QUIE2T Quantum Information Entanglement-Enabled Technologies

Physical approaches and perspectives

An interface between quantum information carriers (quantum states of light) and quantum information storage and processors (atoms, ions, solid state systems) is an integral part of a full-scale quantum information system. Advances with atomic gases and trapped ions have been steady and new efforts on rare earth ions in solids have recently made considerable gains. Recent efforts in the EU project QAP have seen diverse systems making key proof-of-principle demonstrations of long storage times, high efficiency, and high fidelities. An important aspect arising from this work is the need for multiplexing (space, time, frequency) to increase potential distribution rates. In the context of quantum communication, the goal for all of these approaches is integration with photonic (flying qubit) systems and their operation in complete quantum repeater architectures and protocols.

European groups working in this field include: N. Gisin & H. Zbinden (Geneva, CH), E. Polzik (Copenhagen, DK), H. Weinfurter (Munich, D), S. Kröll (Lund, SW), J-L. Le Gouet (Paris, F), E. Giacobino (CNRS, Paris, F), J. Rarity (Bristol, UK), A. Shields, (TREL, UK), M. Mitchell & J. Eschner (ICFO, E), I. Walmsley (Oxford, UK)

State of the art

We have already seen single photons stored in mesoscopic cold atomic ensembles [1] with storage times of order of 10 μs, with a maximum storage and retrieval efficiency of 18%. Heralded entanglement between spatially separated ensembles has been achieved [2] and entanglement between single photons and stored collective spin excitations has been demonstrated [3]. The best retrieval efficiencies demonstrated to date for single stored excitations are 50% in free space [4] and 84% in cavities [5]. Recently, the storage duration of single collective excitations has been improved up to several ms [6], although again with lower retrieval efficiencies (~20%). Storage and retrieval of quantum continuous variables has also been demonstrated in atomic vapours [7] and in cold ensembles [8]. In ensemble based solid-state quantum memories, a light-matter interface at the single photon level has been realised recently [9], importantly with multimode (4-mode) storage and high conditional fidelity (98%). Bright light pulses have been stored for more than 1 s [10] and with efficiencies higher than 45 % (for short storage times) in doped crystals. For quantum dot systems NV-centres in diamond and single molecules in solids, quantum interference between two photons emitted from two remote emitters is still under investigation [11].

Challenges

Europe and the US are both well advanced with a range of architectures under study, however, this remains a fledgling domain within the field of QIFT and the field and the range of architectures and materials under investigation is rapidly expanding so we concentrate here on those most closely focused on quantum communication oriented applications. Key challenges for quantum memories and interfaces are:

• Achieving efficient storage and retrieval for the quantum state;
• Increasing storage time;
• Improving the fidelity of storage;
• Improving multi-mode storage capacity;
• Coupling from the quantum memories to communication channels;
• All of these in one single system. 

[1] T. Chaneliere et al., Nature, 438, 833 (2005); M. D. Eisaman, et al., Nature, 438, 837, (2005); K. S. Choi, et al., Nature, 452, 67 (2008)
[2] C.W. Chou, et al., Nature, 438, 828 (2005)
[3] D.N. Matsukevich, et al., Phys. Rev. Lett., 95, 040405 (2005); H. de Riedmatten, et al., Phys. Rev. Lett., 97, 113603 (2006)
[4] J. Laurat, et al., Opt. Exp. 14, 6912 ( 2006)
[5] J. Simon et al., Phys. Rev. Lett. 98, 183601 (2007)
[6] B. Zhao, et al., Nat Phys 5, 95 (2009); R. Zhao, et al., Nat. Phys., 5, 100, (2009)
[7] B. Julsgaard, et al., Nature, 432, 482, (2004); J. Appel, J., et al., Phys. Rev. Lett., 100, 093602 (2008); J. Cviklinski, et al., Phys. Rev. Lett., 101, 133601 (2008)
[8] K. Akiba, et al., New J. Phys., 11, 013049 (2009)
[9] H. de Riedmatten, et al., Nature 456, 773 (2008)
[10] E. Fraval, et al., Phys. Rev. Lett., 95, 030506 (2005)
[11] K. Sanaka, et al., Phys. Rev. Lett., 103, 053601 (2009); R. Lettow, et al., Opt. Exp., 15, 15842 (2007)