Semiconductor quantum dots (QDs) are commonly considered as artificial atoms and these nanostructures are very promising for the realization of integrated devices such as single photon sources for quantum cryptography applications [1,2]. However, contrarily to genuine atoms, QDs are condensed matter systems and they consequently suffer from the coupling to their environment that acts as a fluctuating reservoir for spectral diffusion [3] and degrades the indistinguishability of the emitted photons. One way to circumvent this coupling in order to reach the so-called fundamental radiative limit is to excite strictly resonantly the QD at low temperature [4-9]. Up to now, the measurements reported in the literature mainly focus on the incoherent photoluminescence signal for achieving single photon emission. In this regime, the linewidth of the photoluminescence spectrum is always limited to the radiative limit. In this paper, we report on the resonant emission (RE) of the exciton in single InAs/GaAs QDs embedded in a planar microcavity, in an orthogonal excitation-detection geometry. We present an original type of single photon source based on the coherent laser light scattering by a single QD [10]. Besides the antibunching effect showing the non-classical nature of the emitted light, we observe that the QD emission spectrum is determined by the spectrum of the resonant excitation laser. We present high-resolution measurements by Fourier transform spectroscopy of the RE where the data are fitted to the first-order correlation function g(1)(τ) of a two-level system. The inverse Fourier transform of the g(1)(τ) function allows a direct comparison of the different RE emission profiles. We show that, by reducing the resonant excitation power, the so-called Mollow triplet shrinks to a single narrow Lorentzian line, while the relative intensity of this narrow peak increases and tends to overwhelm the RE spectrum. This implies that single QDs can produce single photons with a coherence time that is not limited anymore by the QD electronic properties, resulting in what we name an ultra-coherent character [10]. In conclusion, the resulting " ultra-coherent single photon source " promises high degrees of indistinguishability of the emitted photons which is a crucial requirement for quantum information applications. [1] P. Michler et al., Science 290, 2282 (2000). [6] S. Ates et al., PRL 103, 167402 (2009). [2] C. Santori et al., Nature 419, 594 (2002). [7] H. S. Nguyen et al., PRL 108, 057401 (2012). [3] A. Berthelot et al., Nat. Phys. 2, 759 (2006). [8] C. Matthiesen et al., PRL 108, 093602 (2012). [4] A. Muller et al., PRL 99, 187402 (2007). [9] A. Reinhard et al., Nat. Phot. 6, 93 (2012). [5] R. Melet et al., PRB 78, 073301 (2008). [10] H. S. Nguyen et al., APL 99, 261904 (2011).

We report on the resonant emission in coherently-driven single semiconductor quantum dots. We demonstrate that an ultra-weak non-resonant laser acts as an optical gate for the quantum dot resonant response. We show that the gate laser suppresses Coulomb blockade at the origin of a resonant emission quenching, and that the optically-gated quantum dots systematically behave as ideal two-level systems in both regimes of coherent and incoherent resonant emission.

We present an original type of single photon source in solid state, based on the coherent laser light scattering by a single InAs quantum dot. We demonstrate that the coherence of the emitted single photons is tailored by the resonant excitation with a spectral linewidth below the radiative limit. Our ultra-coherent source opens the way for integrated quantum devices dedicated to the generation of single photons with high degrees of indistinguishability.

Single photon emission is one of the most striking optical properties of semiconductor quantum dots (QDs), and the corresponding photon anti-bunching effect was observed by photoluminescence experiments in single QDs under non-resonant excitation [1]. In this configuration, the residual excitation of the QD environment that acts as a fluctuating reservoir for spectral diffusion [2] reduces the coherence time and prevents from the realization of single photons sources with high degree of indistinguishability. Therefore, strictly resonant excitation of the fundamental interband transition [3,4,5] is essential to reduce the dephasing processes and get high-performance single photon sources. In this paper, we report on the resonant Rayleigh scattering (RRS) of the exciton in single InAs/GaAs QDs embedded in a planar microcavity [3]. Strikingly, the most frequent case (98%) corresponds to the absence of resonance in the RRS spectrum, resulting in QDs non-coupled to light. We show that resonant fluorescence of the exciton is restored for all the QDs when an additional non-resonant laser is applied at extremely weak power ranges (few nWs). A clear photon anti-bunching under strictly resonant excitation proves that the optically gated RRS spectrum is dominated by resonant fluorescence of the exciton. A complete study will be presented such as the switching dynamics of the RRS and its power dependence with the resonant and non-resonant lasers. The phenomenon is interpreted within a simple model where the exciton resonant fluorescence is quenched by Coulomb blockade after tunneling of a hole in the QD from a structural deep level. The additional non-resonant laser suppresses Coulomb blockade by switching back the QD into a neutral state. We conclude that the optical response of the QD remains affected by its environment even in case of coherent resonant pumping. An optical control of the QD environment is essential to bring the QD from a "dark" to a "bright" state under strictly resonant excitation. References [1] P. Michler et al., Science 290, 2282 (2000) [2] A. Berthelot et al., Nature Physics 2, 759 (2006) [3] A. Muller et al., PRL 99, 187402 (2007) [4] R. Melet et al., PRB 78, 073301 (2008) [5] S. Ates et al., PRL 103, 167402 (2009)

Single photon emission is one of the most striking optical properties of semiconductor quantum dots (QDs), and the corresponding photon anti-bunching effect was observed by photoluminescence experiments in single QDs under non-resonant excitation [1]. In this configuration, the residual excitation of the QD environment that acts as a fluctuating reservoir for spectral diffusion [2] reduces the coherence time and prevents from the realization of single photons sources with high degree of indistinguishability. Therefore, strictly resonant excitation of the fundamental interband transition [3,4,5] is essential to reduce the dephasing processes and get high-performance single photon sources. In this paper, we report on the resonant Rayleigh scattering (RRS) of the exciton in single InAs/GaAs QDs embedded in a planar microcavity [3]. Strikingly, the most frequent case (98%) corresponds to the absence of resonance in the RRS spectrum, resulting in QDs non-coupled to light. We show that resonant fluorescence of the exciton is restored for all the QDs when an additional non-resonant laser is applied at extremely weak power ranges (few nWs). A clear photon anti-bunching under strictly resonant excitation proves that the optically gated RRS spectrum is dominated by resonant fluorescence of the exciton. A complete study will be presented such as the switching dynamics of the RRS and its power dependence with the resonant and non-resonant lasers. The phenomenon is interpreted within a simple model where the exciton resonant fluorescence is quenched by Coulomb blockade after tunneling of a hole in the QD from a structural deep level. The additional non-resonant laser suppresses Coulomb blockade by switching back the QD into a neutral state. We conclude that the optical response of the QD remains affected by its environment even in case of coherent resonant pumping. An optical control of the QD environment is essential to bring the QD from a "dark" to a "bright" state under strictly resonant excitation. [1] P. Michler et al., A quantum dot single-photon turnstile device, Science 290, 2282 (2000) [2] A. Berthelot et al., Unconventional motional narrowing in the optical spectrum of a semiconductor quantum dot, Nature Physics 2, 759 (2006) [3] A. Muller et al., Resonance fluorescence from a coherently driven semiconductor quantum dot in a cavity, PRL 99, 187402 (2007) [4] R. Melet et al., Resonant excitonic emission of a single quantum dot in the Rabi regime, PRB 78, 073301 (2008) [5] S. Ates et al., Post-selected indistinguishable photons from the resonance fluorescence of a single quantum dot in a microcavity, PRL 103, 167402 (2009) O02B6 48