We report the demonstration of a bulk, intrinsically phase-stable source of polarization- and time-energy-entangled photon pairs at 810nm and 1550nm, directly coupled into single-mode optical fibers. This highly non-degenerate wavelength combination is well suited for hybrid quantum communication networks, enabling low-loss transmission in optical fibers at 1550nm while maintaining efficient free-space propagation and detection at 810nm. The source is based on spontaneous parametric down-conversion in a periodically poled lithium niobate crystal embedded in a polarization Sagnac interferometer, providing inherent stability and dual-degree-of-freedom entanglement. We measure a spectral brightness of B = 4800 pair/s/mW/GHz, with fiber coupling efficiencies exceeding 0.48 at both wavelengths. The entanglement quality is characterized by high-visibility two-photon interference, yielding net visibilities of 0.995 in the polarization basis and 0.991 in the energy-time basis. These performances demonstrate a compact and robust entanglement source compatible with hybrid fiber/free-space quantum key distribution architectures, and suitable for future ground-to-satellite quantum communication links.

Recent advances in quantum photonics have enabled increasingly robust protocols in optical phase estimation, achieving precisions beyond the standard quantum limit and approaching the Heisenberg limit. While intrinsic losses hinder the realization of unconditional super-sensitivity, reaching quantum advantage, defined as sensitivity surpassing that of any classical counterpart with identical resources, remains achievable. Here we experimentally demonstrate such an advantage using a fully fibered Mach-Zehnder-type interferometer operating at telecom wavelengths, free of post-selection. The scheme relies on the conversion of polarization-entangled photon pairs, a degree of freedom commonly favored for experimental convenience, into energy-time entanglement, which is particularly well suited for scalable fiber-based sensors. All system imperfections, including asymmetric losses and detector inefficiencies, are accounted for in the Fisher information analysis, yielding a measured quantum advantage of 10

Chromatic dispersion critically impacts the performance of numerous applications, ranging from telecommunication links to ultrafast optics and nonlinear devices, yet fast and precise measurements are challenging, especially for short length-dispersion products. We present a fully fiber-integrated nonlinear Sagnac interferometer that exploits cascaded second-order processes to generate frequency-anticorrelated idler light and achieve odd-order dispersion cancellation without active stabilization. The measurement is intrinsically self-referenced, as the dispersion-induced phase is extracted from the interference between counter-propagating nonlinear processes within the same Sagnac loop, eliminating the need for an external reference arm or prior calibration. Operating entirely at telecom wavelengths and read out on a standard optical spectrum analyzer, the device produces instantaneous, high-visibility fringes and calibration-free spectra using dual-port normalization. We demonstrate chromatic dispersion measurements on fiber samples ranging from 25 cm to 4 km, spanning short fiber segments to long-haul links. This architecture combines self-stability, broadband compatibility, and rapid acquisition, offering a practical metrology tool for both research and industry.

We report a direct demonstration of quantum-enhanced sensing in the Fourier domain by comparing single- and two-photon interference in a fiber-based interferometer under strictly identical noise conditions. The simultaneous acquisition of both signals provides a common-mode reference that enables a fair and unambiguous benchmark of quantum advantage. Spectral analysis of the interferometric outputs reveals that quantum correlations do not increase the amplitude of the modulation peak, but instead lower the associated noise floor, resulting in the expected 3 dB improvement in signal-to-noise ratio. This enhancement persists in the sub-shot-noise regime, where the classical signal becomes buried in the spectral background while the two-photon contribution remains resolvable. These observations establish Fourier-domain quantum super-sensitivity as an operational and broadly applicable resource for precision interferometric sensing.

Accurate knowledge of the uneven free spectral range of an optical microresonator, which provides direct insight into group velocity dispersion, is essential for understanding and controlling Kerr frequency comb dynamics. In this work, we present a simple and highly precise method for measuring the free spectral range over a 5 THz bandwidth in silicon nitride microresonators, leveraging a wavemeter with 0.4 MHz resolution. Our fully fibered plug-and-play experimental setup enables accurate extraction of resonance frequencies. By carefully analyzing the spectral position of each resonance, we measure both second- and third-order free spectral range expansion coefficients. This approach offers a robust and accessible tool for dispersion characterization in integrated photonic circuits, paving the way for next generation of Kerr comb sources and quantum photonic technologies.

The so-called Quantum Information Networks (QIN) promise to revolutionize the world with new applications based on the interconnection of quantum devices such as quantum computers, quantum sensors and physically secured cryptographic receivers. Such networks employ photons as a propagation mean of quantum information in order to create entanglement between the end-users’ devices. Over long distances, satellites will become mandatory in the network as they offer a better optical losses scaling than fibres. In this paper, we focus on the physical principle at the heart of the two-photon Bell-State Measurement devices used in the QIN to swap entanglement between and inside the network nodes: the Hong-Ou-Mandel effect.

Non-Gaussian quantum states and operations are essential tools for multiple quantum information protocols exploiting light as information career. In this context, a key role is played by schemes operating with continuous wave light, in which non-Gaussian states are obtained by photon subtraction/addition and eventually reconstructed by quantum state tomography. In these configurations, the temporal resolution of the homodyne detection and the digital data processing critically affect our ability to faithfully reconstruct the produced non-Gaussian states. In this work, we apply digital data processing to experimental data to study how the temporal performances of the detection chain affect the acquisition and treatment of tomographic data. This allows understanding how these features impact the quality of quantum states observed by non-ideal detection chains. By doing so, we discuss the actual constraints on the acquisition and reconstruction of non-Gaussian states by taking into account the limitations of realistic experimental resources.

Quantum-enhanced interferometry is often discussed in terms of ideal resources and asymptotic scalings, whereas in practice its performance is set by a delicate interplay between losses, state imbalance, and photon number. We address this interplay in a folded Franson interferometer fed with partially entangled N00N states, treating asymmetric losses and tunable input imbalance on equal footing. From exact detection probabilities we obtain closed-form expressions for the fringe visibility and the Fisher information, and show that these two figures of merit respond very differently to imperfections. In particular, we demonstrate that perfect interference contrast can always be recovered by compensating loss asymmetry with an appropriate input imbalance, while the Fisher information generally peaks at a distinct operating point, reflecting the irreducible trade-off between coherence restoration and signal attenuation. By determining the exact optima and benchmarking against single-photon strategies, we identify the critical loss and minimum entanglement required to maintain a genuine quantum advantage over optimized single-photon strategies under identical loss conditions, and establish their scaling with the photon number N . Beyond delineating the fundamental trade-offs between loss, entanglement, and sensitivity, this work establishes a comprehensive theoretical framework that both underpins and extends the experimental demonstration of quantum advantage reported in [1], providing a unified description of the relevant operating regimes.

Quantum frequency converters are key enabling technologies in photonic quantum information science to bridge the gap between quantum emitters and telecom photons. Here, we report a coherent frequency converter scheme combining a fiber-coupled nonlinear optical lithium niobate waveguide with a fiber-pigtailed single-photon source based on semiconductor quantum dots. Single and indistinguishable photons are converted from 925.7 nm to the telecommunication C-band, with a 48.4% end-to-end efficiency and full preservation of single-photon purity and indistinguishability. The integration of the two fiber-based modules achieving top-level performance represents an important step toward the practical interconnection of future quantum information processing systems operating at different wavelengths.

We demonstrate a 100 km entanglement-based quantum key distribution link with full automation and stability, bridging terrestrial and satellite-compatible infrastructures for future large-scale quantum networks.

A fully fibered source of entangled photon pairs based on a nonlinear Sagnac interferometer is reported. Operating at telecom wavelengths, the source relies exclusively on standard fiber-optic components and periodically poled lithium niobate (PPLN) waveguides, resulting in a compact, robust, and field-deployable architecture. The generation stage supports both polarization and energy-time entanglement without modification, enabling versatile operation depending on the targeted application. Broadband spontaneous parametric down-conversion allows dense wavelength-division multiplexing over the telecom C and L bands. High normalized brightness (10.3 kpairs/s/nm/mW^2) is achieved on a standard 100 GHz ITU channel pair, together with high entanglement quality. Polarization and energy-time encodings are characterized through state tomography and two-photon interference measurements, yielding fidelities, purities, and visibilities exceeding 96

Fiber-optic resonator sensors enable high-sensitivity measurements of various physical parameters. Here, we demonstrate a quantum-enhanced interrogation scheme of an all-fiber-optic Fabry-Pèrot resonator, employing entangled two-photon interference to probe directly strain-induced optical phase shifts. The resonator is interrogated by a narrow-linewidth laser frequency-locked to its resonance mode, thereby exploiting the cavity’s inherent sensitivity to variations in the fiber length and refractive index. To access the phase fluctuations transferred to the interrogation field due to external perturbations on the sensor, we implement an in-fiber Mach–Zehnder interferometer readout. This configuration allows the sensor to operate both with classical coherent light and with entangled photon pairs generated via spontaneous parametric down-conversion. Harnessing two-photon entangled states corresponding to an N = 2 N00N state, we experimentally demonstrate a reduction of the measurement noise below the standard quantum limit. These results provide clear evidence of the quantum advantage in enhancing fiber-optic sensors under practical constraints on optical power

This experimental work demonstrates multipartite quantum correlation in bright frequency combs out of a microresonator integrated on silicon nitride operating above its oscillation threshold. Multipartite features, going beyond so far reported two-mode correlation, naturally arise due to a cascade of nonlinear optical processes, making a single-color laser pump sufficient to initiate their generation. Our results show the transition from two-mode to multipartite correlation, witnessed by noise reductions as low as −2.5dB and −2dB, respectively, compared to corresponding classical levels. A constant of the movement of the nonlinear interaction Hamiltonian is identified and used to asses the multipartite behavior. Reported demonstrations pave the way to next generation on-chip multipartite sources for quantum technologies applications.

The interference between two independent photons stands as a crucial aspect of numerous quantum information protocols and technologies. In this work, we leverage fiber-coupled devices, which encompass fibered photon pair-sources and off-the-shelf optics, to demonstrate Hong-Ou-Mandel interference. We employ two distinct single photon sources, namely a heralded single-photon source and a weak coherent laser source, both operating asynchronously in continuous-wave regime. We record two-photon coincidences, showing a state-of-art visibility of 91.9(5)%. This work, compliant with telecom technology, provides realistic backbones for establishing long-range communication based on quantum teleportation in hybrid quantum networks.

Quantum interferometry methods exploit quantum resources, such as photonic entanglement, to enhance phase estimation beyond classical limits. Nonlinear optics has served as a workhorse for the generation of entangled photon pairs, ensuring both energy and phase conservation, but at the cost of limited rate and degraded signal-to-noise ratio compared to laser-based interferometry approaches. We present a “quantum-like” nonlinear optical method that reaches super-resolution in classical detection regime. This is achieved by replacing photon-pairs by coherent states of light, mimicking quantum properties through classical nonlinear optics processes. Our scheme utilizes two high-brightness lasers. This results in a substantially greater signal-to-noise ratio compared to its quantum counterpart. Such an approach paves the way to significantly reduced acquisition times, providing a pathway to explore signals across a broader range of bandwidth. The need to increase the frequency bandwidth of the quantum sensor significantly motivates the potential applications of this pathway.

Entanglement-based quantum links are the backbone of future quantum internet networks, enabling secure communication between distant cities. Realizing such networks requires addressing multiple practical challenges in long-distance quantum key distribution : time synchronisation, interferometer stabilisation and automation. Here, we report several advances. First, we maintained an operational QKD link continuously for 325hours over 50km between two remotes locations, demonstrating the feasibility of long-duration key generation. We further extended secure key distribution up to a 100km operational link connecting the University of Nice to a ground-based optical station, a setup compatible with future quantum satellite connections. Finally, by employing wavelength demultiplexing to separate photons of entangled pairs, we performed QKD across multiple ITU channels, achieving secure key exchange via the BBM92 protocol and time-energy observables.

We show several quantum-sensing experimental realizations dedicated to the qualification of optical-material key-properties, such as chromatic dispersion and refractive index variations. More specifically, the material under test (typically, an optical fibre or a nonlinear crystal) is introduced in one of the two arms or in the loop of interferometric devices, mainly in Mach-Zehnder and Sagnac configurations. Probing such devices using peculiar quantum, or quantum-mimicking, photonic states creates material-under-test-dependent relative-phase differences that are imprinted on the established coherent superposition. Here, the employed photonic states exhibit specific modal structures, in the frequency and/or spatial domains, that exploit or mimic quantum entanglement, providing access to the relative phases and to the optical properties under investigation. Associated measurement results show unprecedented precision and/or accuracy, reduced overhead, and otherwise unreachable stability and repeatability. Such quantum interferometric methods pave the way to redefining the very way optical materials are characterized, finding repercussion in various fields ranging from fundamental optics to industry-grade developments.

We present the implementation of a time synchronization protocol as part of an experimentally deployed entanglement-based quantum key distribution (QKD) link. The system is deployed over 48 km of optical fibers across the Métropole Côte d'Azur and enables secret cryptographic key exchange between two remote users, with an average rate of 7 kbps. We exploit the time correlation of paired photons generated by a high-quality source of energy-time entanglement implemented in the QKD link to synchronize two rubidium clocks located at the end stations. The level of stability achieved guarantees a time offset between the clocks under 12 ps at all time. We also show that this protocol requires less hardware than a typical synchronization protocol for QKD that would distribute a reference clock signal between the users.

We are developing integrated source-emitting photon pairs in the MIR and IR ranges. This source is the core component of a spectroscopic technique based on non-linear interferometry, which we have successfully demonstrated.