Integrated quantum photonic circuits composed of femtosecond laser direct written waveguides (FLDWs) have allowed implementation of several quantum protocols [1], due to their advantages of fast prototyping and ease of 3D fabrication. Adding a single-photon source based on spontaneous four-wave mixing (SFWM) to such circuits on one chip would eliminate coupling loss and promises 3D all-in-one quantum photonic processors. A key issue for such sources is achieving phase-matching in a low index contrast platform. Nevertheless, waveguide form birefringence was recently used to demonstrate this [2]. Here we use a different approach, exploiting stress birefringence introduced with explicit damage structures in the glass [3]. We note that the control of birefringence via stress tracks has great potential for state engineering [4] as the effective nonlinearity can be modulated by adjusting the damage track offset along the propagation direction while keeping the main waveguide optimised for nonlinear interaction.
Integrated quantum photonic circuits composed of femtosecond laser direct written waveguides (FLDWs) have allowed implementation of several quantum protocols [1], due to their advantages of fast prototyping and ease of 3D fabrication. Adding a single-photon source based on spontaneous four-wave mixing (SFWM) to such circuits on one chip would eliminate coupling loss and promises 3D all-in-one quantum photonic processors. A key issue for such sources is achieving phase-matching in a low index contrast platform. Nevertheless, waveguide form birefringence was recently used to demonstrate this [2]. Here we use a different approach, exploiting stress birefringence introduced with explicit damage structures in the glass [3]. We note that the control of birefringence via stress tracks has great potential for state engineering [4] as the effective nonlinearity can be modulated by adjusting the damage track offset along the propagation direction while keeping the main waveguide optimised for nonlinear interaction.
Direct UV-written silica-on-silicon provides an attractive platform for quantum optics, offering the key benefits of low propagation losses and excellent optical fibre compatibility. This work has aimed to develop the necessary techniques and components for completely integrated quantum optics experiments to be carried out using this platform, on a larger scale than previously possible. Arrays of matched on-chip photon sources based on birefringence-matched spontaneous four-wave mixing (FWM) are demonstrated, both at 800 nm and in the telecommunications C-band, along with progress towards further integration of these sources. Thermo-optic phase shifters, for use in room temperature quantum circuits, have been optimised via modelling, and a range of alternative modulator technologies have been explored. Further work has been concerned with the development of a modular system for quantum optics, comprising a set of duplicate reconfigurable modules and the necessary drive electronics and software, with the intent of simplifying the task of identifying and quantifying manufacturing imperfections in large integrated experiments. Efforts have also been made to improve the detection efficiency of on-chip transition edge sensor (TES) single photon detectors, including the use of longer absorbers and high reflectors for multiple absorption passes; this has resulted in the demonstration of a photon-number-resolving detector with a Bragg grating enhanced quantum efficiency of 87%. Approaches for further increasing the detection efficiency are considered and a device for on-chip Hong-Ou-Mandel and photon subtraction experiments using these detectors is reported.
Heralded single photon sources, based on probabilistic non-linear interactions, currently serve as the de-facto standard for performing multi-photon quantum interference experiments. Already, interesting phenomena may be explored with a handful of such sources [1], whilst in the future, large arrays of identical sources could be multiplexed with a switching network to achieve a truly scalable, near-deterministic emission [2]. However, fabricating such a large number of indistinguishable, pure, heralded single photon sources, operating with low-loss, remains an open challenge. Spontaneous four-wave mixing (SFWM) in a silica waveguide is a promising route to building such a source. The low-loss and excellent mode-matching to optical fibre of silica waveguides allows high preparation efficiency. Guided-wave photonics also allows the precise control of the optical modes needed to engineer emission of photons into a single pair of Schmidt modes: allowing heralded photons of high purity without narrowband filtering [3].
Summary form only given. High-accuracy integrated photonic circuits are desired in many applications, ranging from telecommunications to sensing, especially where many optical modes or many wavelengths are involved. Particularly challenging are the requirements needed for quantum information applications [1], which have recently found in the integrated optics technology a quite unique and powerful platform [2]. In fact, quantum and classical interference effects are ubiquitously exploited in quantum photonic devices, thus requiring a tight fabrication control not only on the power splitting elements inside the circuit, but also on the phase induced by the various paths. Difficulties grow in case quantum simulation tasks [3] are addressed: the need to model other physical systems increases the circuit complexity and many elements need to be integrated without losing in precision.Femtosecond laser microfabrication has recently emerged as an ideal approach to fabricate integrated photonic circuits for quantum applications [4, 5]. *n the one hand it is appreciated for its low costs and fast-prototyping capabilities. *n the other hand, femtosecond laser written waveguides, due to their low birefringence, can support polarization-entangled two-photon states [5], which are useful to implement many quantum information protocols.In this work we demonstrate the capability of femtosecond laser writing technology to fabricate complex waveguide circuits, with tight phase control and polarization independent behaviour. Specifically, we manufacture optical circuits consisting of cascaded directional couplers, which implement discrete-time quantum walks of polarization entangled photon pairs. The scheme of a circuit realizing a four-steps quantum walk on an ordered one-dimensional lattice is shown in Fig. 1(a). To achieve a perfect polarization insensitive behaviour, directional couplers with an innovative three-dimensional geometry are employed (see the blow-up in Fig. 1(a)). In fact, waveguide birefringence causes a slight polarization dependence of the dimension and of the ellipticity of the guided mode, which may affect the coupling coefficients. We show that, by fabricating the two waveguides constituting the coupler on different planes, it is possible to find a condition in which the overlap integrals for differently polarized guided modes are equal, thus providing the desired polarization insensitivity in the coupling. In Fig. 1(b) the experimental two-photon correlations are reported, obtained by injecting polarization-entangled two-photon states with different symmetry, compared with the theoretical ones. This is equivalent to perform quantum walks of quantum particle pairs with bosonic or fermionic symmetry [6]. *ne can note the excellent agreement between the theory and the experiment, which testifies the accuracy of our fabrication process. We will also show how similar circuits, involving more optical modes and implementing arbitrary phase maps, enable to study quantum walks with disorder-induced effects and thus to observe phenomena such as Anderson localization.
The vast development of integrated quantum photonic technology enables the implementation of compact and stable interferometric networks. In particular, laser-written waveguide structures allow for complex 3D circuits and polarization-encoded qubit manipulation. However, the main limitation in the scaling up of integrated quantum devices is the single-photon loss due to mode-profile mismatch when coupling to standard fibers or other optical platforms. Here we demonstrate tapered waveguide structures realized by an adapted femtosecond laser writing technique. We show that coupling to standard single-mode fibers can be enhanced up to 77% while keeping the fabrication effort negligible. This improvement provides an important step for processing multiphoton states on chip.
Quantum light generated through spontaneous four-wave mixing (SFWM) process in nonlinear materials, such as entangled photon pairs and single photons, is an important resource for various emerging quantum applications. Integrated quantum photonics has enabled the generation, manipulation, and detection of quantum states of light with steadily increasing scale and complexity levels. Importantly, the exploration of on-chip integration has accumulated substantial progresses in recent years toward the realization of low-cost, large-scale quantum photonic circuits. Here, we review the underlying mechanism and discuss state-of-the-art SFWM on-chip quantum light sources fabricated with various structures and materials on chip. Furthermore, we enumerate the most appealing applications of on-chip SFWM such as heralding single-photon source, entangled photon source, and system-level integration.
Recent progress in the field of quantum information processing has highlighted the prospects of using integrated optical devices for quantum applications. Integrated quantum photonics offers several advantages compared to free-space setups. Not only the miniaturization, which dramatically reduces the size of the building blocks and allows imprinting or cascading several functions on a single substrate, but also the possibility to reproduce the same photonic circuit many times on the same chip. For instance, this has been exploited to spatially multiplex heralded single-photon, leading to an increase of the single-photon emission rate at constant noise level. More strikingly, one can think of combining, on-chip, several synchronised single photon sources towards engineering large photon-number entangled states. Despite this attractive potential, only few examples of spatial mul- tiplexing have been reported in the literature due to the technological challenge related to fabrication processes. On one hand, lithium nio- bate (LN) is very suitable for quantum integrated photonics since it shows both optic-optic and electro-optic non-linearities, as well as the possibility to integrate low-loss waveguides. However, several parallel and/or cascaded optical functions require various lithographic steps leading to reduced yields. On the other hand, femto-second laser direct-writing (FLDW) technique on glass-type substrates, allows fast fabrication of low-loss waveguide circuits requiring no lithographic masks nor chemical exchange. However, no efficient non-linear processes are available in SiO2 waveguides for photon-pair generation. We discuss here an integrated photonic chip able to generate photon-number states which consists of three photonic chips fabricated on either LN for photon generation or on SiO2 substrates for photon manipulation purposes. Our approach takes advantage of the best features of both worlds.