Quantum-Classical co-existence experiments

QTI Partners: Qunnect and Deutsche Telekom

In June 2025, the CoC3 team of CERN’s QTI started working on joint experiments with Deutsche Telekom and Qunnect in Berlin, using DT’s fiber network. 

A major challenge in long-distance quantum communication is coping with varying conditions in deployed optical fibers. When a classical signal co-propagates with single photons in the same fiber, it experiences identical transmission conditions and can therefore serve as a real-time probe of the link. 

In this research, we aim to investigate the quality of this probe. We multiplex polarized light in the telecom C-band and in the telecom O-band on the same optical fiber, and monitor polarisation changes in both signals. We quantify correlations between polarisation fluctuations at these two wavelengths, such that in future research, polarisation changes in the C-band can act as a trigger to compensate the polarisation an actual quantum signal (single photons) in the O-band, which is highly susceptible to polarisation changes. Precisely, we extend the role of the White Rabbit Precision Time Protocol beyond sub-nanosecond synchronisation, by using this signal – already generated and sent over the link – as classical source of light in the C-band.