QUROPE Quantum Information Processing and Communication in Europe

Physical approaches and perspectives

Fibre

Groups are currently working on fibre systems that encode in polarisation, phase, photon number and time-bins, using both discrete or continuous variables (CV). Weak-pulse encoding schemes are by far the most practical but entanglement based schemes lay a solid foundation for longer distance communication schemes involving repeaters. The extension to multiplexed systems has been a recent but necessary step.

European groups working in this field include: N. Gisin & H. Zbinden (Geneva, CH), A. Shields, (TREL, UK), A. Zeilinger (Vienna, AT), J. Rarity (Bristol, UK), G. Leuchs (Erlangen, D), P. Grangier (Paris, F), P.D. Townsend (Cork, IRL), G. Ribordy (id Quantique, CH).

Free Space

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 quantum information protocols.

European groups working in this field include: A. Zeilinger (Vienna, AT), H. Weinfurter (Munich, D), J. Rarity (Bristol, UK), G. Leuchs, (Erlangen, D).

State of the art

The recent SECOQC QKD network demonstration illustrated the range of different approaches that are currently being developed in Europe [1]. It also demonstrated two other important points: the idea of a trusted-node quantum network; and that different architectures could be made to work transparently on one network. All QKD systems were fully automated, including self-compensation for environmental influences on the fibre link. The demonstration involved one-time pad encrypted telephone communication, a secure (AES encryption protected) video-conference with all deployed nodes and a number of rerouting experiments, highlighting basic mechanisms for quantum network functionality. The average link length was between 20 and 30 km, the longest link 83 km. This is an important interim step (before quantum repeaters) as point-to-point quantum key distribution schemes approach their distance limits. Recent experiments have approached these limits for both weak pulse schemes, with > 144 km for field trials in free-space [2] and > 200km in fibre [3] - as well as > 200km in fibre for entanglement-based schemes [4] and 25km for CV systems [5] in the lab.

Challenges

Europe is a clear leader in this domain with the US and Japan not far behind and China rapidly approaching. The central challenge for point-to-point systems is to increase rates, either simply by higher clock rates or through multiplexing multiple signals or systems. The integration of multiple components for fast, efficient and continuous operation is perhaps the most demanding obstacle. Key challenges are:

• 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;
• Integrated optics design (waveguides/fibres/circuits) for more compact and robust sources and interfaces;
• Extension of GHz clock rates into the MHz continuous secure key distribution regime;
• Multiplexed quantum and classical channels for increased communication bandwidth;
• Fast (> GHz), low-loss (< 1dB) optical switching;
• Invent and investigate new protocols inspired by existing and reliable components, like “decoy states” [6], SARG [7] and COW [3] protocols. Look at systems that combine aspects of discrete and CV operation. This is mainly a matter of the physicists’ imagination;
• 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;
• Quantum communication with entangled states will be important to further develop quantum teleportation and entanglement swapping in view of their possible use in connecting future quantum networks.

[1] M. Peev, et al., New J. Phys. 11 075001 (2009)
[2] T. Schmitt-Manderbach, et al., Phys. Rev. Lett. 98, 010504 (2007)
[3] D. Stucki, et al., Opt. Exp., 17, 13326 (2009)
[4] J. F. Dynes, et al., Opt. Exp., 17, 11440 (2009)
[5] J. Lodewyck et al.,Phys. Rev. A 76, 042305 (2007)
[6] W.-Y. Hwang, Phys. Rev. Lett. 91, 057901 (2003)
[7] V. Scarani, et al., Phys. Rev. Lett. 92, 057901 (2004)