A solid state source of photon triplets based on quantum dot molecules

Time-Resolved Line Shapes of Single Quantum Emitters via Machine Learned Photon Correlations

Solid-state single-photon emitters (SPEs) are quantum light sources that combine atomlike optical properties with solid-state integration and fabrication capabilities. SPEs are hindered by spectral diffusion, where the emitter's surrounding environment induces random energy fluctuations. Timescales of spectral diffusion span nanoseconds to minutes and require probing single emitters to remove ensemble averaging. Photon correlation Fourier spectroscopy (PCFS) can be used to measure time-resolved single emitter line shapes, but is hindered by poor signal-to-noise ratio in the measured correlation functions at early times due to low photon counts. Here, we develop a framework to simulate PCFS correlation functions directly from diffusing spectra that match well with experimental data for single colloidal quantum dots. We use these simulated datasets to train a deep ensemble autoencoder machine learning model that outputs accurate, noiseless, and probabilistic reconstructions of the noisy correlations. Using this model, we obtain reconstructed time-resolved single dot emission line shapes at timescales as low as 10 ns, which are otherwise completely obscured by noise. This enables PCFS to extract optical coherence times on the same timescales as Hong-Ou-Mandel two-photon interference, but with the advantage of providing spectral information in addition to estimates of photon indistinguishability. Our machine learning approach is broadly applicable to different photon correlation spectroscopy techniques and SPE systems, offering an enhanced tool for probing single emitter line shapes on previously inaccessible timescales.

Enhanced TEMPO Algorithm for Quantum Path Integrals with Off-Diagonal System-Bath Coupling: Applications to Photonic Quantum Networks

Multitime system correlation functions are relevant in various areas of physics and science, dealing with system-bath interaction including spectroscopy and quantum optics, where many of these schemes include an off-diagonal system bath interaction. Here we extend the enhanced time-evolving matrix product operator (eTEMPO) algorithm for quantum path integrals using tensor networks [Phys. Rev. Lett. 123, 240602 (2019)PRLTAO0031-900710.1103/PhysRevLett.123.240602 to open quantum systems with off-diagonal coupling beyond a single two level system. We exemplify the approach on a coupled cavity waveguide system with spatially separated quantum two-state emitters, though many other applications in material science are possible, including entangled photon propagation, photosynthesis spectroscopy, and on-chip quantum optics with realistic dissipation.

Magnetic tuning of tunnel coupling between InAsP double quantum dots in InP nanowires

We study experimentally and theoretically the in-plane magnetic field dependence of the coupling between dots forming a vertically stacked double dot molecule. The InAsP molecule is grown epitaxially in an InP nanowire and interrogated optically at millikelvin temperatures. The strength of interdot tunneling, leading to the formation of the bonding-antibonding pair of molecular orbitals, is investigated by adjusting the sample geometry. For specific geometries, we show that the interdot coupling can be controlled in-situ using a magnetic field-mediated redistribution of interdot coupling strengths. This is an important milestone in the development of qubits required in future quantum information technologies.

Proposal for a Deterministic Single-Atom Source of Quasisuperradiant N-Photon Pulses

We propose a single-atom, cavity quantum electrodynamics system, compatible with recently demonstrated, fiber-integrated micro- and nanocavity setups, for the on-demand production of optical number-state, 0N-state, and binomial-code-state pulses. The scheme makes use of Raman transitions within an entire atomic ground-state hyperfine level and operates with laser and cavity fields detuned from the atomic transition by much more than the excited-state hyperfine splitting. This enables reduction of the dynamics to that of a simple, cavity-damped Tavis-Cummings model with the collective spin determined by the total angular momentum of the ground hyperfine level.

Quantum Non-Gaussian Photon Coincidences

Photon coincidences represent an important resource for quantum technologies. They expose nonlinear quantum processes in matter and are essential for sources of entanglement. We derive broadly applicable criteria for quantum non-Gaussian two-photon coincidences that certify a new quality of photon sources. The criteria reject states emerging from Gaussian parametric processes, which often limit applications in quantum technologies. We also analyze the robustness of the quantum non-Gaussian coincidences and compare it to the heralded quantum non-Gaussianity of single photons based on them.

Constructing defect-related subband in silver indium sulfide QDs via pH-dependent oriented aggregation for boosting photocatalytic hydrogen evolution

Surface engineering of quantum dots (QDs) plays critical roles in tailoring carriers' dynamics of I-III-VI QDs via the interplay of QDs in aggregates or assembly, thus influencing their photocatalytic activities. In this work, an aqueous synthesis and the followed pH tuned oriented assembly method are developed to prepare network-like aggregates, dispersion, or sheet-like assembly of GSH-capped Silver Indium Sulfide (AIS). FTIR, DLS, and HRTEM investigation revealed that surface protonation or deprotonation of QDs occurred at pH < 6 or pH > 12 favors the formation of network-like aggregates with various defects or sheet-like assembly with perfect crystal lattice, respectively, via the surface charge induced interaction among AIS QDs. Further UV-vis, steady and transient PL investigation confirm the narrowed band gaps and the prolonged PL lifetime of the acidic network-like aggregates. As a result, the optimized network-like aggregates (3.0-AIS) exhibits superior photocatalytic H₂ evolution (PHE) rates (5.2 mmol·g^-1·h^-1), about 113 times that of alkaline sheet-like assembly (13.0-AIS) or 2.7 times higher than that of dispersed AIS QDs (AIS-8.0). The formation of defects and their roles in PHE mechanisms are discussed. This work is expected to give some new insight for designing efficient non-cadmium/non-novel metal I-III-VI photocatalysts for boosting PHE.

Multiplexed Single-Photon Source Based on Multiple Quantum Dots Embedded within a Single Nanowire

Photonics-based quantum information technologies require efficient, high emission rate sources of single photons. Position-controlled quantum dots embedded within a broadband nanowire waveguide provide a fully scalable route to fabricating highly efficient single-photon sources. However, emission rates for single-photon devices are limited by radiative recombination lifetimes. Here, we demonstrate a multiplexed single-photon source based on a multidot nanowire. Using epitaxially grown nanowires, we incorporate multiple energy-tuned dots, each optimally positioned within the nanowire waveguide, providing single photons with high efficiency. This linear scaling of the single-photon emission rate with number of emitters is demonstrated using a five-dot nanowire with an average multiphoton emission probability of <4% when excited at saturation. This represents the first ever demonstration of multiple single-photon emitters deterministically incorporated in a single photonic device and is a major step toward achieving GHz single-photon emission rates from a scalable multi-quantum-dot system.

Deterministically fabricated solid-state quantum-light sources

The controlled generation of non-classical states of light is a challenging task at the heart of quantum optics. Aside from the mere spirit of science, the related research is strongly driven by applications in photonic quantum technologies, including the fields of quantum communication, quantum computation, and quantum metrology. In this context, the realization of integrated solid-state-based quantum-light sources is of particular interest, due to the prospects for scalability and device integration. This topical review focuses on solid-state quantum-light sources which are fabricated in a deterministic fashion. In this framework we cover quantum emitters represented by semiconductor quantum dots, colour centres in diamond, and defect-/strain-centres in two-dimensional materials. First, we introduce the topic of quantum-light sources and non-classical light generation for applications in photonic quantum technologies, motivating the need for the development of scalable device technologies to push the field towards real-world applications. In the second part, we summarize material systems hosting quantum emitters in the solid-state. The third part reviews deterministic fabrication techniques and comparatively discusses their advantages and disadvantages. The techniques are classified in bottom-up approaches, exploiting the site-controlled positioning of the quantum emitters themselves, and top-down approaches, allowing for the precise alignment of photonic microstructures to pre-selected quantum emitters. Special emphasis is put on the progress achieved in the development of in situ techniques, which significantly pushed the performance of quantum-light sources towards applications. Additionally, we discuss hybrid approaches, exploiting pick-and-place techniques or wafer-bonding. The fourth part presents state-of-the-art quantum-dot quantum-light sources based on the fabrication techniques presented in the previous sections, which feature engineered functionality and enhanced photon collection efficiency. The article closes by highlighting recent applications of deterministic solid-state-based quantum-light sources in the fields of quantum communication, quantum computing, and quantum metrology, and by discussing future perspectives in the field of solid-state quantum-light sources.

Design of Quantum Dot-Nanowire Single-Photon Sources that are Immune to Thermomechanical Decoherence

Nanowire antennas embedding a single quantum dot (QD) have recently emerged as versatile platforms to realize bright sources of quantum light. In this theoretical work, we show that the thermally driven, low-frequency vibrations of the nanowire have a major impact on the QD light emission spectrum. Even at liquid helium temperatures, these prevent the emission of indistinguishable photons. To overcome this intrinsic limitation, we propose three designs that restore photon indistinguishability thanks to a specific engineering of the mechanical properties of the nanowire. We anticipate that such a mechanical optimization will also play a key role in the development of other high-performance light-matter interfaces based on nanostructures.

Faithful Hierarchy of Genuine n-Photon Quantum Non-Gaussian Light

Light is an essential tool for connections between quantum devices and for diagnostic processes in quantum technology. Both applications deal with advanced nonclassical states beyond Gaussian coherent and squeezed states. Current development requires a loss-tolerant diagnostic of such nonclassical aspects. We propose and experimentally verify a faithful hierarchy of genuine n-photon quantum non-Gaussian light. We conclusively witnessed three-photon quantum non-Gaussian light in the experiment. Measured data demonstrate a direct applicability of the hierarchy for a large class of real states.

Real Space Observation of Electronic Coupling between Self-Assembled Quantum Dots

The control of quantum coupling between nano-objects is essential to quantum technologies. Confined nanostructures, such as cavities, resonators, or quantum dots, are designed to enhance interactions between electrons, photons, or phonons, giving rise to new properties, on which devices are developed. The nature and strength of these interactions are often measured indirectly on an assembly of dissimilar objects. Here, we adopt an innovative point of view by directly mapping the coupling of single nanostructures using scanning tunneling microscopy and spectroscopy (STM and STS). We take advantage of the unique capabilities of STM/STS to map simultaneously the nano-object's morphology and electronic density in order to observe in real space the electronic coupling of pairs of In(Ga)As/GaAs self-assembled quantum dots (QDs), forming quantum dot molecules (QDMs). Differential conductance maps d I/d V ( E, x, y) demonstrate the presence of an effective electronic coupling, leading to bonding and antibonding states, even for dissymmetric QDMs. The experimental results are supported by numerical simulations. The actual geometry of the QDMs is taken into account to determine the strength of the coupling, showing the crucial role of quantum dot size and pair separation for device growth optimization.

Nanowire-based sources of non-classical light

Sources of quantum light that utilize photonic nanowire designs have emerged as potential candidates for high efficiency non-classical light generation in quantum information processing. In this review we cover the different platforms used to produce nanowire-based sources, highlighting the importance of waveguide design and material properties in achieving optimal performance. The limitations of the sources are identified and routes to optimization are proposed. State-of-the-art nanowire sources are compared to other solid-state quantum emitter platforms with regard to the key metrics of single photon purity, indistinguishability and entangled-pair fidelity to maximally entangled Bell states. We also discuss the unique ability of the nanowire platform to incorporate multiple emitters in the same optical mode and consider potential applications. Finally, routes to on-chip integration are discussed and the challenges facing the development of a nanowire-based scalable architecture are presented.

Photonic quantum information processing: a review

Photonic quantum technologies represent a promising platform for several applications, ranging from long-distance communications to the simulation of complex phenomena. Indeed, the advantages offered by single photons do make them the candidate of choice for carrying quantum information in a broad variety of areas with a versatile approach. Furthermore, recent technological advances are now enabling first concrete applications of photonic quantum information processing. The goal of this manuscript is to provide the reader with a comprehensive review of the state of the art in this active field, with a due balance between theoretical, experimental and technological results. When more convenient, we will present significant achievements in tables or in schematic figures, in order to convey a global perspective of the several horizons that fall under the name of photonic quantum information.

All-Optical Mapping of the Position of Quantum Dots Embedded in a Nanowire Antenna

Nanowire antennas embedding single quantum dots (QDs) have recently emerged as a versatile solid-state platform for quantum optics. Within the nanowire section, the emitter position simultaneously determines the strength of the light-matter interaction, as well as the coupling to potential decoherence channels. Therefore, to quantitatively understand device performance and guide future optimization, it is highly desirable to map the emitter position with an accuracy much smaller than the waveguide diameter, on the order of a few hundreds of nanometers. We introduce here a nondestructive, all-optical mapping technique that exploits the QD emission into two guided modes with different transverse profiles. These two modes are fed by the same emitter and thus interfere. The resulting intensity pattern, which is highly sensitive to the emitter position, is resolved in the far-field using Fourier microscopy. We demonstrate this technique on a standard microphotoluminescence setup and map the position of individual QDs in a nanowire antenna with a spatial resolution of ±10 nm. This work opens important perspectives for the future development of light-matter interfaces based on nanowire antennas. Beyond single-QD devices, it will also provide a valuable tool for the investigation of collective effects that imply several emitters coupled to an optical waveguide.

Dispersion tuning in sub-micron tapers for third-harmonic and photon triplet generation

Precise control of the dispersion landscape is of crucial importance if optical fibers are to be successfully used for the generation of three-photon states of light-the inverse of third-harmonic generation (THG). Here we report gas-tuning of intermodal phase-matched THG in sub-micron-diameter tapered optical fiber. By adjusting the pressure of the surrounding argon gas up to 50 bars, intermodally phase-matched third-harmonic light can be generated for pump wavelengths within a 15 nm range around 1.38 μm. We also measure the infrared fluorescence generated in the fiber when pumped in the visible and estimate that the accidental coincidence rate in this signal is lower than the predicted detection rate of photon triplets.