Universal photonic processors (UPPs) are fully programmable photonic integrated circuits that are key components in quantum photonics. With this work, we present a novel platform for the realization of low-loss, low-power, and high-fidelity UPPs based on femtosecond laser writing (FLW) and compatible with a large wavelength spectrum. In fact, we demonstrate different UPPs, tailored for operation at 785 nm and 1550 nm, providing similar high-level performances. Moreover, we show that standard calibration techniques applied to FLW-UPPs result in Haar random polarization-insensitive photonic transformations implemented with average amplitude fidelity as high as 0.9979 at 785 nm (0.9970 at 1550 nm), with the possibility of increasing the fidelity over 0.9990 thanks to novel optimization algorithms. Besides being the first demonstrations of polarization-insensitive UPPs, these devices show the highest level of control and reconfigurability ever reported for a FLW circuit. These qualities will be greatly beneficial to applications in quantum information processing.
Universal photonic processors (UPPs) are fully programmable photonic integrated circuits that are key components in quantum photonics. With this work, we present a novel platform for the realization of low-loss, low-power, and high-fidelity UPPs based on femtosecond laser writing (FLW) and compatible with a large wavelength spectrum. In fact, we demonstrate different UPPs, tailored for operation at 785 nm and 1550 nm, providing similar high-level performances. Moreover, we show that standard calibration techniques applied to FLW-UPPs result in Haar random polarization-insensitive photonic transformations implemented with average amplitude fidelity as high as 0.9979 at 785 nm (0.9970 at 1550 nm), with the possibility of increasing the fidelity over 0.9990 thanks to novel optimization algorithms. Besides being the first demonstrations of polarization-insensitive UPPs, these devices show the highest level of control and reconfigurability ever reported for a FLW circuit. These qualities will be greatly beneficial to applications in quantum information processing.
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.
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.
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.
Recent advancements in active reconfigurable photonic devices have spurred interest in quantum information applications, ranging from computation to communications and sensing. Universal photonic processors (UPPs) play a crucial role in this domain, enabling the implementation of arbitrary unitary transformations on input photonic states. Common architectures for UPPs involve intricate interferometric meshes, with the reconfigurable Mach-Zehnder interferometer (MZI) as the fundamental building block. In this work, we present the realization of an 8-mode UPP using direct femtosecond laser writing (FLW) as the fabrication platform. FLW allows rapid and cost-effective prototyping of waveguides in glass-based substrates, achieving low insertion losses (down to 0.13 dB cm−1 for propagation and 0.2 dB per facet for coupling), a critical requirement for quantum applications. By incorporating compact curved deep isolation trenches and stable, efficient thermal phase shifters, we have reduced the size of the MZI unit cell compared to the current state-of-the-art in FLW fabrication. This reduction improves integration density and circuit complexity with respect to the current state-of-the-art devices for this fabrication platform. The phase shifters exhibit minimal power dissipation (∼ 38mW) and thermal crosstalk (∼ 20 %). The device operates at a wavelength of 925 nm, making it compatible with state-of-the-art quantum dot single-photon sources. It features 28 MZIs and 56 thermal phase shifters, with total insertion losses below 3 dB. Additionally, we describe a calibration process combining conventional methods with a machine learning optimization procedure, enabling the realization of unitary transformations with an average amplitude fidelity surpassing 0.99, showcasing the high precision required for quantum photonic applications.
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.
Programmable photonic quantum processors face a critical challenge: despite significant advances in quantum state preparation and manipulation, measurements remain limited to projective techniques. Here, we demonstrate a programmable measurement processor that overcomes this limitation by enabling arbitrary quantum measurements within a scalable circuit framework. Our large-scale integrated photonic architecture achieves precise coherent control of ancillary quantum systems, realizing a universal four-dimensional quantum measurement device. We benchmark the processor by performing measurement tomography on 100 randomly selected measurements, achieving an average fidelity of 97.7%. The processor's performance exceeds the theoretical limits of projective measurements in three key quantum information tasks: state discrimination (with 23 times lower error), state estimation (with 10.6% higher fidelity), and randomness generation (with 37% more randomness yield), demonstrating its high operational quality. This work establishes a fully programmable quantum measurement processor, advancing the development of universal quantum operations for photonic quantum information processing by providing the key missing component.