  Time-bin encoding has been widely used for implementing quantum key
distribution (QKD) on optical fiber channels due to its robustness with respect
to drifts introduced by the optical fiber. However, due to the use of
interferometric structures, achieving stable and low intrinsic Quantum Bit
Error rate (QBER) in time-bin systems can be challenging. A key device for
decoy-state prepare & measure QKD is represented by the state encoder, that
must generate low-error and stable states with different values of mean photon
number. Here we propose the MacZac (Mach-Zehder-Sagnac), a time-bin encoder
with ultra-low intrinsic QBER (<2e-5) and high stability. The device is based
on nested Sagnac and Mach-Zehnder interferometers and uses a single phase
modulator for both decoy and state preparation, greatly simplifying the optical
setup. The encoder does not require any active compensation or feedback system
and it can be scaled for the generation of states with arbitrary dimension. We
experimentally realized and tested the device performances as a stand alone
component and in a complete QKD experiments. Thanks to the capacity to combine
extremely low QBER, high stability and experimental simplicity the proposed
device can be used as a key building block for future high-performance,
low-cost QKD systems.
