Home › Quantum Information Processing and Communication: Strategic report on current status, visions and goals for research in Europe › 4. Assessment of current results and outlook on future efforts › 4.1 Quantum Communication
Tue, 2010-03-02 14:03 - Daniele Binosi
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Physical approaches and perspectives

Sources of quantum light in the discrete variable regime have traditionally relied on spontaneous parametric down-conversion (SPDC) in bulk crystals. This has been extended to periodically poled materials and waveguided devices, which  have significantly higher efficiencies. The development of all-fibre entanglement sources, based on four-wave mixing provide several new approaches ranging from standard fibres to photonic crystal fibres. Deterministic sources that avoid probabilistic multi-pair events, associated with the previous schemes, have advanced to the point where entangled photon pairs can be generated by the optical excitation of the bi-exciton state of a semiconductor quantum dot, although currently the low efficiency of these devices detracts from its potentially deterministic nature. Single photon sources based on NV diamond centres and single molecules in solids have been realised and progress continues on single photon sources in diverse materials for sources ranging from the visible up to 1310 nm. In the continuous variable regime sources of squeezed and entangled light typically rely on either parametric oscillators in bulk crystals or the Kerr effect in optical fibres.

European groups working in this field include: O. Benson (Berlin, D), A. Beveratos (Paris, F), J. Eschner (Saarlandes, D), A. Fiore (Eindhoven, NL), C. Marquardt and G.~Leuchs (Erlangen, D), E. Polzik (Copenhagen, DK),  J. Rarity (Bristol, UK), A. Shields (TREL, UK), C. Silberhorn (Paderborn, D), S. Tanzilli (Nice, F),  R. T. Thew (Geneva, CH), R. Ursin and A. Zeilinger (Vienna, AT), I. Walmsley and B. Smith (Oxford, UK), G. Weihs (Innsbruck, AT). 

State of the art

Two important parameters for quantum light sources are bandwidth (BW) and efficiency -- both creation (brightness) and coupling into other systems. Furthermore, the sources need to be adapted and developed to the desired applications, for example, there are currently few systems that approach quantum memory bandwidths (1-100 MHz). First steps in resolving these limitations have been made for atomic [1-4] and telecom [5] wavelengths. All-fibre entanglement sources based on four-wave mixing [6] provide a high degree of non-degeneracy that may also be useful to bridge telecom wavelength and quantum memory regimes, although for the most common quantum memory systems, the sources are too broad band. These sources are better suited to entanglement distribution in asymmetric architectures or for heralded photon sources. For free-space sources, both entangled photon pairs as well as single photon sources, it is preferable to use shorter wavelengths than for fibre networks. This is to limit the diffraction on the sending aperture, which is especially important for very long optical communication links, e.g. between geo-stationary orbiting satellites as well as the communication to a future moon or even a Mars base. Diverse approaches to continuous variable quantum state sources [7,8], are under development as well as nonlinear interactions in atomic gas cells for discrete and continuous variable non-classical light sources. Single photon sources have made significant advances, in a range of different systems though their performance remains limited and few systems operate into the telecom regime, and then only as far as 1310 nm. Further information can be found in a detailed review of single photon sources that has recently been realised [9].
 

Challenges

Europe is currently leading in efforts towards coupling narrow-band photonic and atomic systems and plays a leading role for CV sources, competing with Australia and Japan. The USA is ahead in terms of pulsed systems in the telecom regime. There are several regimes of operation under study: atomic systems with narrow bandwidths for quantum repeaters,  satellite-based schemes where bandwidth requirements are less critical but the generation rates need to compensate limited transmission time windows due to satellite availability, and in between both of these, pulsed systems for quantum fibre optical networks (teleportation and entanglement swapping) where robustness against fibre length fluctations needs to be balanced with high rates. The increasing complexity and diversity of quantum communication systems has also seen a much more sophisticated approach taken to engineering the sources, and in particular, the nonlinear interactions that are needed. The engineering of factorable, or pure, states of light [10] will be crucial for future quantum communication networks but so far most of this work has taken place in the visible regime- the extension to telecommunication wavelengths will be vital for the next generation of experiments. The main challenges for photon sources are:

  • Photon pair sources capable of high rates with coupling >70% and high fidelity (>90% HOM visibility) between independent sources without spectral filtering;
  • Single photon sources capable of high rates with coupling >50% and high fidelity (>90% HOM visibility) between independent sources without spectral filtering;
  • Development of efficient, stable and pure sources of squeezed, entangled and single photon states that are able to reliably generate and grow large cat states;
  • Narrow band photon pair sources capable of efficiently coupling quantum memories to telecomunication fibre networks.

Key references
[1] M, L. Scholz, et al., Phys. Rev. Lett. 102, 063603 (2009)
[2] A. Haase, et al., Opt. Lett. 34, 55 (2009)
[3] X. H. Bao, et al., Phys. Rev. Lett. 101, 190501 (2008)
[4]  J. S. Neergaard-Nielsen, et al., Opt. Exp., 15, 7940 (2007) 
[5] E. Pomarico, et al., New J. Phys. 11, 113042 (2009)
[6] A. R. McMillan, J. Fulconis, M. Halder, C. Xiong, J. G. Rarity, and W. J. Wadsworth, Opt. Exp., 17  6156 (2009)
[7] H. Vahlbruch et al., Phys. Rev. Lett. 100, 033602 (2008) 
[8] R. Dong et al. Opt. Lett. 33, 116 (2008)
[9] M. D. Eisaman, J. Fan, A. Migdall and S. V. Polyakov,  Rev. Sci. Instrum.  82, 071101  (2011)
[10] P. Mosley, J. S. Lundeen, B. J. Smith, P. Wasylczyk, A. B. U’Ren, C. Silberhorn, and I. A. Walmsley, Phys. Rev. Lett. 100, 133601 (2008)