We demonstrate a high-purity source of indistinguishable single photons using a quantum dot embedded in a nanophotonic waveguide. The source features a near-unity internal coupling efficiency and the collected photons are efficiently coupled off-chip by implementing a taper that adiabatically couples the photons to an optical fiber. By quasi-resonant excitation of the quantum dot, we measure a single-photon purity larger than 99.4 % and a photon indistinguishability of up to 94 ± 1 % by using p-shell excitation combined with spectral filtering to reduce photon jitter. A temperature-dependent study allows pinpointing the residual decoherence processes notably the effect of phonon broadening. Strict resonant excitation is implemented as well as another mean of suppressing photon jitter, and the additional complexity of suppressing the excitation laser source is addressed. The study opens a clear pathway towards the long-standing goal of a fully deterministic source of indistinguishable photons, which is integrated on a planar photonic chip.

Many photonic quantum information processing applications would benefit from a high brightness, fiber-coupled source of triggered single photons. Here, we present a fiber-coupled photonic-crystal waveguide single-photon source relying on evanescent coupling of the light field from a tapered out-coupler to an optical fiber. A two-step approach is taken where the performance of the tapered out-coupler is recorded first on an independent device containing an on-chip reflector. Reflection measurements establish that the chip-to-fiber coupling efficiency exceeds 80 %. The detailed characterization of a high-efficiency photonic-crystal waveguide extended with a tapered out-coupling section is then performed. The corresponding overall single-photon source efficiency is 10.9 % ± 2.3 %, which quantifies the success probability to prepare an exciton in the quantum dot, couple it out as a photon in the waveguide, and subsequently transfer it to the fiber. The applied out-coupling method is robust, stable over time, and broadband over several tens of nanometers, which makes it a highly promising pathway to increase the efficiency and reliability of planar chip-based single-photon sources.

We investigate the optical emission and decay dynamics of excitons confined in large strain-free GaAs quantum dots grown by droplet epitaxy. From time-resolved measurements combined with a theoretical model we show that droplet-epitaxy quantum dots have a quantum efficiency of about 75 % and an oscillator strength between 8 and 10. The quantum dots are found to be fully described by a model for strongly-confined excitons, in contrast to the theoretical prediction that excitons in large quantum dots exhibit the so-called giant oscillator strength. We attribute these findings to localized ground-state excitons in potential minima created by material intermixing during growth. We provide further evidence for the strong-confinement regime of excitons by extracting the size of electron and hole wavefunctions from the phonon-broadened photoluminescence spectra. Furthermore, we explore the temperature dependence of the decay dynamics and, for some quantum dots, observe a pronounced reduction in the effective transition strength with temperature. We quantify and explain these effects as being an intrinsic property of large quantum dots owing to thermal excitation of the ground-state exciton. Our results provide a detailed understanding of the optical properties of large quantum dots in general, and of quantum dots grown by droplet epitaxy in particular.

We report on the observation of single-photon superradiance from an exciton in a semiconductor quantum dot. The confinement by the quantum dot is strong enough for it to mimic a two-level atom, yet sufficiently weak to ensure superradiance. The electrostatic interaction between the electron and the hole comprising the exciton gives rise to an anharmonic spectrum, which we exploit to prepare the superradiant quantum state deterministically with a laser pulse. We observe a five-fold enhancement of the oscillator strength compared to conventional quantum dots. The enhancement is limited by the base temperature of our cryostat and may lead to oscillator strengths above 1000 from a single quantum emitter at optical frequencies.Enhancing and tailoring light-matter interaction is at the heart of modern quantum physics, partly because it enables studying hitherto unexplored realms of physics and partly to meet the steep requirements for quantuminformation science. Photonic nanostructures efficiently tailor the density of optical states and have proven very useful to this end. For example, cavities can reach strong coupling to emitters [1, 2] or mechanical objects [3], and photonic waveguides enable efficient photonic switches [4] and single-photon sources [5]. Another approach to enhancing light-matter interaction concerns tailoring the capability of the emitter to be polarized, i.e., the oscillator strength. This can be achieved with collective effects such as superradiance [6], which has been studied in ensembles of atoms [7], ions [8], Bose-Einstein condensates [9], and superconducting circuits [10]. Collective enhancement can occur at the single-photon level if a single quantum of energy is distributed coherently in an ensemble [6]. This single-photon superradiance (SPS) has been studied so far in ensembles of non-interacting emitters such as nuclei [11], and is central to schemes for robust quantum communication [12] and quantum memories [13]. A drawback of non-interacting systems is their harmonic energy structure, which prohibits deterministic preparation of a particular collective state. Here we show that the fundamental optical excitation of a weakly confining quantum dot is a generalization of SPS. We prepare the collective quantum state deterministically with a laser pulse and demonstrate its superradiant character. Our findings underline the extraordinary potential of weakly confining quantum dots for achieving unprecedented light-matter coupling strengths at optical frequencies, which would improve the radiative efficiency, quantum efficiency, quantum nonlinearities, and coherence of single-photon sources in nanophotonic quantum devices [14].We study quantum dots formed by intentional monolayer fluctuations of a quantum well, which were pioneered by Gammon et al. [15], cf. Fig. 1(a). The subwavelength size of the quantum dot is key to achieving a large collective enhancement; in larger ensembles, such as atomic clouds, the enhancement is reduced by destructive interference [6,16]. The fundamental optical...

Diamond-based microelectromechanical systems (MEMS) enable direct coupling between the quantum states of nitrogen-vacancy (NV) centers and the phonon modes of a mechanical resonator. One example, diamond high-overtone bulk acoustic resonators (HBARs), feature an integrated piezoelectric transducer and support high-quality factor resonance modes into the GHz frequency range. The acoustic modes allow mechanical manipulation of deeply embedded NV centers with long spin and orbital coherence times. Unfortunately, the spin-phonon coupling rate is limited by the large resonator size, > 100 µm, and thus strongly-coupled NV electron-phonon interactions remain out of reach in current diamond BAR devices. Here, we report the design and fabrication of a semi-confocal HBAR (SCHBAR) device on diamond (silicon carbide) with 1 arXiv:1906.06309v1 [cond-mat.mes-hall] 14 Jun 2019 f · Q > 10 12 (> 10 13 ). The semi-confocal geometry confines the phonon mode laterally below 10 µm. This drastic reduction in modal volume enhances defect center electronphonon coupling. For the native NV centers inside the diamond device, we demonstrate mechanically driven spin transitions and show a high strain-driving efficiency with a Rabi frequency of (2π)2.19(14) MHz/V p , which is comparable to a typical microwave antenna at the same microwave power.Defect-based qubits are attractive platforms for solid state quantum technologies. 1 The leading examples are the nitrogen-vacancy (NV) 2 center and the silicon-vacancy (SiV) 3 center in diamond, and the divacancy center 4 and the silicon vacancy center (V Si ) 5 in silicon carbide (SiC). Hybrid quantum systems based on these defect qubits are particularly interesting because they interface the qubit spin to photons or phonons and thus potentially enable the transport of quantum information. For sensing applications, they offer unconventional modalities of quantum control which is a resource for extending the coherence time and thus sensitivity. Coupling spins to mechanical motion could also enable new quantum-enhanced sensors of motion, such as inertial sensing. 6,7 Although solid state spin-photon entanglement has been demonstrated in recent years 8 and has been used to build quantum networks, 9 defect-based spin-mechanical systems have yet to operate at the single phonon quantum level because they are limited by weak electronphonon coupling, g, in existing devices. Considering g ∝ 1/V , where V is the modal volume, one approach to strengthening the coupling is to engineer small mode volume mechanical resonators with high quality factors. Ultimately, defect-based spin-mechanical systems may enable new sensing applications and control of phonon states at the quantum level. 10Defect-based spin-mechanical systems can be classified into two categories: 1) micro-beam resonator systems 11-13 and 2) micro-electromechanical systems (MEMS) 14-17 with integrated thin-film piezoelectric transducers. While the first category minimizes the resonator fabrication to a single material, i.e., diamond, SiC, etc., high...