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4.1.3 Quantum memories and interfaces

Tue, 2010-03-02 14:12 - Daniele Binosi
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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, and subsequently Q-essence, 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: M. Afzelius (Geneva, CH), E. Polzik (Copenhagen, DK), H. Weinfurter, M. Weber and G. Rempe (Munich, D), S. Kroll (Lund, SW), J-L. Le Gouet (Paris, F), J. Laurat  E. Giacobino (CNRS-Paris, F), J. Rarity (Bristol, UK), H. de Riedmatten (ICFO, ES), J. Schmiedmayer (Vienna, AT), A. Shields, (TREL, UK), I. Walmsley and J. Nunn (Oxford, UK).
 
State of the art
 
The first quantum memory in mesoscopic cold atomic ensembles [1-3] achieved storage times of order of 10 micro s, with a maximum storage and retrieval efficiency of 18%. Today, roughly 7 years later, the state of the art in terms of storage time is ~100 ms [4], hence 104 longer. However, the storage efficiency in that experiment was only a few percent. The highest retrieval efficiencies demonstrated to date for single stored excitations are 50% in free space [5] and 84% in cavities [6]. But these high efficiency demonstrations featured short storage times (a few microseconds or less). Recently Bao et al. [7] demonstrated a long-lived and efficient cold gas quantum memory, reaching 73% efficiency and 3 ms storage time. The highest combined write and retrieval efficiency of any quantum memory was achieved in a room-temperature atomic gas, achieving 78% [8]. Another demonstration featured a very large bandwidth of 1.5 GHz [9]. Storage and retrieval of quantum continuous variables has also been demonstrated in room-temperature atomic vapours [10-12]. Unconditional storage fidelities of up to 70% and storage times of a few milliseconds have been reached.
 
Solid-state quantum memories based on rare-earth doped crystals have gained interest, since the first demonstration of a memory at the single photon level in 2008 [13]. Storage efficiencies up to 69% has been achieved [14], for short storage times of a few microseconds. Quantum storage has so far been limited to a few microseconds, although storage of strong classical pulses has reached beyond 1 second [15]. Storage of entanglement has recently been demonstrated in rare-earth crystals [16-17], with storage efficiencies up to 20%. An important feature of these systems is the potential for multimode storage, with demonstrations up to 64 stored modes of weak coherent states has been shown [18] with conditional qubit fidelities of 93%.

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 QIPC. 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:

  • Improving the storage time and fidelity;
  • Improving multi-mode storage capacity;
  • Efficient integration of sources of photonic entanglement and memory devices;
  • Coupling from the quantum memories to communication channels;
  • Reduction of overall experimental complexity for future scalability

Key references

 

[1]  T. Chaneliere et al., Nature  438, 833 (2005)
[2] M. D. Eisaman et al., Nature 438, 837, (2005)
[3] K. S. Choi et al., Nature 452, 67 (2008) 
[4] A. G. Radnaev et al., Nature Phys. 6, 894 (2010)
[5] J. Laurat et al., Opt. Exp. 14, 6912 (2006) 
[6] J. Simon et al., Phys. Rev. Lett. 98, 183601 (2007) 
[7] X.-H. Bao et al., Nature Physics 8, 517 (2012)
[8] M. Hosseini, G. Campbell, B. M. Sparkes, P. K. Lam and B. C. Buchler, Nature Phys. 7, 794 (2011) 
[9] K. F. Reim et al., Phys. Rev. Lett. 107, 053603 (2011)
[10] B. Julsgaard, J.Sherson, J. I. Cirac, J. Fiuraek, E. S. Polzik, Nature 432, 482, (2004)
[11]  J. Appel et al., Phys. Rev. Lett. 100, 093602 (2008)
[12] J. Cviklinski et al., Phys. Rev. Lett. 101, 133601 (2008) 
[13] H. de Riedmatten et al., Nature 456, 773 (2008) 
[14] M. P. Hedges et al. Nature 465, 1052 (2010)
[15] E. Fraval et al., Phys. Rev. Lett. 95, 030506 (2005) 
[16] C. Clausen et al., Nature 469, 508 (2011)
[17] E. Saglamyurek et al. Nature 469, 512 (2011)
[18] I. Usmani et al., Nature Commun. 1, 12 (2010)