Photonic integrated circuits (PICs) are attractive platforms to perform large-scale quantum information processing. While highly-functional PICs (e.g. silicon based photonic-circuits) andhigh-performance single photon sources (SPSs, e.g. compound-semiconductor quantum dots (QDs)) have been independently demonstrated, their combination for single-photon-based applications has still been limited. This is largely due to the complexities of introducing SPSs into existing PIC platforms, which are generally realized with different materials and using distinct fabrication protocols. Here, we report a novel approach to combine SPSs and PICs prepared independently. We employ transfer printing, by which multiple desired SPSs can be integrated in a simple pick-and-place manner with a theoretical waveguide coupling efficiency >99%, fulfilling the demanding requirements of large-scale quantum applications. Experimentally, we demonstrated QD-based SPSs with high waveguide coupling efficiencies, together with the integration of two SPSs into a waveguide. Our approach will accelerate scalable fusion between modern PICs and cutting-edge quantum technologies.

Thresholdless lasing is an outstanding challenge in laser science and is achievable only in devices having near unity quantum efficiency even when not lasing. Such lasers are expected to exhibit featureless linear light output curves. However, such thresholdless behavior hinders identification of the laser transition, triggering a long-lasting argument on how to identify the lasing. Here, we demonstrate thresholdless lasing in a semiconductor quantum dot nanolaser with a photonic crystal nanocavity. We employ cavity resonant excitation for enabling the thresholdless operation via focused carrier injection into high cavity field regions. Under conventional (above bandgap) excitation, the same nanolaser exhibits a typical thresholded lasing transition, thereby facilitating a systematic comparison between the thresholdless and thresholded laser transitions in the single device. Our approach enables a clear verification of the thresholdless lasing and reveals core elements for its realization using quantum dots, paving the way to the development of ultimately energy-efficient nanolasers.

Silicon photonics is a powerful platform for implementing large-scale photonic integrated circuits (PICs), because of its compatibility with mature complementary-metal-oxide-semiconductor (CMOS) technology. Exploiting silicon-based PICs for quantum photonic information processing (or the so-called silicon quantum photonics) provides a promising pathway for large-scale quantum applications. For the development of scalable silicon quantum PICs, a major challenge is integrating on-silicon quantum light sources that deterministically emit single photons. In this regard, the use of epitaxial InAs/GaAs quantum dots (QDs) is a very promising approach, because of their capability of deterministic single-photon emission with high purity and indistinguishability. However, the required hybrid integration is inherently difficult and often lacks the compatibility with CMOS processes. Here, we demonstrate a QD single-photon source (SPS) integrated on a glassclad silicon photonic waveguide processed by a CMOS foundry. Hybrid integration is performed using transfer printing, which enables us to integrate heterogeneous optical components in a simple pick-and-place manner and thus assemble them after the entire CMOS process is completed. We observe single-photon emission from the integrated QD and its efficient coupling into the silicon waveguide. Our transfer-printing-based approach is fully compatible with CMOS back-end processes, and thus will open the possibility for realizing large-scale quantum PICs that leverage CMOS technology. _____________________________ a)

We demonstrate precise and quick detection of the positions of quantum dots (QDs) embedded in two-dimensional photonic crystal nanocavities. We apply this technique to investigate the QD position dependence of the optical coupling between the QD and the nanocavity. We use a scanning electron microscope (SEM) operating at a low acceleration voltage to detect surface bumps induced by the QDs buried underneath. This enables QD detection with a sub-10 nm precision. We then experimentally measure the vacuum Rabi spectra to extract the optical coupling strengths (gs) between single QDs and cavities, and compare them to the values estimated by a combination of the SEM-measured QD positions and electromagnetic cavity field simulations. We found a highly linear relationship between the local cavity field intensities and the QD-cavity gs, suggesting the validity of the point dipole approximation used in the estimation of the gs. The estimation using SEM has a small standard deviation of ±6.2%, which potentially enables the high accuracy prediction of g prior to optical measurements. Our technique will play a key role for deeply understanding the interaction between QDs and photonic nanostructures and for advancing QD-based cavity quantum electrodynamics.

Photonic crystal (PhC) nanocavities with high quality (Q) factors have attracted much attention because of their strong spatial and temporal light confinement capability. The resulting enhanced light-matter interactions are beneficial for diverse photonic applications, ranging from on-chip optical communications to sensing. However, currently achievable Q factors for active PhC nanocavities, which embed active emitters inside, are much lower