Researchers demonstrate a robust, high-fidelity, broadband quantum memory that could be used to realize a long-distance quantum repeater for secure 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.
A long-distance quantum network that could enable distributed quantum computing and unconditionally secure communication will require a quantum memory capable of supporting entanglement swapping over long distances. The memory qubit must preserve its entanglement to the communication qubit sent to a repeater for at least long enough to swap entanglement with an adjacent node. The most important figures-of-merit for a quantum memory are: fidelity, entanglement rate, and memory lifetime. Though many varieties of memories and repeaters are currently being researched (for a review see [1]) it remains an outstanding challenge to develop one that simultaneously meets all the necessary criteria to enable a practical long distance quantum network. We propose to use laser-cooled and trapped rubidium atoms, with Rydberg excitations, coupled to an optical cavity, to address these issues.