Single-photons are key elements of many future quantum technologies, be it for the realisation of large-scale quantum communication networks 1 for quantum simulation of chemical and physical processes 2 or for connecting quantum memories in a quantum computer 3 . Scaling quantum technologies will thus require efficient, on-demand, sources of highly indistinguishable single-photons 4 . Semiconductor quantum dots inserted in photonic structures are ultrabright single photon sources [5][6][7] , but the photon indistinguishability is limited by charge noise induced by nearby surfaces 8 . The current state of the art for indistinguishability are parametric down conversion single-photon sources, but they intrinsically generate multiphoton events and hence must be operated at very low brightness to maintain high single photon purity 9,10 . To date, no technology has proven to be capable of providing a source that simultaneously generates near-unity indistinguishability and pure single-photons with high brightness. Here, we report on such devices made of quantum dots in electrically controlled cavity structures. We demonstrate on-demand, bright and ultra-pure single photon generation. Application of an electrical bias on deterministically fabricated devices 11,12 is shown to fully cancel charge noise effects. Under resonant excitation, an indistinguishability of 0.9956±0.0045 is evidenced with a g (2) (0)=0.0028±0.0012. The photon extraction of 65% and measured brightness of 0.154±0.015 make this source 20 times brighter than any source of equal quality. This new generation of sources open the way to a new level of complexity and scalability in optical quantum manipulation.

Strongly coupled optical microcavities containing different exciton states permit the creation of hybrid-polariton modes that can be described in terms of a linear admixture of cavity-photon and the constituent excitons. Such hybrid states have been predicted to have optical properties that are different from their constituent parts, making them a test bed for the exploration of light-matter coupling. Here, we use strong coupling in an optical microcavity to mix the electronic transitions of two J-aggregated molecular dyes and use both non-resonant photoluminescence emission and photoluminescence excitation spectroscopy to show that hybrid-polariton states act as an efficient and ultrafast energy-transfer pathway between the two exciton states. We argue that this type of structure may act as a model system to study energy-transfer processes in biological light-harvesting complexes.

The desiderata for an ideal photon source are high brightness, high single-photon purity, and high indistinguishability. Defining brightness at the first collection lens, these properties have been simultaneously demonstrated with solid-state sources, however absolute source efficiencies remain close to the 1% level, and indistinguishability only demonstrated for photons emitted consecutively on the few nanosecond scale. Here we employ deterministic quantum dot-micropillar devices to demonstrate solid-state single-photon sources with scalable performance. In one device, an absolute brightness at the output of a single-mode fibre of 14% and purities of 97.1-99.0% are demonstrated. When non-resontantly excited, it emits a long stream of photons that exhibit indistinguishability up to 70%-above the classical limit of 50%-even after 33 consecutively emitted photons, a 400 ns separation between them. Resonant excitation in other devices results in near-optimal indistinguishability values: 96% at short timescales, remaining at 88% in timescales as large as 463 ns, after 39 emitted photons. The performance attained by our devices brings solid-state sources into a regime suitable for scalable implementations.Photon indistinguishability-responsible for unique quantum phenomena with no classical counterpart, notably photon bunching via interference [1]-has been demonstrated in various physical systems [2][3][4][5][6][7][8][9], resulting in a broad range of applications in photonic quantum technologies [10], including quantum teleportation [11,12], generation of entangled photon sources [13][14][15], and linear-optics quantum computation [16,17]. However, achieving conclusive indistinguishability, i..e. above 50% (the classical limit), while simultaneously displaying high single-photon purity and high absolute brightness is experimentally challenging.Semiconductor quantum dots (QDs) inserted in photonic structures [18][19][20][21][22] are a rapidly improving technology for generating bright sources of indistinguishable single-photons. Addressing the excited states of the quantum dot using a non-resonant scheme early showed two-photon interference visibilities in the 70%−80% range [8], yet with limited collection efficiencies. Improvements in the efficiency have been made by deterministically placing the quantum dot in the centre of a photonic micro-cavity. Here the acceleration of photon emission into well defined cavity modes [23], due to Purcell enhancement, has enabled two-photon interference visibilities in the same range, with simultaneous efficiencies at the first collection lens around 80% [9]. Near-unity indistinguishability, in turn, has been achieved in recent years under strictly-resonant excitation of the quantum dot [24][25][26], whereas the recent development of electric control on deterministically coupled devices [27]-thus with scalable fabrication-has now enabled strictlyresonant excitation in combination with Purcell enhancement, resulting in near-optimal single-photon sources [28] with visibilities reach...

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