Reliable quantum interference of transverse spatial modes in fabrication-tolerant inverse-designed photonic devices

Jamika Ann Roque^1*, Daniel Peace^2,4, Simon White^3,5, Emanuele Polino^3,5, Sayantan Das^2,4,5, Farzad Ghafari^3,5, Sergei Slussarenko^3,5, Nora Tischler^3,5, Jacquiline Romero^2,4, Giovanni A Tapang¹

¹National Institute of Physics, University of the Philippines Diliman, Quezon City, Philippines
²School of Mathematics and Physics, The University of Queensland, Queensland, Australia
³Centre for Quantum Dynamics, Griffith University, Queensland, Australia
⁴Australian Research Council Centre of Excellence for Engineered Quantum Systems, Australia
⁵Australian Research Council Centre of Excellence for Quantum Computation and Communication Technology, Australia

* Presenter:Jamika Ann Roque, email:jamikaroque@gmail.com

The transverse spatial mode of photons provides an additional degree of freedom for scaling up quantum photonic circuits. Thus, reliable on-chip quantum interference between modes has to be achieved. Here, we demonstrate repeatable Hong-Ou-Mandel (HOM) interference between transverse spatial modes using inverse-designed ultra-compact photonic components. We designed, fabricated, and characterized 3 μm x 3 μm transverse-mode beamsplitters and multiplexers operating at 1550 nm – representing the smallest transverse-mode beamsplitters reported to date for this wavelength. The devices were designed via gradient-descent topological optimization. Quantum interference between TE0 and TE1 modes yields visibilities up to 99.56 ± 0.62 %, with an average visibility of 99.38 ± 0.41 % across multiple identical devices, highlighting excellent reproducibility. We also develop a generalized model for HOM interference in lossy, asymmetric, and unbalanced beamsplitters that accurately predicts our interference measurements. As part of our proof of fabrication robustness, we simulated fabrication bias conditions (± 5nm) and found that the visibility remains above 85 %, confirming tolerance to typical fabrication variations. Lastly, preliminary simulations show our method can produce three-port mode beamsplitters (tritters) within the same footprint, making our approach compatible with higher-order mode circuits. Our work establishes inverse-designed components as reliable and repeatable building blocks for multimode quantum photonics, paving the way for scalable, high-dimensional on-chip quantum information processing.

Keywords: Integrated photonics, Inverse design, Quantum optics, Hong-Ou-Mandel interference, Transverse spatial modes