Charles Roques-Carmes

While partial coherence often degrades interference and information transfer, its correlations can be engineered to enhance imaging and optical information processing. However, approaches based on tomography and numerical reconstruction do not physically separate unknown fields into their coherence modes—that is, their orthogonal and mutually incoherent components. Here we demonstrate the separation of partially coherent light into coherence modes. We use variational processing in layered self-configuring interferometers on a silicon photonic circuit. This process finds the eigenvectors and eigenvalues of the coherency matrix, characterizing the state while leaving it intact and separated after optimization. We show that spatially orthogonal, mutually incoherent beams can be routed to separate outputs even when their intensity profiles overlap. Our experiment separates the two strongest coherence modes from a nine-input sampling of overlapping fields from two independent lasers; operation beyond two modes is verified in a rank-3 experiment. The number of physical components scales linearly with the rank r of the coherency matrix, through r in situ gradient-based optimizations enabled by electronic drive-frequency multiplexing of the phase shifters. We benchmark performance against an on-chip mixture-based tomographic method. These results establish a scalable framework for programmable coherence analysis and control.

Topological quasiparticles of light, such as classical and quantum optical skyrmions, have so far relied on fully coherent or pure quantum states whose topology is encoded in the entanglement between polarization and two-dimensional spatial modes. Here, we show that skyrmionic topology can emerge directly within the density matrix of a mixed quantum state. We introduce a framework in which a coherence-Stokes vector defines a topological texture over the density matrix, enabling the realization of quantum skyrmions using only a pseudospin and a real or synthetic one-dimensional space of modes. For a single photon, the density matrix is analogous to the coherence matrix of classical light and can be encoded using partially coherent electromagnetic fields. We further analyze the topological texture encoded in a bipartite entangled photon pair, showing how skyrmions arise not only in the full biphoton system but also in its reduced subspaces with any pseudospin-mode combination simultaneously. We then explore the robustness of such skyrmions to environmental noise and discover their persistence in mixed-quantum states of multiple photons. Finally, we propose a feasible experimental route to generate and measure such skyrmions using integrated photonic networks and suggest avenues for similar implementations in other quantum systems. Our work paves the way for robustly encoding classical information on partially coherent light or on mixed quantum states and for encoding topological charges on many-body quantum systems.

We exploit a recently proposed concept for a programmable, mechanically robust, and tunable nonresonant silicon photonic integrated circuit filter and experimentally demonstrate it for tuning an erbium-doped fiber laser ring cavity. This filter integrates forward-only interferometer meshes together with a waveguide delay line array for fully programmable and tunable filtering. Unlike more popular microresonator-based filters, this approach offers transmission function engineering, broad free spectral range and smooth tuning without matching individual resonances, and extension to multiwavelength operation. Moreover, this filter can be readily fabricated in a commercial foundry with standard dispersive components. For a first experimental verification, we designed a filter with a 100 GHz (∼0.8 nm) free spectral range and a 25 GHz (∼0.2 nm) bandpass width at center wavelengths around 1550 nm. We show numerically that dispersion effects are negligible across >40 nm bandwidth aside from grating coupler loss. Experimentally, we confirm single-wavelength laser operation with a continuous tuning range of ∼0.6 nm around 1555 nm and output power up to 25 mW.

Polarization-resolved control and measurement of optical fields are essential for a wide range of photonic systems, including coherent communication, polarimetric sensing, and quantum information processing. We present a photonic integrated circuit architecture that enables the generation and analysis of arbitrary polarization states. The device provides reconfigurable access to the full polarization degree of freedom of coherent light within a single integrated platform. We experimentally demonstrate arbitrary polarization state generation spanning the Poincaré sphere, as well as Stokes vector measurement on a chip. Unlike conventional Stokes measurements that rely on direct detection, the polarization state in this architecture is inferred from the phase settings required to interferometrically combine polarization-demultiplexed optical fields. As a result, the optical signal itself does not need to be detected or absorbed and remains available for subsequent optical-domain processing. The devices are fabricated in a commercial foundry using CMOS-compatible processes, enabling scalable and reproducible integration. By combining polarization generation and analysis in a compact and stable photonic circuit, this work eliminates the need for external polarization optics and provides a foundation for robust, polarization-enabled photonic integrated systems.

Scattering experiments with energetic particles, such as free electrons, have been historically used to reveal the quantum structure of matter. However, realizing coherent interactions between free-electron beams and solid-state quantum systems has remained out of reach, owing to their intrinsically weak coupling. Realizing such coherent control would open up opportunities for hybrid quantum platforms combining free electrons and solid-state qubits for coincident quantum information processing and nanoscale sensing. Here, we present a framework that employs negatively charged nitrogen-vacancy centers (NV ⁻ ) in diamond as quantum sensors of a bunched electron beam. We develop a Lindblad master equation description of the magnetic free-electron–qubit interactions and identify spin relaxometry as a sensitive probe of the interaction. Experimentally, we integrate a confocal fluorescence microscopy setup into a microwave-bunched electron beam line. We monitor charge-state dynamics and assess their impact on key sensing performance metrics (such as spin readout contrast), defining safe operating parameters for quantum sensing experiments. By performing T 1 relaxometry under controlled electron beam exposure, we do not resolve a measurable reduction in T 1 within experimental uncertainty, and instead establish an upper bound on the free-electron–spin coupling strength. Our results establish NV ⁻ centers as quantitative probes of free electrons, providing a metrological benchmark for free-electron–qubit coupling under realistic conditions, and chart a route toward solid-state quantum control with electron beams.

Integrated multimode quantum optics is a promising platform for scalable continuous-variable quantum technologies leveraging multimode squeezing in both the spatial and spectral domains. However, on-chip measurement, routing and processing the relevant “supermodes” over which the squeezing resource is distributed still scales quadratically with the number of modes N , causing a rapid increase in photonic circuit size and number of required measurements. Here, we introduce a variational scheme, relying on self-configuring photonic networks (SCNs) that learn and extract the most-squeezed supermodes sequentially, reducing both the circuit size and the experimental overhead. Using homodyne measurement as a cost function, a sparse SCN discovers the l ≪ N most significant supermodes using O ( l N ) physical elements and optimization steps. We analyze and numerically simulate these architectures for both real-space and frequency-domain implementations, showing a fidelity close to unity between the learned circuit and the supermode decomposition, even in the presence of optical losses and detection noise. In the frequency domain, we show that circuit size can be further reduced by using inverse-designed surrogate networks, which emulate the layers learned thus far. Using two different frequency encoding schemes—uniformly and nonuniformly spaced frequency bins—we reduce an entire network (learning all N supermodes) to O ( N ) and even O ( 1 ) modulated cavities. Our results point toward chip-scale, resource-efficient quantum processing units and demultiplexers for continuous variable processing in multimode quantum optics, with applications ranging from quantum communication, metrology, and computation.

Non-Hermiticity naturally arises in physical systems that exchange energy with their environment. The presence of non-Hermiticity leads to many topological physics phenomena and device applications. In the non-Hermitian energy band theory, the foundation of these physics and applications, both energies and wave vectors take complex values. The energy bands thus become a Riemann surface, and such an energy-band Riemann surface underlies all important signatures of non-Hermitian topology. Despite a long history and recent theoretical interests, the energy-band Riemann surface has not been experimentally studied. Here, we provide a photonic observation of the energy-band Riemann surface of a non-Hermitian system. This is achieved by a tunable imaginary gauge transformation in photonic synthetic frequency dimensions. From measured topologies of the Riemann surface, we reveal the complex-energy winding, the open-boundary-condition spectrum, the generalized Brillouin zone, and the branch points. Our findings demonstrate a unified framework in the studies of diverse effects in non-Hermitian topological physics through an experimental observation of energy-band Riemann surfaces.