@article{54852,
  abstract     = {{<jats:p>The crystal family of potassium titanyl phosphate (KTiOPO4) is a promising material group for applications in quantum and nonlinear optics. The fabrication of low-loss optical waveguides, as well as high-grade periodically poled ferroelectric domain structures, requires a profound understanding of the material properties and crystal structure. In this regard, Raman spectroscopy offers the possibility to study and visualize domain structures, strain, defects, and the local stoichiometry, which are all factors impacting device performance. However, the accurate interpretation of Raman spectra and their changes with respect to extrinsic and intrinsic defects requires a thorough assignment of the Raman modes to their respective crystal features, which to date is only partly conducted based on phenomenological modelling. To address this issue, we calculated the phonon spectra of potassium titanyl phosphate and the related compounds rubidium titanyl phosphate (RbTiOPO4) and potassium titanyl arsenate (KTiOAsO4) based on density functional theory and compared them with experimental data. Overall, this allows us to assign various spectral features to eigenmodes of lattice substructures with improved detail compared to previous assignments. Nevertheless, the analysis also shows that not all features of the spectra can unambigiously be explained yet. A possible explanation might be that defects or long range fields not included in the modeling play a crucial rule for the resulting Raman spectrum. In conclusion, this work provides an improved foundation into the vibrational properties in the KTiOPO4 material family.</jats:p>}},
  author       = {{Neufeld, Sergej and Gerstmann, Uwe and Padberg, Laura and Eigner, Christof and Berth, Gerhard and Silberhorn, Christine and Eng, Lukas M. and Schmidt, Wolf Gero and Rüsing, Michael}},
  issn         = {{2073-4352}},
  journal      = {{Crystals}},
  number       = {{10}},
  publisher    = {{MDPI AG}},
  title        = {{{Vibrational Properties of the Potassium Titanyl Phosphate Crystal Family}}},
  doi          = {{10.3390/cryst13101423}},
  volume       = {{13}},
  year         = {{2023}},
}

@article{45485,
  author       = {{Kruse, Stephan and Serino, Laura and Folge, Patrick Fabian and Echeverria Oviedo, Dana and Bhattacharjee, Abhinandan and Stefszky, Michael and Scheytt, J. Christoph and Brecht, Benjamin and Silberhorn, Christine}},
  issn         = {{1041-1135}},
  journal      = {{IEEE Photonics Technology Letters}},
  keywords     = {{Electrical and Electronic Engineering, Atomic and Molecular Physics, and Optics, Electronic, Optical and Magnetic Materials}},
  number       = {{14}},
  pages        = {{769--772}},
  publisher    = {{Institute of Electrical and Electronics Engineers (IEEE)}},
  title        = {{{A Pulsed Lidar System With Ultimate Quantum Range Accuracy}}},
  doi          = {{10.1109/lpt.2023.3277515}},
  volume       = {{35}},
  year         = {{2023}},
}

@article{45850,
  abstract     = {{Interference between single photons is key for many quantum optics experiments and applications in quantum technologies, such as quantum communication or computation. It is advantageous to operate the systems at telecommunication wavelengths and to integrate the setups for these applications in order to improve stability, compactness and scalability. A new promising material platform for integrated quantum optics is lithium niobate on insulator (LNOI). Here, we realise Hong-Ou-Mandel (HOM) interference between telecom photons from an engineered parametric down-conversion source in an LNOI directional coupler. The coupler has been designed and fabricated in house and provides close to perfect balanced beam splitting. We obtain a raw HOM visibility of (93.5 ± 0.7) %, limited mainly by the source performance and in good agreement with off-chip measurements. This lays the foundation for more sophisticated quantum experiments in LNOI.}},
  author       = {{Babel, Silia and Bollmers, Laura and Massaro, Marcello and Luo, Kai Hong and Stefszky, Michael and Pegoraro, Federico and Held, Philip and Herrmann, Harald and Eigner, Christof and Brecht, Benjamin and Padberg, Laura and Silberhorn, Christine}},
  issn         = {{1094-4087}},
  journal      = {{Optics Express}},
  keywords     = {{Atomic and Molecular Physics, and Optics}},
  number       = {{14}},
  publisher    = {{Optica Publishing Group}},
  title        = {{{Demonstration of Hong-Ou-Mandel interference in an LNOI directional coupler}}},
  doi          = {{10.1364/oe.484126}},
  volume       = {{31}},
  year         = {{2023}},
}

@article{46138,
  abstract     = {{<jats:p>This work reports a fully guided setup for single-mode squeezing on integrated titanium-indiffused periodically poled nonlinear resonators. A continuous-wave laser beam is delivered and the squeezed field is collected by single-mode fibers; up to −3.17(9) dB of useful squeezing is available in fibers. To showcase the usefulness of such a fiber-coupled device, we applied the generated squeezed light in a fiber-based phase sensing experiment, showing a quantum enhancement in the signal-to-noise ratio of 0.35 dB. Moreover, our investigation of the effect of photorefraction on the cavity resonance condition suggests that it causes system instabilities at high powers.</jats:p>}},
  author       = {{Domeneguetti, Renato and Stefszky, Michael and Herrmann, Harald and Silberhorn, Christine and Andersen, Ulrik L. and Neergaard-Nielsen, Jonas S. and Gehring, Tobias}},
  issn         = {{0146-9592}},
  journal      = {{Optics Letters}},
  keywords     = {{Atomic and Molecular Physics, and Optics}},
  number       = {{11}},
  publisher    = {{Optica Publishing Group}},
  title        = {{{Fully guided and phase locked Ti:PPLN waveguide squeezing for applications in quantum sensing}}},
  doi          = {{10.1364/ol.486654}},
  volume       = {{48}},
  year         = {{2023}},
}

@inproceedings{61362,
  abstract     = {{<jats:p>We study the interaction of gray tracking and DC ionic conductivity in Potassium Titanyl Phosphate (KTiOPO<jats:sub>4</jats:sub>, KTP) and present a novel way to reduce conductivity via a potassium nitrate treatment improving the device quality.</jats:p>}},
  author       = {{Eigner, Christof and Padberg, Laura and Quiring, Viktor and Bocchini, Adriana and Santandrea, Matteo and Gerstmann, Uwe and Schmidt, Wolf Gero and Silberhorn, Christine}},
  booktitle    = {{CLEO 2023}},
  publisher    = {{Optica Publishing Group}},
  title        = {{{Potassium Titanyl Phosphate Material Engineering Boosting Integrated Optical Source Performance}}},
  doi          = {{10.1364/cleo_at.2023.jw2a.57}},
  year         = {{2023}},
}

@article{44081,
  author       = {{Serino, Laura and Gil López, Jano and Stefszky, Michael and Ricken, Raimund and Eigner, Christof and Brecht, Benjamin and Silberhorn, Christine}},
  issn         = {{2691-3399}},
  journal      = {{PRX Quantum}},
  keywords     = {{General Physics and Astronomy, Mathematical Physics, Applied Mathematics, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, General Computer Science}},
  number       = {{2}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Realization of a Multi-Output Quantum Pulse Gate for Decoding High-Dimensional Temporal Modes of Single-Photon States}}},
  doi          = {{10.1103/prxquantum.4.020306}},
  volume       = {{4}},
  year         = {{2023}},
}

@article{42648,
  abstract     = {{In real photonic quantum systems losses are an unavoidable factor limiting the scalability to many modes and particles, restraining their application in fields as quantum information and communication. For this reason, a considerable amount of engineering effort has been taken in order to improve the quality of particle sources and system components. At the same time, data analysis and collection methods based on post-selection have been used to mitigate the effect of particle losses. This has allowed for investigating experimentally multi-particle evolutions where the observer lacks knowledge about the system's intermediate propagation states. Nonetheless, the fundamental question how losses affect the behaviour of the surviving subset of a multi-particle system has not been investigated so far. For this reason, here we study the impact of particle losses in a quantum walk of two photons reconstructing the output probability distributions for one photon conditioned on the loss of the other in a known mode and temporal step of our evolution network. We present the underlying theoretical scheme that we have devised in order to model controlled particle losses, we describe an experimental platform capable of implementing our theory in a time multiplexing encoding. In the end we show how localized particle losses change the output distributions without altering their asymptotic spreading properties. Finally we devise a quantum civilization problem, a two walker generalisation of single particle recurrence processes.}},
  author       = {{Pegoraro, Federico and Held, Philip and Barkhofen, Sonja and Brecht, Benjamin and Silberhorn, Christine}},
  issn         = {{0031-8949}},
  journal      = {{Physica Scripta}},
  number       = {{3}},
  publisher    = {{IOP Publishing}},
  title        = {{{Dynamic conditioning of two particle discrete-time quantum walks}}},
  doi          = {{10.1088/1402-4896/acbcaa}},
  volume       = {{98}},
  year         = {{2023}},
}

@article{40273,
  author       = {{Meyer-Scott, Evan and Prasannan, Nidhin and Dhand, Ish and Eigner, Christof and Quiring, Viktor and Barkhofen, Sonja and Brecht, Benjamin and Plenio, Martin B. and Silberhorn, Christine}},
  issn         = {{0031-9007}},
  journal      = {{Physical Review Letters}},
  keywords     = {{General Physics and Astronomy}},
  number       = {{15}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Scalable Generation of Multiphoton Entangled States by Active Feed-Forward and Multiplexing}}},
  doi          = {{10.1103/physrevlett.129.150501}},
  volume       = {{129}},
  year         = {{2022}},
}

@inproceedings{43744,
  abstract     = {{We demonstrate theoretically and experimentally complex correlations in the photon numbers of two-mode quantum states using measurement-induced nonlinearity. For this, we combine the interference of coherent states and single photons with photon sub-traction.}},
  author       = {{Meier, Torsten and Hoepker, Jan Philipp and Protte, Maximilian and Eigner, Christof and Silberhorn, Christine and Sharapova, Polina R. and Sperling, Jan and Bartley, Tim}},
  booktitle    = {{Conference on Lasers and Electro-Optics: Applications and Technology}},
  isbn         = {{978-1-957171-05-0}},
  location     = {{San Jose, California United States}},
  pages        = {{JTu3A. 17}},
  publisher    = {{Optica Publishing Group}},
  title        = {{{Two-Mode Photon-Number Correlations Created by Measurement-Induced Nonlinearity}}},
  doi          = {{10.1364/CLEO_AT.2022.JTu3A.17}},
  year         = {{2022}},
}

@article{33484,
  abstract     = {{We study the DC conductivity in potassium titanyl phosphate (KTiOPO4, KTP) and its isomorphs KTiOAsO4 (KTA) and Rb1%K99%TiOPO4 (RKTP) and introduce a method by which to reduce the overall ionic conductivity in KTP by a potassium nitrate treatment. Furthermore, we create so-called gray tracking in KTP and investigate the ionic conductivity in theses areas. A local unintended reduction of the ionic conductivity is observed in the gray-tracked regions, which also induce additional optical absorption in the material. We show that a thermal treatment in an oxygen-rich atmosphere removes the gray tracking and brings the ionic conductivity as well as the optical transmission back to the original level. These studies can help to choose the best material and treatment for specific applications.}},
  author       = {{Padberg, Laura and Quiring, Viktor and Bocchini, Adriana and Santandrea, Matteo and Gerstmann, Uwe and Schmidt, Wolf Gero and Silberhorn, Christine and Eigner, Christof}},
  issn         = {{2073-4352}},
  journal      = {{Crystals}},
  pages        = {{1359}},
  title        = {{{DC Ionic Conductivity in KTP and Its Isomorphs: Properties, Methods for Suppression, and Its Connection to Gray Tracking}}},
  doi          = {{10.3390/cryst12101359}},
  volume       = {{12}},
  year         = {{2022}},
}

@article{34884,
  author       = {{Prasannan, Nidhin and Sperling, Jan and Brecht, Benjamin and Silberhorn, Christine}},
  issn         = {{0031-9007}},
  journal      = {{Physical Review Letters}},
  keywords     = {{General Physics and Astronomy}},
  number       = {{26}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Direct Measurement of Higher-Order Nonlinear Polarization Squeezing}}},
  doi          = {{10.1103/physrevlett.129.263601}},
  volume       = {{129}},
  year         = {{2022}},
}

@article{30921,
  abstract     = {{Quantum walks function as essential means to implement quantum simulators, allowing one to study complex and often directly inaccessible quantum processes in controllable systems. In this contribution, the notion of a driven Gaussian quantum walk is introduced. In contrast to typically considered quantum walks in optical settings, we describe the operation of the walk in terms of a nonlinear map rather than a unitary operation, e.g., by replacing a beam-splitter-type coin with a two-mode squeezer, being a process that is controlled and driven by a pump field. This opens previously unattainable possibilities for quantum walks that include nonlinear elements as core components of their operation, vastly extending their range of applications. A full framework for driven Gaussian quantum walks is developed, including methods to dynamically characterize nonlinear, quantum, and quantum-nonlinear effects. Moreover, driven Gaussian quantum walks are compared with their classically interfering and linear counterparts, which are based on classical coherence of light rather than quantum superpositions. In particular, the generation and boost of highly multimode entanglement, squeezing, and other quantum effects are studied over the duration of the nonlinear walk. Importantly, we prove the quantumness of the evolution itself, regardless of the input state. A scheme for an experimental realization is proposed. Furthermore, nonlinear properties of driven Gaussian quantum walks are explored, such as amplification that leads to an ever increasing number of correlated quantum particles, constituting a source of new walkers during the walk. Therefore, a concept for quantum walks is proposed that leads to—and even produces—directly accessible quantum phenomena, and that renders the quantum simulation of nonlinear processes possible.}},
  author       = {{Held, Philip and Engelkemeier, Melanie and De, Syamsundar and Barkhofen, Sonja and Sperling, Jan and Silberhorn, Christine}},
  issn         = {{2469-9926}},
  journal      = {{Physical Review A}},
  number       = {{4}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Driven Gaussian quantum walks}}},
  doi          = {{10.1103/physreva.105.042210}},
  volume       = {{105}},
  year         = {{2022}},
}

@article{29524,
  author       = {{De, Syamsundar and Gil López, Jano and Brecht, Benjamin and Silberhorn, Christine and Sánchez-Soto, Luis L. and Hradil, Zdeněk and Řeháček, Jaroslav}},
  issn         = {{2643-1564}},
  journal      = {{Physical Review Research}},
  keywords     = {{General Engineering}},
  number       = {{3}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Effects of coherence on temporal resolution}}},
  doi          = {{10.1103/physrevresearch.3.033082}},
  volume       = {{3}},
  year         = {{2021}},
}

@article{21020,
  author       = {{Ansari, Vahid and Brecht, Benjamin and Gil-Lopez, Jano and Donohue, John M. and Řeháček, Jaroslav and Hradil, Zdeněk and Sánchez-Soto, Luis L. and Silberhorn, Christine}},
  issn         = {{2691-3399}},
  journal      = {{PRX Quantum}},
  title        = {{{Achieving the Ultimate Quantum Timing Resolution}}},
  doi          = {{10.1103/prxquantum.2.010301}},
  volume       = {{2}},
  year         = {{2021}},
}

@article{22259,
  author       = {{Roman-Rodriguez, V and Brecht, Benjamin and Srinivasan, K and Silberhorn, Christine and Treps, N and Diamanti, E and Parigi, V}},
  issn         = {{1367-2630}},
  journal      = {{New Journal of Physics}},
  title        = {{{Continuous variable multimode quantum states via symmetric group velocity matching}}},
  doi          = {{10.1088/1367-2630/abef96}},
  volume       = {{23}},
  year         = {{2021}},
}

@article{23826,
  abstract     = {{<jats:p>Potassium titanyl phosphate (KTP) is a nonlinear optical material with applications in high-power frequency conversion or quasi-phase matching in submicron period domain grids. A prerequisite for these applications is a precise control and understanding of the poling mechanisms to enable the fabrication of high-grade domain grids. In contrast to the widely used material lithium niobate, the domain growth in KTP is less studied, because many standard methods, such as selective etching or polarization microscopy, provides less insight or are not applicable on non-polar surfaces, respectively. In this work, we present results of confocal Raman-spectroscopy of the ferroelectric domain structure in KTP. This analytical method allows for the visualization of domain grids of the non-polar KTP y-face and therefore more insight into the domain-growth and -structure in KTP, which can be used for improved domain fabrication.</jats:p>}},
  author       = {{Brockmeier, Julian and Mackwitz, Peter Walter Martin and Rüsing, Michael and Eigner, Christof and Padberg, Laura and Santandrea, Matteo and Silberhorn, Christine and Zrenner, Artur and Berth, Gerhard}},
  issn         = {{2073-4352}},
  journal      = {{Crystals}},
  title        = {{{Non-Invasive Visualization of Ferroelectric Domain Structures on the Non-Polar y-Surface of KTiOPO4 via Raman Imaging}}},
  doi          = {{10.3390/cryst11091086}},
  year         = {{2021}},
}

@misc{38135,
  author       = {{Padberg, Laura and Eigner, Christof and Santandrea, Matteo  and Silberhorn, Christine}},
  title        = {{{Production of waveguides made of materials from the KTP family}}},
  year         = {{2021}},
}

@article{37936,
  author       = {{Pelucchi, Emanuele and Fagas, Giorgos and Aharonovich, Igor and Englund, Dirk and Figueroa, Eden and Gong, Qihuang and Hannes, Hübel and Liu, Jin and Lu, Chao-Yang and Matsuda, Nobuyuki and Pan, Jian-Wei and Schreck, Florian and Sciarrino, Fabio and Silberhorn, Christine and Wang, Jianwei and Jöns, Klaus}},
  issn         = {{2522-5820}},
  journal      = {{Nature Reviews Physics}},
  keywords     = {{General Physics and Astronomy}},
  number       = {{3}},
  pages        = {{194--208}},
  publisher    = {{Springer Science and Business Media LLC}},
  title        = {{{The potential and global outlook of integrated photonics for quantum technologies}}},
  doi          = {{10.1038/s42254-021-00398-z}},
  volume       = {{4}},
  year         = {{2021}},
}

@article{22770,
  author       = {{Gil López, Jano and Santandrea, Matteo and Roland, Ganaël and Brecht, Benjamin and Eigner, Christof and Ricken, Raimund and Quiring, Viktor and Silberhorn, Christine}},
  issn         = {{1367-2630}},
  journal      = {{New Journal of Physics}},
  title        = {{{Improved non-linear devices for quantum applications}}},
  doi          = {{10.1088/1367-2630/ac09fd}},
  year         = {{2021}},
}

@article{26410,
  author       = {{Gil López, Jano and Teo, Yong Siah and De, Syamsundar and Brecht, Benjamin and Jeong, Hyunseok and Silberhorn, Christine and Sánchez-Soto, Luis L.}},
  issn         = {{2334-2536}},
  journal      = {{Optica}},
  title        = {{{Universal compressive tomography in the time-frequency domain}}},
  doi          = {{10.1364/optica.427645}},
  year         = {{2021}},
}