arXiv subjects

Lea Kopf

Publications and source records attributed to Lea Kopf.

7 recordsLinked to original sources

arXiv2026

Faking entanglement with imperceptible measurement deviations

Quantum entanglement is a central resource underpinning emerging quantum technologies, enabling capabilities beyond those of classical systems. Accurate verification of entanglement is therefore crucial. However, experimental schemes usually rely on the assumption that quantum measurements can be realized exactly. As the complexity of a quantum system grows, this assumption typically becomes increasingly unrealistic, therefore leading to a widening mismatch between theoretical models and experimental implementations. Here we demonstrate that arbitrarily small measurement errors, when adversarially encoded in the measurement apparatus, can lead to the false certification of high-dimensional entanglement in systems that are, in fact, separable. This is achieved by introducing explicit hacking attacks to measurement devices in well-established entanglement verification tests. We further experimentally demonstrate this effect using classical photonic states encoded in the spatial degree of freedom, spanning up to 61 dimensions with measurement fidelity errors as low as 0.23%. Our results uncover a fundamental vulnerability in current methods for high-dimensional entanglement detection, highlighting the susceptibility of complex quantum devices to small adversarial perturbations. The findings underscore the need for developing secure verification of quantum information that is robust to bounded discrepancies between theory and experiment.

quant-ph↗

arXiv2024

Conservation of angular momentum on a single-photon level

Identifying conservation laws is central to every subfield of physics, as they illuminate the underlying symmetries and fundamental principles. A prime example can be found in quantum optics: The conservation of orbital angular momentum (OAM) during spontaneous parametric down-conversion (SPDC) enables the generation of a photon pair with entangled OAM. In this article, we report on the first study of OAM conservation in SPDC pumped by single photons. Our results present the first implementation of cascaded down-conversion without waveguides, setting the stage for experiments on the direct generation of multi-photon high-dimensional entanglement using all degrees of freedom of light.

quant-ph↗

arXiv2024

Higher-order Poincar\'e Spheres and Spatio-Spectral Poincar\'e Beams

The study of fundamental optics effects has been stimulated through the increasing ability to structure light in all its degrees of freedom (DOFs) in sophisticated but simple experimental settings. However, with such an increase in experimental capabilities, it has also become important to study theoretical descriptions for a more intuitive understanding of the underlying concepts. Here, we introduce a visual representation of light that is structured in its transverse space, frequency, and polarization in the form of a higher-order Poincar\'e sphere and discuss interesting links to its fundamental counterpart. We further leverage this connection to discuss and experimentally generate light possessing all possible polarization states across its spatio-spectral shape, which we term spatio-spectral Poincar\'e beams. By invoking all DOFs of light in the powerful description of higher-order Poincar\'e spheres, our work can pave the way for a deeper understanding and beneficial application of structured light as a powerful tool in optics.

physics.optics↗

arXiv2023

Correlating space, wavelength, and polarization of light: Spatio-Spectral Vector Beams

Increasing the complexity of a light field through the advanced manipulation of its degrees of freedom (DoF) provides new opportunities for fundamental studies and technologies. Correlating polarization with the light's spatial or spectral shape results in so-called spatial or spectral vector beams that are fully polarized and have a spatially or spectrally varying polarization structure. Here, we extend the general idea of vector beams by combining both approaches and structuring a novel state of light in three non-separable DoF's, i.e. space, wavelength, and polarization. We study in detail their complex polarization structure, show that the degree of polarization of the field is only unveiled when the field is narrowly defined in space and wavelength, and demonstrate the analogy to the loss of coherence in non-separable quantum systems. Such light fields allow fundamental studies on the non-separable nature of a classical light field and new technological opportunities, e.g. through applications in imaging or spectroscopy.

physics.optics↗

arXiv2021

Talbot self-imaging and two-photon interference in ring-core fibers

Wave propagation on the surface of cylinders exhibits interferometric self imaging, much like the Talbot effect in the near-field diffraction at periodic gratings. We report the experimental observation of the cylindrical Talbot carpet in weakly-guiding ring-core fibers for classical light fields. We further show that the ring-core fiber acts as a high-order optical beamsplitter for single photons, whose output can be controlled by the relative phase between the input light fields. By also demonstrating high-quality two-photon interference between indistinguishable photons sent into the ring-core fiber, our findings open the door to applications in optical telecommunications as a compact beam multiplexer as well as in quantum information processing tasks as a scalable realization of a linear optical network.

physics.optics↗

arXiv2021

Endless Fun in high dimensions -- A Quantum Card Game

Quantum technologies, i.e., technologies benefiting from the features of quantum physics such as objective randomness, superposition, and entanglement, have enabled an entirely different way of distributing and processing information. The enormous progress over the last decades has also led to an urgent need for young professionals and new educational programs. Here, we present a strategic card game in which the building blocks of a quantum computer can be experienced. While playing, participants start with the lowest quantum state, play cards to "program" a quantum computer, and aim to achieve the highest possible quantum state. Thereby they experience quantum features such as superposition, interference, and entanglement. By also including high-dimensional quantum states, i.e., systems that can take more than two possible values, and by developing different multi-player modes, the game can help the players to understand complex quantum state operations and can also be used as an introduction to quantum computational tasks for students. As such, it can also be used in a classroom environment to increase the conceptual understanding, interest, and motivation of a student. Therefore, the presented game contributes to the ongoing efforts on gamifying quantum physics education with a particular focus on the counter-intuitive features which quantum computing is based on.

quant-ph↗

arXiv2021

Spectral Vector Beams for High-Speed Spectroscopic Measurements

Structured light harnessing multiple degrees of freedom has become a powerful approach to use complex states of light in fundamental studies and applications. Here, we investigate the light field of an ultrafast laser beam with a wavelength-depended polarization state, a beam we term spectral vector beam. We demonstrate a simple technique to generate and tune such structured beams and demonstrate their spectroscopic capabilities. By only measuring the polarization state using fast photodetectors, it is possible to track pulse-to-pulse changes in the frequency spectrum caused by, e.g. narrowband transmission or absorption. In our experiments, we reach read-out rates of around 6 MHz, which is limited by our technical ability to modulate the spectrum and can in principle reach GHz read-out rates. In simulations we extend the spectral range to more than 1000 nm by using a supercontinuum light source, thereby paving the way to various applications requiring high-speed spectroscopic measurements.

physics.optics↗