In recent years, femtosecond laser direct writing technology has made great progress in integrated quantum photonic circuits due to its advantages of applicability to an extensive range of materials and true 3-D ability. However, a waveguide prepared via a femtosecond laser shows a non-circular cross section and high birefringence, which seriously restricts the development of polarization encoded quantum photonic integrated chips towards greater integration and higher functions. Here, we propose an amplitude-phase double shaping method to reduce waveguide birefringence, which involves using a cylindrical lens and slits. A waveguide prepared using this method shows a circularity cross-section that reaches up to 97.6% and a birefringence that is as low as 1.49 × 10-6. Based on such an ultralow birefringence waveguide, a polarization independent Hong-Ou-Mandel quantum interferometer was fabricated. Interference visibilities for identical photon pairs in different polarization states were found to be more than 95% with a standard deviation of 0.6%. The ultra-low birefringence single-mode waveguide and the polarization-independent interferometer realized using this approach will play an important role in large-scale polarization encoding integrated quantum photonic chips.
In recent years, femtosecond laser direct writing technology has made great progress in integrated quantum photonic circuits due to its advantages of applicability to an extensive range of materials and true 3-D ability. However, a waveguide prepared via a femtosecond laser shows a non-circular cross section and high birefringence, which seriously restricts the development of polarization encoded quantum photonic integrated chips towards greater integration and higher functions. Here, we propose an amplitude-phase double shaping method to reduce waveguide birefringence, which involves using a cylindrical lens and slits. A waveguide prepared using this method shows a circularity cross-section that reaches up to 97.6% and a birefringence that is as low as 1.49 × 10-6. Based on such an ultralow birefringence waveguide, a polarization independent Hong-Ou-Mandel quantum interferometer was fabricated. Interference visibilities for identical photon pairs in different polarization states were found to be more than 95% with a standard deviation of 0.6%. The ultra-low birefringence single-mode waveguide and the polarization-independent interferometer realized using this approach will play an important role in large-scale polarization encoding integrated quantum photonic chips.
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.
Photonics is a powerful framework for testing in experiments quantum information ideas, which promise significant advantages in computation, cryptography, measurement and simulation tasks. Linear optics is in principle sufficient to achieve universal quantum computation, but stability requirements become severe when experiments have to be implemented with bulk components. Integrated photonic circuits, on the contrary, due to their compact monolithic structure, easily overcome stability and size limitations of bench-top setups. Anyway, for quantum information applications, they have been operated so far only with fixed polarization states of the photons. On the other hand, many important quantum information processes and sources of entangled photon states are based on the polarization degree of freedom. In our work we demonstrate femtosecond laser fabrication of novel integrated components which are able to support and manipulate polarization entangled photons. The low birefringence and the unique possibility of engineering three-dimensional circuit layouts, allow femtosecond laser written waveguides to be eminently suited for quantum optics applications. In fact, this technology enables to realize polarization insensitive circuits which have been employed for entangled Bell state filtration and implementation of discrete quantum walk of entangled photons. Polarization sensitive devices can also be fabricated, such as partially polarizing directional couplers, which have enabled on-chip integration of quantum logic gates reaching high fidelity operation.
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 emerging strategy to overcome the limitations of bulk quantum optics consists of taking advantage of the robustness and compactness achievable by the integrated waveguide technology. Here we report the realization of a directional coupler, fabricated by femtosecond laser waveguide writing, acting as an integrated beam splitter able to support polarization encoded qubits. This maskless and single step technique allows to realize circular transverse waveguide profiles able to support the propagation of Gaussian modes with any polarization state. Using this device, we demonstrate the quantum interference with polarization entangled states.
The emerging strategy to overcome the limitations of bulk quantum optics consists of taking advantage of the robustness and compactness achievable by integrated waveguide technology. Here we report the realization of a directional coupler, fabricated by femtosecond laser waveguide writing, acting as an integrated beam splitter able to support polarization-encoded qubits. This maskless and single step technique allows us to realize circular transverse waveguide profiles which are able to support the propagation of gaussian modes with any polarization state. Using this device, we demonstrate quantum interference with polarization-entangled states and singlet state projection.
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.