The integrated photonic platform has established itself as a versatile tool for experimental implementation of linear optical quantum computations, that base on unitary transformation of the input states. In this paper we investigate the potential application of the waveguide lattice architecture to realization of arbitrary unitary matrices. We propose a waveguide lattice construction and find its parameters using methods of numerical optimization. Also we showed that a necessary lattice parameters are within the reach femtosecond laser writing technology.
The integrated photonic platform has established itself as a versatile tool for experimental implementation of linear optical quantum computations, that base on unitary transformation of the input states. In this paper we investigate the potential application of the waveguide lattice architecture to realization of arbitrary unitary matrices. We propose a waveguide lattice construction and find its parameters using methods of numerical optimization. Also we showed that a necessary lattice parameters are within the reach femtosecond laser writing technology.
Photonic integrated circuits (PICs) are a technology with a growing interest in a wide range of applications in quantum information, from computation to communications and sensing. Amongst the various types of PICs, universal quantum photonic processors (UQPPs) are programmable photonic integrated circuits on which any arbitrary unitary transformation can be implemented on a given input quantum photonic state, sometimes also referred to as quantum photonic FPGAs. Various kinds of UQPPs have been reported, mostly with a reconfigurable Mach-Zehnder interferometer (MZI) as building block, in photonic platforms ranging from silicon nitride to glass-based direct laser writing. Among them, femtosecond laser writing (FLW) is a versatile fabrication technique that allows for cost-effective fabrication of waveguides in glass substrates with low insertion losses (down to 0.1 dB cm−1 for the propagation and 0.2 dB per facet for the coupling) over a wide wavelength range, a key requirement for quantum applications. Moreover, FLW allows for the fabrication of microstructures in the substrate such as trenches, which can act as thermal isolation structures that significantly reduce the power dissipation of thermal phase shifters and their thermal crosstalk. In this work, firstly we present the fabrication of a 6-mode FLW-UQPP in a rectangular MZI mesh layout operating at 785 nm with average amplitude fidelity as high as 0.9963 when implementing switching unitaries and 0.9979 when implementing Haar random unitaries. Secondly, we demonstrate the versatility of the FLW platform by fabricating another 6-mode UQPP with waveguides optimized for operation at 1550 nm wavelength, with similar performance, on which we repeated the implementation of Haar random unitaries with 0.9969 average amplitude fidelity.
Abstract Implementing arbitrary unitary transformations is crucial for applications in quantum computing, signal processing, and machine learning. Unitaries govern quantum state evolution, enabling reversible transformations critical in quantum tasks like cryptography and simulation and playing key roles in classical domains such as dimensionality reduction and signal compression. Integrated optical waveguide arrays have emerged as a promising platform for these transformations, offering scalability for both quantum and classical systems. However, scalable and efficient methods for implementing arbitrary unitaries remain challenging. Here, we present a theoretical framework for realizing arbitrary unitary matrices through programmable waveguide arrays (PWAs). We provide a mathematical proof demonstrating that cascaded PWAs can implement any unitary matrix within practical constraints, along with a numerical optimization method for customized PWA designs. Our results establish PWAs as a universal and scalable architecture for quantum photonic computing, effectively bridging quantum and classical applications, and positioning PWAs as an enabling technology for advancements in quantum simulation, machine learning, secure communication, and signal processing.
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.
Photonic integrated circuits are increasingly used in quantum communications, computing, and sensing. Among them, universal photonic processors (UPPs) are attracting a lot of attention from both the academic and industrial world for their ability to perform any unitary transformation on a quantum photonic state, akin to optical FPGAs. These processors employ meshes of programmable Mach-Zehnder interferometers and are fabricated on platforms like silicon nitride, silicon and glass using various techniques. One such technique, femtosecond laser writing (FLW), is versatile and cost-effective, producing waveguides in glass substrates with high quality and low losses over a wide wavelength range, a feature essential for quantum applications. FLW also allows for the manufacturing of microstructures, such as 2D and 3D void trenches, which improve thermal isolation and reduce power dissipation and thermal crosstalk in thermo-optic phase shifters. With this work, we introduce the first 24-mode FLW-UPP, the most complex UPP reported in the literature. The device is optimized for operation with quantum dot sources emitting at 925 nm and features average total insertion losses of only 4.35 dB, making it suitable for the most demanding multi-photon quantum experiments. Thanks to the realization of suspended waveguides, it operates efficiently with a total power consumption of less than 14W, requiring only a simple thermo-electric cooler for temperature regulation. Preliminary calibration indicates high reproducibility on the splitting ratio distribution of the directional couplers, with amplitude fidelity exceeding 98 % on average for Haar-random unitary transformations. These results highlight the potential of FLW-UPPs in advancing and scaling quantum photonic technologies.
Quantum computers promise to be able to solve tasks beyond the reach of standard computational platforms.Among the others, photonic quantum walks prove to be great candidates for their implementation, since single photon sources, passive linear optics and photo-detectors are sufficient for universal quantum computation.To this aim, a device performing the quantum Fourier transform represents a fundamental building block for quantum algorithms, whose applications are not limited to the field of quantum computation.Recently, an algorithm has been developed to efficiently realize a quantum Fourier transform of an input photonic state by using a quantum walk on elementary linear-optical components.Here we provide a simple operative description of the algorithm, introducing a whole class of quantum transformations achievable through a generalization of this procedure.We finally discuss how femtosecond laser writing technology well represents an efficient and scalable platform for the implementation of this class of photonic quantum walks.
We report the fabrication of the first 24-mode universal photonic processor (UPP) realized through femtosecond laser writing (FLW), marking the most complex UPP demonstrated to date. Optimized for quantum dot emission at 925 nm, the device exhibits total insertion losses averaging only 4.35 dB, enabling its direct application in advanced multi-photon quantum experiments. Leveraging the versatility of FLW, we introduce suspended waveguides and precisely engineered 2D and 3D microstructures, significantly enhancing thermal isolation and minimizing power dissipation. As a result, our processor operates efficiently at less than 10 W, requiring only a simple thermo-electric cooler for stable thermal management. The device exhibits exceptional performance after calibration, implementing Haar-random unitary transformations with an amplitude fidelity of 99.7 %. This work establishes FLW-based integrated photonics as a scalable and robust platform for advancing quantum computing, communication, and sensing technologies.