Title : Simulation-driven design of a passive-basis BB84 QKD receiver on silicon photonics using Tidy3D and PhotonForge

Silicon photonics promises compact and stable quantum key distribution (QKD) receivers, but every decibel of loss and every degree of polarization error directly reduces the secret key rate. We present the simulation-driven design of a passive-basis BB84 receiver at 1550 nm, in which fixed on-chip splitters, rather than an active modulator, choose the measurement basis. The design targets the SiEPIC EBeam PDK on the Applied Nanotools 220 nm SOI electron-beam process. Each component is optimized in Tidy3D, whose open-source Python interface drives FDTD, eigenmode-expansion and heat solvers. The layout, extracted from the simulated geometry, is assembled in PhotonForge and verified with LVS and the open-source SiEPIC design-rule deck in KLayout.

Light from the fiber enters through a perfectly vertical two-dimensional polarization-splitting grating coupler (2D PSGC). It separates the two fiber polarizations into two waveguides with −3.14 dB efficiency and below 0.05 dB polarization-dependent loss, without a bottom mirror. TiW thermo-optic phase shifters and a tunable Mach–Zehnder interferometer (P_π = 23.4 mW, 18 µs response) then undo the polarization rotation of the fiber. In each arm, a biased 1×2 multimode interference (MMI) coupler, designed on a stable 84:16 self-imaging plateau with 0.105 dB loss, sends most photons to the Z-basis detectors. The remaining light from both arms interferes in a 2×2 MMI (0.025 dB imbalance) to measure the X basis. Four inverse-taper edge couplers (0.38 dB loss) carry the outputs to off-chip single-photon detectors. Under fabrication variations, the MMI split ratios move by less than 0.5 %. The grating resonance shifts by 1.65 nm per nm of silicon thickness while its peak efficiency stays between −2.3 and −3.2 dB.

The complete 1.79 × 0.80 mm receiver has 3.8 dB fiber-to-detector loss and an intrinsic QBER below 1.3 × 10⁻⁴. We combine time-domain circuit simulation at 1 GHz, a gradient-descent tracking controller fed by reference pulses, and finite-key decoy-state analysis with superconducting detectors. Under slow fiber drift, the receiver delivers 10.1 Mbps secret key rate at 25 km and reaches 185 km. Under moderate drift it delivers 8.9 Mbps, and the range drops to 90 km because tracking speed becomes the limit. By linking nanoscale device optimization, verified layout, and quantum-link simulation in a single scriptable flow, this work shows that a fabrication-ready QKD receiver can be designed and its secure key rate predicted before any chip is made, giving a reusable path for future quantum photonic circuits.

Irfan Nurhakim Hilmi is an undergraduate student from Institut Teknologi Bandung, Indonesia. He majors in Electrical Engineering and specializes in IC Design and Semiconductor Technology. Irfan has worked on multiple projects related to silicon photonics, from components to circuits. Currently, he is a part of a photonics research team that works on designing a quantum key distribution circuit.