Overview: A simple coating cleans up noisy quantum light
Quantum technologies demand perfection: one photon at a time, every time, with identical energy. Tiny deviations in the number or energy of photons can derail devices, threatening the performance of future quantum computers and a potential quantum internet. Northwestern University engineers have devised a novel strategy to make quantum light sources — devices that emit single photons on demand — more consistent, precise, and reliable.
The trick: coating with PTCDA
In a new study published in Science Advances, the team coated an atomically thin semiconductor, tungsten diselenide, with a sheetlike organic molecule called PTCDA (perylenetetracarboxylic dianhydride). Deposited in a vacuum chamber one molecular layer at a time, the coating covers both sides of the 2D material, creating a uniform environment for the quantum emitters. Importantly, this protective shell does not alter the material’s core electronic properties.
What changed for the photons
With the uniform molecular layer in place, the material’s single-photon emitters become far less sensitive to atmospheric contaminants. The result is a dramatic improvement in spectral purity — up to an 87% increase — along with a controlled shift of photon energy toward lower energy. The coating also lowers the photon activation energy, all while preserving the underlying semiconducting characteristics of tungsten diselenide. In short, the coating produces a cleaner, more reproducible stream of single photons.
Why uniformity matters in quantum devices
In quantum technology, a source must emit one and only one photon at a time, with identical energy. If random contaminants interact with the emitters, energy can drift unpredictably and the photon count can vary, limiting the performance of quantum communications and the precision of quantum sensors. The researchers emphasize that a highly uniform molecular layer shields the emitters from such atmospheric disturbances, delivering repeatable, high-quality single-photon output.
Process details and significance
Two-sided coating of tungsten diselenide was achieved through careful, layer-by-layer deposition of PTCDA. The approach is simple and scalable, leveraging a well-understood organic molecule to stabilize the surface. By protecting surface sites without disturbing the material’s intrinsic properties, the method offers a practical path toward robust quantum light sources suitable for integration into photonic circuits and networks.
Implications for quantum technology
Reliable, bright, and spectrally pure single-photon sources are foundational for secure quantum communication, high-precision sensing, and future quantum networks. The Northwestern team’s strategy could accelerate the transition from isolated quantum devices to interconnected systems — a cornerstone of the envisioned quantum internet. The researchers also plan to test other semiconducting materials and coatings to push control over emission further, and to explore electrically driven emission to better integrate these sources into networked quantum infrastructure.
From single devices to networks
As scientists move from individual quantum computers to distributed networks, stable, tunable, and scalable photon sources are essential building blocks. According to study leader Mark C. Hersam, the coating strategy provides a protective, uniform environment that makes single-photon sources more reproducible without compromising the material’s physics. If refined further, this technique could become a standard step in fabricating quantum light sources for secure communication, ultra-precise sensing, and beyond.
Funding notes: 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.
