An on-demand source of indistinguishable and entangled photon pairs is a fundamental component for different quantum information applications such as optical quantum computing, quantum repeaters, quantum teleportation and quantum communication 1 . Parametric downconversion 2, 3 and four-wave mixing sources 4 of entangled photons have shown high degrees of entanglement and indistinguishability but the probabilistic nature of their generation process also creates zero or multiple photon pairs following a Poissonian distribution. This limits their use in complex algorithms where many qubits and gate operations are required. Here we show simultaneously ultrahigh purity (g (2) (0) < 0.004), high entanglement fidelity (0.81 ± 0.02), high two-photon interference non-post selective visibilities (0.86 ± 0.03 and 0.71 ± 0.04) and on-demand generation of polarization-entangled photon pairs from a single semiconductor quantum dot (QD). Through a twophoton resonant excitation scheme, the biexciton population is deterministically prepared by a π-pulse. Applied on a quantum dot showing no exciton fine structure splitting, this results in the deterministic generation of indistinguishable entangled photon pairs.To date, spontaneous parametric down-conversion (SPDC) and four wave mixing sources have been mostly used for the generation of entangled photon pairs to realize quantum communication protocols and to demonstrate basic quantum logic experiments 5 . However, the photon pair statistics of these sources is described by a Poissonian distribution which implies also the generation of zero and multiple pairs. This leads to errors in the quantum algorithm protocols 6 which effectively limit their usefulness for deterministic quantum technologies. Radiative cascades in single quantum emitters, such as atoms 7 or quantum dots 8 , can in principle emit on demand single pairs of polarization-entangled photons with high generation efficiencies 9 . After optical excitation of two electron-hole pairs (biexciton, called |XX state) in a quantum dot, the biexciton decays through a two-photon cascade ( Fig. 1a). If the fine structure splitting between the intermediate states (excitons called |X ) is smaller than the radiative linewidth, the two decay paths are indistinguishable and the two photons are polarization-entangled which results in a two-photon Bell state |ψ + = 1 √ 2 (|HXX |HX + |VXX |VX ). To ensure the emission of a single pair of entangled photons per excitation pulse the biexcitonic state has to be pumped into saturation. So far, non-resonant pulsed pumping schemes have been successfully applied for entangled photon generation 10, 11 but no simultaneous information on indistinguishability has been provided. Anyhow, it is well known that non-resonant pumping schemes limit the coherence and indistinguishability of the emitted photons making them unfeasible for many quantum information applications. In a recent study, Stevenson and co-workers reported interference and entanglement properties of photons emitted by a QD embedded wi...

We demonstrate a new method to realize the population inversion of a single InGaAs/GaAs quantum dot excited by a laser pulse tuned within the neutral exciton phonon sideband. In contrast to the conventional method of inverting a two-level system by performing coherent Rabi oscillation, the inversion is achieved by rapid thermalization of the optically dressed states via incoherent phonon-assisted relaxation. A maximum exciton population of 0.67±0.06 is measured for a laser tuned 0.83 meV to higher energy. Furthermore, the phonon sideband is mapped using a two-color pump-probe technique, with its spectral form and magnitude in very good agreement with the result of path-integral calculations.

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It is demonstrated how the exciton and the biexciton state of a quantum dot can be prepared with high fidelity on a picosecond time scale by driving the dot with a strong laser pulse that is tuned above the exciton resonance for exciton preparation and in resonance with the exciton transition for biexciton preparation. The proposed protocols make use of the phonon-induced relaxation towards photon dressed states in optically driven quantum dots and combine the simplicity of traditional Rabi oscillation schemes with the robustness of adiabatic rapid passage schemes. Our protocols allow for an on-demand, fast, and almost perfect state preparation even at strong carrier-phonon interaction where other schemes fail. In fact, the performance of the presented protocols is shown to be better the stronger the carrier-phonon interaction is.

We investigate both experimentally and theoretically a simple yet more robust and flexible alternative to Rabi oscillation-type biexciton preparation protocols traditionally used for semiconductor quantum dots. The quantum dot is excited by a strong laser pulse positively detuned from the twophoton resonance yielding an on demand initialization of the biexciton state by making use of the phonon-induced thermalization of the photon dressed states. It is shown that for excitation pulses in the picosecond range, a stable and high fidelity of up to fXX = 0.98 ± 0.01 is reached. Notably, the generated photons show similar coherence properties as measured in the resonant two-photon scheme. This protocol is a powerful tool for the control of complex solid state systems combining radiative cascades, entanglement and resonant cavity modes.

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Abstract. For many applications of semiconductor quantum dots in quantum technology a well controlled state preparation of the quantum dot states is mandatory. Since quantum dots are embedded in the semiconductor matrix, the interaction with phonons plays often a major role in the preparation process. In this review, we discuss the influence of phonons on three basically different optical excitation schemes which can be used for the preparation of exciton, biexciton, and superposition states: a resonant excitation leading to Rabi rotations in the excitonic system, an excitation with chirped pulses exploiting the effect of adiabatic rapid passage, and an off-resonant excitation giving rise to a phononassisted state preparation. We give an overview over experimental and theoretical results showing the role of the phonons and compare the performance of the schemes for state preparation.

We investigate theoretically under which conditions a stable and high-fidelity preparation of the biexciton state in a quantum dot can be realized by means of adiabatic rapid passage in the presence of acoustic phonon coupling. Our analysis is based on a numerically complete real-time path integral approach and comprises different schemes of optical driving using frequency-swept (chirped) pulses. We show that depending on the size of the biexciton binding energy, resonant two-photon excitations or two-color schemes can be favorable. It is demonstrated that the carrier-phonon interaction strongly affects the efficiency of the protocols and that a robust preparation of the biexciton is restricted to positive chirps and low temperatures. A considerable increase of the biexciton yield can be achieved realizing temperatures below 4 K.