In the past decade, the interest in building a photonic quantum computer using single photon sources, linear optics and single photon detectors gained immense interest. A great deal of work has been devoted on the integration of these components in one material platform at a chip-scale size. A material platform of choice for the integration is thin film lithium niobate (TFLN) due to the high integration density and the unique optical properties such as the second order nonlinearity. To show that this material is suited for the realisation of integrated quantum networks, quantum interference between single photons on chip is crucial. Thus, we demonstrate Hong-Ou-Mandel interference (HOMI) of telecom photons on a balanced directional coupler. We achieve a raw HOMI visibility of (93.5±0.7)% with our in-house fabricated coupler. This demonstrates a fundamental building block for integrated quantum networks based on TFLN.

Coherence theory characterizes the statistical randomness in an optical field. Typically, coherence is quantified through a two-point correlation function, which is routinely measured in the spatial domain. In principle, the coherence in the spectral domain can also be quantified by measuring the two-point correlation function. However, the existing pulse characterization techniques do not measure the spectral two-point correlation function. Here, using a quantum pulse gate, we project an unknown input field onto well define superposition of spectral bins and by measuring the QPG output intensity as a function of bin separation, we obtain the two-point correlation function. We demonstrate the measurement of the two-point correlation functions for perfectly and partially coherent pulsed fields.

Generation of controllable multiphoton states is a key element for future developments in the field of quantum algorithms and quantum communication and would enhance the computational power of such evices. Reliable sources of multiple entangled photons would be an essential step towards the implementation of quantum computers.

In this project the quantum valley Hall effect is used as a foundation to design topological valley photonic crystal structures that provide new light guiding possibilities depending on different edge states. The arrangement of different topological areas with specific topological properties allow the customization of waveguides and resonators and to realize on-chip three-photon state generation by nonlinear parametric down conversion.

Photon pair generation and quantum frequency conversion are fundamental in quantum communication. Waveguides and specifically tailored quasi-phase matching have shown an amazing potential for future quantum communication technologies, which established material platforms as LiNbO3 are about to explore. However, one often observes deviations from the expected performance, i.e. a low efficiency and distorted spectral phase-matching curves. We attribute these distortions also to imperfections of the period poling. To gather a profound understanding, how such imperfections impact on the device performance, we conduct theoretical and experimental studies how duty cycle errors affect the conversion characteristic.

Linear optical quantum networks are an increasingly relevant component in many emerging quantum technologies, e.g. gaussian boson sampling. Here, we present a novel resource efficient scheme for the implementation of such networks in a high-dimensional time-frequency encoding. This scheme is based on the use of so called quantum pulse gates (QPG), which allow for time-frequency mode selective frequency conversion. In our scheme the QPG is utilized to coherently filter and superimpose squeezed states from a type-0 PDC source in the high gain regime. This allows for the implementation of fully programmable linear quantum networks while requiring only two non-linear waveguides. In this work we develop a theoretical framework to describe these networks and utilize it to investigate the limits and scalability of our scheme.

A reliable, but cost-effective generation of single-photon states is key for a practical use of quantum communication systems. Stability, affordability, miniaturized design and fiber compatibility are essential requirements for these sources.

Here, we present the first chip-size fully integrated fiber-coupled heralded single photon source (HSPS) module based on a hybrid integration of Lithium-Niobate into a polymer board. A spontaneous parametric down conversion (SPDC) process with a 532 nm pump leads to signal and idler of 810 nm and 1550 nm. The module is fully fibercoupled with one pump input fiber and two output fibers for separated signal and idler and has a size of 2 x 1 cm². The source shows a heralded second-order correlation function of g_h⁽²⁾ = 0.05 with a heralding efficiency of n_h = 4.5 % at low pump powers.

Integrated quantum optics on a chip-scale level gained tremendous interest in the past decade. To achieve this, a material platform of choice is thin film lithium niobate (TFLN) since it features a high integration density as well as unique optical properties such as a wide transparency window, a high second order nonlinearity and high electro-optical coefficients. For the integration of complex circuits on chip, such as a quantum simulator, different components need to be developed. In this poster, we present our TFLN toolbox consisting of waveguides and structures for light routing, periodic poling for efficient frequency conversion and electro-optical modulators for fast on chip modulation of light. This toolbox opens the possibility of designing complex circuits on chip for future quantum optical applications.

Project C11 of Transregio 142 aims at the development of integrated sources for decorrelated photon pairs with high repetition rate. This goal is achieved by the joint integration of high bandwidth electro-optical modulators and a parametric down-conversion (PDC) section in a lithium niobate on insulator (LNOI) platform. With the help of the electro-optical modulators a spectro-temporal shaping of a CW input laser is realized. Two cascaded high-speed Mach-Zehnder modulators, driven by broadband high-voltage swing electronic drivers, generate Nyquist pump pulses with variable repetition rate. These pump pulses are used in a subsequent nonlinear photon pair generation process in the PDC section with specifically tailored phase-matching properties. This is engineered by periodically poled LNOI waveguides. The poster gives an overview of the proposed demonstrator focusing on the system electronics.

The objective of the Photonic Quantum Enhanced Radar System project is to investigate and demonstrate photonic quantum enhanced radar systems using at quantum pulse gate (QPG) as detector. With such a QPG enhanced approach, it is possible to beat the theoretical Rayleigh range resolution limit and the single detection Cramér-Rao bound (CRB) of classic radar systems. This is possible due to the inherent phase-sensitivity of the QPG process. In addition, the low loss property of fiber optical interconnects enables extreme aperture phased arrays and multiple input multiple output (MIMO) radar systems with aperture sizes beyond several meters, even in the millimetre wave and THz domain.

While integrated spontaneous parametric down-conversion (SPDC) is a well-established method for quantum light generation, it is mostly operated at room temperature. This limits the applications to integration with other room-temperature compatible devices since all integrated devices must be functional under the same operation conditions. We demonstrate cryogenic compatibility and customization for SPDC, which paves the way for combined integration with highly efficient superconducting detectors, the golden standard for single-photon detection. Our SPDC source does not only present good cryogenic source performance, but we can also design the spectral properties to obtain degenerate photon pairs in the telecom C-band. We show signal and idler photons centered at (1559.3 ± 0.6) nm from type-II SPDC in a periodically poled titanium in-diffused lithium niobate waveguide. Our results demonstrate a valuable understanding of the cryogenic nonlinear interaction.

As quantum states become larger, it becomes necessary to develop more advanced tools to handle them, as well as refine the techniques used to characterize these tools. Recent advancements have focused on large-scale arrays of single-photon detectors, aimed at over a million pixels. These devices are able to cover a very large Hilbert space. Accurately characterizing these devices quantum mechanically presents considerable challenges. We show an approach that utilizes high-performance computing on a supercomputer to reconstruct the positive-operator-valued measures (POVMs) of a high-dynamic range detector, covering a Hilbert space of 1.2 x 10⁶. The number of variables that need to be determined is in the order of 10⁸. These methods hold potential for application in other fields that involve reconstructing quantum systems of similar mega-scale proportions.

Photonic quantum computing is currently of high interest as it promises to solve a number of problems that are intractable to classical computers. One possible architecture for achieving this goal is Gaussian boson sampling. Gaussian boson sampling requires highly engineered quantum states as a resource, known as single spectral mode single-mode squeezed states (SMSS). Therefore, the ability to produce and characterise these particular squeezed states is paramount. We introduce a new scheme for characterizing SMSSs using photon number resolved measurements with photon number up to 8. This scheme is tested on SMSSs that are produced using a highly engineered KTP waveguide crystal. By characterizing the higher-order photon number correlations in the SMSS produced in this way, we show that the created SMSS is close to optimal and has good qualitative agreement with our theory.

In the field of ultrafast quantum photonics, nonlinear optical elements play a crucial role. Our group focuses on optimizing the use of single-photon pulses by designing new nonlinear devices and engineered waveguides. We realize quantum light sources that generate photon pairs with various properties, such as decorrelated photon pairs for heralded single-photon sources, maximally entangled states with a programmable dimensionality, and ultra-broadband entangled photons for interferometric measurements on extremely short time scales. Additionally, we develop tools for manipulating and detecting single-photon temporal mode based on the so-called quantum pulse gate, which we have extended to perform precise metrology measurements and high-dimensional multi-channel projections.

Quantum computing applications have the potential to revolutionize society by solving many problems that are intractable for classical computers, such as drug discovery. However, the creation of a functional quantum computer is such a technically challenging task, that it is not yet clear how long it will take to develop this technology. To address these challenges, people have begun to investigate other systems, such as quantum simulators and quantum samplers, that do not require the full complexity of a quantum computer. We are pioneering the development of one of these types of systems, a modular test platform for Gaussian boson sampling. Key components, like the squeezing source and detection schemes, are meticulously developed at Paderborn. Meanwhile, collaborative efforts within our consortium focus on crucial elements such as the laser source and data processing. This comprehensive platform serves as a crucial tool to validate and benchmark both the devices and algorithms under development, while simultaneously driving groundbreaking research, ensuring the advancement of quantum computing technologies.

In this work, we present an innovative approach by replacing coaxial cables with single-mode fibers and opto-electronic components. We demonstrate the all-optical operation of an SNSPD using a cryogenic photodiode for power delivery and an electro-optic modulator for signal readout. This decouples the superconducting single photon sensitive circuit from electronic interference. Notably, we achieve low-power SNSPD operation at 1K with a total power consumption of approximately 75µW. Our optical methods pave the way for advanced superconducting circuits that span from room temperature to cryogenic environments. A key aspect is the optimization of signal transformation from single photon detectors to the electro-optic modulator, which enables feed-forward modulation for quantum optic processing.