Leonardo Midolo

Solid-state quantum emitters coupled to optical waveguides provide an efficient and coherent interface for deterministic light-matter interactions, offering a promising route towards large-scale quantum photonic networks. Such networks, however, require active control of the optical phase between emitters, a capability that has remained elusive in integrated photonic circuits. Here we demonstrate a monolithically integrated mirror–phaseshifter–emitter system in a GaAs waveguide. A nano-opto-electro-mechanical slot-waveguide phase shifter is positioned between an InAs quantum dot and its mirror image formed by a photonic-crystal reflector, realizing an effective two-emitter system in which the emitters are exactly on resonance. Tuning the phase between the quantum dot and its mirror image controls the interference between the emitted and reflected fields. This shifts the nodes and antinodes of the resulting standing wave, thereby modifying the local density of optical states experienced by the optical emitter that governs its radiative decay. We observe broadband modulation of quantum dot decay rates, accompanied by a corresponding modulation of the single-photon intensity across the InAs quantum dot emission spectrum. The modulation depth further maps the spatial distribution of the emitters within the waveguide mode, providing a direct probe of emitter–field coupling. Our results establish active phase control in quantum dot waveguide quantum electrodynamics, enabling in situ optimization of emitter–field coupling and opening a path towards programmable interactions between emitters. This functionality provides a scalable foundation for phase-engineered single-photon sources and reconfigurable many-body quantum photonic circuits.

Quantum photonic integrated circuits (QPICs) are a promising platform for scalable quantum technologies. A major outstanding challenge is the efficient interconnection of diverse quantum photonic components, where optical losses must remain below the ∼10% threshold required for advanced quantum applications. In this paper, the optical coupling between GaAs single‐photon sources and a low‐loss SiN interposer is investigated, where reliable high‐efficiency transitions have not yet been achieved. To address this, a fully numerical framework is introduced for the optimization of waveguide mode couplers that directly incorporates experimentally characterized fabrication variations critically impacting performance, including waveguide misalignment, width deviation, and sidewall roughness. Using this approach, low‐loss GaAs/SiN transitions are designed and experimentally demonstrated with reproducible coupling efficiencies of –0.43 dB ± 0.14 dB. These results establish a viable path toward low‐loss heterogeneous QPICs tailored to advanced quantum applications.

Quantum dots have set benchmarks that far surpass other quantum emitters owing to their ability to deliver high-quality, high-rate and pure photons. However, achieving these exceptional capabilities at telecom wavelengths, bridging the gap to fibre-optic infrastructure and scalable silicon photonics, remains a challenge. Overcoming this difficulty demands high-quality quantum materials and devices that, despite extensive efforts, have not yet been realized. Here we demonstrate waveguide-integrated InAs quantum dots and realize a fully quantum-coherent photon-emitter interface operating in the original telecommunication band (or O-band, 1,260-1,360 nm). We record transform-limited linewidths only 8% broader than the inverse lifetime and bright 41.7-MHz emission rate under 80-MHz π-pulse excitation. These findings showcase the potential of quantum dots for scalable quantum networks.

The lack of interactions between single photons prohibits direct nonlinear operations in quantum optical circuits, representing a central obstacle in photonic quantum technologies. Here, we demonstrate multi-mode nonlinear photonic circuits where both linear and direct nonlinear operations can be programmed with high precision at the single-photon level. Nonlinear interaction is realised with a tunable quantum dot embedded in a nanophotonic waveguide mediating interactions between individual photons within a temporal linear optical interferometer. We demonstrate the capability to reprogramme the nonlinear photonic circuits and implement protocols where strong nonlinearities are required, in particular for quantum simulation of anharmonic molecular dynamics, thereby showcasing the new key functionalities enabled by our technology.

We demonstrate a small-footprint electromechanical phase shifter with a compact active length of 10 μ m fabricated on a suspended Ga As membrane, offering versatile integration with quantum-dot single-photon sources. The phase shifter is based on a slot waveguide, whose slot width can be controlled by electrostatic forces, enabling a large effective refractive index change Δ n eff > 0.1 . We observe up to 3 π phase modulation with 10.6 V applied bias, and a figure of merit V π L = 5.7 × 10 − 3 V cm . Integration with a Mach-Zehnder interferometer further allows routing of single photons with an extinction ratio of up to 24 dB at cryogenic temperatures. This device enables advanced manipulation of quantum emitters and the realization of reconfigurable quantum photonic integrated circuits.

Quantum dots stand out as the most advanced and versatile light-matter interface available today. Their ability to deliver high-quality, high-rate, and pure photons has set benchmarks that far surpass other emitters. Yet, a critical frontier has remained elusive: achieving these exceptional capabilities at telecom wavelengths, bridging the gap to fiber-optic infrastructure and scalable silicon photonics. Overcoming this challenge demands high quality quantum materials and devices which, despite extensive efforts, have not been realized yet. Here, we demonstrate waveguide-integrated quantum dots and realize a fully quantum-coherent photon-emitter interface operating in the original telecommunication band. The quality is assessed by recording transform-limited linewidths only 8 % broader than the inverse lifetime and bright 41.7 MHz emission rate under 80 MHz $π$-pulse excitation, unlocking the full potential of quantum dots for scalable quantum networks.

Fusion-based photonic quantum computing architectures rely on two primitives: i) near-deterministic generation and control of constant-size entangled states and ii) probabilistic entangling measurements (photonic fusion gates) between entangled states. Here, we demonstrate these key functionalities by temporally fusing resource states deterministically generated using a solid-state spin-photon interface. Repetitive operation of the source leads to sequential entanglement generation, whereby curiously entanglement is created between the quantum states of the same spin at two different instances in time. Such temporal multiplexing of photonic entanglement provides a resource-efficient route to scaling many-body entangled systems with photons.

Quantum photonic integrated circuits (QPICs) offer a promising path toward scalable quantum technologies. QPICs rely on the integration of many quantum photonic components and interconnecting optical waveguides for generation, manipulation, and detection of single photons. A key challenge in QPICs is the management and minimization of optical losses, which is particularly critical for single-photon applications. In this paper, we investigate optical propagation losses in strip waveguides within suspended gallium arsenide (GaAs) platforms, which can directly host deterministic single-photon sources but suffer high scattering-related losses. We systematically analyze different scattering loss contributions by investigating four key waveguide perturbation types: sidewall roughness, top surface roughness, surface particles, and suspension tethers. Our approach combines rigorous 3D finite-difference time-domain (FDTD) simulations with experimental measurements to decouple and quantify individual contributions to the total propagation loss. We study two suspended GaAs platforms operating at different wavelengths: an established 930 nm platform and an emerging 1300 nm platform in the telecommunication O-band. Based on our findings, we identify the dominant scattering loss mechanisms and propose novel design-time guidelines and concrete strategies to reduce the main loss contributions by factors of 2.5–5. These improvements are crucial for enabling complex QPICs directly within the native platform of the single-photon source, supporting advances in integrated quantum technologies.

The ability to control light-matter interfaces with solid-state photon emitters is a major requirement for the development of quantum photonic integrated circuits. We demonstrate controllable coupling between a quantum dot and an optical mode in a dielectric waveguide using a nano-opto-electro-mechanical phase shifter and a photonic crystal mirror. By controlling the phase, we induce a virtual displacement of the mirror that modifies the local density of states at the position of the emitter, thereby enhancing or suppressing spontaneous emission without an optical cavity. We observe a broadband tuning of the spontaneous emission rate and a modulation of the intensity emitted by the quantum dot in the waveguide. The method reported here could be employed to optimize the emitter-field interaction between quantum dots in-operando, by maximizing a single-photon source generation rate or adjusting its lifetime, as well as a characterization tool for the direct measurement of emitter-photon cooperativity.