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Physical Review Letters

Stopped Light and Image Storage by Electromagnetically Induced Transparency up to the Regime of One Minute

Georg Heinze, Christian Hubrich, Thomas Halfmann

Published Jul 15, 2013• 474 citations
Publisher

Abstract

The maximal storage duration is an important benchmark for memories. In quantized media, storage times are typically limited due to stochastic interactions with the environment. Also, optical memories based on electromagnetically induced transparency (EIT) suffer strongly from such decoherent effects. External magnetic control fields may reduce decoherence and increase EIT storage times considerably but also lead to complicated multilevel structures. These are hard to prepare perfectly in order to push storage times toward the theoretical limit, i.e., the population lifetime T(1). We present a self-learning evolutionary strategy to efficiently drive an EIT-based memory. By combination of the self-learning loop for optimized optical preparation and improved dynamical decoupling, we extend EIT storage times in a doped solid above 40 s. Moreover, we demonstrate storage of images by EIT for 1 min. These ultralong storage times set a new benchmark for EIT-based memories. The concepts are also applicable to other storage protocols.

DOI
10.1103/physrevlett.111.033601

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The maximal storage duration is an important benchmark for memories. In quantized media, storage times are typically limited due to stochastic interactions with the environment. Also, optical memories based on electromagnetically induced transparency (EIT) suffer strongly from such decoherent effects. External magnetic control fields may reduce decoherence and increase EIT storage times considerably but also lead to complicated multilevel structures. These are hard to prepare perfectly in order to push storage times toward the theoretical limit, i.e., the population lifetime T(1). We present a self-learning evolutionary strategy to efficiently drive an EIT-based memory. By combination of the self-learning loop for optimized optical preparation and improved dynamical decoupling, we extend EIT storage times in a doped solid above 40 s. Moreover, we demonstrate storage of images by EIT for 1 min. These ultralong storage times set a new benchmark for EIT-based memories. The concepts are also applicable to other storage protocols.
Abstract Efficient and long-term storage of quantum information encoded in single photons is crucial for applications of optical quantum technologies, e.g. quantum repeaters in communication networks. Obviously, the maximal storage time is an important benchmark for such memories, as it defines the distances covered by the network or the applicability of quantum communication protocols therein. In this paper, we present the implementation of an optical memory driven by electromagnetically induced transparency, permitting the storage of weak coherent pulses containing on average a single photon with a signal-to-noise ratio of 1.3 ( 3 ) for a long storage time of one second. To achieve this goal, we apply decoherence control by static magnetic fields and robust dynamical decoupling sequences to prolong the coherence time in a rare-earth ion doped crystal to 14 s. A novel optical preparation scheme serves to increase the optical depth of the medium, which enables a light storage efficiency of 12.7 ( 5 ) % at the single photon level and one second storage time.
We report on experiments dealing with feedback-controlled pulse shaping to optimize the efficiency of light storage by electromagnetically induced transparency (EIT) in a Pr3+{}^{3+}3+:Y2{}_{2}2​SiO5{}_{5}5​ crystal. A learning loop in combination with an evolutionary algorithm permits the automatic determination of optimal temporal profiles of intensities and frequencies in the driving laser pulses (i.e., the probe and coupling pulses). As a main advantage, the technique finds optimal solutions even in the complicated multilevel excitation scheme of a doped solid, involving large inhomogeneous broadening. The learning loop experimentally determines optimal temporal intensity profiles of the coupling pulses for a given probe pulse. The optimized intensity pulse shapes enhance the light-storage efficiency in the doped solid by a factor of 222. The learning loop also determines a fast and efficient preparation pulse sequence, which serves to optically prepare the crystal prior to light-storage experiments. The optimized preparation sequence is 5 times faster than standard preparation sequences. Moreover, the optimized preparation sequence enhances the optical depth in the medium by a factor of 555. As a consequence, the efficiency of light storage also increases by another factor of 333. Our experimental data clearly demonstrate the considerable potential of feedback-controlled pulse shaping, applied to EIT-driven light storage in solid media.
Abstract We present the experimental demonstration of light storage towards the single photon level at a long storage time by electromagnetically induced transparency in a rare-earth ion-doped Pr3+:Y2SiO5 crystal. We apply decoherence control by static magnetic fields and appropriately designed radio-frequency composite pulse sequences to prolong the storage time in the memory. A rare-earth ion-doped filter crystal prepared by optical pumping serves to efficiently separate the signal at the single photon level from optical noise. Multipass setups around the memory and the filter crystal improve the storage efficiency and filter selectivity. Already without decoherence control, the setup permits storage of single photons in the microsecond regime at a storage efficiency of 42%. With decoherence control we demonstrate storage of weak coherent pulses containing some 10 photons for up to 10 s at a storage efficiency of several percent. The experimental data clearly demonstrate the applicability of EIT light storage to implement a true quantum memory in Pr3+:Y2SiO5 at long storage times. The scientific findings and technical developments are of relevance also to other protocols and media for quantum information storage.
Quantum memories, devices that can store and retrieve photonic quantum states on demand, are essential components for scalable quantum technologies. It is desirable to push the memory towards the broadband regime in order to increase the data rate. Here, we present a theoretical and experimental study of broadband optical memory based on the electromagnetically induced transparency (EIT) protocol. We first provide a theoretical analysis of the issues and requirements needed to achieve broadband EIT memory. We then present our experimental efforts for the movement of EIT memory in cold atoms towards the broadband or short-pulse regime. A storage efficiency of $\ensuremath{\sim}80%$ with a pulse duration of 30 ns (corresponding to a bandwidth of 14.7 MHz) is realized. Due to limitations of the available intensity of the control beam, we could not achieve an optimal storage for the even shorter pulses but were still able to obtain an efficiency of greater than 5050%50 with a pulse duration of 14 ns (31.4 MHz). The time-bandwidth product achieved at an efficiency of 5050%50 is 1267(89).
We demonstrate efficient storage and retrieval of light pulses by electromagnetically induced transparency (EIT) in a Pr^{3+}:Y_{2}SiO_{5} crystal. Using a ring-type multipass configuration, we increase the optical depth (OD) of the medium up to a factor of 16 towards OD≈96. Combining the large optical depth with optimized conditions for EIT, we reach a light storage efficiency of (76.3±3.5)%. In addition, we perform extended systematic measurements of the storage efficiency versus optical depth, control Rabi frequency, and probe pulse duration. The data confirm the theoretically expected behavior of an EIT-driven solid-state memory.
Optical quantum memories are important components in the long-distance quantum communication based on quantum repeater protocol. To outperform the direct transmission of light with quantum repeaters, it is crucial to develop quantum memories with high fidelity, high efficiency and long storage time. Recently, we demonstrate that it is feasible to achieve a high storage efficiency of 92% for electromagnetically-induced-transparency (EIT)-based memory with weak coherent signal pulses in cold atomic ensembles [1]. To realize the highly-efficient memory with quantum light, we have built a bright and narrowband photon-pair source which can be locked to atomic transition based on the cavity-enhanced spontaneous parametric down conversion [2]. Here, we present our results on the storage of single photons generated by such a source in EIT-based memories. A storage efficiency of 36% is obtained in initial runs. Future improvements toward a high efficiency are discussed. Such a development paves the way for the applications of photon-pair-based quantum repeater and multi-photon synchronization.