Indistinguishable Tunable Single Photons Emitted by Spin-Flip Raman Transitions in InGaAs Quantum Dots Yu He (贺煜), 1 Yu-Ming He (何玉明), 1 Y.-J. Wei, 1 X. Jiang, 1 M.-C. Chen, 1 F.-L. Xiong, 1 Y. Zhao, 1 Christian Schneider, 2 Martin Kamp, 2 Sven Ho ¨fling, 2,1,3 Chao-Yang Lu, 1 and Jian-Wei Pan 1 1 Hefei National Laboratory for Physical Sciences at Microscale and Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026, China 2 Technische Physik, Physikalisches Institut and Wilhelm Conrad Ro ¨ntgen-Center for Complex Material Systems, Universita ¨t Wu ¨rzburg, Am Hubland, D-97074 Wu ¨zburg, Germany 3 SUPA, School of Physics and Astronomy, University of St. Andrews, St. Andrews KY16 9SS, United Kingdom (Received 3 July 2013; published 4 December 2013) This Letter reports all-optically tunable and highly indistinguishable single Raman photons from a driven single quantum dot spin. The frequency, linewidth, and lifetime of the Raman photons are tunable by varying the driving field power and detuning. Under continuous-wave excitation, subnatural linewidth single photons from off-resonant Raman scattering show an indistinguishability of 0.98(3). Under  pulse excitation, spin- and time-tagged Raman fluorescence photons show an almost vanishing multiphoton emission probability of 0.01(2) and a two-photon quantum interference visibility of 0.95(3). Lastly, Hong- Ou-Mandel interference is demonstrated between two single photons emitted from remote, independent quantum dots with an unprecedented visibility of 0.87(4). DOI: 10.1103/PhysRevLett.111.237403 PACS numbers: 78.67.Hc, 42.50.Nn, 78.20.Ls, 78.30.j Optically active self-assembled quantum dots (QDs) are of particular interest for physical realization of quantum information [1]. They promise to serve a dual role as solid- state traps for single electron spins and sources of single photon qubits, as well as provide a quantum interface linking them. The confined electron spin has shown long relaxation time [2] and coherence time [3] and can be optically controlled for high-fidelity spin-state initializa- tion [4], ultrafast rotation [5,6], and readout [7]. The recent demonstrations of QD spin-photon entanglement [8] opened the door for spin-based quantum communication [9] and distributed quantum computing [10] protocols. An outstanding challenge is the realization of entanglement between remote QD spins [11] through quantum interfer- ence of two single photons, in a similar way as achieved for trapped ions [12], atomic ensembles [13], and nitrogen vacancy in diamonds [14]. To this end, it is necessary to obtain a high degree of indistinguishability between the coherently scattered, spin-tagged Raman photons. Spontaneous Raman fluorescence from a single QD has been observed [15] and used for spin readout [8]; however, no two-photon Hong-Ou-Mandel (HOM) interference [16] experiment with the Raman photons has been reported. Meanwhile, QD single-photon sources are attractive for applications such as optical quantum computing and quan- tum metrology [17]. Thus far, most experiments generating single photons from QDs are based on two-level systems, exciting above the band gap [18], from p shell [19], and more recently, resonant s shell [20–23]. Moving to a richer, three-level  system is expected to bring practical advan- tages, for instance, an improved photon indistinguishabil- ity, as it is insensitive to excited-state dephasing at large detunings [24]. In this direction, an interesting goal would be controlled and deterministic generation of single pho- tons with tailorable waveforms in a coupled QD-cavity system using stimulated Raman adiabatic passage [24,25], as previously realized in trapped atoms [26] and ions [27]. Toward these goals, it is important to perform a detailed investigation on QD Raman-scattered photons. In this Letter, we report the generation of highly indis- tinguishable and bandwidth-tunable single photons from spin-flip Raman transitions under both cw and pulsed excitations. We have also demonstrated high-visibility quantum interference between two single photons from two coherently driven QDs at a distance. The experiments are carried out at 4.2 K on single self- assembled InGaAs QDs embedded in a planar microcavity [22] and charged with a single excess electron in the con- duction band. A magnetic field is applied perpendicular to the optical axis (Voigt geometry), which results in a double  system [see Fig. 1(a)]. The magnetic field splits the electron-spin ( j"i and j#i) ground states and the trion (con- sisting of two electrons in a spin singlet and a hole, i.e., j"#*i and j#"+i) excited states according to the in-plane electron and hole g factors (here, g e ¼ 0:483, g h ¼ 0:082). A confocal microscope allows laser excitation of a single QD and collection of emitted Raman photons. In the cw excitation experiment, the bichromatic excitation consists of one laser red detuned from the j#i$j#"+i transition by  and the other one blue detuned from the j"i$j"#*i transition by  [see Fig. 1(a)]. The two lasers serve for optical pumping and repumping of the electron spin [4], enabling fast spin restoration which is necessary for high repetition rate generations of Raman photons. PRL 111, 237403 (2013) PHYSICAL REVIEW LETTERS week ending 6 DECEMBER 2013 0031-9007= 13=111(23)=237403(5) 237403-1 Ó 2013 American Physical Society