointing by tracking, where a downlink beacon from the satellite is used a...
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