Frank Setzpfandt

We investigated the spectral properties of squeezed light generated via parametric down-conversion in the high-gain regime, considering both unapodized and apodized dispersion-engineered waveguides. The gain-dependent evolution of these states is examined starting from the low-gain regime, which includes both highly correlated and nearly uncorrelated cases. For the unapodized configuration, our simulated results show a monotonic increase in spectral purity with gain, whereas for some of the apodized configuration, they exhibit a nonmonotonic dependence, initially decreasing and then recovering at higher gain. By combining Schmidt-mode analysis with a group-velocity-based interpretation, we explain why different dispersion conditions exhibit distinct gain-dependent behavior, specifically that rapid purification occurs when the pump group velocity lies between those of the signal and idler. Our study shows that the evolution of spectral purity is governed primarily by the underlying dispersion of the waveguide. These results demonstrate that dispersion engineering and parametric gain can be jointly exploited to tailor the spectral-mode structure of squeezed-light sources, enabling their optimization for a broad range of quantum applications.

Quantum imaging with undetected light has recently emerged as a technique in which quantum correlations and nonlinear interferometry are combined to decouple illumination and detection paths. This approach has been more recently extended and combined with digital phase‐shifting holography and off‐axis holography to extract both the amplitude and phase information of a sample relying on single‐photon interference. Despite these advantages, implementing the technique in real‐world scenarios where the observed system is subject to environmental noise and dynamic variations remains challenging. The primary limitation lies in the inability of quantum imaging systems to retrieve object information in real time under high‐noise conditions. Here, real‐time amplitude and phase imaging in noisy environments is experimentally demonstrated, building upon our previous implementation of quantum off‐axis holography. Our results demonstrate real‐time imaging at acquisition rates up to 4 Hz, even when the noise level exceeds the signal by an order of magnitude.

Quantum phase imaging enables the analysis of transparent samples with thickness and refractive index variations in scenarios requiring precise measurements under low-light conditions. Here, we present a compact quantum phase-gradient imaging system integrating a lithium niobate (LiNbO ₃ ) metasurface for generating spatially entangled photon pairs and a silicon (Si) metasurface for phase gradient extraction. By leveraging nonlocal resonances, the LiNbO ₃ metasurface enables efficient spontaneous parametric down-conversion (SPDC) with all-optically angularly tunable emission, while the Si metasurface provides a nearly linear optical transfer function (OTF) that differentiates the photon wavefunction and extracts phase gradients. Experimental proof-of-concept results demonstrate the imaging of up to 25 rad/mm phase gradients, achieving 89% similarity with the reference values. The pixel resolution of the system can be potentially enhanced by orders of magnitude by increasing the metasurface dimensions and resonance quality factor. Our work showcases the application of metasurfaces in both generating and detecting quantum states and establishes a paradigm for portable quantum phase-gradient imaging, with potential applications in quantum sensing, microscopy, and LiDAR technology.

Polarimetry with quantum light promises improved measurements for various scenarios. However, fundamental understanding of quantum photonic state transport in complex, real media, and tools to interpret the state after interaction with the sample are still lacking. Here, we theoretically and experimentally explore the evolution of polarization‐entangled states in a turbid medium on example of tissue phantoms. By elaborating mathematical relationship between Wolf's coherency matrix and density matrix, we introduce a versatile framework describing the transfer of entangled photons in turbid environments with polarization tracking and resulting quantum state representation with the density operator. Experimentally, we reveal a robust trend in the state evolution depending on the reduced scattering coefficient of the medium. Our theoretical predictions correlate with experimental findings, while the model extends the study by photonic states with different degrees of entanglement. The presented results pave the way for quantitative quantum photonic sensing enabling applications ranging from biomedical diagnostics to remote sensing.

The mid‐infrared (MIR) spectral region, spanning from 2‐ to 25‐, provides access to rich chemical information through the vibrational modes of functional groups, lipids, and other complex molecules. However, conventional imaging in this range remains limited by challenges in generating MIR light and by the lack of mature detection technologies. Quantum imaging with undetected light (QIUL) offers an alternative approach by using photon‐pair sources in which the sample is illuminated with MIR light while detection and image reconstruction are carried out at visible wavelengths. We report the first QIUL technique that achieves diffraction‐limited resolution without relying on spatial correlations of the photon‐pair, thereby overcoming a fundamental limitation of all previous implementations. This work establishes induced coherence without induced emission as the essential physical resource underlying QIUL and redefines its operational boundary, extending its applicability to regimes where spatial correlations are weak or absent. Our results represent a conceptual advance in quantum imaging and open the way toward practical and cost‐effective MIR microscopy based on visible detection.

Quantum nonlinear interferometers play an important role in quantum imaging and sensing applications. Here, we demonstrate the tailoring of the frequency-angular spectrum of such a device, where we use a linear dispersive medium as a phase corrector. By changing the thickness of the dispersive medium, we tune the phase of the interferometer, enabling control over the frequency-angular spectrum and visibility of the interference fringes. Utilizing the enhanced visibility achieved through our proposed technique, we show the application of a simple interferometer in spectroscopy with an undetected photon scheme. We distinguish different polymers by measuring their characteristic absorption features in the mid-infrared spectral range above 6 μm while detecting only photons in the visible range.

Quantum photon pairs play a pivotal role in many quantum applications. Metasurfaces, two-dimensional arrays of nanostructures, have been studied intensively to enhance and control pair generation via spontaneous parametric downconversion (SPDC). Van der Waals (VdW) layered materials have emerged as promising candidates for nonlinear materials in quantum light sources, owing to their high nonlinear susceptibility and compatibility with on-chip integration. In this work, we present the first demonstration of SPDC from a metasurface composed of the VdW material 3R-MoS2. The nanoresonators support quasi-bound states in the continuum (qBIC) with a quality factor of up to 120, enhancing light-matter interactions. This design achieves a 20-fold increase in SPDC rate compared to an unstructured film and significantly higher brightness, resulting in enhanced quantum photon-pair generation. This work establishes a new approach for utilizing van der Waals metasurfaces in the generation of quantum photon pairs, opening avenues for advanced quantum applications.

Quantum photon pairs play a pivotal role in many quantum applications. Metasurfaces, two-dimensional arrays of nanostructures, have been studied intensively to enhance and control pair generation via spontaneous parametric downconversion (SPDC). Van der Waals (VdW) layered materials have emerged as promising candidates for nonlinear materials in quantum light sources, owing to their high nonlinear susceptibility and compatibility with on-chip integration. In this work, we present the first demonstration of SPDC from a metasurface composed of the VdW material 3R-MoS₂. The nanoresonators support quasi-bound states in the continuum (qBIC) with a quality factor of up to 120, enhancing light-matter interactions. This design achieves a 20-fold increase in SPDC rate compared to an unstructured film and significantly higher brightness, resulting in enhanced quantum photon-pair generation. This work establishes a new approach for utilizing van der Waals metasurfaces in the generation of quantum photon pairs, opening avenues for advanced quantum applications.

Tunable biphoton quantum entanglement generated from nonlinear flat optics is highly desirable for cutting-edge quantum technologies, yet its tunability is substantially constrained by the symmetry of material nonlinear tensors. Here, we overcome this constraint by introducing symmetry breaking in nonlinear polarization via resonant metasurfaces. While asymmetric optical responses have enabled breakthroughs in classical applications like nonreciprocal light transmission, we report the experimental demonstration of asymmetric nonlinear responses for biphoton entanglement. Using a structural-asymmetric semiconductor metasurface incorporating [110] InGaP nanoresonators, we realize continuous tuning of polarization entanglement from partially entangled states to a Bell state by adjusting the pump wavelength. We also observe pronounced spatial anti-correlations and theoretically confirm that this approach can extend to tailor hyperentanglement. Furthermore, our nanoscale entanglement source features an ultrahigh coincidence-to-accidental ratio of ≈7 × 10 ⁴ , outperforming existing semiconductor flat optics by two orders of magnitude. Introducing asymmetric nonlinear response in quantum metasurfaces opens directions for tailoring on-demand quantum states.