# In-Field Quantum-Protected Control-Based Key Distribution with a Lossy Urban Fiber Link
## 3 Experimental Setup  
The experimental setup of the implemented QCKD is depicted in Figure 1. In [PERSON]'s module, the laser-generated light undergoes phase randomization via a phase modulator (PM), which is driven by a random signal generator (RSG). The RSG relies on the avalanche breakdown mechanism, as detailed in Refs. [25; 26]. The phase randomization [27; 28; 29; 30] mitigates the potential of the eavesdropping attacks [31], while preserving the statistical distribution of [PERSON]'s measurement results. Following phase randomization, the light passes through an amplitude modulator (AM), which shapes it into optical signals--namely bit-carrying pulses, synchronization pulses, and sinusoidal transmitometry signal. The AM is based on a Mach-Zehnder-type interferometer embedded in a lithium niobate crystal; applying a voltage across radio-frequency (RF) electrodes controls the optical path length. The AM's operation is managed by a field-programmable gate array (FPGA) interfaced with a computer that hosts a quantum random number generator (QRNG). Specifically, the FPGA produces 2.5 ns- and 20 ns-long voltage pulses for bit-encoding and synchronization, respectively. These pulses are fine-tuned in amplitude by an additional electrical circuit--notated in the scheme as the bit-forming device (BFD)--before being amplified and applied to the AM's RF ports. A separate direct-current (DC) port on the AM is employed to stabilize the bias point and compensate for unwanted phase drifts caused by thermal fluctuations and inhomogeneities, photorefractive effects, and electrostatic charge accumulation, see Ref. [32]. This stabilization is achieved via a dedicated bias stabilizer, which provides an offset voltage determined from a power-level feedback loop, using a monitoring detector integrated into a bias control device. [PERSON]'s optical signals shaped by the AM are transmitted to [PERSON] through the 4-km-long urban G.652.D single-mode optical fiber link, see Figure 1. Upon reaching [PERSON]'s module, the incoming light first passes through a 50 GHz passband filter, is then amplified by a variable erbium-doped fiber amplifier (EDFA), and subsequently traverses a narrow-band optical filter with an 8.5 GHz bandwidth. The latter filter, employing the FBG [33], suppresses the amplified spontaneous emission (ASE) generated by the EDFA. Because the FBG is highly sensitive to temperature changes, we use a custom thermostat to maintain the filter within its specified bandwidth. The filtered optical signal is then detected by a photodetector converting it into a voltage, which is then digitized by an analog-to-digital converter (ADC), and finally processed by [PERSON]'s computer. The optical time-domain reflectometer is connected to the fiber line via an optical switch, as illustrated in Figure 1. In short, the OTDR involves sending a short and intense probing pulse down the line and monitoring the backscattered signal. By analyzing both the power of the returned light and its time of flight, one can determine the magnitude and precise locations of potential leakages (for more details see Refs. [19; 20]). Figure 2 presents the reflectogram obtained for the urban fiber link under study. Local losses are primarily attributed to connectors and splices. The feature highlighted in blue corresponds to the optical switch through which the reflectometer is connected on [PERSON]'s  
Figure 1: Schematics of the setup. On [PERSON]’s side, a distributed-feedback (DFB) laser emits light at a wavelength of 1530.33 nm, with its phase randomized by a phase modulator (PM) connected to a random signal generator (RSG). The light is modulated into bit-carrying pulses, synchronization pulses, or a sinusoidal signal by an amplitude modulator (AM), which is controlled by [PERSON]’s computational hardware. The modulated signal is attenuated by a variable optical attenuator (VOA) and passes through an isolator into a 4-km-long urban fiber link.