The observation of extreme dynamics within quantum simulators based on photonic circuits is typically precluded by optical losses, exponentially increasing with the system depth or, equivalently, with the number of optical components. This is a natural consequence of the standard approach to photonic simulations of quantum dynamics, where the complexity of the setup grows with the extension of the evolution in time. By focusing on simple protocols of discrete-time quantum walks, we show that it is possible to compress homogeneous evolutions within only three liquid-crystal metasurfaces, encompassing up to a few hundreds of time steps. By exploiting spin-orbit effects, these devices implement space-dependent polarization transformations that mix circularly polarized optical modes carrying quantized transverse momentum, mimicking the target quantum dynamics with high efficiency and accuracy. Being extremely versatile, our compact platform will pave the way to the simulations of extreme regimes of more exotic dynamics.
Helical modes of light, that is those optical spatial modes carrying a definite amount of orbital angular momentum, are proving a crucial resource in modern photonics. In all applications that rely on these modes and on their combination, a fundamental role is played by the scheme that is used to characterise the complex structure of the light beam. Here we describe a technique that applies the concept of digital holography to the study of the orbital angular momentum content of structured light [1]. In particular, the interference pattern formed by the light beam under investigation and a reference field is analysed digitally, and the complete electric field is obtained. A decomposition in term of helical modes allows one to get the orbital angular momentum spectrum of the beam in a few steps, with the possibility of retrieving also the complex radial profile associated with each mode. Requiring a simple setup and a limited number of measurements, this technique could provide a convenient strategy for the characterisation of structured light beams.
Emerged as the quantum counterpart of classical random walks, quantum walks are established precious resources in a variety of quantum sciences. Recent studies have shown that quantum walks may be characterized by topological invariants, in close analogy to condensed matter systems exhibiting topological order. Exploiting these features, quantum walks are currently used to simulate topological systems and to probe their exotic features. Here we present the implementation of a one-dimensional quantum walk protocol based on the orbital angular momentum of light, manifesting the topological phases that characterize time-periodic systems (Floquet topological insulators) showing chiral symmetry. By considering the orbital angular momentum spectrum of a light beam undergoing this quantum evolution, we show that the associated statistical moments have marked differences in distinct phases and contain information on the system topology. While varying a control parameter determining the value of the invariants, these moments in the large step-number limit exhibit a sharp variation at the phase changes. We show that these phenomena arise from the singular behavior of the dispersion relation at the transition points. The extension of our results to systems featuring different symmetries, or characterized by higher spatial dimensions, may unveil novel intriguing features associated with these complex systems.
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