Engineering Quantum Links: Noise and Quantum-State-Degradation Metrics over Metropolitan Fiber Network

Deploying quantum networks over existing network infrastructures requires the same engineering foundations that underpin classical communications: quantitative models of the channel’s noise and of the impairments it imposes on the transmitted information. In this work, we build such a foundation on experimental measurements, grounding the quantum-network counterparts of the two cornerstone metrics of classical link characterization – namely, the SINR and the BER – on a ≃ 7.3 km deployed metropolitan-scale fiber-loop interconnecting two campuses of the University of Naples Federico II within the national QuantumInternet.it testbed. On the noise side, we adopt a photon-counting quantum analog of the SINR – in which dark counts constitute the intrinsic noise and the photons generated by classical traffic (through either spontaneous Raman scattering or inter-fiber crosstalk) constitute the interference – and we quantify each contribution directly on the deployed loop. On the bit-error side, we consider the main degrees-of-freedom available to encode a quantum state within an optical photon – namely, polarization, time, and frequency – and we quantify for each degree the channel-induced degradation and its drift over time. The experimentally derived drift models capture the temporal behavior of the investigated metropolitan fiber link, establishing a quantitative basis for characterizing and predicting channel-induced impairments in deployed quantum networks. These results show that a quantum fiber link, like its classical counterpart, can be captured by a small set of measurable parameters, turning quantum networking over deployed fiber from a physics demonstration into an engineering design problem. Together, they provide the key ingredients of a quantum link budget for the Quantum Internet.