A quantum emitter efficiently coupled to a nanophotonic waveguide constitutes a promising system for the realization of single-photon transistors, quantum-logic gates based on giant single-photon nonlinearities, and high bit-rate deterministic single-photon sources. The key figure of merit for such devices is the β-factor, which is the probability for an emitted single photon to be channeled into a desired waveguide mode. We report on the experimental achievement of β = 98.43 ± 0.04% for a quantum dot coupled to a photonic-crystal waveguide, corresponding to a single-emitter cooperativity of η = 62.7 ± 1.5. This constitutes a nearly ideal photon-matter interface where the quantum dot acts effectively as a 1D "artificial" atom, since it interacts almost exclusively with just a single propagating optical mode. The β-factor is found to be remarkably robust to variations in position and emission wavelength of the quantum dots. Our work demonstrates the extraordinary potential of photonic-crystal waveguides for highly efficient single-photon generation and on-chip photon-photon interaction. The proposals of quantum communication [1] and linear-optics quantum computing [2] have been major driving forces for the development of efficient singlephoton (SP) sources [3][4][5]. Furthermore, the access to photon nonlinearities that are sensitive at the SP level [6, 7] would open for novel opportunities of constructing highly efficient deterministic quantum gates [7][8][9][10][11][12][13]. A single quantum emitter that is efficiently coupled to a photonic waveguide [14] would facilitate such a SP nonlinearity, enabling the realization of single-photon switches and diodes [7][8][9], as well as serve as a highly efficient single-photon source. Waveguide-based schemes offer highly efficient and broadband channeling of SPs into a directly usable propagating mode where even the photon detection can be integrated on-chip [15]. The associated SP nonlinearity constitutes a very promising and robust alternative to the technologically demanding schemes based on the anharmonicity of the stronglycoupled emitter-cavity system [16][17][18][19].In the present work we consider a single quantum dot (QD) embedded in a photonic-crystal waveguide (PCW). The important figure of merit is the β-factor:which gives the probability for a single exciton in the QD to recombine by emitting a single photon into the waveguide mode. Γ wg and Γ rad are the rate of decay of the QD into either the guided mode or non-guided radiation modes, whereas Γ nr denotes the intrinsic nonradiative decay rate of the QD. The β-factor is related to the single-emitter cooperativity η = β/(1 − β).[20] Experimentally, the β-factor can be obtained by recording the decay rate of a QD that is coupled to the waveguide Γ c = Γ wg + Γ rad + Γ nr and the rate of an uncoupled QD Γ uc = Γ rad + Γ nr in the case where the difference between the total loss rates (Γ rad + Γ nr ) of the two QDs is negligible. Recent proposals have indicated that the β-factor in PCWs may approach unity ...

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Lodahl, P. (2014). Efficient out-coupling of high-purity single photons from a coherent quantum dot in a photonic-crystal cavity. Physical Review B, 90(15) We demonstrate a single-photon collection efficiency of (44.3 ± 2.1)% from a quantum dot in a low-Q mode of a photonic-crystal cavity with a single-photon purity of g (2) (0) = (4 ± 5)% recorded above the saturation power. The high efficiency is directly confirmed by detecting up to 962 ± 46 kilocounts per second on a single-photon detector on another quantum dot coupled to the cavity mode. The high collection efficiency is found to be broadband, as is explained by detailed numerical simulations. Cavity-enhanced efficient excitation of quantum dots is obtained through phonon-mediated excitation and under these conditions, single-photon indistinguishability measurements reveal long coherence times reaching 0.77 ± 0.19 ns in a weak-excitation regime. Our work demonstrates that photonic crystals provide a very promising platform for highly integrated generation of coherent single photons including the efficient out-coupling of the photons from the photonic chip.