Researchers led by NIST sent pairs of entangled photons through roughly 62 kilometers of existing fiber-optic cable. The work marks a practical step toward quantum networks that could one day connect quantum devices for sharper scientific measurements, highly secure communications, and more powerful distributed computing.
Quantum networks gain their special capabilities from entangled particles, usually photons. Building dedicated fiber solely for them would be extremely costly, so the researchers tested whether ordinary internet infrastructure could carry the signals. Most of the cable hung from poles, where temperature changes cause expansion and contraction, wind moves the lines, and birds occasionally land. These disturbances readily twist the polarization of the photons and thereby threaten to destroy the entanglement.
Classical networks that carry ordinary phone calls and internet traffic encode information in ways that remain largely unaffected by such mechanical and thermal noise. Quantum signals, however, rely on the delicate polarization states and are far more vulnerable.
Keeping the quantum connection stable
To protect the entanglement, the researchers used commercial equipment to generate photon pairs whose polarizations were linked. One photon from each pair stayed in the laboratory for measurement while the other traveled the full fiber distance. Devices continuously sent reference light through the same cable, measured how the polarization had been distorted, and applied the exact opposite correction to the experimental photons in real time. This active stabilization kept the shared quantum state intact.
Over a 24-hour period the system distributed about 1,500 entangled photon pairs each second and maintained the link 92.8 percent of the time, with only a small fraction of time devoted to corrections. Statistical checks confirmed that the photons arriving at each end remained entangled. Although longer underground fiber links have been demonstrated elsewhere, this experiment stands out because it used noisy aerial cable that closely resembles the conditions future real-world quantum networks will face. The successful transmission shows that quantum networking methods can function outside carefully controlled laboratory settings.
This research is published in Journal of Optical Communications and Networking.