NIST Sent Entangled Photons 62 Kilometers Through Ordinary Telecom Fiber Strung Above Maryland Suburbs

Physicists at the National Institute of Standards and Technology have pushed quantum entanglement 62 kilometers — about 38.5 miles — through ordinary commercial telecom fiber strung mostly above ground across the Maryland suburbs of Washington, and kept it alive for nearly a full day.

The distinction matters more than the distance. Entanglement has been sent much farther in carefully controlled settings: buried fiber, dedicated dark links, cryogenic labs, even satellites. What has repeatedly failed is doing it on the cable that already exists. Aerial fiber sags and sways in wind, expands and contracts with temperature swings, and picks up vibration from the roads it runs alongside. Every one of those effects rotates the polarization of a photon in flight, and polarization is exactly the property the experiment uses to carry the entangled state.

"This is a stress test of quantum networking systems," said Yicheng Shi, the NIST physicist who led the work. "We put this to an extreme test in an environment that's really noisy."

The link carried about 1,500 entangled photon pairs per second and maintained verified entanglement for 92.8% of a 24-hour run. The remaining 7.2% of the time was not downtime in the usual sense — it was the system pausing to re-measure and correct for polarization drift, then resuming. That the correction overhead landed under 8% on aerial fiber is the headline number for anyone trying to build a network out of infrastructure that is already in the ground and on the poles.

The experiment was run by NIST together with the Joint Quantum Institute, a partnership between NIST and the University of Maryland, and Qunnect, a New York company building hardware for quantum networking. Oliver Slattery, a NIST physicist, was a co-author. The results appear in the Journal of Optical Communications and Networking.

Why anyone wants this: entanglement is the resource that makes quantum networks different from fast classical ones. Two entangled photons share a correlation that cannot be copied or intercepted without destroying it, which is the physical basis for eavesdropping-proof key distribution. Distributed entanglement is also the connective tissue that would let separate quantum processors act as one larger machine, and the timing reference that would let distant atomic clocks and telescopes be compared with far greater precision than classical links allow.

None of that works if entanglement only survives inside a laboratory. A metropolitan-scale quantum network will have to run on the same aerial and underground fiber that carries streaming video, through the same weather. The Maryland run is evidence that the noisy, unglamorous version of the problem — the one involving utility poles — is tractable with equipment that exists now.