Brookhaven and Stony Brook Just Linked a Quantum Network Through Open Air

In my last post, I laid out the fiber problem: quantum networks can't use classical repeaters because the no-cloning theorem forbids copying an unknown quantum state, so every photon has to survive, unassisted, from source to destination. Fiber compounds that: it loses about 0.2 dB/km, and its only efficient band (1550 nm) doesn't match the wavelengths most atomic memories emit, forcing lossy conversion steps at each end.

Commercial telecom has been married to one wavelength band for as long as I've been in the field, going back to writing the telecommunications curriculum for Verizon's NextStep program in 1995 and later directing NSF-funded Centers of Excellence in telecommunications and optics and photonics. My dad worked the generation before that, retiring in 1984 back when the network still ran on copper and fiber hadn't arrived yet. Brookhaven National Laboratory and Stony Brook University just did something a little different. 

Photons travel 13 miles through open air from Stony Brook's Quantum Watchtower to Brookhaven's Quantum Lighthouse, using adaptive optics instead of fiber. Link connects to existing 161 mile fiber network, with a 30 mile extension to Yale planned next.

On August 21, researchers sent single photons and entangled photon pairs 13 miles through open air, from Stony Brook's Quantum Watchtower to Brookhaven's Quantum Lighthouse in Upton, New York, no fiber involved for that leg of the trip. The daytime event was a formal demonstration for DOE and state officials. The actual first detection happened two days earlier, in the dark: at 12:26 a.m. on August 19, the Lighthouse recorded entangled photon pairs arriving from the Watchtower, confirmed through phase folded photon analysis comparing on-phase detection counts against background noise rates.

Both facilities exist to solve one problem: Brookhaven and Stony Brook need an unbroken line of sight to each other, which is why the Lighthouse sits on the only Brookhaven building with that sightline, a seven story rooftop installation, while the Watchtower sits atop Stony Brook's Health Sciences Center. Fog blocks the link outright. Bright daylight is a subtler problem. It raises atmospheric turbulence and buries the faint photon signal in background light, a limitation the Brookhaven team has compared to trying to spot a flashlight beam from a rooftop in broad daylight.

The optics came out of Brookhaven's Instrumentation Department, built on adaptive optics designs borrowed from the Vera C. Rubin Observatory. Photons leave the Watchtower through a fiber core five microns wide, about a tenth the width of a human hair. A telescope expands that pinprick of light into a 25 inch, 0.6 meter beam to match the primary mirror, while deformable mirrors correct for atmospheric turbulence in real time at kilohertz frequencies. At the Lighthouse, the process runs in reverse: the beam narrows back down and threads into a matching five micron fiber core for detection.

Commercial fiber networks are locked into wavelengths near 1550 nanometers because that band travels through glass with the least loss over distance. A free-space link carries no such requirement. Researchers can transmit infrared wavelengths native to the atomic systems and quantum processors themselves, opening a direct channel to entangle remote atomic memories without a wavelength conversion step.

The new link folds into an existing fiber network spanning 161 miles and eight nodes across Long Island and the New York City area, the longest metropolitan quantum network in the country. DOE Under Secretary for Science Dario Gil cut the ribbon on the receiving aperture at the August 21 event, framing the connection as a step toward linking individual quantum computers into something larger.

A third facility, functionally identical to the Lighthouse and Watchtower, is already built at Yale University in New Haven, Connecticut, with a 30 mile free-space link across Long Island Sound planned to connect Stony Brook and Yale directly. Past that, the team plans to repurpose the same rooftop telescope infrastructure to track low-earth-orbit satellites, laying groundwork for satellite-based quantum key distribution and a longer-term global quantum network. Funding comes from DOE's Office of Science, the National Science Foundation, and $300 million in New York Empire State Development money tied to Stony Brook's Quantum Innovation initiative.

This ties directly to Chapter 1 of Quantum from the Ground Up, which opens on the fiber problem: how to move quantum information over distance without destroying the fragile state that makes it quantum in the first place. Fiber solved part of that by forcing everything into a wavelength band it can carry efficiently. This link solves a different part by removing the requirement to force anything at all. This post will roll into the next quarterly edition of the book, out September 1. You can read the current edition here.