Geometrically-controlled polarisation processing in an integrated photonic platform

The polarisation of light is a powerful and widely used degree of freedom to encode information, both in classical and quantum applications. In particular, quantum information technologies based on photons are being revolutionised by the use of integrated photonic circuits. It is therefore very important to be able to manipulate the polarisation of photons in such circuits. We experimentally demonstrate the fabrication by femtosecond laser micromachining of components such as polarisation insensitive or polarising directional couplers, operating at 1550 nm wavelength, where the two opposite behaviours are achieved just by controlling the geometric layout of the photonic circuits, being the waveguides fabricated with the same irradiation recipe. We expect to employ this approach in complex integrated photonic devices, capable of a full control of the photons polarisation for quantum cryptography, quantum computation and quantum teleportation experiments.Comment: 9 pages, 7 figure

Symmetric polarization insensitive directional couplers fabricated by femtosecond laser waveguide writing

We study analytically the polarization behaviour of directional couplers composed of birefringent waveguides, showing that they can induce polarization transformations that depend on the specific input-output path considered. On the basis of this study, we propose and demonstrate experimentally, by femtosecond laser writing, directional couplers that are free from this problem and also yield a polarization independent power-splitting ratio. More in detail, we devise two different approaches to realize such devices: the first one is based on local birefringence engineering, while the second one exploits ultra-low birefringence waveguides obtained by thermal annealing

Photonic simulation of entanglement growth and engineering after a spin chain quench

The time evolution of quantum many-body systems is one of the most important processes for benchmarking quantum simulators. The most curious feature of such dynamics is the growth of quantum entanglement to an amount proportional to the system size (volume law) even when interactions are local. This phenomenon has great ramifications for fundamental aspects, while its optimisation clearly has an impact on technology (e.g., for on-chip quantum networking). Here we use an integrated photonic chip with a circuit-based approach to simulate the dynamics of a spin chain and maximise the entanglement generation. The resulting entanglement is certified by constructing a second chip, which measures the entanglement between multiple distant pairs of simulated spins, as well as the block entanglement entropy. This is the first photonic simulation and optimisation of the extensive growth of entanglement in a spin chain, and opens up the use of photonic circuits for optimising quantum devices

Extreme events in complex linear and nonlinear photonic media

Ocean rogue waves (RW) are huge solitary waves that have for long triggered the interest of scientists. The RWs emerge in a complex environment and it is still under investigation if they are due to linear or nonlinear processes. Recent works have demonstrated that RWs appear in various other physical systems such as microwaves, nonlinear crystals, cold atoms, etc. In this work we investigate optical wave propagation in strongly scattering random lattices embedded in the bulk of transparent glasses. In the linear regime we observe the appearance of extreme waves, RW-type, that depend solely on the scattering properties of the medium. Interestingly, the addition of nonlinearity does not modify the RW statistics, while as the nonlinearities are increased multiple-filamentation and intensity clamping destroy the RW statistics. Numerical simulations agree nicely with the experimental findings and altogether prove that optical rogue waves are generated through the linear strong scattering in such complex environments

The development of an image processing algorithm for the precise monitoring of a laser-polymer interaction via third harmonic generation microscopy measurements

Insight into the processes taking place upon ultraviolet (UV) laser ablation of polymers is of high importance. In this study, a new algorithm for the precise analysis of third harmonic generation (THG) images from irradiated polymers has been developed. In particular, high accuracy qualitative and quantitative morphological information concerning the induced swelling following UV laser irradiation has been obtained by employing the proposed algorithm. Furthermore, compensation for the curvature of the field, correction of the apparent depth distortion and delineation capability of the total swelling area at a high resolution has been achieved. The above advantages allow a precise and comprehensive monitoring (via the detection of THG signals) of the dynamics during the interaction of laser radiation with polymeric materials to an extent that is not possible with other standard techniques such as profilometry or electron microscopy

Isotropic sub-100nm direct laser writing using spherical reflector-enabled 4Pi excitation at 405nm wavelength

We demonstrate a direct laser writing setup combining 405 nm multi-photon lithography with 4Pi excitation enabled by a spherical reflector (SR) refocussing the transmitted excitation. The SR provides a simplified implementation of the 4Pi geometry, avoiding the need for an additional objective and its interferometrically stabilised excitation beam path, while also recycling the beam power. The reflected beam position is measured by imaging the reflected beam and is controlled by a feedback loop to 10nm in all three dimensions. Using this instrument, the fabrication of sinusoidally modulated nanowires and helicoids with sub-100nm near-isotropic cross-section is demonstrated

Symmetric polarization-insensitive directional couplers fabricated by femtosecond laser writing

We study analytically the polarization behaviour of directional couplers composed of birefringent waveguides, showing that they can induce polarization transformations that depend on the specific input-output path considered. On the basis of this study, we propose and demonstrate experimentally, by femtosecond laser writing, directional couplers that yield a polarization-independent power splitting and, at the same time, preserve the polarization state of the propagating light. More in detail, we devise two different approaches to realize such devices: the first one is based on local birefringence engineering by additional refractive index modification tracks, while the second one exploits ultra-low birefringence waveguides (b = 1.2 × 10−6), obtained by thermal annealing

Experimental quantum homomorphic encryption

10.1038/s41534-020-00340-8npj Quantum Information712

Integrated-optics circuits for validation of non-classicality

Summary form only given. Contrarily to the classical physics picture, according to quantum mechanics the observable properties of the objects do not yield defined values, until a measurement is performed. The measurement outcome depends indeed also on the set of observables that is being measured [1]. Such a fundamental aspect of Nature is named quantum contextuality and it has been studied in several experimental systems, including single particles [2, 3]. Interestingly, it was recently suggested that even the non-classical power of quantum computing originates from contextuality [4]. Therefore, it is highly relevant to find experimental evidence of this aspect in technological platforms that may be adopted in future quantum computing devices, such as integrated photonics [5-7]. In this work we investigate experimentally quantum contextuality of a single photon on-a-chip, employing reconfigurable photonic circuits fabricated by femtosecond laser waveguide writing [7, 8]