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Relevant metrological scenarios involve the simultaneous estimation of multiple parameters. The fundamental ingredient to achieve quantum-enhanced performances is based on the use of appropriately tailored quantum probes. However, reaching the ultimate resolution allowed by physical laws requires non trivial estimation strategies both from a theoretical and a practical point of view. A crucial tool for this purpose is the application of adaptive learning techniques. Indeed, adaptive strategies provide a flexible approach to obtain optimal parameter-independent performances, and optimize convergence to the fundamental bounds with limited amount of resources. Here, we combine on the same platform quantum-enhanced multiparameter estimation attaining the corresponding quantum limit and adaptive techniques. We demonstrate the simultaneous estimation of three optical phases in a programmable integrated photonic circuit, in the limited resource regime. The obtained results show the possibility of successfully combining different fundamental methodologies towards transition to quantum sensors applications.

Quantum parameter estimation offers solid conceptual grounds for the design of sensors enjoying quantum advantage. This is realised not only by means of hardware supporting and exploiting quantum properties, but data analysis has its impact and relevance, too. In this respect, Bayesian methods have emerged as an effective and elegant solution, with the perk of incorporating naturally the availability of a priori information. In this article we present an evaluation of Bayesian methods for multiple phase estimation, assessed based on bounds that work beyond the usual limit of large samples assumed in parameter estimation. Importantly, such methods are applied to experimental data generated from the output statistics of a three-arm interferometer seeded by single photons. Our studies provide a blueprint for a more comprehensive data analysis in quantum metrology., Comment: 8 pages, 6 figures

We demonstrate the storage of light-matter entanglement between a telecom photon and an integrated solid-state quantum memory. The quantum memory is a fibre-coupled waveguide written in a rare-earth doped crystal.

Space-based technologies find in optical quantum communication a challenging and promising application. In fact, quantum networks relying on optical fibers are affected by non negligible losses, and there is no possibility of amplifying quantum states. For this reason, free-space optical transmissions will be necessary for the creation of a world-wide optical quantum network. In this respect, satellite-based quantum technologies represent an appealing implementation [1] , [2] . However, during space missions, the equipment should withstand harsh conditions, like exposure to high-energy protons and γ-rays, periodic temperature cycling, vibrations and mechanical shocks. For this reason, it is necessary to qualify any component before sending it into space [3] . Moreover, due to the limited available volume in a satellite, all the equipment should be as compact as possible, so the use of integrated optical devices could be beneficial for scaling the complexity of the performed tasks.

We will present details on how optical crystals such as yttrium aluminum garnet (YAG) and sapphire can be nanostructured on the three dimensions within their volume, with feature sizes down to the 100 nm level and device footprints on the cm scale, with the aim of fabricating photonic crystal type embedded waveguides, sub-wavelength diffraction gratings, and other nanoscale light control devices [1] .

Nanostructuring hard optical crystals has so far been exclusively feasible at their surface, as stress induced crack formation and propagation has rendered high precision volume processes ineffective. We show that the inner chemical etching reactivity of a crystal can be enhanced at the nanoscale by more than five orders of magnitude by means of direct laser writing. The process allows to produce cm-scale arbitrary three-dimensional nanostructures with 100 nm feature sizes inside large crystals in absence of brittle fracture. To showcase the unique potential of the technique, we fabricate photonic structures such as sub-wavelength diffraction gratings and nanostructured optical waveguides capable of sustaining sub-wavelength propagating modes inside yttrium aluminum garnet crystals. This technique could enable the transfer of concepts from nanophotonics to the fields of solid state lasers and crystal optics., Comment: Submitted Manuscript and Supplementary Information

Satellite-based optical quantum technologies represent a promising field for obtaining a worldwide quantum network. However, due to the limited size of satellites and the adverse conditions of a space environment, only compact and resistant devices can be used for this purpose. In this respect, we present for the first time the space qualification of integrated photonic circuits fabricated by Ultrafast Laser Writing. By inscribing different straight waveguides, directional couplers and Mach-Zehnder interferometer, and by exposing them to appropriate proton and gamma ray irradiations, we show that our integrated devices are suited for performing quantum experiments in a low Earth orbit.

Technological quantum sensors requires the development of a calibration procedure that is self-consistent and easily adaptable to different scenarios. Neural networks provide a handy solution in particular when dealing with large systems operating in a noisy environment.

Calibration of sensors is a fundamental step in validating their operation. This can be a demanding task, as it relies on acquiring detailed modeling of the device, which can be aggravated by its possible dependence upon multiple parameters. Machine learning provides a handy solution to this issue, operating a mapping between the parameters and the device response, without needing additional specific information on its functioning. Here, we demonstrate the application of a neural-network-based algorithm for the calibration of integrated photonic devices depending on two parameters. We show that a reliable characterization is achievable by carefully selecting an appropriate network training strategy. These results show the viability of this approach as an effective tool for the multiparameter calibration of sensors characterized by complex transduction functions. Furthermore, the approach is proven to be versatile and promising for mass production, as the same neural network is able to calibrate different devices that have the same structure.

In long-baseline interferometry, over the last few decades integrated optics beam combiners have become at- tractive technological solutions for new-generation instruments operating at infrared wavelengths. We have investigated different architectures of discrete beam combiners (DBC), which are 2D lattice arrangement of channel waveguides that can be fabricated by exploiting the 3D capability of the ultrafast laser inscription (ULI) fabrication techniques. Here, we present the first interferometric on-sky results of an integrated optics beam combiner based on a coherent pupil remapper and 4 input/23 output zig-zag based DBC, both written monolith- ically in a single borosilicate glass. We show the preliminary results of visibility amplitudes and closure phases obtained from the Vega star by using the previously calibrated transfer matrix of the device.

Femtosecond laser pulses have proven, in the recent years, their formidable potential as amicromachining tool applicable to a variety of materials. In particular, femtosecond laser processinghas shown unique capabilities in altering the optical properties of the bulk of transparent dielectricsubstrates in a permanent fashion and with a micrometric resolution, thus allowing the manufactureof innovative integrated devices for photonics applications. [...]

Stellar interferometry performed in integrated photonic devices allows to increase the angular resolution of a ground-based telescope. Here we present the fabrication and characterization of a low-loss polarization insensitive photonic circuit for astrophotonics, whose geometry was engineered to combine interferometrically up to eight input beams. The employed fabrication technique consisted in the femtosecond laser micromachining followed by a thermal annealing to reduce the birefringence of the waveguides. The fabricated device was characterized to validate its functioning in terms of polarization insensitivity, good transmission and proper beam combination, thus benchmarking its suitability with real on-sky observations.

Astrophotonics is an emerging tool for increasing the angular resolution in ground-based sky observations. Due to the unpolarized nature of celestial light, it is necessary to operate with fully polarization insensitive integrated devices. In this respect, here we show that a thermal annealing after the femtosecond laser writing of waveguides reduces their birefringence of more than order of magnitude, providing integrated circuits whose behaviour is insensitive to the polarization of the input light. As a validation of this technique, we present the successful fabrication of a low-loss integrated device for performing stellar interferometry of up to 8 input beams.

Achieving ultimate bounds in estimation processes is the main objective of quantum metrology. In this context, several problems require measurement of multiple parameters by employing only a limited amount of resources. To this end, adaptive protocols, exploiting additional control parameters, provide a tool to optimize the performance of a quantum sensor to work in such limited data regime. Finding the optimal strategies to tune the control parameters during the estimation process is a non-trivial problem, and machine learning techniques are a natural solution to address such task. Here, we investigate and implement experimentally for the first time an adaptive Bayesian multiparameter estimation technique tailored to reach optimal performances with very limited data. We employ a compact and flexible integrated photonic circuit, fabricated by femtosecond laser writing, which allows to implement different strategies with high degree of control. The obtained results show that adaptive strategies can become a viable approach for realistic sensors working with a limited amount of resources., 10+4 pages, 6+2 figures

We will show the first results for a pupil remapping device with an integrated optics discrete beam combiner. Our expected monochromatic visibility functions are in good agreement with simulation and experiment. The device will be used for our upcoming on-sky tests at 4-m Willian-Herschel Telescope (WHT) in canary islands.

We present the first on-sky results of a four-telescope integrated optics discrete beam combiner (DBC) tested at the 4.2 m William Herschel Telescope. The device consists of a four-input pupil remapper followed by a DBC and a 23-output reformatter. The whole device was written monolithically in a single alumino-borosilicate substrate using ultrafast laser inscription. The device was operated at astronomical H-band (1.6 µm), and a deformable mirror along with a microlens array was used to inject stellar photons into the device. We report the measured visibility amplitudes and closure phases obtained on Vega and Altair that are retrieved using the calibrated transfer matrix of the device. While the coherence function can be reconstructed, the on-sky results show significant dispersion from the expected values. Based on the analysis of comparable simulations, we find that such dispersion is largely caused by the limited signal-to-noise ratio of our observations. This constitutes a first step toward an improved validation of the DBC as a possible beam combination scheme for long-baseline interferometry.

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 and 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.

Nanophotonics, where light control is achieved by means of subwavelength-structured media, are so far mostly fabricated by surface lithographic techniques. Inevitably, this dictates that all nanophotonic elements have planar geometry, and lay at (or close to) the surface of dissimilar material substrates. Planarity is an eminent design restriction against the three-dimensional nature of light propagation, and the use of diverse materials imposes limitations to device robustness against environmental factors, such as temperature changes, vibrations and stress. Regardless of these limitations, two "killer" applications of planar nanophotonics, photonic integrated circuits (PIC) (based on suborn waveguides and 2D photonic bandgap crystals) and optical metasurfaces (i.e. wavefront shaping devices based on subwavelength-structured wavelength-thick surfaces) are already revolutionizing the communications and sensing industries. Such devices are typically based on silicon (or silicon nitride) on lower index silicon dioxide, thus providing good compatibility with the semiconductor CMOS industry. Applying surface lithography or other methods to directly nanostructure dielectric optical materials in the three dimensions has however been out of reach, so far. As a result, the general fields of solid-state lasers and crystal optics still have a very weak connection to the field of nanophotonics. In fact, the use of crystals for either laser light generation or non-linear frequency conversion is fundamentally equal today as it was in 1960: a homogenous crystal is put inside an optical setup.

Sources of entangled photons are cornerstones of photonic quantum technologies, providing the basic resources for a wide range of quantum protocols for computing, simulation and sensing. A typical technique to generate entangled photon pairs exploits spontaneous parametric down conversion (SPDC) in non-linear crystals. In this context, an integrated-optics approach based on non-linear waveguides [1] can boost source brightness due to the enhanced mode confinement, and can take great advantage from the unique compactness and stability of on-chip devices. However, integrated devices are often specialized to produce a given quantum state.

We will review the development in the last decade of discrete beam combiners (DBC), phase sensors based on the propagation of light in photonic lattices. The latest results on the development of DBC for astronomical applications will be presented, along with a new application for the complete tomography of modes at the tip of a multi-mode fiber. The possible use of the DBC in monitoring and controlling modal instabilities in high power lasers will be discussed.

A compact, robust and high quality on-chip source of entangled photons pair in telecom wavelength is presented. The entangled output state can be changed by means of the chip reconfigurability and its hybrid structure.

Multiparameter estimation is a general problem that aims at measuring unknown physical quantities, obtaining high precision in the process. In this context, the adoption of quantum resources promises a substantial boost in the achievable performances with respect to the classical case. However, several open problems remain to be addressed in the multiparameter scenario. A crucial requirement is the identification of suitable platforms to develop and experimentally test novel efficient methodologies that can be employed in this general framework. We report the experimental implementation of a reconfigurable integrated multimode interferometer designed for the simultaneous estimation of two optical phases. We verify the high-fidelity operation of the implemented device, and demonstrate quantum-enhanced performances in two-phase estimation with respect to the best classical case, post-selected to the number of detected coincidences. This device can be employed to test general adaptive multiphase protocols due to its high reconfigurability level, and represents a powerful platform to investigate the multiparameter estimation scenario., 10+7 pages, 7+4 figures

The astronomical J-band (1.25 micrometres) is a relatively untapped wave-band in long-baseline infrared interferometry. It allows access to the photosphere in giant and super-giant stars relatively free from opacities of molecular bands. The J-band can potentially be used for imaging spots in the 1350 nm ionised iron line on slowly rotating magnetically-active stars through spectro-interferometry. In addition, the access to the 1080 nanometres He I line may probe out flows and funnel-flows in T-Tauri stars and allow the study of the star-disk interaction. We present the progress in the development of a six-inputs, J-band interferometric beam combiner based on the discrete beam combiner (DBC) concept. DBCs are periodic arrays of evanescent coupled waveguides which can be used to retrieve simultaneously the complex visibility of every baseline from a multi-aperture interferometer. Existing, planned or future interferometric facilities combine or will combine six or more telescopes at the time, thus increasing the snapshot uv coverage from the interferometric measurements. A better uv coverage will consequently enhance the accuracy of the image reconstruction. DBCs are part of the wider project Integrated astrophotonics that aims to validates photonic technologies for utilisation in astronomy. Before manufacturing the component we performed extensive numerical simulations with a coupled modes model of the DBC to identify the best input configuration and array length. The 41 waveguides were arranged on a zig-zag array that allows a simple optical setup for dispersing the light at the output of the waveguides. The component we are currently developing is manufactured in borosilicate glass using the technique of multi-pass ultrafast laser inscription (ULI), using a mode-locked Yb:KYW laser at the wavelength of 1030 nm, pulse duration of 300 fs and repetition rate of 1 MHz. After annealing, the written components showed a propagation loss less than 0.3 dB/cm and a negligible birefringence at a wavelength of 1310 nm, which makes the components suitable for un-polarized light operation. A single mode fiber-to-component insertion loss of 0.9 dB was measured. Work is currently in progress to characterize the components in spectro-interferometric mode with white light covering the J-band spectrum.

Efficient and long-lived interfaces between light and matter are crucial for the development of quantum information technologies. Integrated photonics solutions for quantum storage devices offer improved performances due to light confinement and enable more complex and scalable designs. We demonstrate a novel platform for quantum light storage based on laser written waveguides. The new writing regime adopted allows us to attain waveguides with improved confining capabilities compared to previous demonstrations. We report the first demonstration of single photon storage in laser written waveguides. While we achieve storage efficiencies comparable to those observed in massive samples, the power involved for the memory preparation is strongly reduced, by a factor 100, due to an enhancement of the light-matter interaction of almost one order of magnitude. Moreover, we demonstrate excited state storage times 100 times longer than previous realizations with single photons in integrated quantum memories. Our system promises to effectively fulfill the requirements for efficient and scalable integrated quantum storage devices., 15 pages including Appendix, 8 figures

Entangled photons generation is an interesting field of research, since progress in this area will directly affect the development of photonic quantum technologies, including quantum computing, simulation and sensing. Several methods have been sifted to increase the performances of entangled photon sources and the integrated optics approach represents a promising strategy. In particular, integrated waveguide sources represent a robust tool, thanks to their stability and the enhancement of nonlinear light-crystal interaction provided by waveguide field confinement. Here, we show the versatility of a hybrid approach, realizing an integrated optical source for the generation of entangled photon-pairs at telecom wavelength. The nonlinear active medium used is lithium niobate, while the routing and manipulation of the generated signal is performed in aluminum-borosilicate glass photonic circuits. The system is composed of three cascaded devices. First, a balanced directional coupler at the fundamental wavelength equally splits the pump in the lithium niobate waveguides, which generate single-photon pairs through type 0 spontaneous parametric down-conversion process. A third chip, encompassing directional couplers and waveplates, closes the interferometer and recombines the generated photons, thus giving access to different quantum states of light: path-entangled or polarization-entangled states. A thermal phase shifter, which controls the relative phase between the interferometer arms, gives an additional degree of freedom for engineering the output state of the presented photon pairs source. All these components are entirely fabricated by femtosecond laser micromachining, a direct and very versatile technique that allows to process different kind of materials and realize high quality optical circuits.

Topological insulators are materials that have a gapped bulk energy spectrum but contain protected in-gap states appearing at their surface. These states exhibit remarkable properties such as unidirectional propagation and robustness to noise that offer an opportunity to improve the performance and scalability of quantum technologies. For quantum applications, it is essential that the topological states are indistinguishable. We report high-visibility quantum interference of single-photon topological states in an integrated photonic circuit. Two topological boundary states, initially at opposite edges of a coupled waveguide array, are brought into proximity, where they interfere and undergo a beamsplitter operation. We observe Hong-Ou-Mandel interference with 93.1 ± 2.8% visibility, a hallmark nonclassical effect that is at the heart of linear optics–based quantum computation. Our work shows that it is feasible to generate and control highly indistinguishable single-photon topological states, opening pathways to enhanced photonic quantum technology with topological properties, and to study quantum effects in topological materials., Science Advances, 4 (9), ISSN:2375-2548

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 x 10(-6)), obtained by thermal annealing. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.

Entanglement is a fundamental resource at the basis of quantum-enhanced performances in several applications, such as quantum algorithms and quantum metrology. In these contexts, Fourier interferometers implement a relevant class of unitary evolutions which can be embedded in a large variety of protocols. For instance, in the single-particle regime it can be adopted to implement the quantum Fourier transform, while in the multi-particle scenario it can be employed to generate quantum states possessing useful entanglement for quantum phase estimation purposes, or as a tool to verify genuine multi-photon interference. In this article, we study experimentally the dynamics of single-photon and two-photon input states during the evolution provided by a 8-mode Fourier transformation, implemented by exploiting a three-dimensional architecture enabled by the femtosecond laser micromachining technology. In such a way, we fabricated three devices to study the evolution after each step of the decomposition. We observe that the probability distributions obey a step-by-step majorization relationship, where the quantum state occupies a progressively larger portion of the Hilbert space. Such behaviour can be related to the majorization principle, which has been conjectured as a necessary condition for quantum speedup.

Currently most quantum key distribution (QKD) experiments are focusing on efficient long-distance implementations. Yet the recent development of miniaturized photonic modules and integrated quantum optics circuits could open new perspectives toward secure short-distance communication for daily-life applications. Here, we present the design of a new integrated optics architecture with an effective size of 25 mm $\, {\times } \,$ 2 mm $\, {\times } \,$ 1 $\,$ mm. Our objective is to obtain an ultraflat microoptics QKD add-on suitable for integration into handheld platforms such as smartphones. In this context, we evaluated the suitability of various optical subsystems. We tested an array of four vertical cavity surface emitting lasers (VCSEL) with highly similar emission properties capable of producing subnanosecond near-infrared pulses at 100-MHz repetition rate. As short pulses exhibit a low polarization degree, their polarization can be externally controlled by a micropolarizer array. The fabrication of such elements is quite straightforward using standard lithographic techniques and extinction ratios up to 29 $\,$ dB have been measured. To guarantee spatial indistinguishability of the qubits, we investigate the option of using low-birefringence, single-mode waveguide array manufactured via femtosecond laser micromachining.

The reversible mapping of quantum states of light in cryogenically cooled rare earth doped crystals, represents one of the most promising routes towards the realization of efficient and high fidelity quantum memories. The miniaturization of these devices in robust and monolithic integrated-optics platforms would be beneficial both in terms of experimental scalability and of enhanced light-matter interaction, arising from the waveguide field confinement. Here, for the first time, we fabricate single mode channel waveguides for visible light at 606 nm in a Praseodymium-doped Yttrium Orthosilicate crystal, which is one of the most employed materials for light storage experiments, thanks to its excellent coherence properties. For the waveguide fabrication, we use the direct technique called femtosecond laser micromachining, in which a femtosecond laser beam is focused inside the crystal volume, and produces a permanent and very localized material modification. In particular, we fabricate the waveguide cladding by inscribing a pair of parallel damage tracks which confine light in the in-between region. With this approach, the waveguide core is not directly exposed to the laser irradiation and consequently its bulk properties result only marginally affected. Measurements of the optical coherence time in waveguide gave results comparable to those obtained in a bulk sample and this confirms that the fabrication procedure does not affect the coherence of the active ions. We performed the storage and the on-demand recall of bright coherent pulses in waveguide, using the atomic frequency comb (AFC) protocol extended to the ground state.

Quantum networks of growing complexity play a key role as resources for quantum computation; the ability to identify the quality of their internal correlations will play a crucial role in addressing the building stage of such states. We introduce a novel diagnostic scheme for multipartite networks of entangled particles, aimed at assessing the quality of the gates used for the engineering of their state. Using the information gathered from a set of suitably chosen multiparticle Bell tests, we identify conditions bounding the quality of the entangled bonds among the elements of a register. We demonstrate the effectiveness, flexibility, and diagnostic power of the proposed methodology by characterizing a quantum resource engineered combining two-photon hyperentanglement and photonic-chip technology. Our approach is feasible for medium-sized networks due to the intrinsically modular nature of cluster states, and paves the way to section-by-section analysis of larger photonics resources.

Encoding many qubits in different degrees of freedom (DOFs) of single photons is one of the routes towards enlarging the Hilbert space spanned by a photonic quantum state. Hyperentangled photon states (i.e. states showing entanglement in multiple DOFs) have demonstrated significant implications for both fundamental physics tests and quantum communication and computation. Increasing the number of qubits of photonic experiments requires miniaturization and integration of the basic elements and functions to guarantee the set-up stability. This motivates the development of technologies allowing the precise control of different photonic DOFs on a chip. We demonstrate the contextual use of path and polarization qubits propagating within an integrated quantum circuit. We tested the properties of four-qubit linear cluster states built on both DOFs. Our results pave the way towards the full integration on a chip of hybrid multiqubit multiphoton states., Comment: 7 pages, 7 figures, RevTex4-1, Light: Science & Applications AAP:http://aap.nature-lsa.cn:8080/cms/accessory/files/AAP-lsa201664.pdf

The transfer of data is a fundamental task in information systems. Microprocessors contain dedicated data buses that transmit bits across different locations and implement sophisticated routing protocols. Transferring quantum information with high fidelity is a challenging task, due to the intrinsic fragility of quantum states. Here we report on the implementation of the perfect state transfer protocol applied to a photonic qubit entangled with another qubit at a different location. On a single device we perform three routing procedures on entangled states, preserving the encoded quantum state with an average fidelity of 97.1%, measuring in the coincidence basis. Our protocol extends the regular perfect state transfer by maintaining quantum information encoded in the polarization state of the photonic qubit. Our results demonstrate the key principle of perfect state transfer, opening a route towards data transfer for quantum computing systems., Transferring quantum information is a fundamental task, but doing so with high fidelity is a challenging task. Here, the authors implement the perfect state transfer protocol to a photonic qubit, entangled with a second one in a different location, across eleven coupled waveguides.

Unlike currently implemented encryption schemes, Quantum Key Distribution provides a secure way of generating and distributing a key among two parties. Although a multitude of research platforms has been developed, the integration of QKD units within classical communication systems remains a tremendous challenge. The recently achieved maturity of integrated photonic technologies could be exploited to create miniature QKD add-ons that could extend the primary function of various existing systems such as mobile devices or optical stations. In this work we report on an integrated optics module enabling secure short-distance communication for, e.g., quantum access schemes. Using BB84-like protocols, Alice's mobile low-cost device can exchange secure key and information everywhere within a trusted node network. The new optics platform (35×20×8mm) compatible with current smartphone's technology generates NIR faint polarised laser pulses with 100MHz repetition rate. Fully automated beam tracking and live basis-alignment on Bob's side ensure user-friendly operation with a quantum link efficiency as high as 50% stable over a few seconds.

Photon entanglement is at the basis of many protocols in photonic quantum technologies, from quantum computing to simulation and sensing. The generation of entangled photons in integrated waveguides is particularly advantageous due to the enhanced stability and more efficient nonlinear interaction. Here we realize an integrated source of entangled wavelength-degenerate photons based on the hybrid interfacing of photonic circuits in different materials, all inscribed by femtosecond laser pulses. We show that our source, based on spontaneous parametric down-conversion at the telecom wavelength, gives access to different classes of output states, allowing us to switch from path-entangled to polarization-entangled states with net visibilities above 0.92 for all selected combinations of integrated devices. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

The application of integrated photonic technologies to quantum optics has recently enabled a wealth of breakthrough experiments in several quantum information areas. In particular, femtosecond laser written optical circuits revealed to be the ideal tool for investigating the features of polarization encoded qubits. However, the difficulty of integrating half and quarter wave plates in such circuits avoids the possibility to perform arbitrary rotations of the polarization state of photons on chip. Femtosecond laser written waveguides intrinsically exhibit a certain degree of birefringence and thus they could be exploited as integrated waveplates. In practice, the direction of the birefringence axes of the waveguides is the same of the propagation direction of the writing femtosecond laser beam, namely perpendicular to the substrate surface. Its fine rotation in a controlled fashion, preserving the accuracy of the positioning of the laser focal spot required by the fabrication process, is extremely challenging. In order to achieve this goal, we combine a high NA (1.4) focusing objective partially filled with a reduced diameter writing beam. In this way, the translation of the beam with respect to the objective center produces a rotation of the focusing direction, without altering the focal spot position. With this method we are able to tilt the birefringence axes of the waveguides up to 45°, and thus to use them as integrated light polarization rotators. In order to demonstrate the effectiveness of these components, we developed a fully integrated device capable to perform the quantum tomography of an arbitrary two-photon polarization state.

Controlling and manipulating the polarization state of a light beam is crucial in applications ranging from optical sensing to optical communications, both in the classical and quantum regime, and ultimately whenever interference phenomena are to be exploited. In addition, many of these applications present severe requirements of phase stability and greatly benefit from a monolithic integrated-optics approach. However, integrated devices that allow arbitrary transformations of the polarization state are very difficult to produce with conventional lithographic technologies. Here we demonstrate waveguide-based optical waveplates, with arbitrarily rotated birefringence axis, fabricated by femtosecond laser pulses. To validate our approach, we exploit this component to realize a compact device for the quantum state tomography of two polarization-entangled photons. This work opens perspectives for integrated manipulation of polarization-encoded information with relevant applications ranging from integrated polarimetric sensing to quantum key distribution., Integrated photonic circuits with arbitrary control over the light polarization state are important in quantum information applications. Corrielli et al. realize compact quantum state tomography of polarization-entangled photons using waveguide-integrated waveplates fabricated by femtosecond laser inscription.

We introduce and experimentally demonstrate a class of surface bound states with algebraic decay in a one-dimensional tight-binding lattice. Such states have an energy embedded in the spectrum of scattered states and are structurally stable against perturbations of lattice parameters. Experimental demonstration of surface states with algebraic localization is presented in an array of evanescently-coupled optical waveguides with tailored coupling rates., revised version with Supplemental Material, to appear in Phys. Rev. Lett

We report an experiment demonstrating quantum frequency conversion of weak light pulses compatible with atomic quantum memories to telecommunication wavelengths. We use a PPLN nonlinear waveguide to convert weak coherent states at the single photon level with a duration of 30ns from a wavelength of 780nm to 1552nm. We measure a maximal waveguide internal (external) conversion efficiency eta_int = 0.41 (eta_ext = 0.25), and we show that the signal to noise ratio (SNR) is good enough to reduce the input photon number below 1. In addition, we show that the noise generated by the pump beam in the crystal is proportional to the spectral bandwidth of the device, suggesting that narrower filtering could significantly increase the SNR. Finally, we demonstrate that the quantum frequency converter can operate in the quantum regime by converting a time-bin qubit and measuring the qubit fidelity after conversion., 15 pages, 5 figures (To appear in Optics Express)