title: Quantum-dot spin-photon entanglement via frequency downconversion to telecom wavelength.
abstract: Long-distance quantum teleportation and quantum repeater technologies require entanglement between a single matter quantum bit (qubit) and a telecommunications (telecom)-wavelength photonic qubit. Electron spins in III-V semiconductor quantum dots are among the matter qubits that allow for the fastest spin manipulation and photon emission, but entanglement between a single quantum-dot spin qubit and a flying (propagating) photonic qubit has yet to be demonstrated. Moreover, many quantum dots emit single photons at visible to near-infrared wavelengths, where silica fibre losses are so high that long-distance quantum communication protocols become difficult to implement. Here we demonstrate entanglement between an InAs quantum-dot electron spin qubit and a photonic qubit, by frequency downconversion of a spontaneously emitted photon from a singly charged quantum dot to a wavelength of 1,560 nanometres. The use of sub-10-picosecond pulses at a wavelength of 2.2 micrometres in the frequency downconversion process provides the necessary quantum erasure to eliminate which-path information in the photon energy. Together with previously demonstrated indistinguishable single-photon emission at high repetition rates, the present technique advances the III-V semiconductor quantum-dot spin system as a promising platform for long-distance quantum communication.
authors.affiliation: E. L. Ginzton Laboratory, Stanford University, Stanford, California 94305, USA. kdegreve@stanford.edu
authors.forename: Kristiaan
authors.initials: K
authors.lastname: De Greve
authors.forename: Leo
authors.initials: L
authors.lastname: Yu
authors.forename: Peter L
authors.initials: PL
authors.lastname: McMahon
authors.forename: Jason S
authors.initials: JS
authors.lastname: Pelc
authors.forename: Chandra M
authors.initials: CM
authors.lastname: Natarajan
authors.forename: Na Young
authors.initials: NY
authors.lastname: Kim
authors.forename: Eisuke
authors.initials: E
authors.lastname: Abe
authors.forename: Sebastian
authors.initials: S
authors.lastname: Maier
authors.forename: Christian
authors.initials: C
authors.lastname: Schneider
authors.forename: Martin
authors.initials: M
authors.lastname: Kamp
authors.forename: Sven
authors.initials: S
authors.lastname: Höfling
authors.forename: Robert H
authors.initials: RH
authors.lastname: Hadfield
authors.forename: Alfred
authors.initials: A
authors.lastname: Forchel
authors.forename: M M
authors.initials: MM
authors.lastname: Fejer
authors.forename: Yoshihisa
authors.initials: Y
authors.lastname: Yamamoto