Solid State Quantum Memories for Quantum Repeaters
Hugues de Riedmatten, Imam Usmani, Björn Lauritzen, Christoph Clausen, Jiří Minář, Nicolas Sangouard, Christoph Simon, A. Amari, Andreas Walther, Stefan Kröll, Mikael Afzelius, Nicolas Gisin
Quantum memories are necessary for the implementation of quantum networks and repeaters. Recent progress towards photonic quantum storage in solid state atomic ensembles using photon echo techniques will be presented.
Abstract Many applications of quantum communication crucially depend on reversible transfer of quantum states between light and matter. Motivated by rapid recent developments in theory and experiment, we review research related to quantum memory based on a photon‐echo approach in solid state material with emphasis on use in a quantum repeater. After introducing quantum communication, the quantum repeater concept, and properties of a quantum memory required to be useful in a quantum repeater, we describe the historical development from spin echoes, discovered in 1950, to photon‐echo quantum memory. We present a simple theoretical description of the ideal protocol, and comment on the impact of a non‐ideal realization on its quantum nature. We extensively discuss rare‐earth‐ion doped crystals and glasses as material candidates, elaborate on traditional photon‐echo experiments as a test‐bed for quantum state storage, and describe the current state‐of‐the‐art of photon‐echo quantum memory. Finally, we give a brief outlook on current research.
The ability of storing and retrieving quantum states of light is an important experimental challenge in quantum information science. A powerful quantum memory for light is required in a quantum repeater, which would allow long distance (>500km) quantum communications. To be implemented in such application, the quantum memory must allow on-demand readout, with high fidelity and efficiency, and a long storage time. Additionally a multimode capacity (for temporal or spatial modes) would allow multiplexing. Our approaches focus on rare-earth doped crystals, i.e. solid state quantum memory. I present, in this work, our contributions for a solid-state quantum storage, with good performances in every criteria. In particular, I present the preservation of quantum entanglement during the storage, which paves the way for the implementation of quantum memories in quantum repeaters.
Over the last decade, the record distance for quantum communications remains in the range of 100 km due to losses and decoherence in the quantum channel. To achieve long distance, H.J. Briegel et al proposed a quantum repeater protocol, which combine quantum memory, entanglement purification and entanglement swapping [1]. Due to the lack of perfect quantum memory, the quantum repeater has not yet been implemented experimentally though much efforts have been made over many years. Here we present a solid-state repeater scheme using a quantum-dot spin in an optical microcavity [2,3]. This repeater has several advantages: (1) deterministic (if optimized); (2) complete and loss-resistant Bell-state analyzer with a built-in spin memory; (3) the storage time for spin memory could be significantly prolonged using proposed spin-echo techniques; (4) compatible with standard semiconductor processing techniques. [1] H. -J. Briegel et al, Phys. Rev. Lett. 81, 5932 (1998). [2] C.Y. Hu et al, Phys. Rev. B, 78, 085307 (2008); 78, 125316 (2008); 80, 205326 (2009). [3] C.Y. Hu and J.G. Rarity, ArXiv: quant-ph 1005.5545 (2010). [4] S. Reitzenstein et al, Appl. Phys. Lett. 90, 251109 (2007).
Quantum communication founds on the possibility to encode quantum states into photons and allows, for instance, the provable secure distribution of encryption keys. Despite a lot of progress during the past two decades, quantum communication still faces a distance barrier, which can be overcome by quantum repeaters whose currently most challenging ingredient is a memory for quantum states of light. In addition to robustness and ease of use, the requirements for such a quantum memory include high fidelity quantum state storage and the preservation of entanglement, sufficiently long storage time with on-demand recall, high efficiency, large bandwidth, and high multimode capacity. Six years ago, some of these properties had already been demonstrated using atomic vapor. However, practical solid-state memories were virtually unexplored in view of quantum state storage. The main goal of this thesis was to realize a solid-state quantum memory for light with emphasis on meeting requirements for long distance quantum communication. First, we identified a potential material candidate, a thulium doped lithium niobate waveguide, and conducted spectroscopic investigations to assess its suitability for quantum state storage using a photon-echo type approach. Next, we demonstrated that our storage device is suitable for high fidelity quantum state storage and for the storage of photonic entanglement, and that its large spectral multimode capacity allows for on-demand selective recall in conjunction with standard spectral filters and frequency shifters. In addition, by performing two-photon interference experiments and Bell-State measurements, we found that our memory preserves not only quantum information, but, more generally, the entire photonic wave function, which further confirms its suitability for quantum repeaters as well as for linear optics quantum computers. Finally, we showed that our integrated device also allows for general temporal and spectral manipulation of individual quantum optical pulses, which paves the way towards on-chip quantum optical processors. While more work remains to be done, in particular to improve memory efficiency and storage time, the large number of achievements, together with known ways to overcome the remaining obstacles, makes us confident that a quantum memory suitable for quantum repeaters will soon be built.