Establishing a Quantum Link Over Guanabara Bay
Researchers at Fluminense Federal University in Brazil have successfully transmitted twin photons across a 7-kilometer stretch of Guanabara Bay. This effort represents a significant step toward developing free-space quantum communication. The team generated pairs of light particles in a laboratory in Niterói and tracked one member of each pair as it traveled through the atmosphere to Urca. While one photon remained at the source, the other traversed the bay to a second station.
Physicist Antonio Zelaquett Khoury, the project coordinator, described the results as a breakthrough in signal synchronization. The team recorded 7,000 coincident detections within a five-second window. These measurements suggest that the particles successfully navigated atmospheric turbulence, which typically acts as a barrier to such sensitive quantum transmissions. The experiment provides a platform for future tests regarding whether quantum entanglement persists throughout the journey.
The Technical Hurdles of Open-Air Transmission
Moving quantum experiments out of the lab introduces complex variables. Atmospheric factors like moisture, dust, and gas molecules disrupt light beams over long distances. The team utilized a specialized light source to generate these twin photons through a process called spontaneous parametric down-conversion. A laser at 405 nanometers strikes a nonlinear crystal to split one high-energy photon into two lower-energy counterparts. These twins share correlated physical properties.
Maintaining the connection required precision beyond standard optics. The research group developed an active stabilization system to keep their telescopes perfectly aimed. Because even minor temperature fluctuations or mechanical shifts can move a beam off target over 7 kilometers, the system uses a reference light and cameras to trigger automatic corrections. Postdoctoral researcher Amanda Kronhardt Fritsch and doctoral student Marcos Gil de Oliveira led the development of this infrastructure. The team also secured a dedicated radio link from MLS Wireless to ensure the timing of detections matched across both sides of the bay.
Future Implications for Secure Networks
Verification of these photon pairs marks the primary technical bottleneck for the project. The researchers confirmed the success of their synchronization on August 20, 2026, which coincided with the 60th anniversary of the Brazilian Physical Society. The data shows a clear peak in coincidences, confirming the devices captured twin pairs rather than environmental noise. This temporal alignment allows for future investigation into whether these particles remain entangled at the destination.
If entanglement is confirmed, the implications for information security are substantial. Quantum entanglement allows for the distribution of cryptographic keys that remain secure against interception, as any measurement attempt changes the quantum state. The team plans to test the polarization of the received photons next. Success would confirm that this specific property of light can survive the journey across the bay, opening the door for ground-to-satellite quantum communication networks.