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Quantum-dot spin–photon entanglement via frequency downconversion to telecom wavelength


Long-distance quantum teleportation and quantum repeater technologies require entanglement between a single matter quantum bit (qubit) and a telecommunications (telecom)-wavelength photonic qubit1,2,3,4,5. Electron spins in III–V semiconductor quantum dots are among the matter qubits that allow for the fastest spin manipulation6,7 and photon emission8,9, 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 implement10. 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 rates11,12, the present technique advances the III–V semiconductor quantum-dot spin system as a promising platform for long-distance quantum communication.

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Figure 1: Level structure of quantum dot and spin manipulation.
Figure 2: Ultrafast conversion to 1,560 nm.
Figure 3: Quantum-dot manipulation scheme for spin–photon entanglement verification.
Figure 4: Spin–photon entanglement verification.

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We thank D. Press, T. Ladd, D. Sleiter, S. Tawfeeq, S. Rumley, D. Werthimer, A. Langman, C. Langrock, Q. Zhang, N. Namekata, S. Inoue, T. Inagaki and H. Kosaka for discussions, comments and technical assistance. We thank V. Zwiller and S. Dorenbos (TU Delft) for providing the superconducting detector samples used. This work was supported by the JSPS through its FIRST programme, NICT, NSF CCR-08 29694, NIST 60NANB9D9170, Special Coordination Funds for Promoting Science and Technology, and the State of Bavaria. J.S.P. and M.M.F. were supported by the United States AFOSR (grant FA9550-12-1-0110). Other authors were supported as follows: K.D.G. by a Herb and Jane Dwight Stanford Graduate Fellowship; P.L.M. by a David Cheriton Stanford Graduate Fellowship; J.S.P. by a Robert N. Noyce Stanford Graduate Fellowship; C.M.N. by a SU2P Entrepreneurial Fellowship; and R.H.H. by a Royal Society University Research Fellowship.

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S.M., C.S., M.K. and S.H. grew and fabricated the samples. K.D.G. and Y.Y. designed the experiment. K.D.G., J.S.P., L.Y., P.L.M., C.M.N. and N.Y.K. performed the optical experiments. J.S.P. designed and fabricated the PPLN waveguides. J.S.P. and L.Y. developed the 2.2-µm set-up and the 1,560-nm filtering design. C.M.N. and R.H.H. packaged, characterized and implemented the SNSPD detectors. Y.Y., M.M.F., E.A. and A.F. guided the work. K.D.G. wrote the manuscript with input from all authors.

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Correspondence to Kristiaan De Greve.

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De Greve, K., Yu, L., McMahon, P. et al. Quantum-dot spin–photon entanglement via frequency downconversion to telecom wavelength. Nature 491, 421–425 (2012).

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