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Experimental demonstration of quantum memory for light


The information carrier of today's communications, a weak pulse of light, is an intrinsically quantum object. As a consequence, complete information about the pulse cannot be perfectly recorded in a classical memory, even in principle. In the field of quantum information, this has led to the long-standing challenge of how to achieve a high-fidelity transfer of an independently prepared quantum state of light onto an atomic quantum state1,2,3,4. Here we propose and experimentally demonstrate a protocol for such a quantum memory based on atomic ensembles. Recording of an externally provided quantum state of light onto the atomic quantum memory is achieved with 70 per cent fidelity, significantly higher than the limit for classical recording. Quantum storage of light is achieved in three steps: first, interaction of the input pulse and an entangling field with spin-polarized caesium atoms; second, subsequent measurement of the transmitted light; and third, feedback onto the atoms using a radio-frequency magnetic pulse conditioned on the measurement result. The density of recorded states is 33 per cent higher than the best classical recording of light onto atoms, with a quantum memory lifetime of up to 4 milliseconds.

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Figure 1: Experimental set-up.
Figure 2: An example of the atomic memory performance.
Figure 3: Quantum noise of the stored state and the fidelity of quantum memory as a function of time.


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We are grateful to N. Cerf and K. Hammerer for discussions. This research was funded by the Danish National Research Foundation, by EU grants QUICOV, COVAQIAL and CHIC, and by the project ‘Research Center for Optics’ of the Czech Ministry of Education. I.C. and E.S.P. acknowledge the hospitality of the Institute for Photonic Sciences, Barcelona, where part of this work was initiated.

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Correspondence to Eugene S. Polzik.

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Supplementary information

Supplementary Methods

Describes the calibration of the projection noise level of a coherent spin state and the determination of the parameter k2. Also contains the legend for Supplementary Figure 1. (DOC 34 kb)

Supplementary Figure 1

Presents experimental data used to calibrate the projection noise contribution. (JPG 20 kb)

Supplementary Notes

Contains the proof of the feedback equation valid for an arbitrary quantum state. (DOC 36 kb)

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Julsgaard, B., Sherson, J., Cirac, J. et al. Experimental demonstration of quantum memory for light. Nature 432, 482–486 (2004).

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