Producing advanced quantum states of light is a priority in quantum information technologies. In this context, experimental realizations of multipartite photon states would enable improved tests of the foundations of quantum mechanics as well as implementations of complex quantum optical networks and protocols. It is favourable to directly generate these states using solid state systems, for simpler handling and the promise of reversible transfer of quantum information between stationary and flying qubits. Here we use the ground states of two optically active coupled quantum dots to directly produce photon triplets. The formation of a triexciton in these ground states leads to a triple cascade recombination and sequential emission of three photons with strong correlations. We record 65.62 photon triplets per minute under continuous-wave pumping, surpassing rates of earlier reported sources. Our structure and data pave the way towards implementing multipartite photon entanglement and multi-qubit readout schemes in solid state devices.

A hyperentangled state of light represents a valuable tool capable of reducing the experimental requirements and resource overheads, and it can improve the success rate of quantum information protocols. Here, we report on demonstration of polarization and time-bin hyperentangled photon pairs emitted from a single quantum dot. We achieved this result by applying resonant and coherent excitation on a quantum dot system with marginal fine structure splitting. Our results yield fidelities to the maximally entangled state of 0.81(6) and 0.87(4) in polarization and time bin, respectively.

We report on the generation of time-bin entangled photon pairs from a semiconductor quantum dot via pulsed resonant biexciton generation. Based on theoretical modeling we optimized the duration of the excitation pulse to minimize the laser-induced dephasing and increase the biexcitonto-background single exciton occupation probability. This results in a high degree of entanglement with a concurrence of up to 0.78(6) and a 0.88(3) overlap with a maximally entangled state. Theoretical simulations also indicate a power dependent nature of the dephasing during the laser excitation that limits the coherence of the excitation process.Single semiconductor quantum dots, due to their discrete energy structure, constitute an antibunched single photon source at a well defined frequency and with inherently sub-Poissonian statistics [1]. They generate single photons through a recombination of an exciton, a quasi particle formed by a Coulomb-bound electron from the conduction band and a hole from the valence band. In a more refined operation mode employing biexcitons, the Coulomb-bound four-carrier states containing two electrons and two holes, quantum dots can provide pairs of photons emitted in a fast cascade very similar to the original atomic cascade experiment by Aspect et al. [2]. It has been demonstrated that in the absence of the fine structure splitting of the bright exciton levels, such a cascade exhibits polarization entanglement [3][4][5][6][7][8]. Entanglement of photons is a fundamental resource for long distance quantum communications [9,10], where it forms the central part of various quantum communication protocols like teleportation [11] and entanglement swapping [12]. In addition, it is an essential element of linear optical quantum computing [13].The ability to achieve entanglement of photons from a quantum dot is not limited to polarization. Recently, it has been shown that the biexciton-exciton cascade can also be entangled in its emission time (time-bin) [14]. This type of entanglement (encoding) is important for optical-fibre based quantum communication [15] due to the fact that polarization entanglement can suffer from degradation in an optical fibre outside laboratory conditions [16]. In addition, a method to perform linear optical quantum computing with photons entangled in time-bin has been demonstrated recently [17]. Apart from the obvious goal to generate entangled photon pairs, there are further reasons for investigating the time-bin entanglement in photons emerging from a quantum dot. Namely, this method of entanglement calls for a coherent excitation and therefore is an excellent tool for investigating the coherence properties of a quantum dot system. It is precisely resonant excitation (especially two photonresonant excitation of the biexciton [18,19]) combined with a quantum dot system that exhibits a high degree of coherence, that is a sine qua non for optimal use of quantum dot photons in quantum information processing.Here, we report on an unprecedented degree of timebin entanglement from a...

Semiconductor Bragg-reflection waveguides are well-established sources of correlated photon pairs as well as promising candidates for building up integrated quantum optics devices. Here, we use such a source with optimized non-linearity for preparing time-bin entangled photons in the telecommunication wavelength range. By taking advantage of pulsed state preparation and efficient free-running single-photon detection, we drive our source at low pump powers, which results in a strong photon-pair correlation. The tomographic reconstruction of the state's density matrix reveals that our source exhibits a high degree of entanglement. We extract a concurrence of 88.9 ± 1.8% and a fidelity of 94.2 ± 0.9% with respect to a Bell state.

High-fidelity polarization-entangled photons are a powerful resource for quantum communication, distributing entanglement and quantum teleportation. The Bell-CHSH inequality S\leq2S≤2 is violated by bipartite entanglement and only maximally entangled states can achieve S=2\sqrt{2}S=22, the Tsirelson bound. Spontaneous parametric down-conversion sources can produce entangled photons with correlations close to the Tsirelson bound. Sagnac configurations offer intrinsic stability, compact footprint and high collection efficiency, however, there is often a trade off between source brightness and entanglement visibility. Here, we present a Sagnac polarization-entangled source with 2\sqrt{2}-S=(5.65\pm0.57\times10^{-3})22−S=(5.65±0.57×10−3), on-par with the highest SS parameters recorded, while generating and detecting (4660\pm70)pairs/s/mW(4660±70)pairs/s/mW, which is a substantially higher brightness than previously reported for Sagnac sources and around two orders of magnitude brighter than for traditional cone sources with the highest SS parameters. Our source records 0.9953\pm0.00030.9953±0.0003 concurrence and 0.99743\pm0.000140.99743±0.00014 fidelity to an ideal Bell state. By studying systematic errors in Sagnac sources, we identify that the precision of the collection focal point inside the crystal plays the largest role in reducing the SS parameter in our experiment. We provide a pathway that could enable the highest SS parameter recorded with a Sagnac source to-date while maintaining very high brightness.