Somewhere above the pavements of suburban Maryland, Einstein’s so-called spooky action just passed a stress test involving wind, passing lorries, temperature swings and the occasional bird.
Researchers at the National Institute of Standards and Technology, the Joint Quantum Institute and the New York firm Qunnect reported that they distributed entangled photons through 62 kilometres, about 38.5 miles, of ordinary commercial fibre. The link ran from NIST’s campus in Gaithersburg to the University of Maryland in College Park, and most of it dangled from utility poles overhead. NIST announced the result in early August, and the paper appeared in the Journal of Optical Communications and Networking.
Why aerial fibre is a nightmare
Entanglement is the effect Einstein grumbled about, where two photons share a single quantum state so that measuring one instantly settles the other, however far apart they sit. The catch is that entangled states are encoded in a photon’s polarisation, the direction its electric field vibrates, and polarisation is fragile. Fibres strung between poles expand in the day’s heat, contract at night, sway in the wind and shift when birds land on them. Every wobble twists the polarisation and threatens to destroy the entanglement.
‘It is about as bad a connection as you can possibly have,’ said NIST physicist Oliver Slattery, one of the authors. Classical networks that carry your calls and video streams shrug this off, because they encode information in the power of the light rather than its delicate polarisation. Quantum links do not get that luxury.
The clever fix
The team used a commercial source to generate entangled photon pairs. One photon of each pair stayed in the NIST lab for measurement, while its partner travelled the full 62 kilometres to College Park. To keep the states intact, Qunnect’s devices fired reference beams of light down the same fibre, measured how the journey had scrambled their polarisation, then applied the exact inverse correction to the real photons in real time.
The results are genuinely tidy. The system moved 1,500 entangled photons per second and kept entanglement alive for 92.8% of a 24-hour run, needing only 7.2% of the time to correct for polarisation drift. A statistical test confirmed the photons at each end stayed entangled throughout.
Not a record, and that is the point
This was never about distance. A European group sent entangled photons through 248 kilometres of underground fibre back in 2022. What makes this one notable is that the fibre was mostly above ground and fully exposed to the mess of the real world. ‘I would call this a stress test of quantum networking systems,’ said lead author Yicheng Shi. ‘We put this to an extreme test in an environment that is really noisy. Amazingly, it turned out it still worked.’
That matters because a future quantum internet, one that could link quantum computers, enable tap-proof communications, sharpen telescopes and sense faint tremors, will have to run on the messy fibre already bolted to poles across the country, not on pristine cable laid just for physicists. Proving the trick survives outside the lab is the unglamorous work that gets the field from demo to deployment.
Did you know: ‘spooky action at a distance’ is a rough translation of Einstein’s German phrase ‘spukhafte Fernwirkung,’ which he meant as an insult to the idea, not a compliment.
Sources
- NIST: Spooky Particles Transit DC Suburbs
- The Quantum Insider: NIST Demonstrates Entangled Photon Transmission
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