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.
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.
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.
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.
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.
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.
To date, most integrated quantum photonics experiments rely on single-photon detectors operating at cryogenic temperatures coupled to photonic integrated circuits (PICs) through single-mode optical fibers. This approach presents significant challenges due to the detection complexity, as cryogenic conditions hinder the development of scalable systems. In addition, going towards fully-integrated devices or, at least, removing the optical fibers would be also advantageous to develop compact and cost-efficient solutions featuring a high number of optical modes. This work reports on the direct coupling of a PIC, fabricated by femtosecond laser writing (FLW), and a silicon single-photon avalanche diode (SPAD) array, fabricated in a custom planar technology and compatible with the operation at room temperature. The effectiveness of this solution is shown by achieving perfect coupling and a system detection efficiency as high as 41.0% at a wavelength of 561 nm, which is the highest value reported to date among both heterogeneous/hybrid integrated and directly coupled systems. We also show the robustness of the coupling to misalignments, demonstrating that costly alignment procedures are not needed. Finally, we exploit the SPAD array to characterize a reconfigurable Mach-Zehnder interferometer, i.e., the basic building block of multimode reconfigurable PICs. This solution provides a new avenue to the design and implementation of quantum photonics experiments, especially effective when compact and cost-efficient systems are needed.
The importance of integrated quantum photonics in the telecom band is based on the possibility of interfacing with the optical network infrastructure that was developed for classical communications. In this framework, femtosecond laser-written integrated photonic circuits, which have already been assessed for use in quantum information experiments in the 800-nm wavelength range, have great potential. In fact, these circuits, being written in glass, can be perfectly mode-matched at telecom wavelength to the in/out coupling fibers, which is a key requirement for a low-loss processing node in future quantum optical networks. In addition, for several applications, quantum photonic devices must be dynamically reconfigurable. Here, we experimentally demonstrate the high performance of femtosecond laser-written photonic circuits for use in quantum experiments in the telecom band, and we demonstrate the use of thermal shifters, which were also fabricated using the same femtosecond laser, to accurately tune such circuits. State-of-the-art manipulation of single- and two-photon states is demonstrated, with fringe visibilities greater than 95%. The results of this work open the way to the realization of reconfigurable quantum photonic circuits based on this technological platform. An integrated quantum photonic circuit that can be thermally reconfigured has been fabricated by femtosecond laser micromachining. Thermally reconfigurable quantum photonic circuits would permit quantum protocols to be implemented that require dynamically changing circuit functionality. A team in Italy has used femtosecond laser micromachining to inscribe waveguide Mach–Zehnder interferometers that operate at telecommunication wavelengths on a glass chip and also to pattern gold resistive heaters on the chip surface, which can be used to accurately tune the circuit. The researchers demonstrated high-quality manipulation of single- and two-photon states with fringe visibilities exceeding 95%. They also fully characterized the thermal response of the interferometric circuit and found good agreement with that expected based on the fabrication design. This device could potentially be used as a building block for reconfigurable quantum circuits.