QUROPE Quantum Information Processing and Communication in Europe

Physical approaches and perspectives

Point-to-Point (P2P) QKD systems are the most advanced quantum communication technologies, and as such, much of the progress here will help to reinforce their commercialisation and that of emerging quantum technologies. Nonetheless there are significant and fundamental research efforts ranging from multiplexed fibre optic systems to satellite distribution schemes currently under investigation and taking advantage of both discrete and continous variable encoding schemes.

Fibre Systems

Groups are currently working on fibre QKD systems that encode in polarisation, phase, photon number and time-bins, using both discrete or continuous variables (CV). Weak-pulse (laser pulses attenuated to the single photon level) encoding schemes are by far the most practical and advanced schemes.  Commercial systems still have relatively low rates and hence the research pursuit is primairly directed at novel protocols, improved detector performance and greater integration, both on the quantum level as well as the quantum classical interface and information processing. The extension to multiplexed systems has been a recent but necessary step opening up new operating scenarios and facilitating implementation in installed fibre optic networks. 

European groups working in this field include:  R. Alleaume (SeQureNet, F), J. Capmany (Valencia, E), N. Gisin and H. Zbinden (Geneva, CH), V. Martin (Madrid, E),  A. Poppe (AIT, AT), A. Shields (TREL, UK),  G. Leuchs (Erlangen, D), P. Grangier (Paris, F),  G. Ribordy (IDQ, CH), P. Tombesi (Camerino, I), A. Zeilinger (Vienna, AT).

Free Space Systems

Many current free-space systems focus on polarisation-based encoding. Traditionally dominated by discrete variable systems, work on CV systems has recently been reinvigorated. The CV squeezed states offer potentially higher key rates and longer distances than coherent state CV protocols. The potential for using non-Gaussian states and higher dimensional Hilbert spaces (complex spatial modes/polarisation patterns) may increase the efficiency and capacity of  these quantum information protocols. The European Space Agency ESA has supported various studies in the field of quantum physics and quantum information science in space for several years \cite{Ursin:2009a}. The mission proposal Space-QUEST (Quantum Entanglement for Space Experiments) has the objective of performing space-to-ground quantum communication tests from the International Space Station (ISS). The launch plan is compatible with 2014.

State of the art

P2P schemes face fundamental distance limits and recent experiments have approached these for both fibre and free space schemes. In fibre optic systems, >200 km has been achieved in a field trial for weak-pulse schemes [1] as well as >200 km in fibre for entanglement-based schemes [2] in the lab. Demonstrations that QKD and classical communication channels can co-exist have also been made [3] using DWDM channels. CV systems are much more sensitive to distance though 25 km has been realised in the lab  [4]. An important milestone, in terms of rates, has also been realised with 1 Mbps secure key rate over 20 km [5]. There are also extensive field trials taking place in the Canary Island involving several leading European groups as part of an European Space Agency feasibility study for quantum communcation via satellite [6]. A weak pulse schemes has demonstrated a >144 km free-space QKD [7].  In a related experiment, the Matera Laser Ranging Observatory (MLRO) in Southern Italy served as transceiver station for faint-pulse exchange with a low-earth orbit retro-reflecting satellite at a perigree of 1485 km [8]. 

Challenges

• Faster electronics for increased application-dependent performance incorporating sources, detectors, QRNGs, low-loss phase and amplitude modulators and their integration. This is mainly a (non-trivial "quantum opto-electronics") engineering problem;
• Determine the benefit of free space links using polarisation variables- these prevent dephasing of the CV quantum states with respect to local oscillator phases used in homodyne measurements and straylight is effectively filtered by the homodyne measurement, thus facilitating the implementation of daylight links;
• Demonstration of passive and programmable multiplexing of multiple quantum channels as well as quantum and classical channels to increase rates and reduce infrastructure costs associated with fibre bandwidth and network architectures;
• Study complex spatial mode structures as decoherence-free states in free space channels, both, discrete and CV, that don't suffer due to turbulence and diffraction;
• Development of compact and robust high photon flux sources- depending on the architecture and protocol, the systems must operate in short burst when satellites are in view;
• Robust systems that can be space certified and withstand launch g-forces as well as being immune against the radiation in space;
• Demonstration of practical, autonomous, systems capable of performing continuous secure key distribution > MHz rates over increased distances.