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High-fidelity entanglement between a quantum dot spin and a single photon via time-resolved downconversion to telecom wavelength

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Kristiaan De Greve1, Peter L. McMahon1, Leo Yu1, Jason S. Pelc1, Chandra M. Natarajan1,4, David Press1, Na Young Kim1, Eisuke Abe1,2, Dirk Bisping3, Sebastian Maier3, Christian Schneider3, Martin Kamp3, Sven Höfling1,3, Robert H. Hadfield4, Alfred Forchel3, M. M. Fejer1 & Yoshihisa Yamamoto1,2

1E. L. Ginzton Laboratory, Stanford University, Stanford, California 94305, USA 2National Institute of Informatics, Hitotsubashi 2-1-2, Chiyoda-ku, Tokyo 101-8403, Japan 3Technische Physik, Physikalisches Institut, Wilhelm Conrad Röntgen Research Center for Complex Material Systems, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany 4School of Engineering, University of Glasgow, Glasgow, G12 8QQ , United Kingdom Currently at Harvard University, department of Physics, 11 Oxford Street, Cambridge MA 02138 , USA

Entanglement between light and matter is at the heart of many quantum communication schemes that involve built-in quantum memories (such as quantum repeaters). In this presentation, I will demonstrate recent results involving entanglement between a single photon and an electron spin qubit in an InAs quantum dot1,2,3,4.

We developed a time-resolved frequency conversion technique5,6 that allows measurement of the arrival time of single photon with few-ps timing resolution. With this tool, we can perform stroboscopic measurements of the phase evolution of frequency-superposed photons, or, alternatively, quantum erase frequency-which-path information in polarization- and frequency-entangled photons.

Applying this technique to the case of spin-photon entanglement in InAs quantum dots6,7, we obtain a lower bound of the entanglement fidelity of 92%, still limited by the timing resolution of the frequency conversion. In addition, the pump pulses in the conversion tool were chosen to yield converted photons at 1550 nm, which is the lowest-loss wavelength for fiber communication, and the internal quantum efficiency of the process is well over 70%. CW implementations of the conversion technique should therefore be able to help in extending the distance between adjacent nodes in quantum repeaters.

References:

1D. Press, T. Ladd et al, Nature 456, 218 (2008) 2J. Berevosky, M. Mikkelsen et al., Science 320, 349 (2008) 3D. Press, K. De Greve et al, Nat. Phot. 4, 367 (2010) 4K. De Greve, P. McMahon et al, Nature Physics 7, 872 (2011) 5J. S. Pelc, L. Yu et al, Opt. Express 20, 27510 (2012) 6K. De Greve, L. Yu et al, Nature 491, 421 (2012) 7K. De Greve, P. McMahon et al, Nature Commun. 4, 2228 (2013)

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