Source-linked AI summary
In-field entanglement distribution over a 96 km-long submarine optical fibre
Sören Wengerowsky, Siddarth Koduru Joshi, Fabian Steinlechner, Julien R. Zichi, Sergiy. M. Dobrovolskiy, René van der Molen, Johannes W. N. Los, Val Zwiller, Marijn A. M. Versteegh, Alberto Mura, Davide Calonico, Massimo Inguscio, Hannes Hübel, Anton Zeilinger, André Xuereb, Rupert Ursin
TL;DR
The paper addresses whether polarisation-entangled photons can be distributed through deployed submarine telecommunications fibre outside laboratory infrastructure. It sends entangled photons between Malta and Sicily and observes high visibility, Bell-inequality violation, and key-generation-relevant rates, demonstrating feasibility over such links.
Problem
Evidence was needed on whether deployed submarine telecommunications optical fibres could support long-distance polarisation-entanglement distribution for quantum communication.
Method
The experiment transmitted polarisation-entangled photons through a deployed submarine fibre between Malta and Sicily and characterised them using coincidence measurements and CHSH Bell tests.
Results
257 ± 4 counts per second produced an approximately 30-bit-per-second secure key rate, while the measured CHSH value was 2.421 ± 0.008 at optimal settings.
Takeaways & Limitations
The results demonstrate that deployed submarine telecommunications fibres can serve as long-distance quantum channels for polarisation-entangled photons and support secure quantum communication.
Abstract
from arXiv · showhide
Techniques for the distribution of quantum-secured cryptographic keys have reached a level of maturity allowing them to be implemented in all kinds of environments, away from any form of laboratory infrastructure. Here, we detail the distribution of entanglement between Malta and Sicily over a 96 km-long submarine telecommunications optical fibre cable. We used this standard telecommunications fibre as a quantum channel to distribute polarisation-entangled photons and were able to observe around 257 photon pairs per second, with a polarisation visibility above 90%. Our experiment demonstrates the feasibility of using deployed submarine telecommunications optical fibres as long-distance quantum channels for polarisation-entangled photons. This opens up a plethora of possibilities for future experiments and technological applications using existing infrastructure.
I. RESULTS
The experiment distributed polarisation-entangled photon pairs through a deployed submarine fibre between Malta and Sicily, characterising transmission with coincidence scans and Bell tests. The transmitted state retained high visibility and violated the CHSH inequality, supporting long-distance entanglement distribution outside laboratory infrastructure.
- The source used bidirectional pumping of an MgO:ppLN crystal in a Sagnac-type setup to produce polarisation-entangled photon pairs, with signal and idler channels separated by narrow filters.
- Coincidences were identified by cross-correlating independently recorded arrival times from Malta and Sicily after transmission through the submarine fibre.The experiment used separate local detection modules and a synchronisation fibre.
- The transmitted coincidence data showed visibilities of 86.8% ± 0.8% in the HV basis and 94.1% ± 0.2% in the DA basis.
- The fitted coincidence data gave a maximum CHSH value of −2.534 ± 0.08, approximately 90% of the Tsirelson bound.The maximum occurred at φM = 63.5°, offset from the theoretical optimum.
- At theoretically optimal analyser settings, the direct CHSH measurement was 2.421 ± 0.008, beyond local-realistic bounds.Data were accumulated for 600 seconds per measurement setting and analysed in 39 blocks.
- 257 ± 4 counts per second yielded an approximately 30-bit-per-second secure key rate, with QBER of 5% ± 0.5%.The measured QBER was below the cited 11% minimum requirement.
II. DISCUSSION
The experiment demonstrates stable polarisation-entanglement distribution over a deployed 96 km submarine fibre link, meeting key prerequisites for quantum key distribution. The results also motivate polarisation entanglement for future fibre-based quantum networks.
- 86% CHSH violation (S = 2.421), QBER below 5.5%, and estimated secure key rates of 30 bits per second demonstrate the link’s suitability for secure quantum communication.The QBER remained constant for over two-and-a-half hours without active polarisation stabilisation.
- The 96 km deployed submarine link preserved polarisation entanglement and constituted the first international submarine quantum link reported by the authors.The network was part of an existing telecommunications infrastructure linking Malta and Sicily.
- Polarisation entanglement avoids interferometric measurements and state preparation, external time synchronisation, and special pulse-broadening compensation techniques.These are identified as practical advantages for entanglement-based quantum key distribution networks.
- The authors propose that polarisation entanglement may be a preferred choice for future entanglement-based quantum key distribution networks.This proposal reflects their reported field-trial results and technological progress.
- Polarisation entanglement can interface between free-space and fibre links and connect with quantum-repeater and quantum-networking schemes.The authors identify these properties as routes toward extending QKD range and client reach.
III. METHODS
The methods compensate fibre-induced polarisation changes and quantify entanglement using coincidence-based polarisation measurements. The setup also uses superconducting detectors and CHSH measurements across four basis settings.
- Fibre birefringence compensation: The experiment neutralised the quantum channel’s polarisation rotation by alternately transmitting H and D states from a laser connected in Sicily.The fibre-birefringence compensation was performed so the state could be detected at the far end of the 96 km link.
- Single-photon counting in Malta: The Malta detectors used superconducting nanowires, and their polarisation-dependent efficiency was optimised with a three-paddle fibre polarisation controller.The detector system operated continuously for two weeks in a data-centre environment without performance degradation.
- CHSH Measurements: CHSH measurements combined four basis settings, with Malta and Sicily analyser angles separated by 45°.Coincidence counts between all four detectors were used to compute the S-value.
- CHSH Measurements: The correlation functions E(a_i, b_j) were calculated from coincidence counts at the selected analyser angles and their perpendicular outputs.The perpendicular output corresponds to the second polarising-beam-splitter output.