Communicating Quantum States with Alice on a Satellite
Authors: Giuseppe Vallone1, Davide Bacco1, Daniele Dequal1, Simone Gaiarin1, Vincenza Luceri2, Giuseppe Bianco3, Paolo Villoresi1
1Dipartimento di Ingegneria dell’Informazione, Università degli Studi di Padova, Italy
2e-GEOS spa, Matera, Italy
3Matera Laser Ranging Observatory, Agenzia Spaziale Italiana, Matera, Italy.
The exchange of quantum bits – or qubits – is a fundamental process in all Quantum Information protocols. The faithful transport of the fragile quantum content of a photon is needed inside the prototypes of photonic quantum computers as well for the teleportation of a given state.
Moreover, in the relevant application of Quantum-Key-Distribution (QKD), which allows to create a private key between two terminals exploiting the laws of Quantum Physics, such exchange of qubits is expected to cover very long distances . Indeed, in order to connect with secure communications two embassies, two corporate branches and so on, effective quantum communication schemes on a planetary scale are needed. The fibre channels were investigated first for the realization of QKD, such that now several commercial devices based on optical cables are already in operations. Fibers are very efficient up to about 100 km, and the present limit for QKD in fiber is 300 km as demonstrated in a recent experiment ; beyond that scale there is the need of quantum repeaters, which presently are in development in advanced research labs. A radically different approach is to go along a Space channels, and exploit a satellite as the sender or the receiver. From the link budget analysis and the effect of turbulence in the propagation, it is evident that the transmitter (Alice) is most conveniently located on the satellite and the receiver (Bob) on the ground [3,4,5].
The first attempt of quantum communication in Space, was made in 2008 by Villoresi et al [6,7], where photons launched from a ground station were reflected by CCRs (corner cube retroreflectors) and aimed back to the Earth. In that case it was used the Japanese satellite Ajisai for emulate an optical transmitter in space. In that work it was demonstrated the application of spectral, spatial and temporal filtering capable to point out the return photons with global losses in the up- and down-link as strong as 157 dB.
In the present experiment, reported recently in Ref. , we introduced novel schemes for temporal synchronization and the optical interface, realizing a significant improvement in SNR (signal to noise ratio), dark counts and total transmissivity. We proved that a generic qubit with polarization encoding preserves its characteristics in a channel starting from a source realized again by a retroreflector in orbit and measured on ground by a state analyzer connected to an astronomical telescope designed for satellite-laser ranging . Moreover we were able to prove a communication protocol measuring not only one polarization degree, but a complete set of four polarization states required for protocols as QKD .
A very important parameter in QCs (Quantum communications) is represented by the QBER factor (Quantum bit error rate), defined as the number of wrong bits received in a slot time. In case this factor is too high (the threshold depends on the chosen protocol, and on the sending rate), the security of the generated key is not guaranteed . In our experiment we showed, by using some LEO (Low Earth Orbit) satellites (Starlette, Stella, Larets, Jason-2), that our method and setup allows a secure communication in a very long distance scenario (~2000 km). The measured QBER in different runs results of the order of a few percent. It was possible to attest that even with high losses, variable attenuation, and high background a quantum key distribution system works, and an unconditionally secure key, needful for encryption, can be generated also in this case.
For the first time qubits bouncing from space were measured and analyzed in different polarization states. Moreover, all the results were obtained with existing satellites naturally used for geodetic studies and other activities usually equipped with CCRs. The optical setup used in the experiment is yet completely integrable in a lot of OGS (optical ground station) and present an easy interface between quantum and classical signals. Furthermore, the technology of SLR and classical satellite communications was exploited for synchronizing the transmitter and the receiver, even though the synchronization process is not so easy with satellite in motion.
What’s next? The possibility of sending and receiving single photons in very long distances paves the way to a lot of future experiments and brings Quantum Physics and Quantum Communication in a privilege position. Firstly, the big effort made by the Governments and by the University is surely compensated. From a scientific point of view these experimental results are very fascinating because they allow new experiments based on this technology. In particular, QKD could be realized with a small and compact device capable of changing polarization of photons, creating a base element for quantum two-way protocols. Additionally, experiments like entanglement distribution involving long distances, Bell inequality and teleportation protocol could be possible in next few years.
With this experiment, it was demonstrated that, not only free-space quantum key distribution is a ready technology, but also the quantum satellite communication is nowadays possible and realizable. The results open the way to look towards a global space quantum network, where OGS could talk with satellite and vice-versa creating a global secure network.
Acknowledgments: The work was carried out within QuantumFuture, one of ten Strategic Projects funded by the University of Padova in 2009. Coordinated by Prof. Villoresi, the project has established the Quantum Communication Laboratory and engaged four research groups in a joint activity: Quantum Communications, Quantum Control Theory, Quantum Astronomy and Quantum Optics.
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