Quantum technologies are often described as the future of computing and communication, but their success depends on achieving an extraordinary level of precision.
At the heart of these systems are quantum light sources—materials that release single photons, one at a time, all with the same energy.
If even a small error occurs, such as multiple photons being emitted at once or photons varying slightly in energy, the result can disrupt the entire process.
That makes developing pure and reliable photon sources one of the biggest challenges in building quantum computers, sensors, and eventually a quantum internet.
Now, engineers at Northwestern University have discovered a simple but powerful way to make quantum light sources more consistent and reliable.
In a new study published in Science Advances, the researchers report that coating an ultra-thin semiconductor with a molecular film dramatically improves its ability to emit clean, identical photons.
The semiconductor at the center of this breakthrough is tungsten diselenide, a two-dimensional material just one atom thick.
When tungsten diselenide has tiny defects—missing atoms in its structure—these defects can act as “quantum emitters” that release individual photons.
The problem is that these emitters are extremely sensitive to their surroundings.
Even exposure to oxygen in the air can alter their performance, creating noisy or inconsistent photon signals.
“Any variability in the number or energy of the emitted photons limits the performance of quantum technologies,” explained Mark C. Hersam, the study’s senior author and chair of Northwestern’s Department of Materials Science and Engineering.
“By adding a highly uniform molecular layer, we protect the single-photon emitters from unwanted contaminants.”
Hersam’s team turned to PTCDA, an organic molecule commonly found in pigments and dyes. In a vacuum chamber, they coated both sides of tungsten diselenide with a single molecular layer of PTCDA, applied evenly and precisely.
The result was a protective shield that preserved the defects’ ability to emit photons while blocking random environmental contaminants from interfering.
The impact was striking. The coating improved the photons’ spectral purity by 87 percent, meaning the photons were nearly identical in energy and wavelength.
It also shifted the photons to slightly lower energy levels, which can be especially useful for quantum communication systems. Importantly, the process achieved this without altering the semiconductor’s underlying electronic properties.
“It’s a molecularly perfect coating, which presents a uniform environment for the single-photon-emitting sites,” Hersam said. “Uniformity is the key to getting reproducibility in quantum devices.”
This approach could be a game-changer for technologies that depend on secure and precise photon signals. In quantum communication, for instance, extra photons could undermine cybersecurity. In quantum sensing, variations in photon energy could reduce accuracy.
With this new coating technique, researchers now have a pathway toward developing single-photon sources that are stable, tunable, and scalable.
Looking ahead, Hersam’s group plans to test other semiconducting materials and experiment with different molecular coatings to gain even finer control over photon emission.
They are also exploring ways to power photon emission using electric currents, which could make it easier to link quantum computers together into a larger network.
“The big idea is to move from isolated quantum computers to quantum networks, and ultimately, to a quantum internet,” Hersam said. “Our technology will help make that vision possible by building photon sources that are stable, reliable, and ready for real-world use.”
