A Breakthrough for Stable Quantum Light
Quantum technologies hinge on one unerring rule: every photon must be identical, produced one at a time, with the same energy. Tiny deviations in photon number or energy can derail quantum computers, secure communications, and ultra-precise sensors. Northwestern University engineers have unveiled a practical solution: coating an atomically thin semiconductor with a single, uniform organic layer that cleans up noisy quantum light.
In a study published in Science Advances, the team applied a sheetlike molecule called PTCDA (perylenetetracarboxylic dianhydride) to both sides of tungsten diselenide (WSe2), a two-dimensional material known to host atom-scale defects that emit single photons. Deposited one molecular layer at a time in a vacuum, the coating creates a uniform environment around the quantum emitters without altering the material’s core electronic structure. The result is more reliable, reproducible single-photon output that researchers can harness for quantum technologies.
How the Molecular Coating Works
The coating acts as a protective, uniform surround for the defect sites that produce photons. Atmospheric contaminants — even trace oxygen — can interact with these emitters and introduce variability, undermining the precision required for quantum operations. By shielding the emitters with a molecular layer, the researchers found a dramatic boost in spectral purity and a controlled shift in photon energy, achieved without compromising the semiconductor’s intrinsic properties.
“It’s a molecularly perfect coating, which presents a uniform environment for the single-photon-emitting sites,” said Mark C. Hersam, Northwestern’s chair of Materials Science and Engineering. “The coating protects the sensitive quantum emitters from being corrupted by atmospheric contaminants.”
Significant Gains for Photon Quality
The protective layer led to a notable 87% improvement in spectral purity of the emitted photons and caused a uniform, tunable shift to lower energy. This energy shift is particularly advantageous for quantum communication, where compatibility with other components and improved control over photon wavelengths are essential. Importantly, these gains come without altering the underlying semiconducting properties of tungsten diselenide, preserving its advantageous electronic structure while delivering higher-quality photons.
As Hersam explains, uniform interaction from the coating is key. “While the coating does interact with the quantum emitting defects, it shifts the photon energy in a uniform way. In contrast, random contaminants cause unpredictable shifts. Uniformity is the key to reproducibility in quantum devices.”
Implications for Quantum Technologies
The drive for scalable, high-quality single-photon sources is central to building quantum networks and, ultimately, a quantum internet. Quantum communication relies on photons as carriers of information, and any extra photons or energy variability can compromise security and fidelity. Similarly, in quantum sensing, consistent photon energy enhances measurement precision. The Northwestern approach offers a practical route to brighter, purer, and more tunable photon sources that are also robust against environmental fluctuations.
Future Directions
Looking ahead, the team plans to explore other semiconducting materials and experiment with additional molecular coatings to push control over single-photon-emitting sites even further. They also aim to drive quantum emission with an electric current, a step toward networking individual quantum devices. “The big idea is to go from isolated quantum computers to quantum networks and, eventually, a quantum internet,” Hersam said. “Our technology helps build stable, tunable, scalable single-photon sources—the essential components for that vision.”
About the Study
The study was primarily supported by the U.S. Department of Energy, with additional support from the National Science Foundation and the Army Research Office. The findings underscore how a simple, scalable coating can dramatically improve the reliability of quantum light sources, bringing practical quantum technologies closer to reality.
