Femtosecond-laser written universal quantum photonic processors

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.