Quantum dot spin-photon entanglement through ultrafast frequency downconversion to 1560 nm Kristiaan De Greve 1 , Leo Yu 1,* , Peter L. McMahon 1,* , Jason S. Pelc 1,* , Chandra M. Natarajan 1,4 , Na Young Kim 1 , Eisuke Abe 1,2 , Sebastian Maier 3 , Christian Schneider 3 , Martin Kamp 3 , Sven H ¨ ofling 1,3 , Robert H. Hadfield 4 , Alfred Forchel 3 , M. M. Fejer 1 & Yoshihisa Yamamoto 1,2 1 E. L. Ginzton Laboratory, Stanford University, Stanford, California 94305, USA 2 National Institute of Informatics, Hitotsubashi 2-1-2, Chiyoda-ku, Tokyo 101-8403, Japan 3 Technische Physik, Physikalisches Institut, Wilhelm Conrad R¨ ontgen Research Center for Com- plex Material Systems, Universit¨ at W ¨ urzburg, Am Hubland, D-97074 W¨ urzburg, Germany 4 Scottish Universities Physics Alliance and School of Engineering and Physical Sciences, Heriot- Watt University, Edinburgh EH14 4AS, UK * These authors contributed equally to this work Long-distance quantum teleportation and quantum repeater technologies require entangle- ment between a single matter qubit and a telecom-wavelength photonic qubit 1–5 . Electron spins in III-V semiconductor quantum dots are among the matter qubits that allow for the fastest spin manipulation 6, 7 and photon emission 8, 9 , but entanglement between a single quan- tum dot spin qubit and a flying photonic qubit has yet to be demonstrated. Moreover, many quantum dots emit single photons at visible to near-infrared wavelengths, where silica fiber losses are so high that long-distance quantum communication protocols become difficult to implement 10 . Here, we demonstrate entanglement between an InAs quantum dot electron 1