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arXiv:1301.0636 · Physical Review Letters

Efficient Quantum Memory Using a Weakly Absorbing Sample

Mahmood Sabooni, Qian Li, Stefan Kröll, Lars Rippe

Published Mar 26, 2013• 210 citations
arXivHTMLPDF

Abstract

A light-storage experiment with a total (storage and retrieval) efficiency η=56% is carried out by enclosing a sample, with a single-pass absorption of 10%, in an impedance-matched cavity. The experiment is carried out using the atomic frequency comb (AFC) technique in a praseodymium-doped crystal (0.05%Pr(3+):Y2SiO5) and the cavity is created by depositing reflection coatings directly onto the crystal surfaces. The AFC technique has previously by far demonstrated the highest multimode capacity of all quantum memory concepts tested experimentally. We claim that the present work shows that it is realistic to create efficient, on-demand, long storage time AFC memories.

DOI
10.1103/physrevlett.110.133604

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A light-storage experiment with a total (storage and retrieval) efficiency η=56% is carried out by enclosing a sample, with a single-pass absorption of 10%, in an impedance-matched cavity. The experiment is carried out using the atomic frequency comb (AFC) technique in a praseodymium-doped crystal (0.05%Pr(3+):Y2SiO5) and the cavity is created by depositing reflection coatings directly onto the crystal surfaces. The AFC technique has previously by far demonstrated the highest multimode capacity of all quantum memory concepts tested experimentally. We claim that the present work shows that it is realistic to create efficient, on-demand, long storage time AFC memories.
We report on the high-efficiency storage and retrieval of weak coherent optical pulses and photonic qubits in a cavity-enhanced solid-state quantum memory. By using an atomic frequency comb (AFC) memory in a Pr 3+ :Y 2 SiO 5 crystal embedded in an impedance-matched cavity, we stored weak coherent pulses at the single photon level with up to 62% efficiency for a pre-determined storage time of 2 µs. We also confirmed that the impedance-matched cavity enhances the efficiency for longer storage times up to 70 µs. Harnessing the temporal multimodality of the AFC scheme, we stored weak coherent time-bin qubits with a record (51 ± 2%) efficiency and a fidelity over (94.8 ± 1.4)%, limited by imperfections in the qubits creation and measurement.
We report on the realization of an efficient solid-state spin-wave quantum memory, with on-demand readout, using the full atomic frequency comb (AFC) scheme in a Pr^{3+}:Y_{2}SiO_{5} crystal embedded in an impedance-matched cavity. We demonstrate operation at the single-photon level by storing weak coherent states with an efficiency up to (40±2)% and a signal-to-noise ratio of 14 for an input photon number of 0.42 photons per pulse. We also investigated the enhancement of the incoherent noise due to the impedance-matched cavity and characterized the quantum memory performance, showing a two-way transfer from excited to spin states and back of up to 83%. Finally, we confirmed the quantum nature of our memory by storing nonclassical states of light, i.e., a heralded single photon from a nondegenerate spontaneous parametric down-conversion source, and achieved nonclassical correlations between the heralding and the stored-and-retrieved photon. These results demonstrate that impedance-matched AFC spin-wave quantum memories with on-demand readout can be used for experiments involving the storage of photonic quantum states. They also open the door to solid-state on-demand quantum memories with very high efficiencies, serving as a key resource for quantum networks and quantum repeaters.
We report on the experimental demonstration of an optical spin-wave memory, based on the atomic frequency comb (AFC) scheme, where the storage efficiency is strongly enhanced by an optical cavity. The cavity is of low finesse, but operated in an impedance matching regime to achieve high absorption in our intrinsically low-absorbing Eu 3+ :Y 2 SiO 5 crystal. For storage of optical pulses as an optical excitation (AFC echoes), we reach efficiencies of 53% and 28% for 2 μ s and 10 μ s delays, respectively. For a complete AFC spin-wave memory we reach an efficiency of 12%, including spin-wave dephasing, which is a 12-fold increase with respect to previous results in this material. This result is an important step towards the goal of making efficient and long-lived quantum memories based on spin waves, in the context of quantum repeaters and quantum networks.
This thesis describes experiments aimed at developing a key component in applications such as quantum networks and long-distance quantum communication systems: quantum memories. Quantum memories for light using photon-echo and coherent rephrasing techniques have been developed. The work is carried out on rare-earth-ion-doped inorganic crystals. Several properties are associated with these crystals such as long coherence times and long-lived ground state sublevels that are particularly interesting in connection applications in the area of quantum information science. It is demonstrated the preparation of optical inhomogeneous absorption profiles such that the light only interacts with a specific transition at a selected frequency. Narrow absorbing structures with widths of <100 kHz have been prepared with no absorption in the surrounding spectral interval. Most of the experiments were carried out using Pr3+:Y2SiO5. However, in our search for suitable a Λ-system in the material under consideration, we also performed spectral hole-burning spectroscopy in Nd:Y2SiVO4. As part of the search for host materials for Pr, Pr:La2(WO4)3 crystal was characterized using a variety of methods, such as, hole burning and photon-echoes. Electromagnetically induced transparency was also observed in this crystal. The most important criteria for evaluating quantum memory performance and the most important physical requirements are discussed. After investigating the standard photon echo techniques and controlled reversible inhomogeneous broadening (CRIB) for quantum state storage, we considered an efficient multimode quantum memory protocol; the atomic frequency comb (AFC) protocol. Combs with peaks of widths 100-300 kHz and an optical depth of~ 10 were created by optical pumping inside an emptied region of the inhomogeneous profile of Pr3+:Y2SiO5, allowing us to improve the efficiency of light storage. Light storage combining a photon-echo technique based on an AFC and spin-wave storage have been demonstrated, and an experimental AFC scheme for the storage of weak coherent light pulses in Pr3+:Y2SiO5 is presented. Finally, we have investigated superradiance and slow light effects. Both these effects occur in the high optical depth regime and can influence the memory performance. Under certain conditions superradiance results in the immediate re-emission of the stored light in a single burst of coherent radiation, and inside spectrally structured materials, slow light effects may result when storage is performed.
Abstract Ensemble-based quantum memories are key to developing multiplexed quantum repeaters, able to overcome the intrinsic rate limitation imposed by finite communication times over long distances. Rare-earth ion doped crystals are main candidates for highly multimode quantum memories, where time, frequency and spatial multiplexing can be exploited to store multiple modes. In this context the atomic frequency comb (AFC) quantum memory provides large temporal multimode capacity, which can readily be combined with multiplexing in frequency and space. In this article, we derive theoretical formulas for quantifying the temporal multimode capacity of AFC-based memories, for both optical memories with fixed storage time and spin-wave memories with longer storage times and on-demand read out. The temporal multimode capacity is expressed in key memory parameters, such as AFC bandwidth, fixed-delay storage time, memory efficiency, and control field Rabi frequency. Current experiments in europium- and praseodymium-doped Y2SiO5 are analyzed within this theoretical framework, which is also tested with newly acquired data, as prospects for higher temporal capacity in these materials are considered. In addition we consider the possibility of spectral and spatial multiplexing to further increase the mode capacity, with examples given for praseodymium doped Y2SiO5.
High-efficiency optical storage in rare-earth-ion-doped crystals is challenging because of their typically weak optical absorption,which makes cavity-enhanced memories highly sensitive to parasitic intracavity loss. Here we demonstrate a 90.1(5)% storage efficiency for coherent optical pulses in an impedance-matched cavity atomic-frequency-comb memory based on Eu3+:Y2SiO5. The Brewster-angle configuration of the crystal and fully cryogenic Fabry-Pérot cavity reduce the round-trip intracavity loss to 0.37%. Quantitative modeling shows that further improvement is limited primarily by AFC preparation and its associated dispersive response rather than by residual cavity loss,identifying the principal requirements for further approaching unity efficiency.