Thermally reconfigurable quantum photonic circuits at telecom wavelength by femtosecond laser micromachining
Fulvio Flamini, Lorenzo Magrini, Adil S Rab, Nicolò Spagnolo, Vincenzo D’Ambrosio, Paolo Mataloni, Fabio Sciarrino, Tommaso Zandrini, Andrea Crespi, Roberta Ramponi, Roberto Osellame
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.
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.
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.
Entangled photons generation is an interesting field of research, since progress in this area will directly affect the development of photonic quantum technologies, including quantum computing, simulation and sensing. Several methods have been sifted to increase the performances of entangled photon sources and the integrated optics approach represents a promising strategy. In particular, integrated waveguide sources represent a robust tool, thanks to their stability and the enhancement of nonlinear light-crystal interaction provided by waveguide field confinement. Here, we show the versatility of a hybrid approach, realizing an integrated optical source for the generation of entangled photon-pairs at telecom wavelength. The nonlinear active medium used is lithium niobate, while the routing and manipulation of the generated signal is performed in aluminum-borosilicate glass photonic circuits. The system is composed of three cascaded devices. First, a balanced directional coupler at the fundamental wavelength equally splits the pump in the lithium niobate waveguides, which generate single-photon pairs through type 0 spontaneous parametric down-conversion process. A third chip, encompassing directional couplers and waveplates, closes the interferometer and recombines the generated photons, thus giving access to different quantum states of light: path-entangled or polarization-entangled states. A thermal phase shifter, which controls the relative phase between the interferometer arms, gives an additional degree of freedom for engineering the output state of the presented photon pairs source. All these components are entirely fabricated by femtosecond laser micromachining, a direct and very versatile technique that allows to process different kind of materials and realize high quality optical circuits.
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.
Abstract Integrated quantum photonics, i.e. the generation, manipulation, and detection of quantum states of light in integrated photonic chips, is revolutionizing the field of quantum information in all applications, from communications to computing. Although many different platforms are being currently developed, from silicon photonics to lithium niobate photonic circuits, none of them has shown the versatility of femtosecond laser micromachining (FLM) in producing all the components of a complete quantum system, encompassing quantum sources, reconfigurable state manipulation, quantum memories, and detection. It is in fact evident that FLM has been a key enabling tool in the first‐time demonstration of many quantum devices and functionalities. Although FLM cannot achieve the same level of miniaturization of other platforms, it still has many unique advantages for integrated quantum photonics. In particular, in the last five years, FLM has greatly expanded its range of quantum applications with several scientific breakthroughs achieved. For these reasons, we believe that a review article on this topic is very timely and could further promote the development of this field by convincing end‐users of the great potentials of this technological platform and by stimulating more research groups in FLM to direct their efforts to the exciting field of quantum 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.