@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}},
}

@unpublished{58544,
  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 being 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 and Offen, Christian and Ares, Laura and Wembe Moafo, Boris Edgar and Ober-Blöbaum, Sina and Meier, Torsten and Sharapova, Polina and Sperling, Jan}},
  title        = {{{Multiphoton, multimode state classification for nonlinear optical circuits }}},
  year         = {{2025}},
}

@article{58519,
  abstract     = {{<jats:p>A unified theoretical approach to describe the properties of multimode squeezed light generated in a lossy medium is presented. This approach is valid for Markovian environments and includes both a model of discrete losses based on the beamsplitter approach and a generalized continuous loss model based on the spatial Langevin equation. For an important class of Gaussian states, we derive master equations for the second-order correlation functions and illustrate their solution for both frequency-independent and frequency-dependent losses. Studying the mode structure, we demonstrate that in a lossy environment no broadband basis without quadrature correlations between the different broadband modes exists. Therefore, various techniques and strategies to introduce broadband modes can be considered. We show that the Mercer expansion and the Williamson-Euler decomposition do not provide modes in which the maximal squeezing contained in the system can be measured. In turn, we find a new broadband basis that maximizes squeezing in the lossy system and present an algorithm to construct it.</jats:p>}},
  author       = {{Kopylov, Denis A. and Meier, Torsten and Sharapova, Polina R.}},
  issn         = {{2521-327X}},
  journal      = {{Quantum}},
  publisher    = {{Verein zur Forderung des Open Access Publizierens in den Quantenwissenschaften}},
  title        = {{{Theory of Multimode Squeezed Light Generation in Lossy Media}}},
  doi          = {{10.22331/q-2025-02-04-1621}},
  volume       = {{9}},
  year         = {{2025}},
}

@article{62912,
  abstract     = {{<jats:p>We investigate the dynamics of wave packets in a parabolic optical lattice formed by combining an optical lattice with a global parabolic trap. Our study examines the phase space representation of the system's eigenstates by comparing them to the classical phase space of a pendulum, to which the system effectively maps. The analysis reveals that quantum states can exhibit mixed dynamics by straddling the separatrix. A key finding is that the dynamics around the separatrix enables the controlled creation of highly nonclassical states, distinguishing them from the classical oscillatory or rotational dynamics of the pendulum. By considering a finite momentum of the initial wave packet, we demonstrate various dynamical regimes. Furthermore, a slight energy mismatch between nearly degenerate states localized at opposite turning points of the trap potential results in controlled long-range dynamical tunneling. These results can be interpreted as quantum beating between a clockwise rotating and a counterclockwise rotating pendulum.</jats:p>}},
  author       = {{Ali, Usman and Holthaus, Martin and Meier, Torsten}},
  issn         = {{2643-1564}},
  journal      = {{Physical Review Research}},
  number       = {{1}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Wave packet dynamics in parabolic optical lattices: From Bloch oscillations to long-range dynamical tunneling}}},
  doi          = {{10.1103/physrevresearch.7.013141}},
  volume       = {{7}},
  year         = {{2025}},
}

@article{62911,
  abstract     = {{<jats:p>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.</jats:p>}},
  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}},
}

@inproceedings{62913,
  author       = {{Hunstig, Anna and Peitz, Sebastian and Rose, Hendrik and Meier, Torsten}},
  booktitle    = {{2024 IEEE 63rd Conference on Decision and Control (CDC)}},
  publisher    = {{IEEE}},
  title        = {{{Accelerating the analysis of optical quantum systems using the Koopman operator}}},
  doi          = {{10.1109/cdc56724.2024.10886589}},
  year         = {{2025}},
}

@article{62980,
  abstract     = {{<jats:p>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.</jats:p>}},
  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}},
}

@unpublished{62979,
  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 being 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       = {{Meier, Torsten and Sharapova, Polina R. and Sperling, Jan and Ober-Blöbaum, Sina and Wembe Moafo, Boris Edgar and Offen, Christian}},
  title        = {{{Multiphoton, multimode state classification for nonlinear optical circuits}}},
  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}},
}

@article{63562,
  abstract     = {{<jats:p>Entangled two-mode Gaussian states constitute an important building block for continuous variable quantum computing and communication protocols. In this work, we theoretically study two-mode bipartite states, which are extracted from multimode light generated via type-II parametric downconversion (PDC) in lossy waveguides. For these states, we demonstrate that the squeezing quantifies entanglement and we construct a measurement basis, which results in the maximal bipartite entanglement. We illustrate our findings by numerically solving the spatial master equation for PDC in a Markovian environment. The optimal measurement modes are compared with two widely used broadband bases: the Mercer–Wolf basis (the first-order coherence basis) and the Williamson–Euler basis.</jats:p>}},
  author       = {{Kopylov, Denis and Meier, Torsten and Sharapova, Polina R.}},
  issn         = {{2835-0103}},
  journal      = {{APL Quantum}},
  number       = {{4}},
  publisher    = {{AIP Publishing}},
  title        = {{{Bipartite entanglement extracted from multimode squeezed light generated in lossy waveguides}}},
  doi          = {{10.1063/5.0293116}},
  volume       = {{2}},
  year         = {{2025}},
}

@article{55264,
  abstract     = {{<jats:p>Tunneling ionization is a crucial process in the interaction between strong laser fields and matter which initiates numerous nonlinear phenomena including high-order harmonic generation, photoelectron holography, etc. Both adiabatic and nonadiabatic tunneling ionization are well understood in atomic systems. However, the tunneling dynamics in solids, especially nonadiabatic tunneling, has not yet been fully understood. Here, we study the sub-cycle resolved strong-field tunneling dynamics in solids via a complex saddle-point method. We compare the instantaneous momentum at the moment of tunneling and the tunneling distances over a range of Keldysh parameters. Our results demonstrate that for nonadiabatic tunneling, tunneling ionization away from Γ point is possible. When this happens the electron has a nonzero initial velocity when it emerges in the conduction band. Moreover, consistent with atomic tunneling, a reduced tunneling distance as compared to the quasi-static case is found. Our results provide remarkable insight into the basic physics governing the sub-cycle electron tunneling dynamics with significant implications for understanding subsequent strong-field nonlinear phenomena in solids.</jats:p>}},
  author       = {{Yang, Shidong and Liu, Xiwang and Zhang, Hongdan and Song, Xiaohong and Zuo, Ruixin and Meier, Torsten and Yang, Weifeng}},
  issn         = {{1094-4087}},
  journal      = {{Optics Express}},
  number       = {{9}},
  publisher    = {{Optica Publishing Group}},
  title        = {{{Sub-cycle strong-field tunneling dynamics in solids}}},
  doi          = {{10.1364/oe.521207}},
  volume       = {{32}},
  year         = {{2024}},
}

@article{55267,
  author       = {{Schäfer, F. and Trautmann, A. and Ngo, C. and Steiner, J. T. and Fuchs, C. and Volz, K. and Dobener, F. and Stein, M. and Meier, Torsten and Chatterjee, S.}},
  issn         = {{2469-9950}},
  journal      = {{Physical Review B}},
  number       = {{7}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Optical Stark effect in type-II semiconductor heterostructures}}},
  doi          = {{10.1103/physrevb.109.075301}},
  volume       = {{109}},
  year         = {{2024}},
}

@misc{54405,
  abstract     = {{Dataset of the publication "Microscopic simulations of the dynamics of excitonic many-body correlations coupled to quantum light" H. Rose, P. R. Sharapova, and T. Meier, Proc. SPIE 12884, Ultrafast Phenomena and Nanophotonics XXVIII, 1288403 (2024). ( https://doi.org/10.1117/12.2690245 ). The zip file includes the data on which the plots shown in figures 1 and 2 are based.}},
  author       = {{Rose, Hendrik and Sharapova, Polina and Meier, Torsten}},
  publisher    = {{LibreCat University}},
  title        = {{{Microscopic simulations of the dynamics of excitonic many-body correlations coupled to quantum light}}},
  doi          = {{10.5281/ZENODO.10817980}},
  year         = {{2024}},
}

@inproceedings{55268,
  author       = {{Rose, Hendrik and Sharapova, Polina R. and Meier, Torsten}},
  booktitle    = {{Ultrafast Phenomena and Nanophotonics XXVIII}},
  editor       = {{Betz, Markus and Elezzabi, Abdulhakem Y.}},
  publisher    = {{SPIE}},
  title        = {{{Microscopic simulations of the dynamics of excitonic many-body correlations coupled to quantum light}}},
  doi          = {{10.1117/12.2690245}},
  year         = {{2024}},
}

@article{57410,
  author       = {{Röder, J. and Gerhard, M. and Fuchs, C. and Stolz, W. and Heimbrodt, W. and Koch, M. and Ngo, C. and Steiner, J. T. and Meier, Torsten}},
  issn         = {{2469-9950}},
  journal      = {{Physical Review B}},
  number       = {{19}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Charge transfer magnetoexcitons in magnetoabsorption spectra of asymmetric type-II double quantum wells}}},
  doi          = {{10.1103/physrevb.110.195306}},
  volume       = {{110}},
  year         = {{2024}},
}

@article{57839,
  abstract     = {{<jats:title>Abstract</jats:title>
               <jats:p>Experiments with ultracold atoms in optical lattices usually involve a weak parabolic trapping potential which merely serves to confine the atoms, but otherwise remains negligible. In contrast, we suggest a different class of experiments in which the presence of a stronger trap is an essential part of the set-up. Because the trap-modified on-site energies exhibit a slowly varying level spacing, similar to that of an anharmonic oscillator, an additional time-periodic trap modulation with judiciously chosen parameters creates nonlinear resonances which enable efficient Floquet engineering. We employ a Mathieu approximation for constructing the near-resonant Floquet states in an accurate manner and demonstrate the emergence of effective ground states from the resonant trap eigenstates. Moreover, we show that the population of the Floquet states is strongly affected by the phase of a sudden turn-on of the trap modulation, which leads to significantly modified and rich dynamics. As a guideline for further studies, we argue that the deliberate population of only the resonance-induced effective ground states will allow one to realize Floquet condensates which follow classical periodic orbits, thus providing challenging future perspectives for the investigation of the quantum–classical correspondence.</jats:p>}},
  author       = {{Ali, Usman and Holthaus, Martin and Meier, Torsten}},
  issn         = {{1367-2630}},
  journal      = {{New Journal of Physics}},
  number       = {{12}},
  publisher    = {{IOP Publishing}},
  title        = {{{Floquet dynamics of ultracold atoms in optical lattices with a parametrically modulated trapping potential}}},
  doi          = {{10.1088/1367-2630/ad9b47}},
  volume       = {{26}},
  year         = {{2024}},
}

@inbook{62916,
  author       = {{Zhang, Hongdan and Zuo, Ruixin and Yang, Shidong and Trautmann, Alexander and Song, Xiaohong and Meier, Torsten and Yang, Weifeng}},
  booktitle    = {{High-Order Harmonic Generation in Solids}},
  isbn         = {{9789811279553}},
  publisher    = {{WORLD SCIENTIFIC}},
  title        = {{{Analyzing High-Order Harmonic Generation in Solids Based on Semi-Classical Recollision Models}}},
  doi          = {{10.1142/9789811279560_0006}},
  year         = {{2024}},
}

@inbook{62917,
  author       = {{Reichelt, Matthias and Zuo, Ruixin and Song, Xiaohong and Yang, Weifeng and Meier, Torsten}},
  booktitle    = {{High-Order Harmonic Generation in Solids}},
  isbn         = {{9789811279553}},
  publisher    = {{WORLD SCIENTIFIC}},
  title        = {{{High-Order Harmonic Generation in Semiconductors with Excitonic Effects}}},
  doi          = {{10.1142/9789811279560_0009}},
  year         = {{2024}},
}

@misc{62915,
  author       = {{Meier, Torsten and Ali, Usman and Holthaus, Martin}},
  publisher    = {{LibreCat University}},
  title        = {{{Floquet dynamics of ultracold atoms in optical lattices with a parametrically modulated trapping potential}}},
  doi          = {{10.5281/ZENODO.11935146}},
  year         = {{2024}},
}

@unpublished{48502,
  abstract     = {{The prediction of photon echoes is an important technique for gaining an understanding of optical quantum systems. However, this requires a large number of simulations with varying parameters and/or input pulses, which renders numerical studies expensive. This article investigates how we can use data-driven surrogate models based on the Koopman operator to accelerate this process. In order to be successful, we require a model that is accurate over a large number of time steps. To this end, we employ a bilinear Koopman model using extended dynamic mode decomposition and simulate the optical Bloch equations for an ensemble of inhomogeneously broadened two-level systems. Such systems are well suited to describe the excitation of excitonic resonances in semiconductor nanostructures, for example, ensembles of semiconductor quantum dots. We perform a detailed study on the required number of system simulations such that the resulting data-driven Koopman model is sufficiently accurate for a wide range of parameter settings. We analyze the L2 error and the relative error of the photon echo peak and investigate how the control positions relate to the stabilization. After proper training, the dynamics of the quantum ensemble can be predicted accurately and numerically very efficiently by our methods.}},
  author       = {{Peitz, Sebastian and Hunstig, Anna and Rose, Hendrik and Meier, Torsten}},
  title        = {{{Accelerating the analysis of optical quantum systems using the Koopman operator}}},
  year         = {{2023}},
}