Picosecond-resolved entanglement distribution over an urban free-space channel

Quantum networks require precise synchronization between remote nodes to distribute and utilize entangled states. While standard entanglement distribution is well-studied, distributing time-evolving entangled states—where the specific correlation between particles changes over time—requires timing precision far beyond what traditional GPS-disciplined oscillators can provide. This study investigates whether a custom-built, picosecond-resolved synchronization system can maintain the necessary timing accuracy to distribute such states across a noisy, real-world urban free-space channel.

The authors utilized a semiconductor quantum dot (QD) as an on-demand source of entangled photon pairs. Due to the fine structure splitting (FSS) of the quantum dot, the polarization state of the emitted photons oscillates over time. To preserve these correlations during transmission, the team developed a synchronization device that uses a modulated 852 nm laser beacon sent alongside the quantum signal. This beacon provides a common clock reference to both the transmitter and receiver, enabling sub-50 ps timing jitter. The system was tested over a 270-meter free-space link between two buildings at Sapienza University in Rome, incorporating active stabilization to counteract atmospheric turbulence.

The study successfully resolved the Bell state oscillations of the distributed photon pairs, which were previously impossible to track using standard GPS-based synchronization. By achieving a timing jitter of 39 ps, the researchers maintained a high degree of entanglement, reaching a fully entangled fraction (FEF) of up to 89%. The results confirm that the system can reliably transmit time-evolving quantum states through a noisy urban environment, providing a critical benchmark for future satellite-based quantum communication networks.