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A single-atom quantum memory


The faithful storage of a quantum bit (qubit) of light is essential for long-distance quantum communication, quantum networking and distributed quantum computing1. The required optical quantum memory must be able to receive and recreate the photonic qubit; additionally, it must store an unknown quantum state of light better than any classical device. So far, these two requirements have been met only by ensembles of material particles that store the information in collective excitations2,3,4,5,6,7. Recent developments, however, have paved the way for an approach in which the information exchange occurs between single quanta of light and matter8,9,10,11,12,13. This single-particle approach allows the material qubit to be addressed, which has fundamental advantages for realistic implementations. First, it enables a heralding mechanism that signals the successful storage of a photon by means of state detection14,15,16; this can be used to combat inevitable losses and finite efficiencies. Second, it allows for individual qubit manipulations, opening up avenues for in situ processing of the stored quantum information. Here we demonstrate the most fundamental implementation of such a quantum memory, by mapping arbitrary polarization states of light into and out of a single atom trapped inside an optical cavity. The memory performance is tested with weak coherent pulses and analysed using full quantum process tomography. The average fidelity is measured to be 93%, and low decoherence rates result in qubit coherence times exceeding 180 microseconds. This makes our system a versatile quantum node with excellent prospects for applications in optical quantum gates17 and quantum repeaters18.

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Figure 1: Single-atom quantum memory.
Figure 2: Write and read processes of the memory.
Figure 3: Tomography of the storage process for a storage time of 2 µs.
Figure 4: Storage time.


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We thank N. Kiesel for discussions and A. Neuzner for experimental assistance. This work was supported by the Deutsche Forschungsgemeinschaft (Research Unit 635), by the European Union (Collaborative Project AQUTE) and by the Bundesministerium für Bildung und Forschung via IKT 2020 (QK_QuOReP). E.F. acknowledges support from the Alexander von Humboldt Foundation.

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All authors contributed to the experiment, the analysis of the results and the writing of the manuscript.

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Correspondence to Stephan Ritter.

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The authors declare no competing financial interests.

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Specht, H., Nölleke, C., Reiserer, A. et al. A single-atom quantum memory. Nature 473, 190–193 (2011).

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