arXiv subjects

Alexander Szameit

Publications and source records attributed to Alexander Szameit.

At least 55 records · Page 3Linked to original sources

arXiv2021

Observation-dependent suppression and enhancement of two-photon coincidences by tailored losses

The uncanny ability of multiple particles to interfere with one another is one of the core principles of quantum mechanics, and serves as foundation for quantum information processing. In particular, the interplay of constructive and destructive interference with the characteristic exchange statistics of indistinguishable particles give rise to the Hong-Ou-Mandel (HOM) effect, where the bunching of bosons can lead to a perfect suppression of two-particle coincidences between the output ports of a balanced beam splitter. Conversely, in the case of two fermions, anti-bunching can systematically enhance these coincidences up to twice the baseline value of distinguishable particles. As such, the respective emergence of dips or peaks in the HOM experiment may at first glance appear to be indicative of the bosonic/fermionic nature of the incident particles. In this work, we demonstrate experimentally that the two-particle coincidence statistics of two bosons can instead be seamlessly tuned from the expected case of suppression to substantial enhancement by an appropriate choice of the observation basis. To this end, our photonic setting leverages birefringent polarization couplers to selectively introduce dissipation in the photons polarization degree of freedom. Notably, the mechanism underpinning this this highly unusual behaviour does not act on the individual phases accumulated by pairs of particles along specific paths, but instead allows them to jointly evade losses, while indistinguishable photons are prevented from being simultaneously detected in orthogonal modes. Our findings reveal a new approach to harnessing non-Hermitian settings for the manipulation of multi-particle quantum states and as functional elements in quantum simulation and information processing.

quant-ph↗

arXiv2021

Towards probing for hypercomplex quantum mechanics in a waveguide interferometer

We experimentally investigate the suitability of a multi-path waveguide interferometer with mechanical shutters for performing a test for hypercomplex quantum mechanics. Probing the interferometer with coherent light we systematically analyse the influence of experimental imperfections that could lead to a false-positive test result. In particular, we analyse the effects of detector nonlinearity, input-power and phase fluctuations on different timescales, closed-state transmissivity of shutters and crosstalk between different interferometer paths. In our experiment, a seemingly small shutter transmissivity in the order of about $2 \times 10^{-4}$ is the main source of systematic error, which suggests that this is a key imperfection to monitor and mitigate in future experiments.

quant-ph↗

arXiv2021

Nonlinear second-order photonic topological insulators

Higher-order topological insulators (HOTI) are a novel topological phase beyond the framework of the conventional bulk-boundary correspondence. In these peculiar systems, the topologically nontrivial boundary modes are characterized by a co-dimension of at least two. Despite several promising preliminary considerations regarding the impact of nonlinearity in such systems, the flourishing field of experimental HOTI research has thus far been confined to the linear evolution of topological states. As such, the observation of the interplay between nonlinearity and the dynamics of higher-order topological phases in conservative systems remains elusive. In our work, we experimentally demonstrate nonlinear higher-order topological corner states. Our photonic platform enables us to observe nonlinear topological corner states as well as the formation of solitons in such topological structures. Our work paves the way towards the exploration of topological properties of matter in the nonlinear regime, and may herald a new class of compact devices that harnesses the intriguing features of topology in an on-demand fashion.

physics.optics↗

arXiv2021

Photonic Topological Anderson Insulators

The hallmark property of two-dimensional topological materials is the incredible robustness of the quantized Hall conductivity to disorder. That robustness arises from the fact that in the topological band gap, transport can occur only along the edges modes, which are immune to scattering. However, for sufficiently strong disorder, the band gap closes and the system becomes topologically trivial as all states become localized, such that all transport vanishes -- in accordance with Anderson localization. It therefore came as a surprise when it was suggested that, for a two-dimensional quantum spin-Hall topological system, the opposite could occur. In so-called topological Anderson insulators, the emergence of protected edge states and quantized transport is caused by the introduction of disorder. However, to date, the observation of the topological Anderson insulator phase has been elusive. In this article, we report the first experimental demonstration of a topological Anderson insulator. We do that in a photonic implementation: an array of helical, evanescently-coupled waveguides in a detuned honeycomb geometry. Under proper conditions, adding on-site disorder, in the form of random variations in the refractive index contrast defining the waveguides, drives the system from a trivial phase into a topological state.

cond-mat.mes-hall↗

arXiv2021

Topological dipole Floquet solitons

We theoretically introduce a new type of topological dipole solitons propagating in a Floquet topological insulator based on a kagome array of helical waveguides. Such solitons bifurcate from two edge states belonging to different topological gaps and have bright envelopes of different symmetries: fundamental for one component, and dipole for the other. The formation of dipole solitons is enabled by unique spectral features of the kagome array which allow the simultaneous coexistence of two topological edge states from different gaps at the same boundary. Notably, these states have equal and nearly vanishing group velocities as well as the same sign of the effective dispersion coefficients. We derive envelope equations describing components of dipole solitons and demonstrate in full continuous simulations that such states indeed can survive over hundreds of helix periods without any noticeable radiation into the bulk.

nlin.PS↗

arXiv2021

Symmetry allows for distinguishability in totally destructive many-particle interference

We investigate, in a four photon interference experiment in a laser-written waveguide structure, how symmetries control the suppression of many-body output events of a $J_x$ unitary. We show that totally destructive interference does not require mutual indistinguishability between all, but only between symmetrically paired particles, in agreement with recent theoretical predictions. The outcome of the experiment is well described by a quantitative simulation which accounts for higher order emission of the photon source, imbalances in the scattering network, partial distinguishability, and photon loss.

quant-ph↗

arXiv2020

Topological defect engineering and PT-symmetry in non-Hermitian electrical circuits

We employ electric circuit networks to study topological states of matter in non-Hermitian systems enriched by parity-time symmetry $\mathcal{PT}$ and chiral symmetry anti-$\mathcal{PT}$ ($\mathcal{APT}$). The topological structure manifests itself in the complex admittance bands which yields excellent measurability and signal to noise ratio. We analyze the impact of $\mathcal{PT}$ symmetric gain and loss on localized edge and defect states in a non-Hermitian Su--Schrieffer--Heeger (SSH) circuit. We realize all three symmetry phases of the system, including the $\mathcal{APT}$ symmetric regime that occurs at large gain and loss. We measure the admittance spectrum and eigenstates for arbitrary boundary conditions, which allows us to resolve not only topological edge states, but also a novel $\mathcal{PT}$ symmetric $\mathbb{Z}_2$ invariant of the bulk. We discover the distinct properties of topological edge states and defect states in the phase diagram. In the regime that is not $\mathcal{PT}$ symmetric, the topological defect state disappears and only reemerges when $\mathcal{APT}$ symmetry is reached, while the topological edge states always prevail and only experience a shift in eigenvalue. Our findings unveil a future route for topological defect engineering and tuning in non-Hermitian systems of arbitrary dimension.

physics.app-ph↗

arXiv2020

Nonlinear control of PT-symmetry and non-Hermitian topological states

Advances in topological photonics and non-Hermitian optics have drastically changed our perception on how interdisciplinary concepts may empower unprecedented applications. Bridging the two areas could uncover the reciprocity between topology and non-Hermiticity in complex systems. So far, such endeavors have focused mainly on linear-optics regime. Here, we establish a nonlinear non-Hermitian topological platform for control of parity-time (PT) symmetry and topological edge states. Experimentally, we demonstrate that optical nonlinearity effectively modulates the gain and loss of a topological interface waveguide in a non-Hermitian Su-Schrieffer-Heeger lattice, leading to switching between PT and non-PT-symmetric regimes accompanied by destruction and restoration of topological zero modes. Theoretically, we examine the fundamental issue of the interplay between two antagonistic effects: the sensitivity close to exceptional points and the robustness of non-Hermitian topological modes. Realizing single-channel control of global PT-symmetry via local nonlinearity may herald new possibilities for light manipulation and unconventional device applications.

physics.optics↗

arXiv2020

Nonlinearity-induced photonic topological insulator

The hallmark feature of topological insulators renders edge transport virtually impervious to scattering at defects and lattice disorder. In our work, we experimentally demonstrate a topological system, using a photonic platform, in which the very existence of the topological phase is brought about by nonlinearity. Whereas in the linear regime, the lattice structure remains topologically trivial, light beams launched above a certain power threshold drive the system into its transient topological regime, and thereby define a nonlinear unidirectional channel along its edge. Our work studies topological properties of matter in the nonlinear regime, and may pave the way towards compact devices that harness topological features in an on-demand fashion.

physics.optics↗

arXiv2020

Exploring complex graphs using three-dimensional quantum walks of correlated photons

Graph representations are a powerful concept for solving complex problems across natural science, as patterns of connectivity can give rise to a multitude of emergent phenomena. Graph-based approaches have proven particularly fruitful in quantum communication and quantum search algorithms in highly branched quantum networks. Here we introduce a new paradigm for the direct experimental realization of excitation dynamics associated with three-dimensional networks by exploiting the hybrid action of spatial and polarization degrees of freedom of photon pairs in complex waveguide circuits with tailored birefringence. This novel testbed for the experimental exploration of multi-particle quantum walks on complex, highly connected graphs paves the way towards exploiting the applicative potential of fermionic dynamics in integrated quantum photonics.

physics.optics↗

arXiv2020

Non-Hermitian Anderson Transport

Andersons groundbreaking discovery that the presence of stochastic imperfections in a crystal may result in a sudden breakdown of conductivity revolutionized our understanding of disordered media. After stimulating decades of lively studies, Anderson localization has found intriguing applications in various areas of physics, such mesoscopic physics, strongly-correlated systems, light localization, cavity quantum electrodynamics, random lasers, and topological phases of matter. However, a fundamental assumption in Andersons treatment is that no energy is exchanged with the environment, in contrast to the common knowledge that every real system is subject to dissipation. Recently, a growing number of theoretical studies has addressed disordered media with dissipation. In particular it has been predicted that in such systems all eigenstates exponentially localize, similar to the original case without dissipation that Anderson considered. However, in dissipative systems an eigenstate analysis is insufficient for characterizing the transport dynamics of wave, in stark contrast to Hermitian systems, where the localization of all eigenstates necessarily suppresses transport. In our work, we show in theory and experiment that systems with dissipative disorder allow for a new type of spatial transport, despite the fact that all eigenstates are exponentially localized. This Anderson transport is characterized by super-diffusive spreading and ultra-far spatial jumps between localized states. We anticipate our findings to mark the starting point towards novel phenomena in dissipative media, which are subject to general non-Hermitian disorder.

physics.optics↗

arXiv2020

Nontrivial coupling of light into a defect: the interplay of nonlinearity and topology

The flourishing of topological photonics in the last decade was achieved mainly due to developments in linear topological photonic structures. However, when nonlinearity is introduced, many intriguing questions arise. For example, are there universal fingerprints of underlying topology when modes are coupled by nonlinearity, and what can happen to topological invariants during nonlinear propagation? To explore these questions, here we experimentally demonstrate nonlinearity-induced coupling to topologically protected edge states using a photonic platform, and theoretically develop a general framework for interpreting the mode-coupling dynamics in nonlinear topological systems. Performed in laser-written photonic Su-Schrieffer-Heeger lattices, our experiments reveal nonlinear coupling of light into a nontrivial edge or interface defect channel otherwise not permissible due to topological protection. Our theory explains well all the observations. Furthermore, we introduce the concepts of inherited and emergent nonlinear topological phenomena, and a protocol capable of unveiling the interplay of nonlinearity and topology. These concepts are applicable for other nonlinear topological systems, either in higher dimensions or beyond our photonic platform.

physics.optics↗

arXiv2020

Bragg solitons in topological Floquet insulators

We consider a topological Floquet insulator consisting of two honeycomb arrays of identical waveguides having opposite helicities. The interface between the arrays supports two distinct topological edge states, which can be resonantly coupled by additional weak longitudinal refractive index modulation with a period larger than the helix period. In the presence of Kerr nonlinearity, such coupled edge states enable topological Bragg solitons. Theory and examples of such solitons are presented.

physics.optics↗

arXiv2020

Efficient Light Funneling based on the non-Hermitian Skin Effect

In the last two decades, the ubiquitous effect of dissipation has proven to entail astonishing non-Hermitian features, rather than just being an inescapable nuisance. As an alternative route to non-Hermiticity, we tailor the anisotropy of a lattice, which constitutes an, up to now, barely exploited degree of freedom. In this case, the appearance of an interface dramatically alters the entire eigenmode spectrum, leading to the exponential localization of all modes at the interface, which goes beyond the expectations for Hermitian systems. This effect is dubbed "non-Hermitian skin effect". We experimentally demonstrate it by studying the propagation of light in a large scale photonic mesh lattice. For arbitrary excitations, we find that light is always transported to the interface, realizing a highly efficient funnel for light.

physics.optics↗

arXiv2020

Possible link between the distribution of atomic spectral lines and the radiation-matter-equilibrium in the early universe

We report on the uncanny resemblance of the global distribution of all experimentally known atomic spectral lines to the Planckian spectral distribution associated with black body radiation at a temperature of $T\approx9000\,\mathrm{K}$. This value coincides with the critical temperature of equilibrium between the respective densities of radiation and matter in the early universe.

physics.gen-ph↗

arXiv2020

Edge Solitons in Lieb Topological Floquet Insulators

We describe topological edge solitons in a continuous dislocated Lieb array of helical waveguides. The linear Floquet spectrum of this structure is characterized by the presence of two topological gaps with edge states residing in them. A focusing nonlinearity enables families of topological edge solitons bifurcating from the linear edge states. Such solitons are localized both along and across the edge of the array. Due to the non-monotonic dependence of the propagation constant of the edge states on the Bloch momentum, one can construct topological edge solitons that either propagate in different directions along the same boundary or do not move. This allows us to study collisions of edge solitons moving in the opposite directions. Such solitons always interpenetrate each other without noticeable radiative losses; however, they exhibit a spatial shift that depends on the initial phase difference.

physics.optics↗

arXiv2020

Vector Topological Edge Solitons in Floquet Insulators

We introduce topological vector edge solitons in a Floquet insulator, consisting of two honeycomb arrays of helical waveguides with opposite directions of rotation in a focusing nonlinear optical medium. Zigzag edges of two arrays placed in contact create a zigzag-zigzag interface between two structures with different topology. A characteristic feature of such a photonic insulator is that, in the linear limit, it simultaneously supports two topologically protected chiral edge states at the interface between the two arrays. In the presence of nonlinearity, either bright or dark scalar Floquet edge soliton can bifurcate from a linear topological edge state. Such solitons are unidirectional and are localized in both directions, along the interface due to nonlinear self-action, and across the interface as being an edge state. The presence of two edge states with equal averaged group velocities enables the existence of stable topological vector edge solitons. In our case, these are nonlinearly coupled bright and dark solitons bifurcating from different branches of the topological Floquet edge states. Here we put forward a new mathematical description of scalar and vector small-amplitude Floquet envelope solitons in the above-mentioned continuous system. Importantly for the design of future photonic devices based on Floquet edge solitons, we find that the latter can be described by nonlinear Schrodinger equations for the mode envelopes obtained by averaging over one rotation period in the evolution coordinate.

physics.optics↗

arXiv2020

Quantum Random Number Generation using a Solid-State Single-Photon Source

Quantum random number generation (QRNG) harnesses the intrinsic randomness of quantum mechanical phenomena. Demonstrations of such processes have, however, been limited to probabilistic sources, for instance, spontaneous parametric down-conversion or faint lasers, which cannot be triggered deterministically. Here, we demonstrate QRNG with a quantum emitter in hexagonal boron nitride; an emerging solid-state quantum source that can generate single photons on demand and operates at room temperature. We achieve true random number generation through the measurement of single photons exiting one of four integrated photonic waveguides, and subsequently, verify the randomness of the sequences in accordance with the National Institute of Standards and Technology benchmark suite. Our results open a new avenue to the fabrication of on-chip deterministic random number generators and other solid-state-based quantum-optical devices.

physics.optics↗