The strong confinement of semiconductor excitons in a quantum dot gives rise to atomlike behavior. The full benefit of such a structure is best observed in resonant excitation where the excited state can be deterministically populated and coherently manipulated. Because of the large refractive index and device geometry it remains challenging to observe resonantly excited emission that is free from laser scattering in III/V self-assembled quantum dots. Here we exploit the biexciton binding energy to create an extremely clean single photon source via two-photon resonant excitation of an InAs/GaAs quantum dot. We observe complete suppression of the excitation laser and multiphoton emissions. Additionally, we perform full coherent control of the ground-biexciton state qubit and observe an extended coherence time using an all-optical echo technique. The deterministic coherent photon pair creation makes this system suitable for the generation of time-bin entanglement and experiments on the interaction of photons from dissimilar sources.

Long distance quantum communication is one of the prime goals in the field of quantum information science. With information encoded in the quantum state of photons, existing telecommunication fibre networks can be effectively used as a transport medium. To achieve this goal, a source of robust entangled single photon pairs is required. Here, we report the realization of a source of time-bin entangled photon pairs utilizing the biexciton-exciton cascade in a III/V self-assembled quantum dot. We analyse the generated photon pairs by an inherently phase-stable interferometry technique, facilitating uninterrupted long integration times. We confirm the entanglement by performing quantum state tomography of the emitted photons, which yields a fidelity of 0.69(3) and a concurrence of 0.41(6) for our realization of time-energy entanglement from a single quantum emitter.

Within the established theoretical framework of quantum mechanics, interference always occurs between pairs of paths through an interferometer. Higher order interferences with multiple constituents are excluded by Born's rule and can only exist in generalized probabilistic theories. Thus, high-precision experiments searching for such higher order interferences are a powerful method to distinguish between quantum mechanics and more general theories. Here, we perform such a test in an optical multi-path interferometer, which avoids crucial systematic errors, has access to the entire phase space and is more stable than previous experiments. Our results are in accordance with quantum mechanics and rule out the existence of higher order interference terms in optical interferometry to an extent that is more than four orders of magnitude smaller than the expected pairwise interference, refining previous bounds by two orders of magnitude.

Multiparticle quantum interference is critical for our understanding and exploitation of quantum information, and for fundamental tests of quantum mechanics. A remarkable example of multipartite correlations is exhibited by the Greenberger-Horne-Zeilinger (GHZ) state. In a GHZ state, three particles are correlated while no pairwise correlation is found. The manifestation of these strong correlations in an interferometric setting has been studied theoretically since 1990 but no three-photon GHZ interferometer has been realized experimentally. Here we demonstrate threephoton interference that does not originate from two-photon or single photon interference. We observe phase-dependent variation of three-photon coincidences with (92.7±4.6) % visibility in a generalized Franson interferometer using energy-time entangled photon triplets. The demonstration of these strong correlations in an interferometric setting provides new avenues for multiphoton interferometry, fundamental tests of quantum mechanics and quantum information applications in higher dimensions.

The development of linear quantum computing within integrated circuits demands high quality semiconductor single photon sources. In particular, for a reliable single photon source it is not sufficient to have a low multi-photon component, but also to possess high efficiency. We investigate the photon statistics of the emission from a single quantum dot with a method that is able to sensitively detect the trade-off between the efficiency and the multi-photon contribution. Our measurements show, that the light emitted from the quantum dot when it is resonantly excited possess a very low multi-photon content. Additionally, we demonstrated, for the first time, the non-Gaussian nature of the quantum state emitted from a single quantum dot.The ideal single photon state is quantum mechanically represented by the Fock state |1 , a quantum counterpart of the classical particle. The particle nature of a single photon is traditionally verified by observing an anticorrelation effect on a beam splitter [1]. This measurement of the intensity autocorrelation is commonly accepted as the way to test a light source for non-classicality [2]. Nonetheless, such a measurement is an intensitynormalized measurement and therefore completely insensitive to the vacuum contribution, |0 . In practice, however, all single-photon sources emit a stochastic mixture of the vacuum and the Fock state |1 . For many applications, which do not depend on the efficiency of the singlephoton source the intensity autocorrelation is a sufficient test. On the other hand, applications like linear optical quantum computing [3] crucially depend on the overall source and detector efficiency [4][5][6].Seen from this perspective, the characterization of a quantum state produced by realistic single photon sources would strongly benefit from a measurement that is sensitive to the vacuum contribution as well as the multiphoton component. By measuring the density matrix one obtains the full information about the quantum state of a real single photon source; regrettably, such a full state tomography is usually quite challenging. An alternative approach is to measure a quantum phase-space description of the state, its so-called Wigner function [7]. It is well known that the single photon state |1 has a negative Wigner function [8,9]. Furthermore, there is a whole class of states that possess negative Wigner functions (so called non-Gaussian states of light) as shown by Hudson's theorem [10]. The Wigner function is usually measured by homodyne tomography. This is a very tedious procedure and in some cases even impossible. However, when it is possible, it can detect the negativity of the states Wigner function, as was demonstrated for heralded single photon sources [11]. The practical obstacle for such a measurement is that the negativity depends crucially on the overall source and detector efficiency η. Namely, if η < 0.5 the measured Wigner function is positive, even though the photon source may give a perfectly antibunched intensity autocorrelation measurement. Theref...