Phase Stabilization for a real world deployed quantum network at the Fermilab Quantum Network

We present the design and implementation of an active stabilization system for a long-baseline Mach-Zehnder interferometer that incorporates a 4 km underground optical fiber link at Fermilab. The system employs a coherent self-homodyne detection scheme with a 90-degree optical hybrid to generate in-phase (I) and quadrature (Q) signals, which provide an error signal for a high-bandwidth feedback loop. A low-latency, FPGA controller executes a PID control algorithm, applying real-time phase corrections via an electro-optic modulator to counteract environmental phase noise. We will present a full characterization of the free-drifting phase noise and demonstrate a significant reduction in the phase drift rate, quantified by comparing the statistical distribution of the phase before and after stabilization. The successful implementation of this feedback system is expected to achieve robust, long-term phase-locking, establishing a stable interferometric platform for future quantum networking experiments.