We demonstrate efficient (>30%) quantum frequency conversion of visible single photons (711 nm) emitted by a quantum dot to a telecom wavelength (1313 nm). Analysis of the first- and second-order coherence before and after wavelength conversion clearly proves that pivotal properties, such as the coherence time and photon antibunching, are fully conserved during the frequency translation process. Our findings underline the great potential of single photon sources on demand in combination with quantum frequency conversion as a promising technique that may pave the way for a number of new applications in quantum technology.

1Emission from a resonantly excited quantum emitter is a fascinating research topic within quantum optics and a useful source for different types of quantum light fields. The resonance spectrum consists of a single spectral line below saturation of a quantum emitter which develops into a triplet at powers above saturation of the emitter [1][2][3]. The spectral properties of the triplet strongly depends on pump power [4,5] and detuning of the excitation laser. The three closely spaced photon channels from the resonance fluorescence have different photon statistical signatures [6]. We present a detailed photon-statistics analysis of the resonance fluorescence emission triplet from a solid state-based artificial atom, i.e. a semiconductor quantum dot. The photon correlation measurements demonstrate both 'single' and 'heralded' photon emission from the Mollow triplet sidebands [6]. The ultra-bright and narrowband emission (5.9 MHz into the first lens) can be conveniently frequency-tuned by laser detuning over 15 times its linewidth (∆ν ≈ 1.0 GHz). These unique properties make the Mollow triplet sideband emission a valuable light source for, e.g.quantum light spectroscopy and quantum information applications [7].Generation of non-classical light fields like single-, entangled-and heralded-photons form a vital part of many schemes of quantum information and computation. Atom optics demonstrated the heralded emission of single photons using resonance fluorescence from single atoms [8] and from cold atoms in a cavity [9]. Currently most of the heralded photon schemes use sources based on parametric down conversion [10]. Recently, there has been important developments in fiber-based heralded photon sources using four-wave mixing [11].Both of these techniques present Poissonian statistics with photon bandwidths usually larger than 100 GHz. The increased two-photon yield in these photon sources is at the expense of fidelity. Semiconductor QDs have demonstrated single- [12], entangled- [13,14] and heralded-photon sources [15] but mostly under non-resonant excitation schemes. Though, coherent excitation conditions of those quantum emitters are an important precondition since these promise to minimize most of the dephasing caused by the non-resonant, i.e. incoherent processes [7]. Coherent control of QD excitons has been demonstrated in a variety of experiments exhibiting Rabi splitting [16], Rabi oscillations [17], and resonant absorption [18,19]. Observations of oscillations in the first-order correlation [20], charac-2 teristic emission spectra in the frequency domain [21], and oscillations of the second-order photon correlation function [2] were all measured directly on the resonance emission from the QD. In particular, single-photon emission from a single fluorescence line (below the emitter saturation) along with photon indistinguishability as high as 90% was demonstrated [3]. Also the AC Stark shift of an exciton was used to bring the initially split QD exciton components into resonance with each other, thus forming a ...

Abstract:We present both experimental and theoretical investigations of a laser-driven quantum dot (QD) in the dressed-state regime of resonance fluorescence. We explore the role of phonon scattering and pure dephasing on the detuning-dependence of the Mollow triplet and show that the triplet sidebands may spectrally broaden or narrow with increasing detuning. Based on a polaron master equation approach which includes electron-phonon interaction nonperturbatively, we derive a fully analytical expression for the spectrum. With respect to detuning dependence, we identify a crossover between the regimes of spectral sideband narrowing or broadening. A comparison of the theoretical predictions to detailed experimental studies on the laser detuning-dependence of Mollow triplet resonance emission from single In(Ga)As QDs reveals excellent agreement. Kuhn, "Long-time dynamics and stationary nonequilibrium of an optically driven strongly confined quantum dot coupled to phonons," Phys. Rev. B 84, 195311 (2011 Lapointe, R. Cheriton, and R. L. Williams, " Influence of electron-acoustic phonon scattering on off-resonant cavity feeding within a strongly coupled quantum-dot cavity system," Phys. Rev. B 83, 165313 (2011). 39. Dara P. S. McCutcheon, and Ahsan Nazir, "Emission properties of a driven artificial atom: increased coherent scattering and off-resonant sideband narrowing," arXiv:1208.4620v1

By metal-organic vapor-phase epitaxy, we have fabricated InAs quantum dots (QDs) on InGaAs/GaAs metamorphic buffer layers on a GaAs substrate with area densities that allow addressing single quantum dots. The photoluminescence emission from the quantum dots is shifted to the telecom C-band at 1.55 μm with a high yield due to the reduced stress in the quantum dots. The lowered residual strain at the surface of the metamorphic buffer layer results in a reduced lattice mismatch between the quantum dot material and growth surface. The quantum dots exhibit resolution-limited linewidths (mean value: 59 μeV) and low fine-structure splittings. Furthermore, we demonstrate single-photon emission (g(2)(0)=0.003) at 1.55 μm and decay times on the order of 1.4 ns comparable to InAs QDs directly deposited on GaAs substrates. Our results suggest that these quantum dots can not only compete with their counterparts deposited on InP substrates but also constitute an InAs/GaAs-only approach for the development of non-classical light sources in the telecom C-band.

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We present a detailed study of a phonon-assisted incoherent excitation mechanism of single quantum dots. A spectrally-detuned laser couples to a quantum dot transition by mediation of acoustic phonons, whereby excitation efficiencies up to 20 % with respect to strictly resonant excitation can be achieved at T = 9 K. Laser frequency-dependent analysis of the quantum dot intensity distinctly maps the underlying acoustic phonon bath and shows good agreement with our polaron master equation theory. An analytical solution for the photoluminescence is introduced which predicts a broadband incoherent coupling process when electron-phonon scattering is in the strong phonon coupling (polaronic) regime. Additionally, we investigate the coherence properties of the emitted light and study the impact of the relevant pump and phonon bath parameters.

Efficient fiber-based long-distance quantum communication via quantum repeaters relies on deterministic single-photon sources at telecom wavelengths, with the potential to exploit the existing world-wide infrastructures. For upscaling the experimental complexity in quantum networking, two-photon interference (TPI) of remote non-classical emitters in the low-loss telecom bands is of utmost importance. With respect to TPI of distinct emitters, several experiments have been conducted, e.g., using trapped atoms [1], ions [2], NV-centers [3, 4], SiV-centers [5], organic molecules[6] and semiconductor quantum dots (QDs) [7][8][9][10][11][12][13][14]; however, the spectral range was far from the highly desirable telecom C-band. Here, we report on TPI at 1550 nm between down-converted single photons from remote QDs [15], demonstrating quantum frequency conversion [16][17][18] as precise and stable mechanism to erase the frequency difference between independent emitters. On resonance, a TPI-visibility of (29 ± 3) % has been observed, being only limited by spectral diffusion processes of the individual QDs [19,20]. Up to 2-km of additional fiber channel has been introduced in both or individual signal paths with no influence on TPI-visibility, proving negligible photon wave-packet distortion. The present experiment is conducted within a local fiber network covering several rooms between two floors of the building. Our studies pave the way to establish long-distance entanglement distribution between remote solid-state emitters including interfaces with various quantum hybrid systems [21][22][23][24].

Abstract. We report on in-lab free space quantum key distribution (QKD) experiments over 40 cm distance using highly efficient electrically driven quantum dot single-photon sources emitting in the red as well as near-infrared spectral range. In the case of infrared emitting devices, we achieve sifted key rates of 27.2 kbit s −1 (35.4 kbit s −1 ) at a quantum bit error rate (QBER) of 3.9% (3.8%) and a g (2) (0) value of 0.35 (0.49) at moderate (high) excitation. (2) (0) value of 0.49. This first successful proof of principle QKD experiment based on electrically operated semiconductor single-photon sources can be considered as a major step toward practical and efficient quantum cryptography scenarios. Contents