Integrated photonics is increasing in importance for compact, robust, and scalable enabling quantum technologies. This is particularly interesting for developing quantum communication networks, where resources need to be deployed in the field. We exploit photonic chip-based Si 3 N 4 ring microresonators to realise a photon pair source with low-loss, high-noise suppression and coincidence rates of 80×10 3 s −1 . A simple photonic noise characterisation technique is presented that distinguishes linear and nonlinear contributions useful for system design and optimisation. We then demonstrate an all-fibre 750 MHz clock-rate sequential Time-Bin entanglement scheme with raw interference visibilities > 98 %.
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.Integrated optics is a very powerful platform to produce miniaturised complex photonic devices with improved scalability, robustness and suitability for field application 1 . This approach greatly benefited classical optics communications, which is at the basis of today's information society. A similar trend is being adopted also for quantum optical devices, fostering an exponential increase in the layout complexity of integrated photonic circuits for various quantum applications, from computation to simulation [2][3][4][5][6] . Integrated quantum photonics is currently based on consolidated technologies, such as silicon on insulator 3 and silica on silicon 2 , as well as on more innovative approaches as waveguide writing by femtosecond laser micromachining (FLM) [4][5][6] . The two main advantages that FLM introduced in integrated quantum photonics are: the possibility to manipulate polarisation-encoded photons on-chip 5 and unique 3D fabrication capabilities 7 . The former enables the direct on-chip transfer of many protocols already developed in quantum optics and based on polarisation encoding. The latter opens the way to innovative 3D layouts that can produce novel functionalities in more compact geometries.The polarisation sensitivity of the FLM fabricated devices comes from the low (~10 −4 -10 −5 ) optical birefringence that is typically present in this type of waveguides. Depending on the material, birefringence in FLM can originate from one or more of the following sources. It can be due to the creation of periodic nano-structures aligned orthogonally to the writing beam polarisation 8 , it can arise due to asymmetric material stresses induced in the focal volume 9 , or it may be due to ellipticity of the written waveguide cross section 10,11 . While high birefringence is desirable to achieve effective polarisation manipulation in compact devices, on the other hand this same property induces unwanted strong delays between the two main polarisation components also during plain propagation in the chip, thus rapidly destroying the original polarisation state. T...
Integrated photonics represents a technology that could greatly improve quantum communication networks in terms of cost, size, scaling, and robustness. A key benchmark for this is to demonstrate their performance in complex quantum networking protocols, such as entanglement swapping between truly independent photon-pair sources. Here, using two independent, asynchronouslypumped, integrated Si 3 N 4 microring resonator photon-pair sources, operating in the continuouswave regime with time-resolved detections, we obtained state of the art Hong-Ou-Mandel (93.2 ± 1.6%) and entanglement swapping (91.2 ± 3.4%) visibilities, while maintaining high rates. The time-resolved detection facilitates high spectral purities without the need for spectral filtering. Our results demonstrate the potential of such telecom-band sources for practical, real-world quantum communication.
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