Quantum networks enhance quantum communication schemes and link multiple users over large areas. Harnessing high dimensional quantum states - qu-d-its - allows for a denser transfer of information with increased robustness to noise compared to qubits. Frequency encoded qu-d-its can be manipulated at telecom wavelengths with off-the-shelf fibered devices. We use a low free spectral range silicon microresonator to propose, assess and optimize a reconfigurable entanglement-based quantum key distribution network with frequency-bin encoded Bell states of dimension d = 2 and d = 3, using a single fibered hardware. We achieve secure key rates of 1374 bit/s with qutrits (d = 3), and estimate the communication range to 295 km with qubits (d = 2) across 21 parallel two-user quantum channels. This multi-dimensional versatile demonstration is stable beyond 21 h and lays the groundwork for larger dimensionality implementations deployed on metropolitan fiber links.

Quantum states of light with many entangled photons are key resources for photonic quantum computing and quantum communication. In this work, we exploit a highly resource-efficient generation scheme based on a linear optical circuit embedding a fibered delay loop acting as a quantum memory. The single photons are generated with a bright single-photon source based on a semiconductor quantum dot, allowing to perform the entangling scheme up to 6 photons. We demonstrate 2, 3, 4 and 6-photon entanglement generation at respective rates of 6 kHz, 120 Hz, 2.2 Hz, and 2 mHz, corresponding to an average scaling ratio of 46. We introduce a method for real-time control of entanglement generation based on partially post-selected measurements. The visibility of such measurements enables discrimination and correcting for experimental phase drifts or entangling gate fidelity variations, and thus carries faithful information to monitor the entanglement process, an important feature for the practical implementation of photonic measurement-based quantum computation.

We present a comprehensive framework for evaluating the phase sensitivity of spectrally multimode SU(1,1) interferometers probed with a coherent-vacuum input, under both number and homodyne detections. The optical parametric amplifiers (OPAs) are simulated using a periodically polled thin-film lithium niobate waveguide. The theoretical model incorporates the intrinsic multimode spectral nature of waveguide-based OPAs. Under the assumptions of identical OPAs and Schmidt-mode-independent phase shifts, we have shown that the multimode SU(1,1) interferometer is equivalent to a collection of independent single-mode interferometers in the Schmidt basis that operate in parallel, each associated with a Schmidt mode of the parametric process. We further present an optimized waveguide design based on asymmetric group-velocity matching to realize a practical OPA with nearly factorizable joint spectral amplitude (JSA). The Schmidt coefficients extracted from the numerically simulated JSA are incorporated into the theoretical model to evaluate the phase sensitivity. The results show that the sensitivity under a multimode condition depends strongly on the measurement scheme. For number detection, the degradation in sensitivity arises from the redistribution of the available nonlinear resource among the Schmidt modes, whereas homodyne detection exhibits an additional coherence penalty that can be substantially reduced by optimally shaping the local oscillator. Moreover, we show that the performance degradation can be to a large extent mitigated by optimizing the spectrum of the injected signal coherent state and, in the case of homodyne detection, the spectrum of the local oscillator. Altogether, this work provides a unified theoretical and numerical framework for analyzing and optimizing realistic multimode SU(1,1) interferometers with waveguide-based OPAs for quantum-enhanced sensing applications.

We provide a general theoretical derivation of the phase sensitivity achieved by SU(1,1) interferometers under homodyne detection. The general expressions obtained accommodate arbitrary input states and include internal and external losses. In this systematic review, both full SU(1,1) interferometers with two parametric amplifiers and the truncated interferometers with only one parametric amplifier are examined. We investigate scenarios involving both single-output ports and joint homodyne detection, and consider parametric amplifiers with equal gains or with a boosted gain second amplifier. Our analytical formulation provides physical insight and understanding of the improvements in the sensitivity, which are shown to originate from noise reduction and/or signal amplification, depending on the configurations and practical implementations. Surprisingly, the configuration with single-output mode detection and parametric amplifiers with equal gains exhibits the highest robustness to very high internal losses. We finally apply this framework to a ubiquitous |α,0⟩ input two-mode coherent probe state. This approach permits the comparison of different strategies and the optimization of the interferometer performance in the presence of losses. In particular, we determine which amplification and detection configurations provide the best performance, depending on the level of losses. This exemplifies how this general analytical approach provides a powerful tool to design quantum-enhanced interferometers and achieve optimal sensitivity with selected probe states and homodyne detection.

We introduce a framework where light-matter transitions, rather than states, are the primary dynamical objects. Successive compositions of elementary transitions yield multiphoton processes with compact diagrammatic bookkeeping of resonant and off-resonant pathways. This approach enables transparent derivations of effective high-order Hamiltonians in the dispersive regime, foundational to quantum-information applications. Applied to the paradigmatic Jaynes-Cummings model, our framework reveals a photon-number-independent intrinsic Rabi frequency and persistent polaritonic hybridization in the dispersive regime, unifying resonant and dispersive limits.

We propose an adiabatic-elimination formalism in the dispersive regime based on a transition-centric perturbation theory. The perturbative expansion is recast into a diagrammatic framework, while adiabatic elimination is implemented through controlled projections onto transition subspaces. Our approach applies systematically at arbitrary perturbation order, and is suited to multilevel systems and multiple qubits in both cavity and waveguide quantum electrodynamics. It ultimately enables the explicit construction of effective higher-order Hamiltonians while bypassing important limitations of existing techniques, thereby providing a practical toolbox for multiphoton processes in the dispersive regime.

We experimentally demonstrate a non-linear dependence of the spin noise variance on atomic density in a warm alkali vapor. Implementing high-bandwidth spin noise spectroscopy (SNS) near the D2 transition of rubidium, a quadratic spin noise contribution is shown to arise at high densities, in contrast with the linear dependence valid in non-interacting ensembles. This non-linear scaling is shown to crucially depend on the residual optical excitation of the vapor by the probe beam, suggesting it stems from atomic cross-correlations due to resonant dipole-dipole interaction (DDI) in the vapor. We support this claim by introducing an additional experimental protocol to quench the ddi, resulting in a suppression of both the quadratic scaling of the spin variance and the distortions of the spin noise spectrum induced by the interaction. These results extend the applications of SNS to the characterization of many-body correlations in complex quantum systems.

Polarization-encoded spin-photon interfaces constitute promising candidates for the development of stationary nodes used as photon receivers, for quantum communication and distributed quantum computing. Here we introduce a time-resolved tomography approach which allows observing the dynamics of an electron spin, in a semiconductor quantum dot, mapped onto the dynamics of the polarization state of reflected photons. Through a single tomography experiment, we infer all the relevant spin dynamics timescales, including precession, decoherence and relaxation times. We also demonstrate and quantify the measurement back-action induced, on the embedded spin qubit, by the detection of a single reflected photon. We show that the induced population and coherence of the spin state can be tuned by the chosen polarization basis of the measurement. The control of the photon-induced back-action on the embedded spin qubit constitutes a crucial requirement for the use of spin-photon interfaces as quantum receivers.

Multiplexing information in different degrees of freedom is a natural solution to the scalability bottleneck in optical quantum communications and computing. However, for bulk-optics systems, size, cost, stability, and reliability can make scalability either impractical or highly challenging to implement. We present a framework to engineer continuous and discrete-variable entanglement produced through nondegenerate spontaneous parametric down-conversion in χ^{(2)} nonlinear photonic lattices in spatial and spectral degrees of freedom that can alleviate the scalability challenge. We show how spatio-spectral pump shaping produces cluster states that are natively distributable in quantum communication networks and a resource for measurement-based quantum computing.

We probe the origin of hole g -factor anisotropy in InGaAs QDs, using photo-luminescence measurements to construct a spin Hamiltonian model, then using this model to simulate the impact of the anisotropy on spin-photon entanglement generation.

Multiplexing information in different degrees of freedom and use of integrated and fiber-optic components are natural solutions to the scalability bottleneck in optical quantum communications and computing. However, for bulk-optics systems, where size, cost, stability, and reliability are factors, this remains either impractical or highly challenging to implement. In this paper we present a framework to engineer continuous-variable entanglement produced through nondegenerate spontaneous parametric down-conversion in χ^(2) nonlinear photonic lattices in spatial and spectral degrees of freedom that can solve the scalability challenge. We show how spatio-spectral pump shaping produce cluster states that are naturally distributable in quantum communication networks and a resource for measurement-based quantum computing.

Photonic integrated circuits (PICs) are key platforms for the compact and stable manipulation of classical and quantum light. Imperfections arising from fabrication constraints, tolerances, and operation wavelength limit the accuracy of intended operations on light and impede the practical utility of current PICs. In particular, crosstalk between reconfigurable phase shifters is challenging to characterize due to the large number of parameters to estimate and the difficulty in isolating individual parameters. Previous studies have attempted to model crosstalk solely as an interaction between controlled phase shifters, overlooking the broader scope of this issue. We introduce the concept of induced phase shifter, arising from crosstalk on bare waveguide sections as predicted by simulations, resulting in an exhaustive description and systematic analysis of crosstalk. We characterize induced phase shifters in physical devices using a machine learning-based method and propose a mitigation framework. This framework further allows to establish a criterion certifying that a given interferometer has a sufficient number of degrees of freedom adequately laid out to fully mitigate crosstalk. Our approach is experimentally validated on a 12-mode Clements interferometer. We demonstrate the efficacy of our extended crosstalk model to accurately recover physical crosstalk properties of the PIC and cancel induced phase shifters following our mitigation framework.

Light-matter interactions with quantum dots have been extensively studied to harness key quantum properties of photons, such as indistinguishability and entanglement. In this theoretical work, we exploit the atomic-like four-level structure of a quantum dot coupled to a waveguide to model a shaping frequency entangling gate (ShaFrEnGa) for single photons. Our approach is based on the identification of input frequencies and an atomic level structure for which frequency-dependent one-photon transitions are adiabatically eliminated, while frequency-dependent two-photon transitions are resonantly enhanced. The frequency entanglement performance of the gate is analyzed using a Schmidt decomposition for continuous variables, revealing a trade-off between entanglement generation efficiency and entanglement quality. We further demonstrate the use of the ShaFrEnGa for the generation of entangled frequency qudit states.

The quantum interference between a coherent state and a single photon is an important tool in continuous variable optical quantum technologies to characterize and engineer non-Gaussian quantum states. Semiconductor quantum dots (QDs), which have recently emerged as a key platform for efficient single-photon generation, could become interesting assets in this context. An essential parameter for interfering single photons and classical fields is the mean wavepacket overlap between both fields. Here, we report on two homodyne photon-correlation techniques enabling the precise measurement of the overlap between a single photon generated by a QD-cavity device and pulsed laser light. The different statistics of interfering fields lead to specific signatures of the quantum interference on the photon correlations at the output of the interfering beam splitter. We compare the behavior of maximized overlap, measuring either the Hong-Ou-Mandel visibility between both outputs or the photon bunching at a single output. Through careful tailoring of the laser light in various degrees of freedom, we achieve a record overlap of 76% with integrated solid-state sources, which evidences the very low level of noise in our integrated single-photon sources.

Generating identical photons from remote emitter-based bright single-photon sources is an important step for scaling up optical quantum technologies. Here, we study the Hong-Ou-Mandel interference of photons emitted from remote sources based on semiconductor quantum dots (QDs). We make use of a deterministic fabrication technique to position the QDs in a spectrally resonant micropillar cavity and fine-tune their operation wavelength electrically. In doing so, we can match four pairs of sources between five distinct sources, study them under various excitation schemes, and measure their degree of indistinguishability. We demonstrate remote indistinguishabiltity between 44 ± 1% and 69 ± 1% depending on the pair of sources and excitation conditions, record values for QDs in cavities. The relative contribution of pure dephasing and spectral diffusion is then analyzed, revealing that the remaining distinguishability is mostly due to low-frequency noise.

Self-assembled InGaAs/GaAs quantum dots (QDs) are of particular importance for the deterministic generation of spin-photon entanglement. One promising scheme relies on the Larmor precession of a spin in a transverse magnetic field, which is governed by the in-plane $g$-factors of the electron and valence band heavy-hole. We probe the origin of heavy-hole $g$-factor anisotropy with respect to the in-plane magnetic field direction and uncover how it impacts the entanglement generated between the spin and the photon polarization. First, using polarization-resolved photoluminescence measurements on a single QD, we determine that the impact of valence-band mixing dominates over effects due to a confinement-renormalized cubic Luttinger $q$ parameter. From this, we construct a comprehensive hole $g$-tensor model. We then use this model to simulate the concurrence and fidelity of spin-photon entanglement generation with anisotropic hole $g$-factors, which can be tuned via magnetic field angle and excitation polarization. The results demonstrate that post-growth control of the hole $g$-factor can be used to improve spin-photon cluster state generation.

The frequency or color of photons is an attractive degree of freedom to encode and distribute quantum information over long distances. However, the generation of frequency-encoded photonic qubits has so far relied on probabilistic nonlinear single-photon sources and inefficient gates. Here, we demonstrate the deterministic generation of photonic qubits hyper-encoded in frequency and polarization based on a semiconductor quantum dot in a cavity. We exploit the double dipole structure of a neutral exciton and demonstrate the generation of any quantum superposition in amplitude and phase, controlled by the polarization of the pump laser pulse. The source generates frequency-polarization single-photon qubits at a rate of 4 MHz corresponding to a generation probability at the first lens of 28 +/- 2%, with a photon number purity >98%. The photons show an indistinguishability >91% for each dipole and 88% for a balanced quantum superposition of both. The density matrix of the hyper-encoded photonic state is measured by time-resolved polarization tomography, evidencing a fidelity to the target state of 94 +/- 8% and concurrence of 77 +/- 2%, here limited by frequency overlap in our device. Our approach brings the advantages of quantum dot sources to the field of quantum information processing based on frequency encoding.

We create 6-photon linear cluster states using a resource-efficient quantum dot single photon source and linear optics setup. Shorter, high-rate, states created in the process give indications on the quality of alignment and interference for optimization.

Quantum emitters, such as atoms, defects in crystals, or quantum dots, are excellent sources of indistinguishable single-photons for quantum technologies. Upon coherent excitation, however, the emitted photonic state includes a vacuum component in a quantum superposition with the one-photon component. This feature has so far been largely disregarded in the framework of linear optical computing. Here we experimentally and theoretically study how the presence of photon-number coherence alters the foundation of photon-photon gates: the Hong-Ou-Mandel interference. We show that the presence of vacuum coherence not only introduces errors to standard photon indistinguishability measurements, but also results in complex quantum interference phenomena. These phenomena lead to additional entanglement that has profound impact on linear computing schemes, as we illustrate by simulating a heralded gate. Our work reveals the rich physics arising from photon-number coherence, which holds the potential to become an asset in future quantum protocols.