Continuous entanglement distribution over a transnational 248 km fiber link

Abstract

Reliable long-distance distribution of entanglement is a key technique for many quantum applications, most notably quantum key distribution. Here, we present a continuously working, trusted-node free international link between Austria and Slovakia, directly distributing polarization-entangled photon pairs via 248 km of deployed telecommunication fiber. Despite 79 dB loss, we observe stable detected pair rates of 9 s−1 over 110 h. We mitigate multi-pair detections with strict temporal filtering, enabled by nonlocal compensation of chromatic dispersion and superconducting nanowire detectors. Fully automatized active polarization stabilization keeps the entangled state’s visibility at 86% for altogether 82 h. In a quantum cryptography context, this corresponds to an asymptotic secure key rate of 1.4 bits/s and 258 kbit of total key, considering finite-key effects. Our work paves the way for low-maintenance, ultra-stable quantum communication over long distances, independent of weather conditions and time of day, thus constituting an important step towards the quantum internet.

High-dimensional quantum key distribution using energy-time entanglement over 242 km partially deployed fiber

“…Enormous efforts have been made to realize entanglement-based QKD under long-distance fiber transmission in the laboratory [25,26] and in the field [27]. Recently, researchers have shown the entanglement distribution of polarization entangled photon pairs along 248 km deployed fiber and calculated its SKRs [28]. Since the distribution of polarization entanglement over optical fibers is sensitive to environmental variations, dedicated polarization calibration is necessary for these works to compensate for polarization drift, and the active calibration process is required from time to time to reduce the quantum bit error rate (QBER) to a sufficiently low level [28,29].…”

“…Recently, researchers have shown the entanglement distribution of polarization entangled photon pairs along 248 km deployed fiber and calculated its SKRs [28]. Since the distribution of polarization entanglement over optical fibers is sensitive to environmental variations, dedicated polarization calibration is necessary for these works to compensate for polarization drift, and the active calibration process is required from time to time to reduce the quantum bit error rate (QBER) to a sufficiently low level [28,29]. Among entanglement on different degrees of freedom (DOFs), the energy-time entanglement and its discrete form, i.e.…”

Entanglement distribution quantum networking within deployed telecommunications fibre-optic infrastructure

“…Such QNs have mainly used photon pair sources to distribute entanglement on a metropolitan scale 1 and across long distance. 10 Some have used single photons from local quantum systems to entangle multiple systems over distance. 11 Progress has been made in generating multi-partite entangled photon states, 12 but there are still challenges in producing a scalable network while connect many users together.…”

Deploying an Inter‐European Quantum Network

“…For example, optical fiber links, [ 5–10 ] satellites, [ 11–13 ] or both, have been used to create a quantum network enhancing secure communications among different cities and two different countries. [ 14,15 ] It is worth noticing that fiber communications over thousands of kilometers are possible only thanks to the trusted‐node scenario (quantum states are measured and then subsequently re‐encoded). [ 16–20 ] In this way, it is possible to extend the maximum haul of a point‐to‐point link and to allow connection of multiple users.…”