Quantum communication relies on the availability of light pulses with strong quantum correlations among photons. An example of such an optical source is a single-photon pulse with a vanishing probability for detecting two or more photons. Using pulsed laser excitation of a single quantum dot, a single-photon turnstile device that generates a train of single-photon pulses was demonstrated. For a spectrally isolated quantum dot, nearly 100% of the excitation pulses lead to emission of a single photon, yielding an ideal single-photon source.

Maxwell's equations successfully describe the statistical properties of fluorescence from an ensemble of atoms or semiconductors in one or more dimensions. But quantization of the radiation field is required to explain the correlations of light generated by a single two-level quantum emitter, such as an atom, ion or single molecule. The observation of photon antibunching in resonance fluorescence from a single atom unequivocally demonstrated the non-classical nature of radiation. Here we report the experimental observation of photon antibunching from an artificial system--a single cadmium selenide quantum dot at room temperature. Apart from providing direct evidence for a solid-state non-classical light source, this result proves that a single quantum dot acts like an artificial atom, with a discrete anharmonic spectrum. In contrast, we find the photon-emission events from a cluster of several dots to be uncorrelated.

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 present measurements of first- and second-order coherence of quantum-dot micropillar lasers together with a semiconductor laser theory. Our results show a broad threshold region for the observed high-beta microcavities. The intensity jump is accompanied by both pronounced photon intensity fluctuations and strong coherence length changes. The investigations clearly visualize a smooth transition from spontaneous to predominantly stimulated emission which becomes harder to determine for high beta. In our theory, a microscopic approach is used to incorporate the semiconductor nature of quantum dots. The results are in agreement with the experimental intensity traces and the photon statistics measurements.

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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.

Detailed properties of resonance fluorescence from a single quantum dot in a micropillar cavity are investigated, with particular focus on emission coherence in the dependence on optical driving field power and detuning. A power-dependent series over a wide range reveals characteristic Mollow triplet spectra with large Rabi splittings of |Ω|≤15  GHz. In particular, the effect of dephasing in terms of systematic spectral broadening ∝Ω(2) of the Mollow sidebands is observed as a strong fingerprint of excitation-induced dephasing. Our results are in excellent agreement with predictions of a recently presented model on phonon-dressed quantum dot Mollow triplet emission in the cavity-QED regime.