# In-Field Quantum-Protected Control-Based Key Distribution with a Lossy Urban Fiber Link  
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Conceptually, the security of the QKD against the intrusions is based on the ability of the communicating users to detect the presence of an eavesdropper, [PERSON], using the fundamental laws of quantum mechanics. The eavesdropper disturbs the quantum states distributed between users in the process of their communications and, therefore, introduces measurable anomalies in the process, such as an increased bit error rate or loss of entanglement. By detecting and evaluating these anomalies in a way specific to each QKD protocol, the legitimate users determine the extent of the information breach. This allows them to take measures protecting secure key, so that none of transmitted bits becomes known to [PERSON]. However, when implementing these protocols in existing fiber-optic infrastructure, relying on such control mechanisms becomes problematic, as the telecommunication fiber lines have a high level of losses. Quantum signals, which usually consist of only a few photons, degrade rapidly in these lossy environments, making it not possible to rely on the state disturbances as on a direct indicator of eavesdropping. Even if the users manage to discover [PERSON]'s presence, attributing to her all the channel losses, this will provide them little or even practically no informational advantage and, therefore, very limited achievable secret key rates. To address these issues, recent works [19; 20; 21] introduced the concept of physical loss control in optical quantum communication. In this approach, users send high-intensity probe signals and analyze the transmitted and reflected components, which enables them to distinguish between localized losses exploitable by [PERSON] and the naturally occurring, uniformly distributed scattering losses on quenched disorder that remain unusable for eavesdropping due to thermodynamic constraints [19]. By precisely determining the fraction of light which an eavesdropper could intercept, users precisely estimate [PERSON]'s potential information gain and apply the optimal postprocessing techniques to the distributed bits to ensure key rates guaranteeing high-secrecy. Physical loss control is a central component of the quantum-protected control-based key distribution (QCKD) [19], a prepare-and-measure protocol that encodes bits into the low-intensity coherent quantum states, \(|\gamma_{0}\rangle\) and \(|\gamma_{1}\rangle\). Because these states are substantially non-orthogonal (\(|\langle\gamma_{0}|\gamma_{1}\rangle|\sim 1\)), neither the legitimate receiver ([PERSON]) nor an eavesdropper ([PERSON]) can reliably distinguish them. To simplify the explanation of the QCKD, users measure the fraction of the stolen signal \(r_{\rm E}\) and verify that [PERSON], receiving \(|\sqrt{r_{\rm E}}\gamma_{0}\rangle\) and \(|\sqrt{r_{\rm E}}\gamma_{1}\rangle\), cannot distinguish them better than [PERSON] can distinguish the states he receives. Once users accurately determine their informational advantage over [PERSON], they postprocess their respective bit strings to produce a secure secret key about which [PERSON] knows nothing (we note that in reality, even if [PERSON] is better in distinguishing bits than [PERSON], users may still have an informational edge on account of postselection). The QCKD has already been experimentally implemented over a 1707 km optical fiber line in a controlled laboratory environment [22]. In this paper, we advance to the first real-world implementation of the protocol over a 4 km lossy urban fiber link within an operational fiber network. Unlike the fiber line in Ref. [22], where sections are joined through low-loss splices, the transmission line in our experiment includes multiple physical connectors that introduce significant losses, which represents the real-world fiber network conditions.