QUROPE Quantum Information Processing and Communication in Europe

Physical approaches and perspectives

In classical communication information is transferred encoded in pulses of light. The pulses are detected by photodetectors, transformed into electrical current pulses, amplified by electronics, and sent to computers, phones, etc. This transformation of light into electrical signals forms a classical light-matter interface. In quantum information processing, this simple approach is inadequate as it destroys the quantum aspect. Quantum communication requires a coherent storage interface – quantum repeaters. There are a significant number of proposals for realising quantum repeaters ranging from atomic ensembles (cold and hot gases and solid state systems) and linear optics – perhaps the simpler and more advanced approach, to atom and ion approaches - that could take advantage of deterministic entanglement swapping operations. Other approaches based on NV centres in diamonds and quantum dots have been proposed as well as hybrid schemes that combine coherent states and individual quantum systems. A detailed review of ensemble approaches using linear optics and discussions on several others can be found here [1].

State of the art

Great progress has been made in this direction in the last few years by European groups, on the photonic side, with real world teleportation (3x2km, field) [2] and entanglement swapping experiments (lab) [3] as well as proof-of-principle repeater links [4] based on atomic ensembles. Significant progress continues on the way towards implementation of a repeater, by US groups (Lukin, Monroe, Kimble, Kuzmich and Vuletich) where a strong experimental effort on Electromagnetically Induced Transparency (EIT) stored light and quantum memories involving ensembles as well as single atoms can be identified and by European groups (Pan, Gisin, Polzik, Weinfurter) covering ensembles, in gas and solids, and single atom systems. Entanglement between single trapped ions/atoms/quantum-dots and single photons [5] at distances of up to 300 m and coherence times of several 100 μs [6] have been shown. Entanglement fidelities up to 90% were reported. Probabilistic entanglement between two single trapped ions at over 1 m distance has been demonstrated via two-photon interference on a beamsplitter [7]. Much of the recent development towards quantum repeaters has focused on quantum memories and interfaces and hence much of the state of the art is already mentioned there.

European groups working in this field include: H. Weinfurter (Munich, D), N. Gisin & H. Zbinden (Geneva, CH), J.W. Pan (Heidelberg, D), Schmiedmayer (Vienna, AT) E. Polzik (Copenhagen, DK), J. Rarity (Bristol, UK), E. Giacobino (CNRS, Paris, F).

Challenges

In the next 5-10 years we should see fibre optic systems that can beat the direct-transmission distance limitation of around 300-400 km. Initially, quantum repeaters that can function over 1-10 km will provide the building blocks for longer transmission systems – it is these building blocks that provide a scalable route towards pan-European and even global scale quantum communication. These distances will obviously need to be extended further, but not necessarily by much. We note that classical communication links are of the order of 50-100 km between amplification stages. The important aspect for quantum repeaters is the scaling of multiple quantum repeater links. Scalable quantum repeater systems will ensure that the concatenation of multiple links will extend quantum communication distances beyond this fundamental (loss-based) limit. Effort in the next few years should be focused on engineering the sources, interfaces and detectors specifically adapted to long distance transmission and working in unison – long coherence lengths, and high fidelity Bell-State measurements and work towards input/output coupling of photons to Quantum Memories etc. Challenges and directions of future work are thus similar to those already mentioned for quantum detectors, sources and memories and while many aspects have been demonstrated, all need to be improved and demonstrated in the one systems, i.e.:

• Extending memory capabilities to single photon/qubit storage in diverse media;
• Exploring hybrid approaches that combine both discrete and CV aspects for improved performance;
• Develop probabilistic repeater schemes, possibly integrated using atoms on chip technology;
• Integrated solutions such as CNOT gates on optical circuits or circuits that connect multiple elements: source; detector; interface, on a chip;
• Incorporate deterministic strategies for sources, storage and entanglement swapping;

[1] N. Sangouard, C. Simon, H. de Riedmatten, N. Gisin, arXiv:0906.2699v2 (2009)
[2] O. Landry, et al., J. Opt. Soc. Am. B, 24, 398 (2007)
[3] M. Halder, et al., Nature Physics, 3, 692 (2007); R. Kaltenbaek, et al., Phys. Rev. A, 79, 040302(R) (2009)
[4] C.W. Chou, et al., Science, 316 1316 (2007); Z-S Yuan, et al., Nature 454, 1098 (2008)
[5] B. B. Blinov, et al., Nature, 428, 153 (2004); J. Volz, et al., Phys. Rev. Lett., 96, 030404 (2006); R. M. Stevenson, et al., Phys. Rev. Lett., 101, 170501 (2008)
[6] W. Rosenfeld, et al., Phys. Rev. Lett., 101, 260403 (2008)
[7] D. N. Matsukevich , et al., Phys. Rev. Lett., 100, 150404 (2008)