Shrinking Quantum Key Distribution onto a Chip
By Ashwath, Ethan, Agastya, Isaac
Quantum key distribution (QKD) allows two parties to establish a shared secret key while detecting whether an eavesdropper has intercepted the communication. In the BB84 protocol, information is encoded using four quantum states divided between two measurement bases. Because measuring a quantum state can disturb it, an eavesdropper attempting to intercept the transmitted information introduces detectable errors. Although BB84 is well established, many QKD systems still rely on separate fiber and bulk optical components arranged across a laboratory setup. These systems can be large, sensitive to alignment, and difficult to scale or reproduce. Integrated photonics offers a way to address these limitations by placing many of the same optical functions onto a compact chip.
Our project will design an integrated photonic circuit that performs the optical operations required for BB84 using dual-rail encoding. The circuit will use waveguides, directional couplers, phase shifters, and Mach-Zehnder interferometers to prepare and measure the four BB84 states. We will first design and simulate individual components in Tidy3D, then combine them into a complete circuit model and optimize parameters such as waveguide dimensions, coupler geometry, optical loss, and phase. The final design will be prepared for fabrication at the U.S. Naval Research Laboratory. If the chip is fabricated, we will measure its loss, splitting ratios, interference visibility, and state error rates and compare the experimental results with our simulations.
A major focus of the project is understanding how small changes in individual photonic components affect the performance of the complete QKD circuit. Parameters such as directional-coupler gaps, waveguide widths, and phase shifts can vary during fabrication, and these variations can change the splitting ratios and interference behavior of the circuit. By systematically modeling these effects, we can determine which parameters have the greatest influence on the circuit's intrinsic error rate and which errors can be compensated after fabrication. Comparing the fabricated chip with the simulated design will also allow us to evaluate how accurately our models predict real hardware and identify which assumptions or fabrication tolerances are most important.
Beyond the immediate experiment, this work contributes to the broader goal of making quantum communication hardware smaller, more stable, and easier to scale. Integrated photonic circuits can replace many alignment-sensitive optical components with structures fabricated together on a single device, which could make future QKD systems more compact and reproducible. Within QLab, the project will also establish a reusable workflow for moving from photonic simulation to circuit design, fabrication, and experimental testing. The models, layouts, test structures, and measurement procedures developed through this project could serve as a foundation for future work in integrated quantum photonics and quantum communication.