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.

A central challenge in developing quantum computers and long-range quantum networks lies in the distribution of entanglement across many individually controllable qubits 1 . Colour centres in diamond have emerged as leading solid-state 'artificial atom' qubits 2,3 , enabling on-demand remote entanglement 4 , coherent control of over 10 ancillae qubits with minute-long coherence times 5 , and memory-enhanced quantum communication 6 . A critical next step is to integrate large numbers of artificial atoms with photonic architectures to enable large-scale quantum information processing systems. To date, these efforts have been stymied by qubit inhomogeneities, low device yield, and complex device requirements. Here, we introduce a process for the high-yield heterogeneous integration of 'quantum micro-chiplets' (QMCs) -diamond waveguide arrays containing highly coherent colour centreswith an aluminium nitride (AlN) photonic integrated circuit (PIC). Our process enables the development of a 72-channel defect-free array of germanium-vacancy (GeV) and silicon-vacancy (SiV) colour centres in a PIC. Photoluminescence spectroscopy reveals long-term stable and narrow average optical linewidths of 54 MHz (146 MHz) for GeV (SiV) emitters, close to the lifetime-limited linewidth of 32 MHz (93 MHz). Additionally, inhomogeneities in the individual qubits can be compensated in situ with integrated tuning of the optical frequencies over 100 GHz. The ability to assemble large numbers of nearly indistinguishable artificial atoms into phase-stable PICs provides an architecture toward multiplexed quantum repeaters 7,8 and general-purpose quantum computers [9][10][11] . Main textArtificial atom qubits in diamond combine minute-scale quantum memory times 5 with efficient spin-photon interfaces 2 , making them attractive for processing and distributing quantum information 1,3 . However, the low device yield of functional qubit systems presents a critical barrier to large-scale quantum information processing (QIP). Furthermore, although individual diamond cavity systems coupled to artificial atoms can now achieve excellent performance, the lack of active chip-integrated photonic components and wafer-scale single crystal diamond currently prohibit scaling to large-scale QIP applications [8][9][10][11] . A promising method to alleviate these constraints is heterogeneous integration (HI), which is increasingly used in advanced microelectronics to assemble separately fabricated sub-components into a single, multifunctional chip. HI approaches have also recently been used to integrate PICs with quantum devices, including quantum dot single-photon sources 12,13 , superconducting nanowire single-photon detectors 14 , and nitrogen-vacancy (NV) centre diamond waveguides 15 . However, these demonstrations assembled components one-by-one, which presents a formidable scaling challenge. The diamond 'quantum micro-chiplet (QMC)' introduced here significantly improves HI assembly yield and accuracy to enable a 72-channel defect-free waveguide-coupled art...

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...

Solid-state quantum emitters that couple coherent optical transitions to long-lived spin states are essential for quantum networks. Here we report on the spin and optical properties of single tin-vacancy (SnV) centers in diamond nanostructures. Through magneto-optical spectroscopy at 4 K, we verify the inversion-symmetric electronic structure of the SnV, identify spin-conserving and spin-flipping transitions, characterize transition linewidths, and measure electron spin lifetimes. We find that the optical transitions are consistent with the radiative lifetime limit and that the spin lifetimes are longer than for other inversion-symmetric color centers under similar conditions. These properties indicate that the SnV is a promising candidate for quantum optics and quantum networking applications.A central goal of quantum information processing is the development of quantum networks consisting of stationary, long-lived matter qubits coupled to flying photonic qubits [1,2], with applications in quantum computing, provably secure cryptography, and quantumenhanced metrology [3]. Among matter qubits, quantum emitters in wide-bandgap semiconductors [4,5] have emerged as leading systems as their coherent, spinselective optical transitions act as an interface between quantum information stored in their spin degrees of freedom and emitted photons. While most work has so far focused on the nitrogen-vacancy (NV) center in diamond [6-8], its relatively poor optical properties, including a low percentage of emission into the coherent zero-phonon-line (ZPL) [9] and large spectral diffusion when located near surfaces [10,11], have fueled the investigation of alternative emitters. These include the group-IV color centers in diamond [12], comprising the silicon-vacancy (SiV) [13][14][15][16], germanium-vacancy (GeV) [17,18], and the recently observed lead-vacancy (PbV) [19] centers. These centers have a large fraction of emission into the ZPL and a crystallographic inversion symmetry that limits spectral diffusion and inhomogeneous broadening [20,21]. Unlike the NV center, however, the electronic spin coherence of SiV and GeV centers is limited by phonon scattering to an upperlying ground-state orbital [22,23], requiring operation at dilution-refrigerator temperatures (∼ 100 mK) [24,25], or controllably induced strain [26] to achieve long coherence times.The tin-vacancy (SnV) center in diamond [27, 28] is a group-IV color center that promises favorable optical properties and long spin coherence time at readily achievable temperatures (liquid helium, ∼ 4 K). DFT calculations predict that the SnV has the same symmetry as the SiV and GeV[9], while experimental measurement of a large ground-state orbital splitting indicates that single-phonon scattering, the dominant spin dephasing mechanism of SiV and GeV centers at liquid helium temperatures, should be suppressed significantly [27]. In this work, we report spectroscopic measurements that are consistent with the conjectured electronic structure of the SnV, demonstrate that its optical...

A strong limitation of linear optical quantum computing is the probabilistic operation of two-quantum-bit gates based on the coalescence of indistinguishable photons. A route to deterministic operation is to exploit the single-photon nonlinearity of an atomic transition. Through engineering of the atom-photon interaction, phase shifters, photon filters and photon-photon gates have been demonstrated with natural atoms. Proofs of concept have been reported with semiconductor quantum dots, yet limited by inefficient atom-photon interfaces and dephasing. Here, we report a highly efficient single-photon filter based on a large optical nonlinearity at the single-photon level, in a near-optimal quantum-dot cavity interface. When probed with coherent light wavepackets, the device shows a record nonlinearity threshold around 0.3 ± 0.1 incident photons. We demonstrate that 80% of the directly reflected light intensity consists of a single-photon Fock state and that the two- and three-photon components are strongly suppressed compared with the single-photon one.

Solid-state emitters are excellent candidates for developing integrated sources of single photons. Yet, phonons degrade the photon indistinguishability both through pure dephasing of the zerophonon line and through phonon-assisted emission. Here, we study theoretically and experimentally the indistinguishability of photons emitted by a semiconductor quantum dot in a microcavity as a function of temperature. We show that a large coupling to a high quality factor cavity can simultaneously reduces the effect of both phonon-induced sources of decoherence. It first limits the effect of pure dephasing on the zero phonon line with indistinguishabilities above 97% up to 18K. Moreover, it efficiently redirects the phonon sidebands into the zero-phonon line and brings the indistinguishability of the full emission spectrum from 87% (resp. 24%) without cavity effect to more than 99% (resp 76%) at 0 K (resp. 20 K). We provide guidelines for optimal cavity designs that further minimize the phonon-induced decoherence.Indistinguishable single photons are the building blocks of optical quantum computation protocols and quantum networks [1][2][3]. This has motivated great efforts to develop devices generating on-demand indistinguishable single photons, using solid-state emitters such as diamond color centres [4, 5], molecules [6] or semiconductor quantum dots (QDs) [7][8][9][10][11][12]. In QDs, understanding the extrinsic sources of decoherence such as spin and charge noise [13] has recently enabled impressive progresses in the performances of these sources [11,12]. Yet, acoustic phonons generally remain an intrinsic and limiting source of dephasing.Indeed acoustic phonons are responsible for two kinds of dephasing processes. First, acoustic phonons induce a rapid and partial decay of coherence [14][15][16]. This non-Markovian dephasing dynamics is the time-domain counterpart of the emitter spectrum consisting of a sharp zero-phonon line (ZPL) sitting on top of a broad phonon sideband (PSB) [17][18][19]. Second, acoustic phonons can assist virtual transitions towards higher energy levels, resulting in a Markovian pure dephasing of the ZPL [3]. Such effects impose two severe limitations to obtain indistinguishable photons: (i) to work at low temperatures, typically below 10 K for QDs and (ii) to use spectral postselection of the ZPL. Indeed, even at zero temperature, phonon emission processes result in the presence of a PSB on the low energy side, fundamentally limiting the indistinguishability. In practice, the indistinguishability has been measured to rapidly drop with temperature even with spectral selection [21], and without it remains further away from unity [22].In typical self-assembled QDs, the emission fraction into the ZPL, η ZPL , represents typically 90% of the emission at 4K, a fraction that rapidly drops with temperature. Moreover, the photons emitted in the PSB are essentially incoherent, due to their broadband nature with respect to the natural linewidth. In a Hong-Ou-Mandel (HOM) experiment, only the fraction η ...

Quantum dots in cavities have been shown to be very bright sources of indistinguishable single photons. Yet the quantum interference between two such bright quantum dot sources, a critical step for photon-based quantum computation, still needs to be investigated. Here, we report on such a measurement, taking advantage of a deterministic fabrication of the devices. We show that cavity quantum electrodynamics can efficiently improve the quantum interference between remote quantum dot sources: Poorly indistinguishable photons can still interfere with good contrast with high quality photons emitted by a source in the strong Purcell regime. Our measurements and calculations show that cavity quantum electrodynamics is a powerful tool for interconnecting several quantum dot devices. Recent years have seen impressive progress in the implementation of quantum functionalities using semiconductor quantum dots (QDs). These artificial atoms [1] have been used to generate flying quantum bits in the form of single photons [2] or entangled photon pairs [3,4], to map and optically manipulate the quantum information encoded onto a single stationary electron [5,6] or hole spin [7], as well as to implement optical logic gates [8][9][10]. The potential of QD-based single photon sources lies in their deterministic and pure quantum statistics, as opposed to the parametric down-conversion sources currently used in quantum optics. At saturation, the QD emits a single photon with a probability close to one and a vanishing probability of emitting a second photon. However, collecting single photons emitted by a QD is challenging: The collection efficiency is limited to 1%-2% for a QD in bulk [11] and to 5%-10% for a QD in a planar cavity structure. New strategies have been developed to reach high collection efficiencies (in the 40%-90% range) such as inserting the QD in one-dimensional photonic crystal or nanowire waveguides [11][12][13][14], in photonic crystal nanocavities [15], microdisk cavities [16], or micropillar cavities [17].Since the first demonstration in 2002 [18], the coalescence of photons successively emitted by a single QD has been widely investigated. Various strategies have been developed to minimize the environment-induced dephasing (phonons, charge noise). One approach consists of using a strictly resonant excitation to create the carriers directly into the QD state and reduce the time jitter of the photon emission [9,[19][20][21][22][23]. Another approach consists of using the Purcell effect by inserting the QD in a microcavity. The acceleration of spontaneous emission reduces the effect of dephasing * Present address: Institut fur Halbleriteroptik und Funktionelle Grenzflachen, Universitat Stuttgart, Germany.† pascale.senellart@lpn.cnrs.fr [17,18,23] and leads to an efficient extraction of photons: Ultrabright sources of highly indistinguishable photons were recently reported using this approach [17]. The scalability of a quantum network based on QDs relies on the possibility of interconnecting two QD devices. Pioneer...

The ability to generate light in a pure quantum state is essential for advances in optical quantum technologies. However, obtaining quantum states with control in the photon-number has remained elusive. Optical light fields with zero and one photon can be produced by single atoms, but so far it has been limited to generating incoherent mixtures, or coherent superpositions with a very small one-photon term. Here, we report on the on-demand generation of quantum superpositions of zero, one, and two photons via pulsed coherent control of a single artificial atom. Driving the system up to full atomic inversion leads to the generation of quantum superpositions of vacuum and one photon, with their relative populations controlled by the driving laser intensity. A stronger driving of the system, with 2π-pulses, results in a coherent superposition of vacuum, one, and two photons, with the two-photon term exceeding the one-photon component, a state allowing phase super-resolving interferometry. Our results open new paths for optical quantum technologies with access to the photon-number degree-of-freedom. arXiv:1810.05170v2 [quant-ph]