Title : Implementation of QKD BB84 as photonics IC using open-source software Tidy3D and photonforge

A quantum key distribution (QKD) circuit utilizing the BB84 protocol has been designed and simulated on a Silicon-on-Insulator (SOI) platform, specifically using SiEPIC Ebeam 220nm technology. The design leverages open-source software tools, Tidy3D and PhotonForge, which are crucial for the efficient and accessible implementation of photonics integrated circuits (PIC). The QKD circuit comprises two principal functional blocks: the 'Alice' circuit, which acts as the quantum transmitter, and the 'Bob' circuit, serving as the quantum receiver. The core of this implementation is the BB84 protocol's quantum encoding, which utilizes four distinct polarization states: 0°, 90°, 45°, and -45°. The implementation of these four polarization states relies on plasmonic rotators, which function as the main components for encoding. These rotators are integrated with Mach-Zehnder Modulator (MZM) switches, which serve a dual purpose as both the selector and combiner circuits within the transmitter. Concurrently, the receiver circuit ('Bob') employs a Polarizing Splitter Rotator (PSR) as its fundamental component for measurement. For the rectilinear basis (0° and 90°), the PSR alone is sufficient to effectively separate the two orthogonal polarization states. However, for the diagonal basis (45° and -45°), an additional 2x2 Multimode Interferometer (MMI) is required to successfully split the polarization states. This is achieved by exploiting the inherent 90-degree phase difference between the polarization components in the MMI. An MZM pair is subsequently used to selectively switch the output between the rectilinear and diagonal detection paths. Both the 'Alice' and 'Bob' circuits are precisely controlled via 'Base' and 'Bit' electrical pads, with a minimum required voltage of 5V necessary to actuate the MZM switches.

The simulation results confirm the high performance and fidelity of the proposed PIC design. Specifically, the transmitter 'Alice' successfully generates all four required polarization states with high purity. The performance for the rectilinear basis (0° and 90°) demonstrated a substantial extinction ratio of 70 dB, accompanied by an acceptable insertion loss of approximately -2 dB. Furthermore, the diagonal basis states exhibited an insertion loss of around -6.5 dB and maintained a clear 180° phase difference between the 45° and -45° states, which is critical for secure encoding. On the receiver side, the 'Bob' circuit demonstrated accurate detection of all four polarization states. The detection performance yielded an extinction ratio of approximately 25 dB for the rectilinear basis and an even higher ratio of up to 50 dB for the diagonal basis. This successful simulation demonstrates the feasibility of implementing a BB84 QKD system using integrated photonics, offering a path toward compact, high-performance, and scalable quantum communication devices.

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.