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Integrated photonic circuit fabrication by femtosecond laser writing for quantum information

Periodic Reporting for period 1 - PhotonFAB (Integrated photonic circuit fabrication by femtosecond laser writing for quantum information)

Reporting period: 2022-04-01 to 2023-09-30

Integrated quantum photonics is widely recognised to be a leading contender across the spectrum of quantum technologies. Existing platforms for integrated quantum photonics build up on a technology (guided-wave optics) originally developed for classical photonics, where the loss of some photons from the circuit is well tolerated. This is not the case for quantum computation, where photon loss can cause fatal errors. The mitigation of the damaging effects of photon losses, if at all possible, requires significant additional resources to protect the information. The extra resources can decrease the competitiveness of integrated quantum photonics with respect to other hardware platforms for quantum technologies. Additionally, most existing
platforms for integrated quantum photonics need complex and costly manufacturing processes (such as the production of masks for photolithography) that are designed for mass production but are not flexible and not good for prototyping. Quantum technologies, on the other hand, are not yet at the mass production stage and their development can benefit from tools that can ensure flexibility and rapid fabrication methods.
Through this ERC PoC we intend to leverage the femtosecond laser writing (FLW) microfabrication capabilities developed by the research team, to build a universal 3D integrated quantum photonic processor that will possess simultaneously unprecedented levels of quantum computational complexity and low optical losses.
In the development of this project we mainly pursued two complementary activities: the increase of the photonic circuit complexity to a large number of optical modes, and the improvement on the control of the operation of complex photonic circuits. The first objective has been achieved by developing 3D photonic circuits that have a better scaling with respect to planar ones as they take advantage of the possibility of adding modes in the third dimension. We demonstrated a 3D photonic circuit with 128 modes (thus largely exceeding the project objective that was set at 64 modes), arranged on a 16x8 rectangular grid, with the exact spacing that can match commercial 2D fiber arrays. We have added reconfiguration capabilities to the circuit by adding thermal shifters on the top surface. In addition, we explored the possibility to laser ablate trenches in glass to position thermal shifters at different depths. The second target was achieved by implementing machine learning algorithms to optimize the calibration of complex, fully reconfigurable devices with up to 8 modes. The use of a data-driven approach gave significantly better results than a standard calibration process, trying to characterize all possible crosstalks between the different thermal shifters. These results are at the forefront of integrated quantum photonics and make the femtosecond laser writing technology appeling for commercial use cases.
In this project we have upscaled the power of our integrated quantum photonic devices by largely increasing the number of optical modes that can be simultaneously processed and the accuracy at which this operation is performed.
The development of a powerful and reliable programmable photonic circuit will have an impact on all applications that require processing quantum information. We envisage the main impact onto quantum computing, where our programmable units can act as the processing core of the system. Nevertheless, information processing will be needed also in quantum communications and quantum sensing. Here, we expect smaller systems, but still needing the same level of accuracy that we are developing for quantum computing. Our programmable photonic processors are essential building blocks in the creation of a quantum photonic infrastructure for the acquisition, manipulation, and transmission of quantum information.
Sketch of the capabilities of femtosecond laser writing in direct writing 3D photonic circuits