@article{64877, author = {{Taheri, Behnood and Kopylov, Denis and Hammer, Manfred and Meier, Torsten and Förstner, Jens and Sharapova, Polina R.}}, journal = {{arXiv}}, title = {{{Gain-induced spectral non-degeneracy in type-II parametric down-conversion}}}, doi = {{10.48550/ARXIV.2603.01656}}, year = {{2026}}, } @article{65575, abstract = {{For the ever-growing field of quantum information processing, large-scale, efficient multiport interferometers serving as photonic processors are required. In this context, the suitability of quantum walks as the interferometric base for universal computation has been theoretically proven. In this work, we bridge the gap between theoretical proposals and state-of-the-art experimental capabilities by providing the recipe for the implementation of a universal photonic processor in discrete-time quantum walks. Specifically, we present the protocol for translating arbitrary linear transformations into the coin and step operator of a quantum walk and map these to the experimental parameters of the established time-multiplexed platform [A. Schreiber , Phys. Rev. Lett. , 050502 (2010)]. We show that our interface is highly scalable and resource efficient due to the hybrid encoding consisting of multiple degrees of freedom. Finally, we prove that our system is highly resilient against experimental imperfections and show that it compares favorably against existing architectures.}}, author = {{Lammers, Jonas and Ares, Laura and Pegoraro, Federico and Held, Philip and Brecht, Benjamin and Sperling, Jan and Silberhorn, Christine}}, issn = {{2331-7019}}, journal = {{Physical Review Applied}}, number = {{5}}, publisher = {{American Physical Society (APS)}}, title = {{{Resource-efficient universal photonic processors based on time-multiplexed hybrid architectures}}}, doi = {{10.1103/x99y-2sms}}, volume = {{25}}, year = {{2026}}, } @article{65574, author = {{Pinske, Julien and Sperling, Jan and Mølmer, Klaus}}, issn = {{2469-9926}}, journal = {{Physical Review A}}, number = {{5}}, publisher = {{American Physical Society (APS)}}, title = {{{Entangling power of nonentangling channels}}}, doi = {{10.1103/vy93-dnc8}}, volume = {{113}}, year = {{2026}}, } @article{65777, abstract = {{Abstract In this work, we address the numerical identification of entanglement in dynamical scenarios. To this end, we consider different programs based on the restriction of the evolution to the set of separable (i.e., non-entangled) states, together with the discretization of the space of variables for numerical computations. As a first approach, we apply linear splitting methods to the restricted, continuous equations of motion derived from variational principles. We utilize an exchange interaction Hamiltonian to confirm that the numerical and analytical solutions coincide in the limit of small time steps. The application to different Hamiltonians shows the wide applicability of the method to detect dynamical entanglement. To avoid the derivation of analytical solutions for complex dynamics, we consider variational, numerical integration schemes, introducing a variational discretization for Lagrangians linear in velocities. Here, we examine and compare two approaches: one in which the system is discretized before the restriction is applied, and another in which the restriction precedes the discretization. We find that the ‘first-discretize-then-restrict’ method becomes numerically unstable, already for the example of an exchange-interaction Hamiltonian, which can be an important consideration for the numerical analysis of constrained quantum dynamics. Thereby, broadly applicable numerical tools, including their limitations, for studying entanglement over time are established for assessing the entangling power of processes that are used in quantum information theory.}}, author = {{Offen, Christian and Wembe Moafo, Boris Edgar and Ares, Laura and Sperling, Jan and Ober-Blöbaum, Sina}}, issn = {{1751-8113}}, journal = {{Journal of Physics A: Mathematical and Theoretical}}, number = {{22}}, publisher = {{IOP Publishing}}, title = {{{Numerical approaches to entangling dynamics from variational principles}}}, doi = {{10.1088/1751-8121/ae6d51}}, volume = {{59}}, year = {{2026}}, } @article{66665, abstract = {{Multimode quantum light has promising applications in many areas of physics, such as quantum communications and quantum computing. However, its multimode nature also makes it challenging to measure its properties. Recently [I. Barakat et al., Optica Quantum 3, 36 (2025)], a technique for the simultaneous measurement of squeezing of multiple broadband modes based on a phase-sensitive amplification approach was experimentally implemented using a setup that effectively corresponds to an SU(1,1) interferometer. Here, we aim to provide a complete theoretical analysis of the modal structure of (generally unbalanced) SU(1,1) interferometers and a detailed theoretical formal derivation of the framework for this technique. Utilizing the joint Schmidt decomposition of the transfer functions, we investigate the shape and phase profiles of the modes of the SU(1,1) interferometer and its components [parametric down-conversion (PDC) sections] for different parametric gain regimes. We discover a complicated interplay between the PDC modes and the modes of the entire interferometer, and analyze it by using their overlap coefficients as a similarity measure. Finally, we develop a rigorous processing method for the aforementioned multimode squeezing measurement technique and discuss necessary approximations to make this method experimentally feasible.}}, author = {{Scharwald, Dennis and Sharapova, Polina}}, issn = {{2643-1564}}, journal = {{Physical Review Research}}, number = {{3}}, publisher = {{American Physical Society (APS)}}, title = {{{Characterization of spatial Schmidt modes in high-gain SU(1,1) interferometers}}}, doi = {{10.1103/ph64-ts39}}, volume = {{8}}, year = {{2026}}, } @article{63744, abstract = {{Orbital angular momentum (OAM) modes are an important resource used in various branches of quantum science and technology due to their unique helical structure and countably infinite basis. Generating light that simultaneously carries high-order orbital angular momenta and exhibits quantum correlations is a challenging task. In this work, we present a theoretical approach to the generation of correlated Schmidt modes carrying OAM via parametric down-conversion (PDC) in cascaded nonlinear systems (nonlinear interferometers) pumped by Laguerre–Gaussian beams. We demonstrate how the number of generated modes and their population can be controlled by varying the pump parameters, the gain of the PDC process, and the distance between the crystals. We investigate the angular displacement measurement uncertainty of these interferometers and demonstrate that it can overcome the classical shot noise limit.}}, author = {{Scharwald, Dennis and Gehse, Lucas and Sharapova, Polina}}, issn = {{2378-0967}}, journal = {{APL Photonics}}, number = {{1}}, publisher = {{AIP Publishing}}, title = {{{Schmidt modes carrying orbital angular momentum generated by cascaded systems pumped with Laguerre–Gaussian beams}}}, doi = {{10.1063/5.0229802}}, volume = {{10}}, year = {{2025}}, } @article{63745, abstract = {{Multimode squeezed light is an increasingly popular tool in photonic quantum technologies, including sensing, imaging, and computation. Meanwhile, the existing methods of its characterization are technically complicated, which reduces the level of squeezing, and mostly deal with a single mode at a time. Here, for the first time, to the best of our knowledge, we employ optical parametric amplification to characterize multiple squeezing eigenmodes simultaneously. We retrieve the shapes and squeezing degrees of all modes at once through direct detection followed by modal decomposition. This method is tolerant to inefficient detection and does not require a local oscillator. For a spectrally and spatially multimode squeezed vacuum, we characterize eight strongest spatial modes, obtaining squeezing and anti-squeezing values of up to −5.2 ± 0.2 dB and 8.6 ± 0.3 dB, respectively, despite the 50% detection loss. This work, being the first exploration of an optical parametric amplifier’s multimode capability for squeezing detection, paves the way for the real-time detection of multimode squeezing.}}, author = {{Barakat, Ismail and Kalash, Mahmoud and Scharwald, Dennis and Sharapova, Polina and Lindlein, Norbert and Chekhova, Maria}}, issn = {{2837-6714}}, journal = {{Optica Quantum}}, number = {{1}}, publisher = {{Optica Publishing Group}}, title = {{{Simultaneous measurement of multimode squeezing through multimode phase-sensitive amplification}}}, doi = {{10.1364/opticaq.524682}}, volume = {{3}}, year = {{2025}}, } @article{61245, author = {{Barkhausen, Franziska and Ares Santos, Laura and Schumacher, Stefan and Sperling, Jan}}, issn = {{2469-9926}}, journal = {{Physical Review A}}, number = {{3}}, publisher = {{American Physical Society (APS)}}, title = {{{Entanglement between dependent degrees of freedom: Quasiparticle correlations}}}, doi = {{10.1103/physreva.111.032404}}, volume = {{111}}, year = {{2025}}, } @article{61351, abstract = {{AbstractThe interaction of water molecules with semiconductor surfaces is relevant to various optoelectronic phenomena and physicochemical processes. Despite advances in fundamental understanding of water‐exposed surfaces, the detailed time‐ and energy‐resolved behavior of excited electrons remains largely unexplored. Here, the effects of water exposure on the near‐surface electron dynamics of phosphorus‐terminated p(2×2)/c(4×2)‐reconstructed indium phosphide (100) (P‐rich InP) are studied experimentally and matched to theoretical calculations. The P‐rich InP surface, consisting of H‐passivated P‐dimers, serves as a model for other P‐containing III‐V semiconductors such as gallium phosphide (GaP) or aluminum indium phosphide (AlInP). Electron dynamics near the surface are probed with femtosecond resolution using time‐resolved two‐photon photoemission (tr‐2PPE), a pump‐probe spectroscopic technique. Pulsed water exposure preserves electronic states and significantly increases lifetimes at the conduction band minimum (CBM). Density‐functional theory (DFT) calculations attribute these findings to suppression of surface vibrational modes in the top P‐layer by water exposure, reducing electronic transition probabilities of near‐band‐gap surface states. The results suggest that many near‐surface state lifetimes reported in ultra‐high vacuum may change significantly upon electrolyte exposure. These states may thus contribute more strongly to surface reactions than traditionally assumed. Demonstrating this effect for the technologically relevant P‐rich InP surface opens new opportunities in this underexplored area of surface electrochemistry.}}, author = {{Diederich, Jonathan and Paszuk, Agnieszka and Ruiz Alvarado, Isaac Azahel and Krenz, Marvin and Zare Pour, Mohammad Amin and Babu, Diwakar Suresh and Velazquez Rojas, Jennifer and Höhn, Christian and Gao, Yuying and Schwarzburg, Klaus and Ostheimer, David and Eichberger, Rainer and Schmidt, Wolf Gero and Hannappel, Thomas and de Krol, Roel van and Friedrich, Dennis}}, issn = {{2196-7350}}, journal = {{Advanced Materials Interfaces}}, number = {{16}}, publisher = {{Wiley}}, title = {{{Ultrafast Electron Dynamics at the Water‐Modified InP(100) Surface}}}, doi = {{10.1002/admi.202500463}}, volume = {{12}}, year = {{2025}}, } @article{61356, abstract = {{First-principles calculations reveal how topological defects in semiconducting carbon nanotubes trap triplet excitons and enable single-photon emission at telecom wavelengths, offering new insights into their potential for photonic devices.}}, author = {{Biktagirov, Timur and Gerstmann, Uwe and Schmidt, Wolf Gero}}, issn = {{2040-3364}}, journal = {{Nanoscale}}, number = {{11}}, pages = {{6884--6891}}, publisher = {{Royal Society of Chemistry (RSC)}}, title = {{{Topological defects in semiconducting carbon nanotubes as triplet exciton traps and single-photon emitters}}}, doi = {{10.1039/d4nr03904a}}, volume = {{17}}, year = {{2025}}, } @article{60891, abstract = {{Straight dielectric waveguide channels made from slabs of thin-film lithium niobate (TFLN), or lithium niobate on insulator (LNOI), are investigated in the linear regime, for channels of rib and strip type with common trapezoidal cross sections, in Z-cut and X-cut samples at varying on-chip orientation. We clarify the theoretical basis for the waveguides with potentially non-diagonal core permittivity. Symmetry classes can be distinguished that differ in their consequences for potential modal degeneracy and polarization conversion. Our rigorous numerical analysis by means of a finite-element solver takes the anisotropy of the lithium niobate cores rigorously into account. We discuss extensive data for effective indices, polarization properties, and hybridization of guided modes, in single- and multimode channels. Scans over the waveguide width and orientation as primary parameters are complemented by a series of illustrations of vectorial mode profiles. These turn out to be essentially complex in cases of X-cut channels at non-crystal-axis-aligned orientations.}}, author = {{Hammer, Manfred and Khan, Shahriar and Taheri, Behnood and Farheen, Henna and Förstner, Jens}}, issn = {{2770-0208}}, journal = {{Optics Continuum}}, keywords = {{tet_topic_waveguide}}, number = {{10}}, pages = {{2356}}, publisher = {{Optica Publishing Group}}, title = {{{TFLN channel waveguides of rib and strip type: Properties of guided modes}}}, doi = {{10.1364/optcon.569959}}, volume = {{4}}, year = {{2025}}, } @inproceedings{61760, abstract = {{We present a topology-optimized silicon nitride (Si3N4) coupler designed to enhance the coupling efficiency between integrated single-photon emitters and photonic waveguide modes. By leveraging inverse design techniques, we optimize the coupler’s geometry to maximize power transfer while maintaining fabrication feasibility by improving mode overlap and directional emission, addressing the challenge of low coupling efficiency caused by size mismatch and material incompatibility. Simulations demonstrate a substantial enhancement in photon extraction and waveguide coupling. This approach can be extended to other photonic devices, offering a versatile framework for improving quantum light-matter interactions in integrated photonics.}}, author = {{Farheen, Henna and Chen, Yuheng and Chen, Peigang and Kryvobok, Artem and Peana, Samuel and Senichev, Alexander and Shalaev, Vladimir M. and Boltasseva, Alexandra and Förstner, Jens and Kildishev, Alexander V.}}, booktitle = {{Photonic Computing: From Materials and Devices to Systems and Applications II}}, editor = {{Ni, Xingjie and Cai, Wenshan}}, keywords = {{tet_topic_waveguide}}, publisher = {{SPIE}}, title = {{{Topology-optimized silicon nitride coupler for integrated single-photon emitters}}}, doi = {{10.1117/12.3065734}}, year = {{2025}}, } @article{62911, abstract = {{In this paper, we theoretically study the spectral and temporal properties of pulsed spontaneous parametric down-conversion (SPDC) generated in lossy waveguides. Our theoretical approach is based on the formalism of Gaussian states and the Langevin equation, which is elaborated for weak parametric down-conversion and photon-number-unresolved click detection. Using the example of frequency-degenerate type-II SPDC generated under the pump-idler group-velocity-matching condition, we show how the joint-spectral intensity, mode structure, normalized second-order correlation function, and Hong-Ou-Mandel interference pattern depend on internal losses of the SPDC process. We found that the joint-spectral intensity is almost insensitive to internal losses, while the second-order correlation function shows a strong dependence on them, being different for the signal and idler beams in the presence of internal losses. Based on the sensitivity of the normalized second-order correlation function, we show how its measurement can be used to experimentally determine internal losses.}}, author = {{Kopylov, Denis A. and Stefszky, Michael and Meier, Torsten and Silberhorn, Christine and Sharapova, Polina R.}}, issn = {{2643-1564}}, journal = {{Physical Review Research}}, number = {{3}}, publisher = {{American Physical Society (APS)}}, title = {{{Spectral and temporal properties of type-II parametric down-conversion: The impact of losses during state generation}}}, doi = {{10.1103/zp72-7qwl}}, volume = {{7}}, year = {{2025}}, } @article{60568, author = {{Bocchini, Adriana and Kollmann, S. and Gerstmann, Uwe and Schmidt, Wolf Gero and Grundmeier, Guido}}, issn = {{0039-6028}}, journal = {{Surface Science}}, publisher = {{Elsevier BV}}, title = {{{Phosphonic acid adsorption on α-Bi2O3 surfaces}}}, doi = {{10.1016/j.susc.2025.122776}}, volume = {{760}}, year = {{2025}}, } @article{61353, abstract = {{Abstract Muonic hydrogen is an exotic atom where a muon instead of an electron is bound to a proton. The comparably high mass of the muon (≈ 207 · me ) has two important effects, (i) the reduced mass of the system becomes more important, and (ii) the muon is localized much closer to the nucleus. Thus, muonic hydrogen is not only excellently suitable for evaluating highly precise quantum electrodynamic (QED) calculations, but may also be used for assessing new approaches including finite nuclear size (FNS) effects to evaluate the proton structure and improve calculation schemes for the hyperfine splittings of many-particle systems, as e.g. to be implemented in density functional theory (DFT) software packages. Here, starting from Dirac’s equation we calculate the relativistic hyperfine splitting of the ground state and several excited states of muonic hydrogen analytically for different charge and magnetization models. The FNS related hyperfine shifts are compared with the differences between QED calculations and experimental measurements. This comparison also allows to unravel the role of the reduced mass, which is on one hand crucial in case of muonic atoms, but on the other hand is by no means well defined in relativistic quantum mechanics.}}, author = {{Franzke, Katharina L. and Schmidt, Wolf Gero and Gerstmann, Uwe}}, issn = {{1742-6588}}, journal = {{Journal of Physics: Conference Series}}, number = {{1}}, publisher = {{IOP Publishing}}, title = {{{Finite-size and relativistic effects onto hyperfine interaction of muonic hydrogen}}}, doi = {{10.1088/1742-6596/3027/1/012001}}, volume = {{3027}}, year = {{2025}}, } @article{60992, abstract = {{Non-Hermitian systems hosting exceptional points (EPs) exhibit enhanced sensitivity and unconventional mode dynamics. Going beyond isolated EPs, here we report on the existence of exceptional rings (ERs) in planar optical resonators with specific form of circular dichroism and TE-TM splitting. Such exceptional rings possess intriguing topologies as discussed earlier for condensed matter systems, but they remain virtually unexplored in presence of nonlinearity, for which our photonic platform is ideal. We find that when Kerr-type nonlinearity (or saturable gain) is introduced, the linear ER splits into two concentric ERs, with the larger-radius ring being a ring of third-order EPs. Transitioning from linear to nonlinear regime, we present a rigorous analysis of spectral topology and report enhanced and adjustable perturbation response in the nonlinear regime. Whereas certain features are specific to our system, the results on non-Hermitian spectral topology and nonlinearity-enhanced perturbation response are generic and equally relevant to a broad class of other nonlinear non-Hermitian systems, providing a universal framework for engineering ERs and EPs in nonlinear non-Hermitian systems.}}, author = {{Wingenbach, Jan and Ares Santos, Laura and Ma, Xuekai and Sperling, Jan and Schumacher, Stefan}}, journal = {{Arxiv}}, publisher = {{Arxiv}}, title = {{{Sensitivity and Topology of Exceptional Rings in Nonlinear Non-Hermitian Planar Optical Microcavities}}}, doi = {{10.48550/ARXIV.2507.07099}}, year = {{2025}}, } @article{62980, abstract = {{We introduce a new classification of multimode states with a fixed number of photons. This classification is based on the factorizability of homogeneous multivariate polynomials and is invariant under unitary transformations. The classes physically correspond to field excitations in terms of single and multiple photons, each of which is in an arbitrary irreducible superposition of quantized modes. We further show how the transitions between classes are rendered possible by photon addition, photon subtraction, and photon-projection nonlinearities. We explicitly put forward a design for a multilayer interferometer in which the states for different classes can be generated with state-of-the-art experimental techniques. Limitations of the proposed designs are analyzed using the introduced classification, providing a benchmark for the robustness of certain states and classes.}}, author = {{Kopylov, Denis A. and Offen, Christian and Ares, Laura and Wembe Moafo, Boris Edgar and Ober-Blöbaum, Sina and Meier, Torsten and Sharapova, Polina R. and Sperling, Jan}}, issn = {{2643-1564}}, journal = {{Physical Review Research}}, number = {{3}}, publisher = {{American Physical Society (APS)}}, title = {{{Multiphoton, multimode state classification for nonlinear optical circuits}}}, doi = {{10.1103/sv6z-v1gk}}, volume = {{7}}, year = {{2025}}, } @article{63021, abstract = {{Bell measurements, entailing the projection onto one of the Bell states, play a key role in quantum information and communication, where the outcome of a variety of protocols crucially depends on the success probability of such measurements. Although in the case of qubit systems, Bell measurements can be implemented using only linear optical components, the same result is no longer true for qudits, where at least the use of ancillary photons is required. In order to circumvent this limitation, one possibility is to introduce nonlinear effects. In this work, we adopt the latter approach and propose a scalable Bell measurement scheme for high-dimensional states, exploiting multiple squeezer devices applied to a linear optical circuit for discriminating the different Bell states. Our approach does not require ancillary photons, is not limited by the dimension of the quantum states, and is experimentally scalable, thus paving the way toward the realization of an effective high-dimensional Bell measurement.}}, author = {{Bianchi, Luca and Marconi, Carlo and Sperling, Jan and Bacco, Davide}}, issn = {{2643-1564}}, journal = {{Physical Review Research}}, number = {{2}}, publisher = {{American Physical Society (APS)}}, title = {{{Predetection squeezing as a resource for high-dimensional Bell-state measurements}}}, doi = {{10.1103/physrevresearch.7.023038}}, volume = {{7}}, year = {{2025}}, } @article{62859, abstract = {{Frequency-filtered photon correlations have been proven to be extremely useful in grasping how the detection process alters photon statistics. Harnessing the spectral correlations also permits refinement of the emission and unraveling of previously hidden strong correlations in a plethora of quantum-optical systems under continuous-wave excitation. In this work, we investigate such correlations for time-dependent excitation and develop a methodology to compute efficiently time-integrated correlations, which are at the heart of the photon-counting theory, and subsequently apply it to analyze the photon emission of pulsed systems. By combining this formalism with the —which facilitates frequency-resolved correlations—we demonstrate how spectral filtering enhances single-photon purity and suppresses multiphoton noise in time-bin-encoded quantum states. Specifically, filtering the central spectral peak of a dynamically driven two-level system boosts temporal coherence and improves the fidelity of time-bin entanglement preparation, even under conditions favoring multiphoton emission. These results establish spectral filtering as a critical tool for tailoring photon statistics in pulsed quantum light sources.}}, author = {{Bermúdez-Feijóo, Santiago and Zubizarreta Casalengua, Eduardo and Müller, Kai and Jöns, Klaus}}, issn = {{2643-1564}}, journal = {{Physical Review Research}}, number = {{3}}, publisher = {{American Physical Society (APS)}}, title = {{{Spectral correlations of dynamical resonance fluorescence}}}, doi = {{10.1103/jmy9-bd3l}}, volume = {{7}}, year = {{2025}}, } @article{63160, author = {{Rose, Hendrik and Schumacher, Stefan and Meier, Torsten}}, issn = {{2469-9950}}, journal = {{Physical Review B}}, number = {{24}}, publisher = {{American Physical Society (APS)}}, title = {{{Microscopic approach to the quantized light-matter interaction in semiconductor nanostructures: Complex coupled dynamics of excitons, biexcitons, and photons}}}, doi = {{10.1103/528f-7smh}}, volume = {{112}}, year = {{2025}}, }