Scientists have discovered a way to convert fluctuating lasers into remarkably stable beams that defy classical physics, opening new doors for photonic technologies that rely on both high power and high precision.

Lasers are essential tools in science, industry and medicine, but increasing their power often results in “noise”—unpredictable fluctuations in intensity that disrupt applications requiring consistent, stable light.

Researchers led by Cornell and the Massachusetts Institute of Technology have demonstrated how noisy, amplified lasers can be transformed into ultra-stable beams through the clever use of optical fibers and filters. The technique was detailed in Nature Photonics.

“What was super surprising is that the noise is so low that there’s no classical laser beam that has those same properties,” said Nicholas Rivera, assistant professor of applied and engineering physics at Cornell Engineering, who co-led the study with Shiekh Uddin, postdoctoral associate at MIT. “It’s in a quantum state that has no classical analog.”

Such quantum light is typically produced using nonamplified, low-power lasers in controlled settings. The new research shows, for the first time, that amplified light with very high noise can be transformed into an “intensity-squeezed” state of light, in which the fluctuations in the number of photons are reduced below a fundamental limit imposed by quantum mechanics.

The key to the discovery lies in a phenomenon the researchers call “noise-immune quantum correlations.” To counter the amplification of noise, the team passed laser pulses through a nonlinear optical fiber, a material in which light waves interact and mix in complex ways. One of those processes, called four-wave mixing, shuffles energy between different colors of light, establishing correlations between them.

The researchers then used a programmable spectral filter to isolate the most stable combinations of these frequencies. Some of the filtered combinations showed noise levels 30 times lower than the original beam, while the filtered light retained high peak intensities of up to 0.1 terawatts per square centimeter.

Rivera said the idea for the project stemmed from a practical need in his own lab: generating quantum light without purchasing an expensive, low-noise system.

“What it means is that now there are so many more laser sources you can use to generate quantum light,” Rivera said. “Amplified sources are super common and they’re the easiest, cheapest way to build a high-power laser.”

To explain the behavior they observed, the researchers developed a new model that extracts quantum noise predictions directly from classical simulations of the laser’s dynamics, and allowed the researchers to understand how to bypass the noise in their laser system. The model gives other researchers a method to apply the technique to their own laser systems, Rivera said.

“What I’m most excited about is scaling this up. We demonstrated this at modestly high intensities, but lasers today go orders of magnitude brighter,” he said. Rivera plans to apply the technique in combination with other laboratory methods designed to produce ultra-powerful lasers.

The research could eventually extend beyond laboratory experiments, such as for communication systems like cross-ocean fiber-optic cables that employ the amplification technique to preserve signal quality.

“Commercially, it’s an extremely exciting prospect,” Rivera said.

Co-authors include researchers from Boston University, Harvard University, Stanford University, the Technion-Israel Institute of Technology and the University of Central Florida.