Entangled Photons Crossed 21 Kilometers of Open Air
Researchers transmitted entangled photons through open air across 21 kilometres between Stony Brook University and Brookhaven National Laboratory. The demonstration adds a wireless segment to an existing fibre quantum network, but it remains an experimental link tested mainly at night, not a complete quantum internet or deployed communications service.
Researchers have sent entangled photons through 21 kilometres of open air between two buildings on Long Island. The experiment connects Stony Brook University’s Quantum Watchtower with Brookhaven National Laboratory’s Quantum Lighthouse, adding a free-space segment to a quantum network that previously relied on fibre.
The 30-second summary
- What happened? A rooftop transmitter at Stony Brook sent faint quantum light across 21 kilometres to a telescope and detector at Brookhaven.
- Why does it matter? Open-air links could connect quantum devices that use wavelengths poorly suited to commercial fibre and may eventually extend networks through satellites.
- What is the catch? The entangled-photon tests were conducted at night and demonstrated reception, not a sustained public network, useful computation or secure customer service.
KEY NUMBER
21 kilometres separated the two rooftop facilities, with photons leaving and entering optical fibres only about 5 micrometres wide.
A quantum link leaves the cable
During a daytime demonstration on August 21, 2026, the team generated quantum states containing only a few photons and sent them from Stony Brook to Brookhaven. An ultrafast camera at the receiving site marked their arrival after the beam crossed the open-air route.
The more significant tests happened at night, when background light was lower. Researchers generated entangled photon pairs in a Stony Brook physics laboratory, carried them by fibre to the rooftop transmitter, sent one part of the quantum signal through open air and detected it at Brookhaven. Their measurements confirmed that the quantum signal reached the receiver.
Why use open air at all?
Commercial optical fibre works especially well at telecommunications wavelengths, but many quantum processors, memories and sensors naturally operate at other wavelengths. A free-space optical link can carry those colours without first converting every photon to a fibre-friendly wavelength.
The experiment complements NewTqnia’s earlier coverage of a device that sent information using highly correlated microwave signals. Both projects explore how unusual physical correlations can move from isolated laboratory setups toward usable links, although the hardware and signals are very different.
Telescopes keep a fragile beam on target
The rooftop system borrows techniques from astronomy. Telescopes collect the faint light, automated alignment keeps the transmitter and receiver pointed at each other, and adaptive optics compensate for atmospheric turbulence that can distort the beam.
The challenge is not merely covering the distance. The beam must couple from a fibre core about 5 micrometres across, survive air movement and background light, then enter another similarly narrow fibre. That precision is necessary because quantum information cannot be copied and amplified in the ordinary way when photons are lost.
Entanglement is also the connection to earlier work on keeping distant qubits linked. The Long Island experiment tackles a different problem: distributing entangled light between sites rather than preserving entanglement inside one controlled system.
Before we overstate the result
- The team reported successful reception and measurement of entangled photons, not continuous high-rate operation or an end-user application.
- The entanglement tests took place at night, when sunlight creates less background noise. Reliable daytime quantum operation has not yet been demonstrated on this route.
- The link connects research facilities with a clear line of sight and specialized telescopes, controls and detectors. Cost and performance outside that setting remain unknown.
- The demonstration does not create an unhackable internet. Practical security depends on complete protocols, devices and implementation, not entanglement alone.
The next crossing is more difficult
The team plans to establish a 48-kilometre link from Stony Brook to Yale University across Long Island Sound. Researchers also want to connect quantum computers and, eventually, transmit quantum information through the atmosphere to satellites.
The useful milestone is therefore specific: entangled photons crossed a 21-kilometre outdoor path and were detected at the other end. The next tests must show stable operation in daylight, quantify loss and error rates, and demonstrate a task that benefits from combining this wireless segment with the existing fibre network.
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