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We investigate theoretically the confinement of light in a three-dimensional (3D) superlattice of cavities that is embedded in a 3D photonic band gap crystal. Such a superlattice serves to trap photons and manipulate their behavior, which is relevant in applications like photovoltaic absorp¬tion enhancement, Anderson localization of light [And58], or photonic computing [Har08]. As a host of the 3D superlattice we choose the so-called inverse woodpile structure that consists of two perpendicular sets of pores in a high-refractive-index backbone. Inverse woodpile crystals made from silicon, as realized in our group, have a broad 3D photonic band gap [Lei11], ideal for cavity confinement. Cavities are defined in the inverse woodpile structure by introducing two proximate defect pores to have smaller radius than the other pores [Wol14]. Near the intersection of the de¬fect pores, excess silicon traps light. Indeed, band structure calculations reveal flat defect bands in the band gap, typical of cavity resonances. For the optimal pore size for the band gap, the single-cavity resonances are identified to have quadrupolar symmetry [Dev19]. When the band gap cavities are weakly coupled in a superlattice, light effectively hops from cavity to cavity, along main Cartesian (x,y,z) directions, hence the name “Cartesian light” [Hac19]. It seems that resonances (and the band gap) are strong functions of the crystal pore radius and the defect pore radius. But this behavior has to date not been mapped, whereas such a map is crucial to interpret ongoing experiments, where real crystals have pore radii that differ from the design, or pore radii that depend on the location where the crystal is probed. Here, we set out to obtain such a map, and study whether the resonances remain quadrupolar. We use the plane-wave expansion with supercells to assess the open questions. In certain parts of parameter space, the number of resonances changes drastically compared to the known behavior: there are sometimes just a few (perhaps displaying dipolar behavior) and in other ranges 11 to 13 resonances that are attributed to higher order multipoles. At the same time our results show good agreement with available experimental data and offer new insights into the physics of light confinement and propagation through 3D nanostructures. REFERENCES [And58] P.W. Anderson, Phys. Rev. 109, 1492 (1958) [Dev19] D. Devashish, et al., Phys. Rev. B 99, 075112 (2019) [Hac19] S.A. Hack, J.J.W. van der Vegt, and W.L. Vos, Phys. Rev. B 99, 115308 (2019) [Ho94] K. Ho, C. Chan, C. Soukoulis, R. Biswas, M. Sigalas, Solid State Commun. 89, 413 (1994) [Har08] M.J. Hartmann, F.G.S.L. Brandao, and M.B. Plenio, Laser & Photon. Rev. 2, 527 (2008) [Lei11] M.D. Leistikow, et al., Phys. Rev. Lett. 107, 193903 (2011) [Wol14] L.A. Woldering, A.P. Mosk, and W.L. Vos, Phys. Rev. B. 90, 115140 (2014)

Recently, we applied wavefront shaping techniques to implement quantum-secure readout of a physical unclonable key for authentication [1]. Unfortunately, spatial wavefront shaping is unsuitable for long distance use. We therefore propose to use wavefront shaping in the time domain. We show temporal focusing of a pulse through a complex medium. Furthermore, we investigate a secure communication method based on physical unclonable functions in the time domain. We will report on the progress. [1] S.A. Goorden et al., Quantum-secure authentication of a physical unclonable key, Optica 1 , 421-424 (2014)

We provide a pedagogical introduction to the concept of the local density of optical states (LDOS), illustrating its application to both the classical and quantum theory of radiation. We show that the LDOS governs the efficiency of a macroscopic classical antenna, determining how the antenna's emission depends on its environment. The LDOS is shown to similarly modify the spontaneous emission rate of a quantum emitter, such as an excited atom, molecule, ion, or quantum dot that is embedded in a nanostructured optical environment. The difference between the number density of optical states, the local density of optical states, and the partial local density of optical states is elaborated and examples are provided for each density of states to illustrate where these are required. We illustrate the universal effect of the LDOS on emission by comparing systems with emission wavelengths that differ by more than 5 orders of magnitude, and systems whose decay rates differ by more than 5 orders of magnitude. To conclude we discuss and resolve an apparent difference between the classical and quantum expressions for the spontaneous emission rate that often seems to be overlooked, and discuss the experimental determination of the LDOS., 80 pages, 19 figures

The identification of a complete three-dimensional (3D) photonic band gap in real crystals always employs theoretical or numerical models that invoke idealized crystal structures. Thus, this approach is prone to false positives (gap wrongly assigned) or false negatives (gap missed). Therefore, we propose a purely experimental probe of the 3D photonic band gap that pertains to many different classes of photonic materials. We study position and polarization-resolved reflectivity spectra of 3D inverse woodpile structures that consist of two perpendicular nanopore arrays etched in silicon. We observe intense reflectivity peaks $(R > 90\%)$ typical of high-quality crystals with broad stopbands. We track the stopband width versus pore radius, which agrees much better with the predicted 3D photonic band gap than with a directional stop gap on account of the large numerical aperture used. A parametric plot of s-polarized versus p-polarized stopband width agrees very well with the 3D band gap and is model-free. This practical probe provides fast feedback on the advanced nanofabrication needed for 3D photonic crystals and stimulates practical applications of band gaps in 3D silicon nanophotonics and photonic integrated circuits, photovoltaics, cavity QED, and quantum information processing., 11 pages, 12 figures

We investigate theoretically the confinement of light in a three-dimensional (3D) superlattice of cavities that is embedded in a 3D photonic band gap crystal. Such a superlattice serves to trap photons and manipulate their behavior, which is relevant in applications like photovoltaic absorp¬tion enhancement, Anderson localization of light [And58], or photonic computing [Har08]. As a host of the 3D superlattice we choose the so-called inverse woodpile structure that consists of two perpendicular sets of pores in a high-refractive-index backbone. Inverse woodpile crystals made from silicon, as realized in our group, have a broad 3D photonic band gap [Lei11], ideal for cavity confinement. Cavities are defined in the inverse woodpile structure by introducing two proximate defect pores to have smaller radius than the other pores [Wol14]. Near the intersection of the de¬fect pores, excess silicon traps light. Indeed, band structure calculations reveal flat defect bands in the band gap, typical of cavity resonances. For the optimal pore size for the band gap, the single-cavity resonances are identified to have quadrupolar symmetry [Dev19]. When the band gap cavities are weakly coupled in a superlattice, light effectively hops from cavity to cavity, along main Cartesian (x,y,z) directions, hence the name “Cartesian light” [Hac19]. It seems that resonances (and the band gap) are strong functions of the crystal pore radius and the defect pore radius. But this behavior has to date not been mapped, whereas such a map is crucial to interpret ongoing experiments, where real crystals have pore radii that differ from the design, or pore radii that depend on the location where the crystal is probed. Here, we set out to obtain such a map, and study whether the resonances remain quadrupolar. We use the plane-wave expansion with supercells to assess the open questions. In certain parts of parameter space, the number of resonances changes drastically compared to the known behavior: there are sometimes just a few (perhaps displaying dipolar behavior) and in other ranges 11 to 13 resonances that are attributed to higher order multipoles. At the same time our results show good agreement with available experimental data and offer new insights into the physics of light confinement and propagation through 3D nanostructures. REFERENCES [And58] P.W. Anderson, Phys. Rev. 109, 1492 (1958) [Dev19] D. Devashish, et al., Phys. Rev. B 99, 075112 (2019) [Hac19] S.A. Hack, J.J.W. van der Vegt, and W.L. Vos, Phys. Rev. B 99, 115308 (2019) [Ho94] K. Ho, C. Chan, C. Soukoulis, R. Biswas, M. Sigalas, Solid State Commun. 89, 413 (1994) [Har08] M.J. Hartmann, F.G.S.L. Brandao, and M.B. Plenio, Laser & Photon. Rev. 2, 527 (2008) [Lei11] M.D. Leistikow, et al., Phys. Rev. Lett. 107, 193903 (2011) [Wol14] L.A. Woldering, A.P. Mosk, and W.L. Vos, Phys. Rev. B. 90, 115140 (2014)

We developed an experimental set-up and procedure to measure spectral and angular solar irradiance to accurately predict and optimize solar power plant performance. The set-up is comprised of a fiber coupled spectrometer mounted to a rotational stage which allows for 360⁰ light capture. The lightweight and flexible design enables irradiance measurements at any location. Measurements were taken in Enschede, Netherlands and in Phoenix, AZ, USA. We find a strong dependence of spectro-angular irradiance on the location, surroundings and cloud coverage which needs to be considered when modelling and optimizing location dependent solar power plant output. Index Terms — Bifacial solar cells, spectral albedo, spectral and angular irradiance, solar irradiance measurements.

Three-dimensional (3D) nanostructures are drawing a fast-growing attention for their advanced functionalities in nanophotonics [1], photovoltaics, and novel 3D integrated circuits and flash memories. The functionalities of such nanostructures are fundamentally determined by their complex internal structure. Inevitably, any fabricated nanostructure differs from its initial design. Hence, the observed functionality differs from the expected one. It is thus critical to assess the structure of a 3D nanomaterial and verify how well it matches the design. Ideally such an inspection technique leaves no traces of the inspection in order to leave the nanostructure fully functional and ready for integration. Here, we introduce traceless X-ray tomography (TXT) as a new methodology in nanotechnology to non-destructively assess the functionality of nanostructures.

Secure communication became extremely important in the Information Age [1]. Quantum communication protocols have been developed to provide absolutely secure transmission of information [2]. Historically, quantum key distribution (QKD) uses the two-dimensional polarization basis to encode information and the key element of long-distance communication is single-mode optical fiber. However, it intrinsically limits the information content to 1 bit per photon. Nowadays, significant effort is devoted to increase the data carrying capacity of a single quantum secure optical line [3]. Large-core multimode fibers support a multitude of transverse optical modes and can potentially transfer information at much higher density.

There is a wide interest in three-dimensional (3D) photonic crystals that radically control the spontaneous emission of embedded quantum emitters and cavity QED, control thermal emission, allow for efficient miniature lasers, or efficient photoelectric conversion in solar cells [1,2]. Curiously, the experimental demonstration of a 3D photonic band gap remains a major challenge. To probe the band gap, spectra or dynamics are studied of emitters positioned inside the crystal, but such experiments are difficult for a number of practical reasons. On the other hand, a band gap is indicated by the overlap of stop bands seen in directional reflectivity or transmission experiments. A reflectivity peak (or transmission trough) may also occur due to other physical reasons. Hence experimentally observed stop bands are typically interpreted by comparing to band structures that, however, only pertain to infinite and perfect crystals. Thus there remains scope for a purely experimental probe of a 3D band gap, especially in view of the unavoidable deviations from perfect periodicity [3].

The confinement of light to a tiny volume is an important drive in nanophotonics. Notable applications such as trapping of photons, sensing for biotechnology, control of spontaneous emission and Cherenkov radiation, lasing, and cavity QED [1-3]. While much work has been done on microspheres, micropillars, microdisks, plasmonic cavities, toroidal rings, or 2D photonic crystals, there are a few studies on cavities in a three-dimensional (3D) photonic band gap crystal. Therefore, we study the transport and storage of light in a cavity in a 3D photonic band gap crystal slab. The crystal has finite support as it is surrounded by free space, as in experiments. We employ the finite element method (on a computer cluster) to model the diamond-like inverse woodpile crystal that consists of two orthogonal arrays of pores in a high-index dielectric such as silicon and that has experimentally been realized by CMOS-compatible methods [4]. The resonant cavity is formed in the proximal region of two selected orthogonal pores with a radius smaller than the ones in the bulk of the crystal [5].

In this work, a novel multidiode model is proposed for optimizing the front grid of multijunction solar cells operating under concentration conditions. The model allows for quickly exploring the maximum achievable efficiency under a wide range of operating conditions and design parameters such as the redirecting capability, period and width of the fingers, the light concentration, and the metal and emitter sheet resistivity. The proposed multidiode model shows to be consistent with experimental data and with more complex modeling approaches such as the simulation program with integrated circuit emphasis (SPICE) model.

It is a major outstanding goal in Nanophotonics to precisely place quantum emitters inside a three-dimensional (3D) metamaterial. It is well-known that such control offers exquisite control over cavity QED, spontaneous and stimulated emission, and even non-linear optics [1]. Theory predicts that the emission of an emitter, e.g. a quantum dot, varies spatially on 100s nm scale [2]. Thus, the challenge is to place emitters with a precision better than Δx < 100 nm. We present our newly developed chemical toolbox to fix the positions of quantum dots with a polymer brush layer with thicknesses in the 10s nm range in silicon nanostructures [3].

We explore the unconventional propagation of light in a three-dimensional (3D) superlattice of coupled resonant cavities in a 3D photonic band gap crystal (Fig. 1). Such a 3D cavity superlattice is the photonic analogue of the Anderson model for spins and electrons in the limit of zero disorder [1]. Using the plane-wave expansion method [2], we calculate the dispersion relations of the 3D cavity superlattice with the cubic inverse woodpile structure [3,4] that reveal five coupled-cavity bands. These bands have quadrupolar symmetry and are the optical analogues of d-orbitals in solid-state physics [4,5]. Therefore, one might naively expect neighboring cavities to couple mostly in diagonal directions.

Deterministic fabrication of random metamaterials requires filling of a space with randomly oriented and randomly positioned chords with an on-average homogenous density and orientation, which is a nontrivial task. We describe a method to generate fillings with such chords, lines that run from edge to edge of the space, in any dimension. We prove that the method leads to random but on-average homogeneous and rotationally invariant fillings of circles, balls and arbitrary-dimensional hyperballs from which other shapes such as rectangles and cuboids can be cut. We briefly sketch the historic context of Bertrand's paradox and Jaynes' solution by the principle of maximum ignorance. We analyse the statistical properties of the produced fillings, mapping out the density profile and the line-length distribution and comparing them to analytic expressions. We study the characteristic dimensions of the space in between the chords by determining the largest enclosed circles and balls in this pore space, finding a lognormal distribution of the pore sizes. We apply the algorithm to the direct-laser-writing fabrication design of optical multiple-scattering samples as three-dimensional cubes of random but homogeneously positioned and oriented chords., Comment: 10 pages, 12 figures; v3: restructured paper, more references, more graphs

We are developing effectively transparent superstrates for perovskite solar cells. These superstrates incorporate triangular-shaped grid fingers with thin transparent conductors, which mitigate shading losses associated with conventional grid fingers and/or thick TCO layers. We have investigated various types of polymers for use as superstrates in both rigid (glass-backed) and flexible applications. We have developed an electroplating process to fabricate the triangular gridlines, which improves their conductivity vs. prior silver ink printing approaches. We will present a summary of perovskite solar cells that were fabricated on these effectively transparent superstrates, including a discussion of their photovoltaic efficiency, spectral response, and LBIC mapping. The results demonstrate that effectively transparent superstrates enable an improvement in the short-circuit current density and fill factor for large-area perovskite solar cells. Perovskite solar cells are of great interest due to their potential for low cost and high performance. One of the challenges to attaining high photovoltaic conversion efficiency, particularly for large-area cells, is the tradeoff between the optical and electrical performance of the top contact. Because perovskite absorbers and selective electrode materials provide very little lateral conductivity for current collection, a transparent conductive oxide (TCO) such as indium tin oxide (ITO) must be used for the front contact. However, TCOs offer a tradeoff between transparency and conductivity, resulting in a compromise between short-circuit current density (due to optical losses) and fill factor (due to resistive losses) for solar cells. A solution is to increase the density of the grid fingers such that thinner TCOs can be used; however, this increases the shading losses. Recently, a method to produce effectively transparent front contact grids has been described (Adv. Optical Mater. 4 (10), 1470-1474 (2016); Photovoltaic Specialists Conference (PVSC) IEEE 43rd, 3612-3615, (2016); Sustainable Energy and Fuels, 1 (3), 593-598, (2017)). This approach yields a relatively dense array of high-aspect-ratio, triangular-shaped front contact fingers, in which light striking the metal is reflected towards the cell. Our current work pertains to the application of this technology to perovskite solar cells.

We explore the visual appearance of microscale silver grids for use in solar windows. The distortion of view is investigated by ray optical computations and experiments. We find that the visual appearance strongly depends on the geometric properties of the microcontacts (i.e., shape, aspect ratio, and periodicity) as well as on the viewing angle. From a perpendicular view, glass substrates appear marginally darker with microcontacts. In the case of effectively transparent triangular cross-section microcontacts, observation of scenes under a steep angle with respect to the substrate leads to the appearance of "ghost images." Scenes are projected through redirection of rays by triangular cross-section microcontacts which can be tuned by adjusting aspect ratio and periodicity.

Laser wakefield acceleration (LWFA) has emerged as a promising concept for the next generation of high energy electron accelerators. The acceleration medium is provided by a target that creates a local well-defined gas-density profile inside a vacuum vessel. Target development and analysis of the resulting gas-density profiles is an important aspect in the further development of LWFA. Gas-jet targets are widely used in regimes where relatively high electron densities over short interaction lengths are required (up to several millimetres interaction length, plasma densities down to ~1018 cm-3). In this paper we report a precise characterisation of such gas-jet targets by a laser interferometry technique. We show that phase shifts down to 4 mrad can be resolved. Tomographic phase reconstruction enables detection of non-axisymmetrical gas-density profiles which indicates defects in cylindrical nozzles, analysis of slit-nozzles and nozzles with an induced shock-wave density step. In a direct comparison between argon and helium jets we show that it cannot automatically be assumed, as is often done, that a nozzle measured with argon will provide the same gas density with helium.

We study the energy density of shaped waves inside a quasi-1D disordered waveguide. We find that the spatial energy density of optimally shaped waves, when expanded in the complete set of eigenfunctions of the diffusion equation, is well described by considering only a few of the lowest eigenfunctions. Taking into account only the fundamental eigenfunction, the total internal energy inside the sample is underestimated by only 2%. The spatial distribution of the shaped energy density is very similar to the fundamental eigenfunction, up to a cosine distance of about 0.01. We obtain the energy density of transmission eigenchannels inside the sample by numerical simulation of the scattering matrix. Computing the transmission-averaged energy density over all transmission channels yields the ensemble averaged energy density of shaped waves. From the averaged energy density, we reconstruct its spatial distribution using the eigenfunctions of the diffusion equation. The results of our study have exciting applications in controlled biomedical imaging, efficient light harvesting in solar cells, enhanced energy conversion in solid-state lighting, and low threshold random lasers.

We explore the unconventional propagation of light in a three-dimensional (3D) superlattice of coupled resonant cavities in a 3D photonic band-gap crystal. Such a 3D cavity superlattice is the photonic analog of the Anderson model for spins and electrons in the limit of zero disorder. Using the plane-wave expansion method, we calculate the dispersion relations of the 3D cavity superlattice with the cubic inverse woodpile structure that reveal five coupled-cavity bands, typical of quadrupole-like resonances. For three out of five bands, we observe that the dispersion bandwidth is significantly larger in the $({k}_{x},{k}_{z})$-diagonal directions than in other directions. To explain the directionality of the dispersion bandwidth, we employ the tight-binding method from which we derive coupling coefficients in three dimensions. For all converged coupled-cavity bands, we find that light hops predominantly in a few high-symmetry directions including the Cartesian $(x,y,z)$ directions, therefore we propose the name ``Cartesian light.'' Such 3D Cartesian hopping of light in a band gap yields propagation as superlattice Bloch modes that differ fundamentally from the conventional 3D spatially extended Bloch wave propagation in crystals, from light tunneling through a band gap, from coupled-resonator optical waveguiding, and also from light diffusing at the edge of a gap.

Gaussian boson sampling is a promising scheme for demonstrating a quantum computational advantage using photonic states that are accessible in a laboratory and, thus, offer scalable sources of quantum light. In this contribution, we study two-point photon-number correlation functions to gain insight into the interference of Gaussian states in optical networks. We investigate the characteristic features of statistical signatures which enable us to distinguish classical from quantum interference. In contrast to the typical implementation of boson sampling, we find additional contributions to the correlators under study which stem from the phase dependence of Gaussian states and which are not observable when Fock states interfere. Using the first three moments, we formulate the tools required to experimentally observe signatures of quantum interference of Gaussian states using two outputs only. By considering the current architectural limitations in realistic experiments, we further show that a statistically significant discrimination between quantum and classical interference is possible even in the presence of loss, noise, and a finite photon-number resolution. Therefore, we formulate and apply a theoretical framework to benchmark the quantum features of Gaussian boson sampling under realistic conditions.

We study numerically the transport and storage of light in a 3D photonic band gap crystal doped by a single embedded resonant cavity. The crystal has finite support since it is surrounded by vacuum, as in experiments. Therefore, we employ the finite element method to model the diamond-like inverse woodpile crystal that consists of two orthogonal arrays of pores in a high-index dielectric such as silicon and that has experimentally been realized by CMOS-compatible methods. A point defect that functions as the resonant cavity is formed in the proximal region of two selected orthogonal pores with a radius smaller than the ones in the bulk of the crystal. We present a field-field cross-correlation method to identify resonances in the finite-support crystal with defect states that appear in the 3D photonic band gap of the infinite crystal. Out of 5 observed angle-independent cavity resonances, one is s-polarized and 4 are p-polarized for light incident in the X or Z directions. It is remarkable that quality factors up to Q = 1000 appear in such thin structures (3 unit cells). We find that the optical energy density is remarkably enhanced at the cavity resonances by up to 2400x the incident energy density in vacuum or up to 1200x the energy density of the equivalent effective medium. We find that an inverse woodpile photonic band gap cavity with a suitably adapted lattice parameter reveals substantial absorption in the visible range. Below the 3D photonic band gap, Fano resonances arise due to interference between the discrete fundamental cavity mode and the continuum light scattered by the photonic crystal. We argue that the five eigenstates of our 3D photonic band gap cavity have quadrupolar symmetry, in analogy to d-like orbitals of transition metals. We conclude that inverse woodpile cavities offer interesting perspectives for applications in optical sensing and photovoltaics., 18 pages, 19 figures

Landauer's erasure principle states that the irreversible erasure of a one-bit memory, embedded in a thermal environment, is accompanied with a work input of at least $k_{\text{B}}T\ln2$. Fundamental to that principle is the assumption that the physical states representing the two possible logical states are close to thermal equilibrium. Here, we propose and theoretically analyze a minimalist mechanical model of a one-bit memory operating with squeezed thermal states. It is shown that the Landauer energy bound is exponentially lowered with increasing squeezing factor. Squeezed thermal states, which may naturally arise in digital electronic circuits operating in a pulse-driven fashion, thus can be exploited to reduce the fundamental energy costs of an erasure operation., 5 pages, 3 figures

Controlling the flow of light is a fundamental requirement for quantum simulations with light. We have recently introduced two novel direct laser writing methods for the microstructuring of mirrors, which allow us to control the transverse flow of light in a high finesse optical microresonator [1]. In particular, it is possible to capture photons onto periodic lattices sites and to control their tunnelling rate. In future work, this will enable optical simulations of classical spin systems. In particular, we aim at a device that is capable of finding the ground state of spin glass systems, which is a well-know computationally hard problem, faster than any other computer. [1] D. Dung, C. Kurtscheid, T. Damm, J. Schmitt, F. Vewinger, M. Weitz, and J. Klaers, “Variable potentials for thermalized light and coupled condensates”, Nature Photonics 11, 565 (2017).

The last decades were beneficial for nanofabrication techniques but validation of the increasingly complex nano-structures became more challenging. Recent advancements of X-ray tomography permit 3D reconstruction down to 20 nm resolution. We propose the use of this non-invasive technique to quantify the fabrication quality of 3D structures made with direct laser writing (DLW). The comparison of X-ray density function with the designed fabrication density map brings into light information of how polymerization takes place throughout the fabricated volume and also gives the opportunity to estimate the impact on the optical response of such media.

In this work, a novel multidiode model is proposed for optimizing the front grid of multijunction solar cells operating under concentration conditions. The model allows for quickly exploring the maximum achievable efficiency under a wide range of operating conditions and design parameters such as the redirecting capability, period and width of the fingers, the light concentration, and the metal and emitter sheet resistivity. The proposed multidiode model shows to be consistent with experimental data and with more complex modeling approaches such as the simulation program with integrated circuit emphasis (SPICE) model.

The tunneling of waves, electrons, photons, and phonons incident on a periodic medium is limited to a certain distance, the Bragg length L B , due to the reflection arising from Bragg interference. While this perfect reflection by a crystal has enabled their application in solar cells [1] and optical memories [2], the limited tunnelling depth does not allow any functionalizing of periodic medium beyond a Bragg length.

Solar energy has sustained incredible technological advancement for several decades, driving down costs to allow rapid deployment of photovoltaic (PV) technology. However, public perception and understanding of these advancements is hard-pressed to keep up. The pace of information dissemination will become a limiting factor, in that public policy and practices will act on outdated information regarding PV tech. The PV community must engage in science communication and education practices appropriate for the new digital age to ensure a continued, increasing pace of PV deployment. To that end, the authors have created a podcast, and will share their findings on impact and best practices.

The confinement of light in three dimensions (3D) is an active research topic in Nanophotonics, since it allows for ultimate control over photons [1]. A powerful tool to this end is a 3D photonic band gap crystal with a tailored defect that acts as a cavity or even a waveguide [2]. When a one-dimensional array of cavities is coupled, an intricate waveguiding system appears, known as a CROW (coupled resonator optical waveguide) [3]. Remarkably, 3D superlattices of coupled cavities that resonate inside a 3D band gap have not been studied to date. Recently, theoretical work has predicted the occurrence of 'Cartesian light', wherein light propagates by hopping only in high symmetry directions in space [4]. This represents the optical analog of the Anderson model for spins or electrons that is relevant for neuromorphic computing and may lead to intricate lasing [5].

Performance of solar cells is usually determined under AM 1.5 conditions rarely existing at power plant sites. We present the dependence of solar cell output on the spectro-angular solar irradiance.

Novel computational tools in machine learning open new perspectives in quantum information systems. Here we adopt the open-source programming library Tensorflow to design multi-level quantum gates including a computing reservoir represented by a random unitary matrix. In optics, the reservoir is a disordered medium or a multimodal fiber. We show that by using trainable operators at the input and at the readout, it is possible to realize multi-level gates. We study single and qudit gates, including the scaling properties of the algorithms with the size of the reservoir.

We show that boson sampling is classically simulable in the presence of linear loss (constant per-photon transmission). We discuss the impact that this has on experimental efforts to demonstrate a quantum advantage.

Integrated universal linear optical networks are essential for the development of quantum information processing (QIP). We demonstrate a universal, reconfigurable, 8×8 photonic processor based on Si3N4 waveguides showing a variety of QIP primitives.

We explore the unconventional propagation of light in a three-dimensional (3D) superlattice of coupled resonant cavities in a 3D photonic band gap crystal. Such a 3D cavity superlattice is the photonic analogue of the Anderson model for spins and electrons in the limit of zero disorder. Using the plane-wave expansion method, we calculate the dispersion relations of the 3D cavity superlattice with the cubic inverse woodpile structure that reveal five coupled-cavity bands, typical of quadrupole-like resonances. For three out of five bands, we observe that the dispersion bandwidth is significantly larger in the ($k_x$,$k_z$)-diagonal directions than in other directions. To explain the directionality of the dispersion bandwidth, we employ the tight-binding method from which we derive coupling coefficients in 3D. For all converged coupled-cavity bands, we find that light hops predominantly in a few high-symmetry directions including the Cartesian (x,y,z) directions, therefore we propose the name "Cartesian light". Such 3D Cartesian hopping of light in a band gap yields propagation as superlattice Bloch modes that differ fundamentally from the conventional 3D spatially-extended Bloch wave propagation in crystals, from light tunneling through a band gap, from coupled-resonator optical waveguiding, and also from light diffusing at the edge of a gap.

We have developed superstrates for perovskite solar cells that feature increased transparency and conductivity due to the incorporation of effectively transparent contacts (ETCs). They increase the short-circuit current density by more than 1 mA/cm2 compared to standard indium tin oxide (ITO) on glass superstrates. These ETC superstrates are composed of sodalime glass with a thin $( \sim 40 \mu \mathrm {m})$ layer of polydimethylsiloxane (PDMS) that features triangular cross-section microscale grooves, which are infilled with a conductive silver ink and subsequently coated by a thin $( \sim 30$ nm) ITO layer such that high lateral conductivity $( < 5 \Omega /$sq) is achieved without altering the surface properties of standard perovskite superstrates.

Resonant cavities with high quality factor and small mode volume provide crucial enhancement of light-matter interactions in nanophotonic devices that transport and process classical and quantum information. The production of functional circuits containing many such cavities remains a major challenge as inevitable imperfections in the fabrication detune the cavities, which strongly affects functionality such as transmission. In photonic crystal waveguides, intrinsic disorder gives rise to high-Q localized resonances through Anderson localization, however their location and resonance frequencies are completely random, which hampers functionality. We present an adaptive holographic method to gain reversible control on these randomly localized modes by locally modifying the refractive index. We show that our method can dynamically form or break highly transmitting necklace states, which is an essential step towards photonic-crystal based quantum networks and signal processing circuits., Comment: 13 pages, 3 figures

Light incident on a layer of scattering material such as a piece of sugar or white paper forms a characteristic speckle pattern in transmission and reflection. The information hidden in the correlations of the speckle pattern with varying frequency, polarization and angle of the incident light can be exploited for applications such as biomedical imaging and high-resolution microscopy. Conventional computational models for multi-frequency optical response involve multiple solution runs of Maxwell's equations with monochromatic sources. Exponential Krylov subspace time solvers are promising candidates for improving efficiency of such models, as single monochromatic solution can be reused for the other frequencies without performing full time-domain computations at each frequency. However, we show that the straightforward implementation appears to have serious limitations. We further propose alternative ways for efficient solution through Krylov subspace methods. Our methods are based on two different splittings of the unknown solution into different parts, each of which can be computed efficiently. Experiments demonstrate a significant gain in computation time with respect to the standard solvers., Comment: 22 pages, 4 figures

Secure communication is of crucial importance for many applications. Currently communication security is based on encryption protocols which are computationally hard to crack by classical computers. However, it is generally believed that quantum computers will be able to decipher these encryption methods. Quantum communication has to potential to solve this threat, but it is vulnerable to man-in-the-middle attacks. Recently, Goorden et al. proposed a scheme which uses a multiple-scattering key as an authentication device, which solves the man-in-the-middle threat.[1] As a next step towards secure quantum communication we propose a Shih-Alley type of entanglement generation in such a scattering key.[2] This would enable us to develop a one-sided device-independent communication protocol. We report on the latest developments in this experiment. [1] S.A. Goorden et al., Optica 1, 421-424 (2014) [2] Y.H. Shih et al., Phys. Rev. Lett. 61, 2921 (1988)

Integrated universal linear optical networks are essential for the development of quantum information processing (QIP). We demonstrate a universal, reconfigurable, 8×8 photonic processor based on Si3N4 waveguides showing a variety of QIP primitives.

Bosonic many-body systems are prominent candidates for a quantum advantage demonstration, with the most popular approaches being either a quantum simulation beyond the reach of current classical computers, or a demonstration of boson sampling. It is a crucial open problem to understand how resilient such quantum advantage demonstrations are to imperfections such as boson loss and particle distinguishability. We partially solve this problem by showing that imperfect multi-boson interference can be efficiently approximated as ideal interference of groups of smaller number of bosons, where the other particles interfere classically. Crucially, the number of bosons undergoing interference in our approxmation only depends on the level of imperfections, but is independent of the actual number of particles. This allows us to construct a simple but stringent benchmark for comparing many-body bosonic technological platforms.

Multimode fibers support a multitude of transverse optical modes. These modes are mixed by the fiber. By complex wavefront shaping through the multimode fiber, we can undo this mixing, making it possible to communicate through the fiber even at very low light levels.