A team at Brookhaven National Laboratory and Stony Brook University has established a 21-kilometer free-space quantum link, transmitting single and entangled photons across the atmosphere to connect metropolitan quantum network nodes under real-world conditions
Researchers at the U.S. Department of Energy's Brookhaven National Laboratory and Stony Brook University have demonstrated a permanent free-space quantum communication link spanning 21 kilometers (13 miles) of open atmosphere. The experiment, which connects Stony Brook's Quantum Watchtower and Brookhaven's Quantum Lighthouse, marks the first time in the United States that single photons and entangled photon pairs have been transmitted continuously across a metropolitan-scale open-air path as part of a functioning quantum network.
Atmospheric Quantum Link Design
The free-space optical (FSO) link integrates adaptive optics, large-aperture telescopes, and real-time wavefront correction to address the severe turbulence and spatial distortion encountered in ground-level atmospheric transmission. The system expands a 5-micrometer fiber output to a 0.6-meter primary mirror, then applies kilohertz-frequency deformable mirror corrections to compensate for thermal gradients, wind, and ground-induced turbulence. At the receiving end, the beam is refocused into a matching 5-micrometer fiber core for detection and analysis. This architecture enables the transmission of quantum states at infrared wavelengths native to atomic systems, bypassing the telecom-band constraints of conventional fiber networks and eliminating the need for wavelength conversion when interfacing with quantum memories or processors.
Entanglement Transmission and Verification
On August 19, 2026, at 12:26 a.m. Eastern Time, the Quantum Lighthouse node at Brookhaven recorded the arrival of entangled photon pairs sent from the Quantum Watchtower at Stony Brook. The team subsequently demonstrated stable single-photon transmission and entanglement verification during daylight hours, validating the system's ability to maintain quantum coherence and spatial alignment under variable atmospheric conditions. The experiment relied on an ultra-fast single-photon camera and real-time beam stabilization to track and correct for atmospheric fluctuations, achieving reliable coupling efficiency over the full 21-kilometer path.
Integration and Expansion Plans
The new free-space link is directly integrated into the existing 259-kilometer (161-mile), eight-node Long Island quantum fiber network, extending its reach and enabling hybrid fiber-free-space quantum networking. A third optical facility has been constructed at Yale University in New Haven, Connecticut, with plans to establish a 48-kilometer (30-mile) free-space entanglement link across Long Island Sound. The infrastructure is also being prepared for quantum and classical satellite tracking, supporting future ground-station operations for satellite-based quantum key distribution and distributed quantum computing. This regional expansion is supported by the DOE Office of Science, the National Science Foundation, and New York's Empire State Development, which has invested $300 million in the SUNY Stony Brook Quantum Innovation Hub.
Technical and Engineering Challenges
Transmitting quantum states through open air introduces significant engineering constraints not present in fiber-based systems. Atmospheric turbulence, thermal gradients, and environmental noise can rapidly degrade spatial coherence and reduce photon coupling efficiency. The Brookhaven-Stony Brook system addresses these challenges with adaptive optics derived from astronomical telescope technology, real-time deformable mirror correction, and high-speed photon detection. While the demonstration establishes a robust metropolitan-scale quantum link, further work is needed to quantify long-term stability, loss rates, and integration with quantum repeater protocols. For context on related advances in quantum device fabrication, see this report on low-temperature tantalum qubit deposition at Cornell.
Entanglement is a uniquely quantum phenomenon in which two or more particles share a correlated state such that measurement outcomes on one system are statistically linked to outcomes on the other, regardless of the distance separating them. In quantum communication, entangled photons can be distributed between distant nodes to enable secure key distribution, quantum teleportation, or distributed quantum computing. Maintaining entanglement over long distances requires careful control of loss, noise, and decoherence, especially in open-air channels where environmental factors can rapidly disrupt quantum correlations. The Brookhaven-Stony Brook demonstration provides experimental evidence that metropolitan-scale free-space entanglement distribution is technically feasible, but scaling to larger networks and integrating with quantum repeaters will require further advances in loss mitigation, error correction, and system stability.
