Low-error encoder for time-bin and decoy states for quantum key distribution

Time-bin encoding is a robust method for implementing quantum key distribution (QKD) on optical fiber channels, minimizing drift-induced errors. However, interferometric structures make achieving stable and low intrinsic Quantum Bit Error Rate (QBER) challenging. A key device for decoy-state QKD is the state encoder, which must produce low-error, stable states with varying photon mean values. Here, we introduce the MacZac (Mach-Zehnder-Sagnac), a time-bin encoder with ultra-low QBER (<2 × 10⁻⁵) and high stability. Based on nested Sagnac and Mach-Zehnder interferometers, it uses a single phase modulator for both decoy and state preparation, simplifying the optical setup. The encoder requires no active compensation and can generate states of arbitrary dimension. We experimentally tested it as a standalone component and in a QKD experiment. With its low QBER, stability, and simplicity, this device is a key building block for high-performance, low-cost QKD systems.