QUROPE Quantum Information Processing and Communication in Europe

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 waveguides in periodically poled materials that have significantly improved performance. The development of all-fibre entanglement sources, based on four-wave mixing are a promising solution that needs further investigation. 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. Other 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 1550nm. 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: J. Rarity (Bristol, UK), A. Zeilinger (Vienna, AT), A. Shields, (TREL, UK), N. Gisin & H. Zbinden (Geneva, CH), I. Walmsley (Oxford, UK), O. Benson (Berlin, D), M. Mitchell & J. Eschner (ICFO, E), J-W. Pan (Heidelberg, D), C. Silberhorn (Erlangen, D), S. Suage (Stockholm, SW), V. Sandoghdar (Zurich, CH), A. Beveratos (Paris, F), A. Fiore (Eindhoven, NL), J. Wrachtrup & F. Jelezko (Stuttgart, D), G. Leuchs (Erlangen, D)

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] and telecom [2] wavelengths. These waveguide sources demonstrate high brightness and are capable of saturated performance (limited by multiple-pair probabilities). All-fibre entanglement sources based on four-wave mixing [3] provide a high degree of non-degeneracy that may also be useful to couple telecom wavelength with quantum memories. 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 [4, 5], are under development as well as nonlinear interactions in atomic gas cells for CV non-classical light sources.

Challenges

There is no clear global leader on high-rate photon-pair sources, however, Europe is leading in efforts towards coupling photonic and atomic systems despite the only report of actual coupling coming from Japan, while Europe plays a leading role for CV sources, competing with Australia and Japan. There are two extremes of operation under study – for atomic systems with narrow bandwidths and for satellite-based schemes where BW requirements are less critical but the generation rates need to compensate limited transmission time windows due to satellite availability. The main challenges for photon sources are:

• High single & photon-pair rates (rates should be BW limited and take into consideration all intrinsic source losses, such as coupling and filtering);
• High fidelity (> 90% HOM visibility) between multiple sources;
• Improved coupling of generated photons into the quantum channel (single > 50% & photon pairs > 70%);
• Match bandwidths with quantum memories;
• Single-photon sources have made spectacular progress in the last years, but there are still open questions as to whether they can realise high repetition rates, high coupling efficiency and electronic cooling (no liquid helium);
• Development of efficient, stable and pure sources of squeezed, entangled and single photon states;
• Combine efficient squeezing and single photon detection to reliably generate and grow large cat states;
• Use quantum relays exploiting quantum teleportation and entanglement swapping. Dividing the connection into sections allows one to open the receiving detector less frequently, thus lowering the dark-count rate. It should be stressed that quantum relays are a necessary stepping-stone towards quantum repeaters;
• The next crucial challenge in this direction will be a field demonstration over tens of km of entanglement swapping and high fidelity (> 90%) Bell-State measurements.

[1] M, L. Scholz, et al., Phys. Rev. Lett., 102, 063603 (2009); A. Haase, et al., Opt. Lett., 34, 55 (2009); X. H. Bao, et al., Phys. Rev. Lett. 101, 190501 (2008); J. S. Neergaard-Nielsen, et al., Opt. Exp., 15, 7940 (2007)
[2] E. Pomarico, et al, New J. Phys., 11 113042 (2009)
[3] J. Fulconis et al., Phys. Rev. Lett. 99, 120501 (2007)
[4] H. Vahlbruch et al, Phys. Rev. Lett. 100, 033602 (2008)
[5] R. Dong et al. Opt. Lett. 33, 116 (2008)