@article{59069,
abstract = {{Stable and bright single photon sources are key components for future quantum applications. A simple fabrication method is an important requirement for such sources. Here, we present a single photon source based on diced ridge waveguides in titanium indiffused LiNbO3. These waveguides can be easily fabricated by combining planar titanium in-diffusion without lithographic patterning and easy-to-handle precision dicing. Such devices have the potential to generate high single photon rates because ridge structures are typically less prone to the photorefractive effect. We achieve waveguide propagation losses <0.4dBcm and a SHG conversion efficiency of about 81%Wcm2. Harnessing a type-0 SPDC process to generate 1550 nm photons, we obtain a SPDC brightness of 3⋅1051s⋅mW⋅nm, with a heralding efficiency of ηh=45% (ηh,wg=77.5% for the waveguide itself excluded setup losses) and a heralded second-order correlation function of gh2(0)<0.003 at low pump powers.}},
author = {{Kießler, Christian and Kirsch, Michelle and Lengeling, Sebastian and Herrmann, Harald and Silberhorn, Christine}},
issn = {{2770-0208}},
journal = {{Optics Continuum}},
number = {{3}},
publisher = {{Optica Publishing Group}},
title = {{{SPDC single-photon source in Ti-indiffused diced ridge LiNbO3 waveguides}}},
doi = {{10.1364/optcon.557439}},
volume = {{4}},
year = {{2025}},
}
@article{62269,
abstract = {{The titanium in-diffused lithium niobate waveguide platform is well-established for reliable prototyping and packaging of many quantum photonic components at room temperature. Nevertheless, compatibility with certain quantum light sources and superconducting detectors requires operation under cryogenic conditions. We characterize alterations in phase-matching and mode guiding of a non-degenerate spontaneous parametric down-conversion process emitting around 1556 nm and 950 nm, under cryogenic conditions. Despite the effects of pyroelectricity and photorefraction, the spectral properties match our theoretical model. Nevertheless, these effects cause small but significant variations within and between cooling cycles. These measurements provide a first benchmark against which other nonlinear photonic integration platforms, such as thin-film lithium niobate, can be compared.}},
author = {{Lange, Nina Amelie and Lengeling, Sebastian and Mues, Philipp and Quiring, Viktor and Ridder, Werner and Eigner, Christof and Herrmann, Harald and Silberhorn, Christine and Bartley, Tim}},
issn = {{1094-4087}},
journal = {{Optics Express}},
number = {{24}},
publisher = {{Optica Publishing Group}},
title = {{{Widely non-degenerate nonlinear frequency conversion in cryogenic titanium in-diffused lithium niobate waveguides}}},
doi = {{10.1364/oe.578108}},
volume = {{33}},
year = {{2025}},
}
@article{60466,
author = {{Brockmeier, Julian and Schapeler, Timon and Lange, Nina Amelie and Höpker, Jan Philipp and Herrmann, Harald and Silberhorn, Christine and Bartley, Tim}},
journal = {{New Journal of Physics}},
title = {{{Harnessing temporal dispersion for integrated pump filtering in spontaneous heralded single-photon generation processes}}},
doi = {{10.1088/1367-2630/ade46c}},
year = {{2025}},
}
@article{63192,
abstract = {{Lithium niobate (LiNbO3) is a widely used material with several desirable physical properties, such as high second-order nonlinear optical and strong electro-optical effects. Thus LiNbO3 is used for various applications such as electro-optic modulation or nonlinear frequency conversion and mixing. But LiNbO3 also exhibits a strong photorefractive effect, which limits the intensity of the optical fields involved. Various approaches to reduce the photorefractive effect have been investigated, such as increasing the temperature, doping the crystal or using different waveguide designs in LiNbO3. Here, we present an analysis of the approach to increase the photorefractive damage threshold by using different waveguide designs. Contrary to previous claims and investigations, our SHG measurements revealed no significant difference in resistance to photorefractive damage when comparing conventional Ti-doped channel waveguides and Ti-doped diced ridge waveguides in LiNbO3. Furthermore, we have investigated the effect of photorefractive cleaning and curing using a light field at 532 nm. Here, we observe a reduction in the photorefractive effect at room temperature during and after SHG measurements, which is an easy alternative to conventional approaches.}},
author = {{Kirsch, Michelle and Kießler, Christian and Lengeling, Sebastian and Stefszky, Michael and Eigner, Christof and Herrmann, Harald and Silberhorn, Christine}},
issn = {{0030-3992}},
journal = {{Optics & Laser Technology}},
publisher = {{Elsevier BV}},
title = {{{Photorefraction and in-situ optical cleaning in various types of LiNbO3 waveguides}}},
doi = {{10.1016/j.optlastec.2025.114260}},
volume = {{193}},
year = {{2025}},
}
@article{63091,
abstract = {{We present the design and characterization of a guided-wave, bright, and highly frequency non-degenerate parametric down-conversion (PDC) source in thin-film lithium niobate. The source generates photon pairs with wavelengths of 815 nm and 1550 nm, linking the visible wavelength regime with telecommunication wavelengths. We confirm the high quality of the generated single photons by determining a value for the heralded second-order correlation function as low as g_h^(2)=(6.7+/-1.1)*10^8-3). Furthermore, we achieve a high spectral brightness of 0.44·10pairs/(smWGHz) which is two orders of magnitude higher than sources based on weakly guiding waveguides. The shape of the PDC spectrum and the strong agreement between the effective and nominal bandwidth highlight our high fabrication quality of periodically poled waveguides. The good agreement between the measured and simulated spectral characteristics of our source demonstrates our excellent understanding of the PDC process. Our result is a valuable step towards practical and scalable quantum communication networks as well as photonic quantum computing.}},
author = {{Babel, Silia and Bollmers, Laura and Roeder, Franz and Ridder, Werner and Golla, Christian and Köthemann, Ronja and Reineke, Bernhard and Herrmann, Harald and Brecht, Benjamin and Eigner, Christof and Padberg, Laura and Silberhorn, Christine}},
issn = {{1094-4087}},
journal = {{Optics Express}},
number = {{25}},
publisher = {{Optica Publishing Group}},
title = {{{Ultrabright, two-color photon pair source based on thin-film lithium niobate for bridging visible and telecom wavelengths}}},
doi = {{10.1364/oe.571605}},
volume = {{33}},
year = {{2025}},
}
@article{51339,
author = {{Babai-Hemati, Jonas and vom Bruch, Felix and Herrmann, Harald and Silberhorn, Christine}},
issn = {{1094-4087}},
journal = {{Optics Express}},
keywords = {{Atomic and Molecular Physics, and Optics}},
publisher = {{Optica Publishing Group}},
title = {{{Tailored second harmonic generation inTi-diffused PPLN waveguides usingmicro-heaters}}},
doi = {{10.1364/oe.510319}},
year = {{2024}},
}
@article{57619,
abstract = {{Ferroelectric liquid crystals exhibiting a chiral smectic C* phase are deposited on z cut periodically poled lithium niobate substrates and investigated by polarized optical microscopy. While the pure substrates placed between crossed polarizers and observed in transmission appear dark, uniformly aligned liquid crystal films deposited on these substrates show alternating domains with varying brightness. This effect can be attributed to the well-known coupling between the direction of the spontaneous polarization and the optical axis in the birefringent ferroelectric smectic C* phase. Quantitative measurements of the tilt angle between the local optical axis and the smectic layer normal confirm antiparallel orientations of spontaneous polarization of the liquid crystal from domain to domain, as expected by the periodic poling of the lithium niobate substrate. This effect provides a valuable non-destructive method of optical inspection of the quality of periodically poled ferroelectric substrates, which plays an important role in achieving quasi-phase-matching in non-linear optical applications.}},
author = {{Meier, Patrick A. and Keuker-Baumann, Susanne and Röder, Thorsten and Herrmann, Harald and Ricken, Raimund and Silberhorn, Christine and Kitzerow, Heinz-Siegfried}},
issn = {{1896-3757}},
journal = {{Opto-Electronics Review}},
pages = {{150611--150611}},
publisher = {{Polish Academy of Sciences Chancellery}},
title = {{{Optical imaging of ferroelectric domains in periodically poled lithium niobate using ferroelectric liquid crystals}}},
doi = {{10.24425/opelre.2024.150611}},
year = {{2024}},
}
@article{51356,
abstract = {{Abstract
Lithium niobate has emerged as a promising platform for integrated quantum optics, enabling efficient generation, manipulation, and detection of quantum states of light. However, integrating single-photon detectors requires cryogenic operating temperatures, since the best performing detectors are based on narrow superconducting wires. While previous studies have demonstrated the operation of quantum light sources and electro-optic modulators in LiNbO3 at cryogenic temperatures, the thermal transition between room temperature and cryogenic conditions introduces additional effects that can significantly influence device performance. In this paper, we investigate the generation of pyroelectric charges and their impact on the optical properties of lithium niobate waveguides when changing from room temperature to 25 K, and vice versa. We measure the generated pyroelectric charge flow and correlate this with fast changes in the birefringence acquired through the Sénarmont-method. Both electrical and optical influence of the pyroelectric effect occur predominantly at temperatures above 100 K.}},
author = {{Thiele, Frederik and Hummel, Thomas and Lange, Nina Amelie and Dreher, Felix and Protte, Maximilian and Bruch, Felix vom and Lengeling, Sebastian and Herrmann, Harald and Eigner, Christof and Silberhorn, Christine and Bartley, Tim}},
issn = {{2633-4356}},
journal = {{Materials for Quantum Technology}},
keywords = {{General Earth and Planetary Sciences, General Environmental Science}},
number = {{1}},
publisher = {{IOP Publishing}},
title = {{{Pyroelectric influence on lithium niobate during the thermal transition for cryogenic integrated photonics}}},
doi = {{10.1088/2633-4356/ad207d}},
volume = {{4}},
year = {{2024}},
}
@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 = {{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.}},
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}},
}
@article{46644,
abstract = {{A reliable, but cost-effective generation of single-photon states is key for practical quantum communication systems. For real-world deployment, waveguide sources offer optimum compatibility with fiber networks and can be embedded in hybrid integrated modules. Here, we present what we believe to be the first chip-size fully integrated fiber-coupled heralded single photon source (HSPS) module based on a hybrid integration of a nonlinear lithium niobate waveguide into a polymer board. Photon pairs at 810 nm (signal) and 1550 nm (idler) are generated via parametric down-conversion pumped at 532 nm in the LiNbO3 waveguide. The pairs are split in the polymer board and routed to separate output ports. The module has a size of (2 × 1) cm^2 and is fully fiber-coupled with one pump input fiber and two output fibers. We measure a heralded second-order correlation function of g_h(2)=0.05 with a heralding efficiency of η_h=3.5% at low pump powers}},
author = {{Kießler, Christian and Conradi, Hauke and Kleinert, Moritz and Quiring, Viktor and Herrmann, Harald and Silberhorn, Christine}},
issn = {{1094-4087}},
journal = {{Optics Express}},
keywords = {{Atomic and Molecular Physics, and Optics}},
number = {{14}},
publisher = {{Optica Publishing Group}},
title = {{{Fiber-coupled plug-and-play heralded single photon source based on Ti:LiNbO3 and polymer technology}}},
doi = {{10.1364/oe.487581}},
volume = {{31}},
year = {{2023}},
}
@article{33672,
abstract = {{Abstract
Lithium niobate is a promising platform for integrated quantum optics. In this platform, we aim to efficiently manipulate and detect quantum states by combining superconducting single photon detectors and modulators. The cryogenic operation of a superconducting single photon detector dictates the optimisation of the electro-optic modulators under the same operating conditions. To that end, we characterise a phase modulator, directional coupler, and polarisation converter at both ambient and cryogenic temperatures. The operation voltage
V
π
/
2
of these modulators increases, due to the decrease in the electro-optic effect, by 74% for the phase modulator, 84% for the directional coupler and 35% for the polarisation converter below 8.5
K
. The phase modulator preserves its broadband nature and modulates light in the characterised wavelength range. The unbiased bar state of the directional coupler changed by a wavelength shift of 85
n
m
while cooling the device down to 5
K
. The polarisation converter uses periodic poling to phasematch the two orthogonal polarisations. The phasematched wavelength of the utilised poling changes by 112
n
m
when cooling to 5
K
.}},
author = {{Thiele, Frederik and vom Bruch, Felix and Brockmeier, Julian and Protte, Maximilian and Hummel, Thomas and Ricken, Raimund and Quiring, Viktor and Lengeling, Sebastian and Herrmann, Harald and Eigner, Christof and Silberhorn, Christine and Bartley, Tim}},
issn = {{2515-7647}},
journal = {{Journal of Physics: Photonics}},
keywords = {{Electrical and Electronic Engineering, Atomic and Molecular Physics, and Optics, Electronic, Optical and Magnetic Materials}},
number = {{3}},
publisher = {{IOP Publishing}},
title = {{{Cryogenic electro-optic modulation in titanium in-diffused lithium niobate waveguides}}},
doi = {{10.1088/2515-7647/ac6c63}},
volume = {{4}},
year = {{2022}},
}
@article{38532,
author = {{Trenti, Alessandro and Achleitner, Martin and Prawits, Florian and Schrenk, Bernhard and Conradi, Hauke and Kleinert, Moritz and Incoronato, Alfonso and Zanetto, Francesco and Zappa, Franco and Luch, Ilaria Di and Cirkinoglu, Ozan and Leijtens, Xaveer and Bonardi, Antonio and Bruynsteen, Cedric and Yin, Xin and Kießler, Christian and Herrmann, Harald and Silberhorn, Christine and Bozzio, Mathieu and Walther, Philip and Thiel, Hannah C. and Weihs, Gregor and Hubel, Hannes}},
issn = {{0733-8724}},
journal = {{Journal of Lightwave Technology}},
keywords = {{General Engineering}},
number = {{23}},
pages = {{7485--7497}},
publisher = {{Institute of Electrical and Electronics Engineers (IEEE)}},
title = {{{On-Chip Quantum Communication Devices}}},
doi = {{10.1109/jlt.2022.3201389}},
volume = {{40}},
year = {{2022}},
}
@article{26221,
author = {{Bartnick, Moritz and Santandrea, Matteo and Höpker, Jan Philipp and Thiele, Frederik and Ricken, Raimund and Quiring, Viktor and Eigner, Christof and Herrmann, Harald and Silberhorn, Christine and Bartley, Tim}},
issn = {{2331-7019}},
journal = {{Physical Review Applied}},
title = {{{Cryogenic Second-Harmonic Generation in Periodically Poled Lithium Niobate Waveguides}}},
doi = {{10.1103/physrevapplied.15.024028}},
year = {{2021}},
}
@article{26889,
author = {{Luo, Kai Hong and Santandrea, Matteo and Stefszky, Michael and Sperling, Jan and Massaro, Marcello and Ferreri, Alessandro and Sharapova, Polina and Herrmann, Harald and Silberhorn, Christine}},
issn = {{2469-9926}},
journal = {{Physical Review A}},
title = {{{Quantum optical coherence: From linear to nonlinear interferometers}}},
doi = {{10.1103/physreva.104.043707}},
year = {{2021}},
}
@article{26077,
abstract = {{Nonlinear SU(1,1) interferometers are fruitful and promising tools for spectral engineering and precise measurements with phase sensitivity below the classical bound. Such interferometers have been successfully realized in bulk and fiber-based configurations. However, rapidly developing integrated technologies provide higher efficiencies, smaller footprints, and pave the way to quantum-enhanced on-chip interferometry. In this work, we theoretically realised an integrated architecture of the multimode SU(1,1) interferometer which can be applied to various integrated platforms. The presented interferometer includes a polarization converter between two photon sources and utilizes a continuous-wave (CW) pump. Based on the potassium titanyl phosphate (KTP) platform, we show that this configuration results in almost perfect destructive interference at the output and supersensitivity regions below the classical limit. In addition, we discuss the fundamental difference between single-mode and highly multimode SU(1,1) interferometers in the properties of phase sensitivity and its limits. Finally, we explore how to improve the phase sensitivity by filtering the output radiation and using different seeding states in different modes with various detection strategies.}},
author = {{Ferreri, Alessandro and Santandrea, Matteo and Stefszky, Michael and Luo, Kai Hong and Herrmann, Harald and Silberhorn, Christine and Sharapova, Polina R.}},
issn = {{2521-327X}},
journal = {{Quantum}},
title = {{{Spectrally multimode integrated SU(1,1) interferometer}}},
doi = {{10.22331/q-2021-05-27-461}},
year = {{2021}},
}
@inproceedings{39027,
abstract = {{We experimentally investigate the generation of continuous-wave optical squeezing from a titanium-indiffused lithium niobate waveguide resonator at low and high frequencies. The device promises integration with different platform chips for more complex optical systems.}},
author = {{Domeneguetti, Renato R. and Conradi, Hauke and Kleinert, Moritz and Kießler, Christian and Stefszky, Michael and Herrmann, Harald and Silberhorn, Christine and Andersen, Ulrik L. and Neergaard-Nielsen, Jonas Schou and Gehring, Tobias}},
booktitle = {{2021 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference}},
keywords = {{Optical systems, Polymer waveguides, Quantum key distribution, Quantum light sources, Squeezed states, Waveguides}},
pages = {{eb_4_1}},
publisher = {{Optica Publishing Group}},
title = {{{Nonlinear waveguides for integrated quantum light source}}},
year = {{2021}},
}
@inproceedings{40374,
abstract = {{We present a frequency multimode integrated SU (1,1) interferometer with a polarization converter and strong signal-idler photon correlations. Phase sensitivity below the shot noise limit is demonstrated, various filtering and seeding strategies are discussed.}},
author = {{Ferreri, A. and Santandrea, Matteo and Stefszky, Michael and Luo, Kai Hong and Herrmann, Harald and Silberhorn, Christine and Sharapova, Polina}},
booktitle = {{Conference on Lasers and Electro-Optics}},
publisher = {{Optica Publishing Group}},
title = {{{Multimode integrated SU(1,1) interferometer}}},
doi = {{10.1364/cleo_qels.2021.ftu1n.6}},
year = {{2021}},
}
@article{22771,
author = {{Stefszky, Michael and Santandrea, Matteo and vom Bruch, Felix and Krapick, S. and Eigner, Christof and Ricken, R. and Quiring, V. and Herrmann, Harald and Silberhorn, Christine}},
issn = {{1094-4087}},
journal = {{Optics Express}},
title = {{{Waveguide resonator with an integrated phase modulator for second harmonic generation}}},
doi = {{10.1364/oe.412824}},
year = {{2020}},
}
@article{20157,
author = {{Thiele, Frederik and vom Bruch, Felix and Quiring, Victor and Ricken, Raimund and Herrmann, Harald and Eigner, Christof and Silberhorn, Christine and Bartley, Tim}},
issn = {{1094-4087}},
journal = {{Optics Express}},
title = {{{Cryogenic electro-optic polarisation conversion in titanium in-diffused lithium niobate waveguides}}},
doi = {{10.1364/oe.399818}},
year = {{2020}},
}