The generation of indistinguishable single photons is a fundamental requirement for photonic quantum technologies. However, spectral fluctuations, often induced by charge noise in epitaxial quantum dots (QDs), lead to exciton dephasing, thereby limiting their practical usage in quantum applications. We present a straightforward approach to mitigate charge noise-induced decoherence in droplet-etched GaAs QDs embedded in an n-i-p diode structure and integrated deterministically into an electrically contacted circular Bragg grating resonator for emission enhancement. The quantum device allows for the stabilization of the charge environment by applying an external electrical field while producing a photon extraction efficiency of (37±2)%. Hong-Ou-Mandel two-photon interference measurements reveal a strong dependence of the exciton dephasing time and interference visibility on the applied bias, in excellent agreement with our theoretical predictions. Notably, the reduction in visibility from a maximum, charge stabilized corrected value of 97% at the optimum bias point follows an inverse square dependence (∝1/I^{2}) with increasing diode current (I) in the forward direction. Under a quasi-resonant excitation scheme, we achieve a maximum exciton dephasing time (T_{2}^{*}) of approximately (6.8±0.5) ns, reaching nearly the Fourier limit (T_{2}=2T_{1}) without the need for complex echo schemes like Ramsey or Carr-Purcell-Meiboom-Gill sequences. These findings are consistent with theoretical predictions from rate equation modeling and quantum optical analysis, as well as voltage-dependent linewidth measurements, demonstrating optimized electrical control of exciton dephasing.

Quantum reservoir computing (QRC) harnesses driven quantum dynamics for time-series processing, yet the mechanisms behind the differing performance levels across its many implementations remain unclear. We show that apparently unrelated approaches-including memory restriction, weak measurements, operation near the edge of quantum chaos, and dissipative dynamics-are in fact governed by the same underlying principle, namely a tunable balance between memory retention and nonlinear response. Using the information processing capacity, a dynamical measure from nonlinear systems theory, we place these behaviors in a unified framework and identify the regimes in which quantum reservoirs surpass the standard protocol. Our results reveal a fundamental connection between memory and nonlinear response. This provides a general design principle for enhanced information processing and enables systematic analysis and optimization inspired by classical dynamical quantifiers.

Tunnel-coupled optically active quantum dot molecules (QDMs), have the potential to operate as spin-photon-interfaces with coupled spins that interact with two different photon frequencies at the same time. A prerequisite is to deterministically prepare two (electron or hole) spins in the QDM and be able to electrically tune the orbital state couplings. Here, we demonstrate the sequential optical charging of a single QDM with two electron spins while simultaneously maintaining the ability to widely tune orbital couplings using static electric fields and optically drive the system for quantum light generation. We optically prepare one- and two-spin states, initialize via optical pumping and explore orbital and spin relaxation dynamics for one and two-spin states as a function of the energy detuning and hybridization of orbital states. For two-spin states, remarkably long S-T relaxation times are observed extending beyond ∼ 100μs with strong dependence on the relative energy of ground and excited two-spin states. Qualitative agreement is observed with 𝐤 · 𝐩 calculations of phonon-mediated spin-relaxation. Our results provide new quantitative understanding of the dynamics of one and two-spin states and confirm their suitability of QDMs for creating multidimensional photonic cluster states by exploiting tunable spin-spin exchange couplings at zero magnetic fields combined with optical driving.

Abstract The scalable integration of solid-state quantum emitters into photonic nanostructures remains a central challenge for quantum photonic technologies. Here, we demonstrate a robust and streamlined integration strategy that tackles the long-standing issue of deterministic fabrication on randomly positioned self-assembled quantum dots (QDs), leveraging a buried-stressor-based site-controlled InGaAs QD platform. We show that this deterministic growth approach enables precise spatial alignment with circular Bragg grating (CBG) resonators for enhanced emission, eliminating the need for complex and time-consuming deterministic lithography techniques. We fabricated a 6 × 6 SCQD-CBG array with 100% device yield, with 35 devices falling within the radial-offset range where the simulated photon-extraction efficiency (PEE) exceeds 20%, underscoring the spatial precision and scalability of our fabrication concept. A systematically selected subset of five devices with varying radial displacements reveals clear offset-dependent trends in PEE, degree of linear polarization, spectral linewidth, and photon indistinguishability, thereby establishing quantitative bounds on spatial alignment tolerances. In the best-aligned QD-CBG device, we achieve a PEE of (47.1 ± 3.8)% (corresponding to an end-to-end system efficiency of 3.4%), a linewidth of (1.41 ± 0.22) GHz, a radiative decay lifetime of (0.80 ± 0.02) ns, a single-photon purity of (99.58 ± 0.18)%, and a Hong-Ou-Mandel two-photon interference visibility of (81 ± 5)% under quasi-resonant excitation at saturation power. We confirm our conceptual understanding of the effect of emitter-position dependent charge-noise fluctuations in terms of a quantum-optical model for the (quantum-)emission properties. The established nanofabrication platform provides a reproducible, lithography-compatible route to scalable, high-performance single-photon sources (SPS), offering a powerful alternative to conventional lithography-based deterministic integration techniques.

Strain-engineered transition-metal dichalcogenide nanobubbles are promising platforms for quantum emission, as revealed by recent experimental observations. In this work, we present an ab initio investigation of MoS2, WS2, MoSe2, and WSe2 nanobubbles, linking their structural and electronic properties to predictions of their optical activity. Inflating forces yield tunable geometries with non-uniform, apex-concentrated strain, which is sensitive to material rigidity. Strain modifies band gaps and universally induces non-dispersive valence states, exhibiting composition-dependent wave-function character, as revealed by an in-depth analysis of band structures and orbital contributions. Crucially, transitions from these apex-localized valence states are predominantly dark. This characteristic is attributed to their localization at the Γ-point, inhibiting transitions to the lowest unoccupied states that reside at the K-valley. While revealing that the herein considered sub-10-nm nanobubbles fall short as single-photon emitters, our findings provide essential understanding of the structure-property relations in emerging quantum materials, providing robust design rules to optimize their characteristics for novel quantum applications.

In a combined experimental and theoretical study, we demonstrate anisotropic polariton transport on the nanoscale in the van der Waals antiferromagnet CrSBr. While effective cavity-polariton formation emerges via the self-hybridization of ultra-high oscillator strength excitons with a thin slab photonic mode, the absence of external mirrors facilitates spectroscopic investigation of these polaritons via cathodoluminescence (CL) on length scales determined by the electron wavelength. This direct access allows us to perform precise charting of the polariton landscape with nanometric resolution, and to probe polariton interference phenomena. The main finding of the work highlights that the coherent polariton transport follows the C_2v symmetry of CrSBr, allowing exclusive transport along the crystallographic a-axis, while no coherent feature is found along the b-axis direction. Our work sets the foundation to use CL spectroscopy in cavity-polaritonics in more advanced landscapes, such as photonic crystals or optical lattices, and establishes the technique as a powerful tool to probe anisotropic expansion and relaxation phenomena on the nanoscale

ABSTRACT Versatile, tunable, and potentially scalable single‐photon sources are a key asset in emergent photonic quantum technologies. In this work, a single‐photon source based on WS 2 micro‐domes, created via hydrogen ion irradiation, is realized and integrated into an open, tunable optical microcavity. Single‐photon emission from the coupled emitter–cavity system is verified via the second‐order correlation measurement, revealing a value of . A detailed analysis of the spectrally selective, cavity enhanced emission features shows the impact of a pronounced acoustic phonon emission sideband, which contributes specifically to the non‐resonant emitter–cavity coupling in this system. The achieved level of cavity–emitter control highlights the potential of open‐cavity systems to tailor the emission properties of atomically thin quantum emitters, advancing their suitability for real‐world quantum technology applications.

The moiré pattern, which emerges due to a relative rotation between two monolayers of transition metal dichalcogenides, features a long lattice period for small twist angles. The resulting band structure modulation acts as an effective potential for interlayer excitons (IXs), which can realize correlated many-body phenomena. Here, we aim for a material-realistic modelling of the exciton-exciton interaction, taking into account lattice reconstructions and an exciton-exciton potential that incorporates the highly anisotropic screening imposed by the two-dimensional bilayer and the dielectric background. We find strong modifications of the on-site interaction induced by the change of the moiré potential during lattice reconstructions, while for long-range interactions on the length scale of the moiré period, anisotropic dielectric screening leads to a crossover from a repulsive to an attractive interaction. The interaction potential and hopping amplitudes serve as parameters for a Bose-Hubbard model on the moiré lattice, which we use to explain correlated behavior of interlayer excitons.

We investigate the short-term temporal dynamics of superradiance in closely spaced quantum emitters. Building on Dickes 1954 framework, we analyze the sequential emergence of coherence, superradiance, and entanglement, revealing a distinct temporal hierarchy in their extremal values: relative coherence develops first, followed by the peak of correlated emission, then minimal entanglement, and subsequently minimal spin-spin correlation is reached. These findings suggest that enhanced relative coherence precedes correlated emission, and when spin correlations are negligible, entanglement and correlated emission become linked in time.

Versatile, tunable, and potentially scalable single-photon sources are a key asset in emergent photonic quantum technologies. In this work, a single-photon source based on WS_2 micro-domes, created via hydrogen ion irradiation, is realized and integrated into an open, tunable optical microcavity. Single-photon emission from the coupled emitter-cavity system is verified via the second-order correlation measurement, revealing a g^(2)(τ=0) value of 0.3. A detailed analysis of the spectrally selective, cavity enhanced emission features shows the impact of a pronounced acoustic phonon emission sideband, which contributes specifically to the non-resonant emitter-cavity coupling in this system. The achieved level of cavity-emitter control highlights the potential of open-cavity systems to tailor the emission properties of atomically thin quantum emitters, advancing their suitability for real-world quantum technology applications.

The fading memory property is a key requirement for reservoir computers – a specific type of recurrent neural network with fixed internal weights. While mostly undesired in gate-based quantum computing, dissipation due to material imperfections or coupling to the environment acts as a natural mechanism intrinsically providing fading memory to reservoir computers based on dynamical open quantum systems. In this work, we unravel a connection between the physical metric of optical absorption and the performance of quantum reservoir computers in terms of their short-term memory capacity. We establish this link by considering a coherent input encoding in conjunction with tunable qubit decay, giving precise control over the fading memory in the quantum reservoir computer. Our analysis enables us to identify a sweet-spot regime for the dissipation strength at which memory performance is maximized.

The interaction of a quantum two-level system with a resonant driving field results in the emergence of Rabi oscillations, which are the hallmark of a controlled manipulation of a quantum state on the Bloch sphere. This all-optical coherent control of solid-state two-level systems is crucial for quantum applications. In this work we study Rabi oscillations emerging in a WSe2 monolayer-based quantum dot. The emitter is driven coherently using picosecond laser pulses to a higher-energy state, while photoluminescence is probed from the ground state. The theoretical treatment based on a three-level exciton model reveals the population transfer between the exciton ground and excited states coupled by Coulomb interaction. Our calculations demonstrate that the resulting exciton ground state population can be controlled by varying driving pulse area and detuning which is evidenced by the experimental data. Our results pave the way towards the coherent control of quantum emitters in atomically thin semiconductors, a crucial ingredient for monolayer-based high-performance, on-demand single photon sources.

Optical resonators are a powerful platform to control the spontaneous emission dynamics of excitons in solid-state nanostructures. We study a MoSe2-WSe2 heterostructure that is integrated in a cryogenic open optical microcavity to gain insights into fundamental optical properties of the emergent interlayer excitons. First, we utilize a low-quality-factor planar open cavity and investigate the modification of the excitonic lifetime as on- and off-resonance conditions are met with consecutive longitudinal modes. Time-resolved photoluminescence measurements revealed a periodic tuning of the interlayer exciton lifetime by 220 ps, which allows us to extract a 0.5 ns free-space radiative lifetime and a quantum efficiency as high as 81.4%±1.4%. We subsequently engineer the local density of optical states by spatially confined and spectrally tunable Tamm-plasmon resonances. The dramatic redistribution of the local optical modes allows us to encounter a significant inhibition of the excitonic spontaneous emission rate by a factor of 3.2. Our open cavity is able to tune the cavity resonances accurately to the emitters to have a robust in situ control of the light-matter coupling. Such a powerful characterization approach can be universally applied to tune the exciton dynamics and measure the quantum efficiencies of more complex van der Waals heterostructures and devices.

Quantum reservoir computing (QRC) offers a promising paradigm for harnessing quantum systems for machine learning tasks, especially in the era of noisy intermediate-scale quantum devices. While information-theoretical benchmarks like short-term memory capacity (STMC) are widely used to evaluate QRC performance, they fail to provide insights into the physical mechanisms underlying these quantum neural networks. We establish a quantitative connection between the optical absorption spectrum of a quantum reservoir and its memory performance, revealing that optimal STMC aligns directly with maximal absorption, providing a physical explanation for the previously reported "sweet-spot" behavior in QRC performance as a function of dissipation. This connection bridges quantum information theory with experimentally accessible physical properties, opening pathways for targeted engineering of quantum reservoir computers with optimized performance for specific tasks.

Quantum Natural Language Processing (QNLP) offers a novel approach to encoding and understanding the complexity of natural languages through the power of quantum computation. This paper presents a pretrained quantum context-sensitive embedding model, called QCSE, that captures context-sensitive word embeddings, leveraging the unique properties of quantum systems to learn contextual relationships in languages. The model introduces quantum-native context learning, enabling the utilization of quantum computers for linguistic tasks. Central to the proposed approach are innovative context matrix computation methods, designed to create unique, representations of words based on their surrounding linguistic context. Five distinct methods are proposed and tested for computing the context matrices, incorporating techniques such as exponential decay, sinusoidal modulation, phase shifts, and hash-based transformations. These methods ensure that the quantum embeddings retain context sensitivity, thereby making them suitable for downstream language tasks where the expressibility and properties of quantum systems are valuable resources. To evaluate the effectiveness of the model and the associated context matrix methods, evaluations are conducted on both a Fulani corpus, a low-resource African language, dataset of small size and an English corpus of slightly larger size. The results demonstrate that QCSE not only captures context sensitivity but also leverages the expressibility of quantum systems for representing rich, context-aware language information. The use of Fulani further highlights the potential of QNLP to mitigate the problem of lack of data for this category of languages. This work underscores the power of quantum computation in natural language processing (NLP) and opens new avenues for applying QNLP to real-world linguistic challenges across various tasks and domains.

Localized excitons in two-dimensional (2D) materials are considered promising sources of single photons on demand. The photon indistinguishability as a key figure of merit for quantum information processing is strongly influenced by the coupling of charge excitations to lattice vibrations of the surrounding semiconductor material. Here, we quantify the impact of exciton-acoustic-phonon interaction and cavity quantum electrodynamics effects on photon indistinguishability in a Hong-Ou-Mandel setup by solving fully quantum-mechanical equations for a coupled quantum dot-cavity-phonon system including non-Markovian effects. We find a strong reduction of indistinguishability compared to 3D systems due to increased exciton-phonon coupling efficiency. Moreover, we show that the coherence properties of photons are significantly influenced by the finite phonon lifetime in the surrounding material giving rise to pure dephasing. If these limitations are overcome, localized excitons in 2D semiconductors can become a new avenue for quantum light sources.

Recent advancements in machine learning have led to an exponential increase in computational demands, driving the need for innovative computing platforms. Quantum computing, with its Hilbert space scaling exponentially with the number of particles, emerges as a promising solution. In this work, we implement a quantum extreme machine learning (QELM) protocol leveraging indistinguishable photon pairs and multimode fiber as a random densly connected layer. We experimentally study QELM performance based on photon coincidences – for distinguishable and indistinguishable photons – on an image classification task. Simulations further show that increasing the number of photons reveals a clear quantum advantage. We relate this improved performance to the enhanced dimensionality and expressivity of the feature space, as indicated by the increased rank of the feature matrix in both experiment and simulation.

Using Hamiltonian encoding to inject an input into parameterized quantum circuits (PQCs), the output of the PQC can be written as truncated Fourier series. In recent years, the expressivity of PQCs was established as the number of frequencies contained in this Fourier series. While this concept has also been applied to other quantum machine learning (QML) paradigms, a clear notion of expressivity for temporal information processing with quantum systems is still lacking. Here, we introduce such a notion to the field of quantum reservoir computing (QRC). We analytically derive an expression for the readouts showing that the output of a QRC can be interpreted as a multi-dimensional Fourier series. We give a formula for the growth of expressivity induced by the sequential information injection, which we corroborate with numerical simulations, calculating explicitly the number of multi-dimensional output functions which can be generated from the readouts. Our results show that the specific interplay between system size, input encoding, and memory time gives rise to a boundary on the system size beyond which it is obstructive to further increase the reservoir size in extreme scrambling systems. We propose a recipe for determining this maximal system size for a given QRC setup.