Periodic Reporting for period 1 - ESSENS (Efficient Detection of Squeezed Light on Nanophotonic Chips using Subwavelength-Engineered Superconducting Nanowire Avalanche Photodetectors)
Reporting period: 2022-10-04 to 2024-10-03
The ESSENS project aims at implementing a photonic integrated chip that features high system detection efficiency to exploit the potential of squeezed states of light for quantum simulation, communication and sensing applications. Light in a squeezed state exhibits reduced quantum uncertainty, which can, for example, be employed in super-sensitive gravitational-wave detectors and has been utilized for Gaussian boson sampling experiments. However, squeezed light is very delicate, as interaction with the environment (i.e. photon loss) rapidly degrades quantum correlations. This inconvenience limits the scalability and stability of systems based on bulk optical components such as beamsplitters and mirrors. Photonic integrated circuits offer novel means for overcoming such current limitations because recent advances in design and nanofabrication benefit compact realizations of processing loss-sensitive squeezed light on silicon chips: on the one hand, very low propagation loss is achievable in wideband-transparent moderate-index-contrast platforms (e.g. silicon nitride on insulator); on the other hand, monolithic chips provide interferometric stability even when the component count increases. Thus, in the ESSENS project a photonic integrated system is developed that paves the way for exploiting squeezed states of light on a chip. The key components of such a system are a surface grating coupler with sub-decibel coupling efficiency and a waveguide-integrated superconducting nanowire single-photon detector (SNSPD) with high on-chip detection efficiency (OCDE) and low jitter. The specific goals of the project align with the independent design of both key components. In order to achieve the required high performance, direct-laser-writing (DLW) fabrication technology and subwavelength-grating (SWG) metamaterial engineering are considered. The former facilitates the fast fabrication of any 3D shape, thereby expanding the design space beyond planar geometries that result from standard electron-beam lithography and reactive ion etching processes; the latter enable the synthesis of anisotropic metamaterials with tailorable optical properties, which also expands the design space by enlarging the range of usable refractive indices.
In its first stage, the ESSENS project focused on the development of waveguides with subwavelength-grating (SWG) lateral claddings in the silicon nitride–on-insulator platform, as these are the fundamental building blocks for embedding the targeted superconducting nanowire single-photon detectors (SNSPDs). By using a finite-element-method (FEM) solver and 3D finite-difference time-domain (FDTD) simulations, the dimensions of the lateral SWG waveguides and homogeneous-to-SWG waveguide transitions were determined. Further fabrication, based on electron-beam lithography, and experimental characterization confirmed the suitability of the designed structures.
The supported mode of a homogenized lateral SWG waveguide with a nm-thin NbTiN nanowire on top was estimated by FEM simulations, revealing an absorption of ~5% per nanowire, in agreement with reports found in the literature. The fabrication process of the nanowires on top of the SWG waveguides was carefully applied, comprising the following steps: (1) sputtering of a thin NbTiN film on a silicon nitride wafer, (2) lithographic definition of gold electrical pads, (3) deposition of gold and lift-off, (4) lithographic definition of superconducting nanowires and etching, (5) lithographic definition of nanophotonic components, including SWG waveguides, and etching. The experimental characterization of the nanowires on the SWG waveguides yielded an absorption of ~5% at room temperature, in excellent agreement with the simulations.
Finally, taking advantage of the knowledge acquired throughout the previous work, a photonic integrated chip including waveguide-integrated cavity-based SWG-engineered SNSPDs was developed and characterized at cryogenic temperature. The attained results overcome the performance of state-of-the-art cavity-based NbTiN SNSPDs.
The supported mode of a homogenized lateral SWG waveguide with a nm-thin NbTiN nanowire on top was estimated by FEM simulations, revealing an absorption of ~5% per nanowire, in agreement with reports found in the literature. The fabrication process of the nanowires on top of the SWG waveguides was carefully applied, comprising the following steps: (1) sputtering of a thin NbTiN film on a silicon nitride wafer, (2) lithographic definition of gold electrical pads, (3) deposition of gold and lift-off, (4) lithographic definition of superconducting nanowires and etching, (5) lithographic definition of nanophotonic components, including SWG waveguides, and etching. The experimental characterization of the nanowires on the SWG waveguides yielded an absorption of ~5% at room temperature, in excellent agreement with the simulations.
Finally, taking advantage of the knowledge acquired throughout the previous work, a photonic integrated chip including waveguide-integrated cavity-based SWG-engineered SNSPDs was developed and characterized at cryogenic temperature. The attained results overcome the performance of state-of-the-art cavity-based NbTiN SNSPDs.
In the ESSENS project, bowtie-shaped superconducting nanowire single-photon detectors (SNSPDs) were integrated into a waveguide-coupled subwavelength-engineered Fabry-Pérot cavity, yielding an on-chip detection efficiency (OCDE) exceeding 80% at telecom wavelengths, negligible dark counts, detector dead times of ~1 ns (enabling high repetition rates of up to ~1 GHz), and a timing uncertainty (jitter) below 20 ps. This is, to the best of our knowledge, the highest experimentally demonstrated OCDE and lowest jitter for high-speed bowtie-shaped NbTiN SNSPDs.