arXiv subjects

Paola Savarese

Publications and source records attributed to Paola Savarese.

4 recordsLinked to original sources

arXiv2026

Universal free-space photonic circuits for polarized structured light

Traditional linear-optical schemes for implementing discrete unitary transformations rely on multi-channel interferometers, typically composed of regular meshes of beam splitters and phase shifters. These architectures are well-suited to integrated photonics, where scalability and miniaturization are central requirements. In free space, universal transformations of optical modes have often been pursued through propagation in structured or inverse-designed optical systems, offering advantages in terms of addressable optical modes, reconfigurability, and flexible detection schemes. Here, we present a complementary approach: a collinear free-space photonic circuit that implements the standard interferometric paradigm in a Hilbert space of polarized structured-light modes. The circuit exploits spin-orbit coupling between circular polarization and optical modes carrying quantized transverse momentum, so that conventional beam splitters and phase shifters are replaced by structured optical materials acting directly on selected spin-orbit modes. In our demonstration, liquid-crystal metasurfaces serve as mode beam splitters and phase shifters, and we experimentally realize representative gates in a four-mode space. We further show how the architecture generalizes to larger numbers of modes, providing a scalable route to arbitrary unitary transformations in free space. Numerical optimization over random target unitaries supports the universality of the proposed scheme. These results establish a framework for universal free-space photonic circuits encoded in structured light and structured matter.

physics.optics↗

arXiv2025

Tomographic characterization of non-Hermitian Hamiltonians in reciprocal space

Non-Hermitian Hamiltonians enrich quantum physics by extending conventional phase diagrams, enabling novel topological phenomena, and realizing exceptional points with potential applications in quantum sensing. Here, we present an experimental photonic platform capable of simulating a non-unitary quantum walk generated by a peculiar type of non-Hermitian Hamiltonian, largely unexplored in the literature. The novelty of this platform lies in its direct access to the reciprocal space, which enables us to scan the quasi-momentum across the entire Brillouin zone and thus achieve a precise tomographic reconstruction of the underlying non-Hermitian Hamiltonian, indicated by the comparison between theoretical predictions and experimental measurements. From the inferred Hamiltonian, it is possible to retrieve complex-valued band structures, resolve exceptional points in momentum space, and detect the associated parity-time symmetry breaking through eigenvector coalescence. Our results, presented entirely in quasi-momentum space, represent a substantial shift in perspective in the study of non-Hermitian phenomena.

quant-ph↗

arXiv2025

Programmable non-Hermitian photonic quantum walks via dichroic metasurfaces

The evolution of a closed quantum system is described by a unitary operator generated by a Hermitian Hamiltonian. However, when certain degrees of freedom are coupled to an environment, the relevant dynamics can be captured by non-unitary evolution operators, arising from non-Hermitian Hamiltonians. Here we introduce a photonic platform that implements non-unitary quantum walks, commonly used to emulate open-system dynamics, in the synthetic space of light transverse momentum. These walks are realized by propagating light through a series of dichroic liquid-crystal metasurfaces, that impart polarization-dependent momentum shifts. The non-unitary behavior stems from dichroic dye molecules with polarization-dependent absorption, whose orientation is coupled to that of the liquid crystals. We demonstrate multiple walks up to five time steps, with adjustable levels of dichroism set by the metasurface voltage, which is controlled remotely. This discrete-time process maps onto two-band tight-binding models with reciprocal yet non-Hermitian nearest-neighbor couplings, corresponding to a less-studied class of non-Hermitian systems. Our platform broadens the range of optical simulators for controlled investigations of non-Hermitian quantum dynamics.

quant-ph↗

arXiv2025

Electrically tunable liquid-crystal metasurfaces with patterned birefringence and dichroism

Light propagation through artificially patterned anisotropic materials, such as dielectric metasurfaces, enables precise control of the spatio-vectorial properties of optical fields using highly transparent, thin, and flat optical elements. Liquid-crystal cells are a common realization of such devices. Optical losses are typically assumed to be polarization-independent and are therefore often overlooked in modeling these systems. In this work, we introduce electrically tunable liquid-crystal metasurfaces with patterned birefringence and dichroism, achieved by incorporating dichroic dye molecules into the liquid-crystal mixture. These dye molecules align with the liquid crystal, effectively coupling birefringence and dichroism effects. The behavior of these metasurfaces is described using non-unitary Jones matrices, validated through polarimetric measurements. In the case of devices that are patterned to form polarization gratings, we also characterize the diffraction efficiency as a function of the dichroism and birefringence parameters, which can be tuned jointly by applying an electric field across the cell. This study not only introduces a new class of optical components but also deepens our understanding of light propagation through anisotropic materials, where dichroism can naturally arise from bulk material properties or from reflection and transmission laws at their interfaces.

physics.optics↗