New photon purification technique may unlock faster, more secure quantum networks

Single photons are the backbone of photonic quantum systems. Unlike classical bits, which toggle between ones and zeroes, quantum systems rely on qubits—often individual particles of light—to process and transmit information.

But producing photons one at a time, without strays or duplicates, has proven notoriously difficult. Two persistent issues have slowed progress. One is laser scatter, where a laser used to excite an atom unintentionally produces extra photons. The other occurs when atoms emit more than one photon at once, disrupting the delicate single-photon stream needed for quantum fidelity.

Instead of trying to suppress these effects independently, the Iowa team took a counterintuitive approach: let the noise fight itself. Their theoretical work shows that the unwanted photons from laser scatter and multi-photon emission share nearly identical wavelength and waveform signatures.

By carefully tuning the laser’s properties, the two sources of noise can be made to cancel each other out, leaving behind a much purer photon stream. The researchers say the idea could remove two major barriers to scaling photonic quantum hardware. Experimental tests are planned next. Turning noise useful “We have shown that stray laser scatter, typically considered a nuisance, can be harnessed to cancel out unwanted, multi-photon emission,” says Ravitej Uppu, assistant professor in the Department of Physics and Astronomy and the study’s corresponding author.

“This theoretical breakthrough could turn a long-standing problem into a powerful new tool for advancing quantum technologies.” At the heart of the discovery is work by graduate student Matthew Nelson, who identified the spectral overlap between the unwanted photons and the laser light itself. That insight opened the door to using precise laser control—such as beam angle and shape—to suppress excess photon emissions. “If we can control exactly how the laser beam shines on an atom — the angle at which it’s coming, the shape of the beam, and so on — you can actually make it cancel out all the additional photons that the atom likes to emit,” Uppu explains.“We would be left with a stream that is actually very pure.” Why purity matters Photon purity is more than a technical detail.

In photonic quantum computing, orderly single-photon streams are easier to control, synchronize, and scale. They also reduce the risk of errors and interference that can derail quantum operations. There are security implications as well. Single-photon communication channels are harder to intercept or eavesdrop on, making them attractive for quantum encryption and secure data transfer.

A clean, predictable photon source strengthens those protections. Photonic approaches are gaining momentum across the quantum industry, with several startups betting that light-based systems will outperform electronics in speed and energy efficiency.

But without reliable single-photon sources, those ambitions face hard limits. The Iowa team s work remains theoretical for now, but the researchers are preparing laboratory experiments to validate the model. The project was supported by the U.S. Department of Defense and internal University of Iowa research funding.

If confirmed experimentally, the method could reshape how engineers think about noise in quantum systems—not as something to eliminate, but something to exploit, according to the study published in Optica Quantum.