Quantum mutual information of an entangled state propagating through a fast-light medium


It is widely accepted that information cannot travel faster than c, the speed of light in vacuum1,2,3. Here, we investigate the behaviour of quantum correlations and information in the presence of dispersion. To do so we send one half of an entangled state of light through a gain-assisted slow- or fast-light medium and detect the transmitted quantum correlations and quantum mutual information4,5,6. We show that quantum correlations can be advanced by a small fraction of the correlation time, even in the presence of noise added by phase-insensitive gain. Additionally, although the peak of the quantum mutual information between the modes can be advanced, we find that the degradation of the mutual information due to added noise appears to prevent an advancement of the leading edge. In contrast, we demonstrate a significant delay of both the leading and trailing edges of the mutual information in a slow-light system.

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Figure 1: Experimental set-up.
Figure 2: Persistence of correlations associated with entanglement in the presence of anomalous dispersion.
Figure 3: Observed advance in the quantum correlations.
Figure 4: Comparison of computed quantum mutual information between the c.w. probe and conjugate as a function of relative delay for fast and slow light.


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This research was supported by the Physics Frontiers Center at the Joint Quantum Institute and the Air Force Office of Scientific Research. Q.G. performed this work with the support of the Marie Curie IOF FP7 Program (Multimem–300632), while U.V. was supported by the Alexander von Humboldt Foundation and R.T.G. was supported by a National Research Council Research Associateship Award at NIST. J.B.C. acknowledges support from the National Science Foundation.

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J.B.C., R.T.G., Q.G. and U.V. analysed the data. J.B.C., R.T.G., U.V. and P.D.L. conceived and designed the experiments. J.B.C., R.T.G., Q.G. and U.V. contributed materials and analysis tools. J.B.C., R.T.G., Q.G., U.V. and T.L. performed the experiments. J.B.C., R.T.G., Q.G., U.V., K.M.J. and P.D.L. wrote the paper.

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Correspondence to Paul D. Lett.

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Clark, J., Glasser, R., Glorieux, Q. et al. Quantum mutual information of an entangled state propagating through a fast-light medium. Nature Photon 8, 515–519 (2014). https://doi.org/10.1038/nphoton.2014.112

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