Robert Keil

• Dr.
• University of Innsbruck

We measure the nonlinearity of a telecom-wavelength superconducting nanowire single-photon detector via incoherent beam combination. At typical photon count rates and detector bias current, the observed relative deviation from a perfectly linear response is in the order of 0.1% when the flux is doubled. This arises from a balance between the counte...

Entanglement and interference are both hallmark effects of quantum physics. Particularly rich dynamics arise when multiple (at least partially) indistinguishable particles are subjected to either of these phenomena. By combining both entanglement and many-particle interference, we propose an interferometric setting through which N -particle interfe...

We measure the nonlinearity of a telecom-wavelength superconducting nanowire single-photon detector via incoherent beam combination. At typical photon count rates and detector bias current, the observed relative deviation from a perfectly linear response is in the order of 0.1% when the flux is doubled. This arises from a balance between the counte...

We introduce a novel quantum phenomenon combining entanglement and many-body interference, enabling the observation of N -particle interference while suppressing lower-order interferences. A four-photon experiment demonstrates high-visibility (nonlocal) interference patterns based on a collective four-particle phase.

Much of the recent development of single-photon sources is driven by the desire to apply them to protocols and technologies that use the interference of two or more photons, such as quantum repeaters or boson sampling. In all of these cases, the indistinguishability of the produced photons is a key requirement. For those applications that want to s...

We report on a multi-photon source based on active demultiplexing of single photons emitted from a resonantly excited GaAs quantum dot. Active temporal-to-spatial mode demultipexing is implemented via resonantly enhanced free-space electro-optic modulators, making it possible to route individual photons at high switching rates of 38 MHz. We demonst...

We report on a multi-photon source based on active demultiplexing of single photons emitted from a resonantly excited GaAs quantum dot. Active temporal-to-spatial mode demultipexing is implemented via resonantly enhanced free-space electro-optic modulators, making it possible to route individual photons at high switching rates of 38 MHz. We demonst...

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 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 wit...

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 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 w...

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 pr...

High-fidelity polarization-entangled photons are a powerful resource for quantum communication, distributing entanglement and quantum teleportation. The Bell-CHSH inequality S\leq2 S ≤ 2 is violated by bipartite entanglement and only maximally entangled states can achieve S=2\sqrt{2} S = 2 2 , the Tsirelson bound. Spontaneous parametric down-conver...

High-fidelity polarization-entangled photons are a powerful resource for quantum communication, distributing entanglement and quantum teleportation. The Bell-CHSH inequality $S\leq2$ is violated by bipartite entanglement and only maximally entangled states can achieve $S=2\sqrt{2}$, the Tsirelson bound. Spontaneous parametric down-conversion source...

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...

We study three-dimensional quantum walks on complex graphs arising from the hybrid action of the spatial and polarization degrees of freedom for single photons in photonic waveguide circuits with tailored birefringence.

Photonic circuits naturally implement boson sampling, a quantum algorithm that is classically hard to solve. Four photon pairs produced and processed within a single silicon chip have now been used to run it, a step towards besting classical computers.

Several distinct classes of unitary mode transformations have been known to exhibit the strict suppression of a large set of transmission events, as a consequence of totally destructive many-particle interference. In [Dittel et al., Totally Destructive Many-Particle Interference] we unite these cases by identifying a general class of unitary matric...

In a general, multi-mode scattering setup, we show how the permutation symmetry of a many-particle input state determines those scattering unitaries which exhibit strictly suppressed many-particle transition events. We formulate purely algebraic suppression laws that identify these events and show that the many-particle interference at their origin...

Several distinct classes of unitary mode transformations have been known to exhibit the strict suppression of a large set of transmission events, as a consequence of totally destructive many-particle interference. In another work [Dittel et al., Phys. Rev. Lett. 120, 240404 (2018)] we unite these cases by identifying a general class of unitary matr...

In a general, multi-mode scattering setup, we show how the permutation symmetry of a many-particle input state determines those scattering unitaries which exhibit strictly suppressed many-particle transition events. We formulate purely algebraic suppression laws that identify these events and show that the many-particle interference at their origin...

We conceive an all-optical representation of the dynamics of two distinct types of interacting bosons in a double well, with tunable interaction strengths and relative abundance, by an array of evanescently coupled photonic waveguides. We show that many-particle interference effects can be readily identified by monitoring the propagation of the lig...

We conceive an all-optical representation of the dynamics of two distinct types of interacting bosons in a double well by an array of evanescently coupled photonic waveguides. Many-particle interference effects are probed for various interaction strengths by changing the relative abundance of the particle species and can be readily identified by mo...

Simulating the evolution of a nonrelativistic quantum-mechanical particle in a periodic potential by propagating an optical wave packet in an array of evanescently coupled waveguides has received continuous and increasing attention in recent years. However, one can also simulate the evolution of a relativistic quantum particle in free space, as des...

Within the established theoretical framework of quantum mechanics, interference always occurs between pairs of paths through an interferometer. Higher order interferences with multiple constituents are excluded by Born's rule and can only exist in generalized probabilistic theories. Thus, high-precision experiments searching for such higher order i...

Beyond the regime of distinguishable particles, many-body quantum interferences influence quantum transport in an intricate manner. However, symmetries of the single-particle transformation matrix alleviate this complexity and even allow the analytic formulation of suppression laws, which predict final states to occur with a vanishing probability d...

In this work we experimentally demonstrate the realization of the discrete fractional Fourier transforms (DFrFT) in both the classical and quantum realm. Our approach is fully integrated and free of bulk optical components.

We design and realise a hybrid interferometer consisting of three paths based on integrated as well as on bulk optical components. This hybrid construction offers a good compromise between stability and footprint on one side and means of intervention on the other. As experimentally verified by the absence of higher-order interferences, amplitude an...

We report measurements of Hanbury Brown and Twiss correlation of coherent light transmitted through disordered one-dimensional photonic lattices. Although such a lattice exhibits transverse Anderson localization when a single input site is excited, uniform excitation precludes its observation. By examining the Hanbury Brown–Twiss correlation for a...

We design and realise a hybrid interferometer consisting of three paths based on integrated as well as on bulk optical components. This hybrid construction offers a good compromise between stability and footprint on one side and means of intervention on the other. As experimentally verified by the absence of higher-order interferences, amplitude an...

Fourier transforms are ubiquitous mathematical tools in basic and applied sciences. We here report classical and quantum optical realizations of the discrete fractional Fourier transform, a generalization of the Fourier transform. In the integrated configuration used in our experiments, the order of the transform is mapped onto the longitudinal coo...

Supplementary Figures 1-3, Supplementary Notes 1 – 2 and Supplementary References.

We present a method for controlling the sign of the coupling term for individual links in arbitrary tight-binding lattices. The scheme employs defect insertion and eigenmode matching and is experimentally verified in photonic lattice implementations.

The charged version of Majorana particles are unphysical and due to the violation of charge conservation they can not exist. However, we experimentally emulate the dynamics of charged Majorana particles in a tailored waveguide chip.

We experimentally demonstrate deterministic tuning of photon statistics and controlled bunching of photons in off-diagonal disordered lattices. We achieve this by gradually activating the chiral mode pairs with a coherent structured illumination.

We experimentally demonstrate deterministic control of Mandel's Q-parameter in off-diagonal disordered lattices while the mean photon number remains fixed. We achieve this by gradually breaking the excitation symmetry of the chiral mode pairs.

We present a method of locally inverting the sign of the coupling term in tight-binding systems, by means of inserting a judiciously designed ancillary site and eigenmode matching of the resulting vertex triplet. Our technique can be universally applied to all lattice configurations, as long as the individual sites can be detuned. We experimentally...

We measure the next-nearest-neighbour coupling in an array of coupled optical waveguides directly via an integrated eigenmode interferometer. In contrast to light propagation experiments, the technique is insensitive to nearest-neighbour dynamics. Our results show that second-order coupling in a linear configuration can be suppressed well below the...

Within the established theoretical framework of quantum mechanics, interference always occurs between pairs of trajectories. Higher order interferences with multiple constituents are, however, excluded by Born's rule and can only exist in generalized probabilistic theories. Thus, high-precision experiments searching for such higher order interferen...

Fourier transforms are ubiquitous mathematical tools in basic and applied sciences. We here report classical and quantum optical realizations of the discrete fractional Fourier transform, a generalization of the Fourier transform. In the integrated configuration used in our experiments, the order of the transform is mapped onto the longitudinal coo...

Unphysical solutions are ruled out in physical equations, as they lead to behavior that violates fundamental physical laws. One of the celebrated equations that allows unphysical solutions is the relativistic Majorana equation, thought to describe neutrinos and other exotic particles predicted in theories beyond the standard model. The neutrally ch...

Quantum superposition is the quantum-mechanical property of a particle whereby it inhabits several of its possible quantum states simultaneously. Ideally, this permissible coexistence of quantum states, as defined on any degree of freedom, whether spin, frequency or spatial, can be used to fully exploit the information capacity of the associated ph...

We present an innovative approach for ultra-precise phase manipulation in integrated photonic quantum circuits. To this end, we employ generalized directional couplers that utilize a detuning of the propagation constant in optical waveguides by the overlap of adjacent waveguide modes. We demonstrate our findings in experiments with classical as wel...

We introduce a new perfect state transfer protocol based on single-photon W-eigenstates of photonic lattices. Such W-eigenstates appear as impulse response of the system, e.g., when single photons are launched into single sites.

Two photons in an entangled, spatially correlated (anti-correlated) state transmitted through an Anderson disordered lattice maintain their correlation (anti-correlation) but exhibit coincidence-map speckle in the Fourier plane.

We demonstrate quantum walks of correlated photons in a two-dimensional network of directly laser written waveguides coupled in a "swiss cross" arrangement. The correlated detection events show high-visibility quantum interference and unique composite behavior: strong correlation and independence of the quantum walkers, between and within the plane...

We experimentally demonstrate the generation of high-order photon-encoded W-states involving up to 16 spatial optical modes. Additionally, we utilize the inherent probabilistic properties of these multipartite entangled W-states for the generation of genuine random numbers.

We introduce special photonic lattices where we can, in principle, selectively excite the constitutive quantum eigenstates. Hence, through the excitation of such systems with single-photon W-states we can readily generate entanglement of the so-called W-eigenstates.

We report the observation of near-perfect light wave transfer by emulating quantum state transfer on a lattice with Hamiltonian dynamics, i.e., time-dependent intersite couplings. The structure transferring a single waveguide excitation over 11 sites with a fidelity of 0.93 works for classical light as well as single photons. As our implementation...

We describe theoretically and observe experimentally the formation of a surface state in a semi-infinite waveguide array with a side-coupled waveguide, designed to simultaneously achieve Fano and Fabry-Perot resonances. We demonstrate that the surface mode is compact, with all energy concentrated in a few waveguides at the edge and no field penetra...

In 1958, Philip Anderson suggested that a quantum mechanical wave function may undergo localization in a disordered lattice as a result of interference between different paths arising from multiple scattering. While direct evidence of this localization remains elusive in solid state physics, its optical analogue may be realized in random arrays of...

We experimentally and theoretically investigate the evolution of different types of two photon entangled states in disordered photonic waveguide arrays. We present a transition from Anderson co-localization to Anderson anti-co-localization.

form only given. The manipulation of the phase of classical and quantum light is a major key to quantum photonics [1]. Integrated optics has several advantages over bulk optical systems, most notably in robustness, handling and size. Writing three-dimensional waveguides circuits into transparent materials using an ultrafast laser is thereby a new e...

The coherent transport of quantum states between distant qubits is one of the key milestones towards the realisation of large-scale quantum computers. For static qubits, this state transfer is often envisioned to be carried out only by the internal dynamics of the system, which has the great advantage that detrimental influences of the environment...

Quantum computation with large numbers of qubits is often envisioned to be facilitated by the division of one large register into several smaller units. The efficient information processing then relies on a coherent transfer of qubits between these units. Chains of spin-1/2-particles possessing nearest-neighbor interactions have been proposed as a...

The full text of this article is available in the PDF provided.

We demonstrate quantum walks of a photon pair in a spatially extended Einstein-Podolsky-Rosen state coupled into an on-chip multiport photonic lattice. By varying the degree of entanglement we observe Anderson localization for pairs in a separable state and Anderson colocalization for pairs in an Einstein-Podolsky-Rosen entangled state. In the form...

A novel approach to investigate the dynamics of indistinguishable particles in non-Hermitian lattice systems is presented, allowing an efficient calculation of quantum correlations between these particles in the presence of losses. Particular attention is paid to quasi-parity-time-symmetric systems, for which we numerically analyze two-particle qua...

We demonstrate that perfect transfer of path-entangled photons as well as of single-photon states is possible in a certain class of spin inspired optical systems—the so-called Jx photonic lattices. In these fully integrable optical arrangements, perfect cyclic transitions from correlated states to totally anticorrelated states can naturally occur....

Long-range correlation-the non-local interdependence of distant events-is a crucial feature in many natural and artificial environments. In the context of solid state physics, impurity spins in doped spin chains and ladders with antiferromagnetic interaction are a prominent manifestation of this phenomenon, which is the physical origin of the unusu...

We propose photonic lattices with segmentation-based linear self imaging as integrated optical limiters. Starting from unity transmission in the linear regime, nonlinear delocalization leads to a continuous decrease of the overall transmission for increasing input powers. The diffractive propagation between input and output port offers the addition...

We report the first experimental implementation of Glauber–Fock oscillator lattices. Bloch-like revivals are observed in these optical structures in spite of the fact that the photonic array is effectively semi-infinite and the waveguide coupling is not uniform. This behavior is entirely analogous to the dynamics exhibited by a driven quantum harmo...

Quantum entanglement became essential in understanding the non-locality of quantum mechanics. In optics, this non-locality can be demonstrated on impressively large length scales, as photons travel with the speed of light and interact only weakly with their environment. Spontaneous parametric down-conversion (SPDC) in nonlinear crystals provides an...

Transferring quantum states efficiently between distant nodes of an information processing circuit is of paramount importance for scalable quantum computing. We report on the first observation of a perfect state transfer protocol on a lattice, thereby demonstrating the general concept of trans- porting arbitrary quantum information with high fideli...

DOI:https://doi.org/10.1103/PhysRevA.86.019903

We demonstrate that light propagating in an appropriately designed lattice can exhibit dynamics akin to that expected from massless relativistic particles as governed by the one-dimensional Dirac equation. This is accomplished by employing a waveguide array with alternating positive and negative effective coupling coefficients, having a band struct...

Nonlinearity and disorder in discrete systems give rise to fascinating dynamics in various fields of physics. Photonic lattices allow investigation of them in an optical context. The very nature of discrete propagation allows perfect reconstruction of arbitrary initial wave packets by introducing phase shifts to specific lattice sites. We investiga...

We suggest and demonstrate experimentally that evolution of classical light can simulate bi-photon generation through spontaneous parametric down-conversion and correlated quantum walks in waveguide arrays, including violation of Bell's inequality.

We introduce a multiport waveguide array beam-splitter that emulates the action of the angular momentum matrix Jx. Quantum transformations carried out by such devices are discussed and pertinent examples are provided.

It is experimentally demonstrated that perfect imaging is possible in disordered wave guiding media, provided that the disorder is off-diagonal, i.e., that only the spacing varies randomly between the otherwise identical lattice sites. On-diagonal disorder or Kerr nonlinearity destroys the imaging.

We report on the experimental demonstration of negative coupling constants between defect guides in a waveguide lattice. We find that coupling can only be negative if the defects are negative and an odd number of lattice sites is between the defect guides.

We demonstrate that single-photon as well as biphoton revivals are possible in a new class of dynamic optical systems—the so-called Glauber-Fock oscillator lattices. In these arrays, both Bloch-like oscillations and dynamic delocalization can occur which can be described in closed form. The bunching and antibunching response of path-entangled photo...

We suggest and realize experimentally an optical platform where linear evolution of classical light simulates-biphoton generation through spontaneous parametric down-­conversion and correlated quantum walks in waveguide arrays, including the regime of violated Bell's inequality.

We present an experimental observation of Klein tunneling of light waves in lattices of evanescently coupled waveguides with a superimposed potential step. The incident wave packet "mass" which is a characteristic feature of Klein tunneling is generated by a minigap in the band structure of the lattice. We studied different masses and measured the...

Multi-photon quantum walks on integrated circuits are emonstrated, showing non-classical correlations, in one and two dimensional networks. Time evolution of quantum walks and a scheme for simulating fermionic quantum walks using entanglement are demonstrated.

We report the first observation of classical Bloch-like oscillations and revivals of light in a new class of dynamic optical systems-the so-called Glauber-Fock oscillator lattices.

We report the first observation of Anderson co-localization of spatially entangled photon pairs propagating through random optical waveguide lattices with controllable off-diagonal disorder.

We experimentally demonstrate negative coupling between two defect guides in a waveguide lattice and elaborate the required conditions to explain, why this effect can only be found for negative defects and certain geometric devices.

We experimentally realized a waveguide device with alternating positive and negative coupling and show that this geometry is an optical simulator of the conditions found for a massless relativistic particle described by the one-dimensional Dirac-equations.

We investigate the impact of nonlinearity on the perfect imaging by segmentation in photonic lattices with disorder. We find the presence of strongly localized Anderson modes renders the imaging significantly more susceptible to nonlinear perturbations.

We report on the experimental observation of Bloch oscillations of an optical wave packet in a lattice with second-order coupling. To this end, we employ zigzag waveguide arrays, in which the second-order coupling can be precisely tuned.

The ability to efficiently transmit and rapidly process huge amounts of data has become almost indispensable to our daily lives. It turned out that all-optical networks provide a very promising platform to deal with this task. Within such networks opto-optical switches, where light is directed by light, are a crucial building block for an effective...

Supplementary Online Information

Signal_Y_with blocker

Signal_T

Signal_T_with blocker

Signal_Y

Coherent states and their generalizations, displaced Fock states, are of fundamental importance to quantum optics. Here we present a direct observation of a classical analogue for the emergence of these states from the eigenstates of the harmonic oscillator. To this end, the light propagation in a Glauber-Fock waveguide lattice serves as equivalent...

In recent years, photonic lattices fabricated by the femtosecond laser direct writing technique in fused silica have emerged as the tool of choice for the experimental investigation of light propagation in discrete optical systems with Kerr nonlinearity. In this article, we review the recent results of our research conducted in this field and prese...

Coherent states and their generalizations, displaced Fock states, are of fundamental importance to quantum optics. Here we present a direct observation of a classical analogue for the emergence of these states from the eigenstates of the harmonic oscillator. To this end, the light propagation in a Glauber-Fock waveguide lattice serves as equivalent...

We show that the geometrically induced potential existing in undulated slab waveguides dramatically affects the properties of solitons. In particular, whereas solitons residing in the potential maxima do not feature power thresholds and are stable, their counterparts residing in the potential minima are unstable and may exhibit a power threshold fo...

We report on the experimental observation of discrete-continuous three-dimensional X-waves. This type of an optical space-time dynamical wave emerges due to the interplay of discrete diffraction, normal dispersion and focusing Kerr nonlinearity.

We report on the observation of Anderson wave localization in one-dimensional waveguide arrays with off-diagonal disorder. The waveguide elements are inscribed in silica glass, and a uniform random distribution of coupling parameters is achieved by a precise variation of the relative waveguide positions. In the absence of disorder we observe ballis...

In this work we experimentally demonstrate nontrivial Bloch oscillations in lattices particularly with second-order coupling; an effect which has been proposed theoretically only recently.

We demonstrate numerically and experimentally how the presence of so-called Topo logical Defects represented by junctions within otherwise periodical planar photonic lattices influences the formation of discrete solitons similar to conventional defects constituted by de tuned waveguides.

Discrete light propagation in lattices of evanescently coupled waveguides represents a very elegant and noise-insensitive realisation of continuous-time single-particle quantum random walks [1,2]. On the other hand, two or more indistinguishable walkers exhibit nonclassical correlations, and therefore increase the graph size substantially. Such wal...