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Quantum Memory For Quantum Repeaters

Erhan Sağlamyürek, Neil E. Sinclair, Jeongwan Jin, Joshua A. Slater, Daniel Oblak, Félix Bussières, Mathew George, Raimund Ricken, W. Sohler, Wolfgang Tittel

Published Jan 1, 2011• 0 citations
Publisher

Abstract

The implementation of quantum memory, i.e. a quantum interface between light and matter, is central to advanced applications of quantum information processing and communication [1]. Notably, quantum memories are key ingredients in quantum repeaters [2], which promise extending quantum communication beyond its current distance limit. After a brief review of the working principle of a quantum repeater and the state-of-the-art of experimental investigations into quantum memory, we will present the reversible transfer of photon-photon entanglement into entanglement between a photon and an excitation in a thulium-doped lithium-niobate waveguide cooled to 3K. Our finding constitutes an important step towards quantum repeaters and fully quantum-enabled networks.

DOI
10.1364/iqec.2011.i93

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The implementation of quantum memory, i.e. a quantum interface between light and matter, is central to advanced applications of quantum information processing and communication [1]. Notably, quantum memories are key ingredients in quantum repeaters [2], which promise extending quantum communication beyond its current distance limit. After a brief review of the working principle of a quantum repeater and the state-of-the-art of experimental investigations into quantum memory, we will present the reversible transfer of photon-photon entanglement into entanglement between a photon and an excitation in a thulium-doped lithium-niobate waveguide cooled to 3K. Our finding constitutes an important step towards quantum repeaters and fully quantum-enabled networks.
The implementation of quantum memory, i.e. a quantum interface between light and matter, is central to advanced applications of quantum information processing and communication [1]. Notably, quantum memories are key ingredients in quantum repeaters [2], which promise extending quantum communication beyond its current distance limit. After a brief review of the working principle of a quantum repeater and the state-of-the-art of experimental investigations into quantum memory, we will present the reversible transfer of photon-photon entanglement into entanglement between a photon and an excitation in a thulium-doped lithium-niobate waveguide cooled to 3K. Our finding constitutes an important step towards quantum repeaters and fully quantum-enabled networks.
For advanced applications in quantum information and communication the reversible transfer of quantum information between light and matter is essential. A key property of such a light-matter quantum interface is the faithful mapping of entanglement. We report the reversible transfer of photon-photon entanglement into entanglement between a photon and a collective atomic excitation in a thulium-doped lithium niobate waveguide. We use an Atomic Frequency Comb (AFC) memory protocol yielding a 5 GHz acceptance bandwidth of our memory. We reconstruct the bi-photon quantum state before and after the reversible transfer and asses the entanglement-preserving nature of our memory. We demonstrate a near perfect mapping of the entangled state. Additionally, we measure a violation of the CHSH Bell inequality before and after storage, thus directly verifying the sufficiency of the entanglement for quantum communication.
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.