Northwestern University researchers have developed a new method to improve the consistency and reliability of quantum light sources, which are essential for emerging quantum technologies. These technologies require the emission of single photons—particles of light—that are identical in energy, but even minor variations can impact the performance of devices such as quantum computers and communication systems.
In their recent study published in Science Advances on October 3, the research team coated atomically thin tungsten diselenide with a sheetlike organic molecule known as PTCDA. This molecular coating significantly improved the spectral purity of emitted photons by 87%, enabled controlled color shifts, and reduced photon activation energy without altering the semiconductor’s core properties.
“When there are defects, such as missing atoms, in tungsten diselenide, the material can emit single photons,” said Mark C. Hersam, chair of Northwestern’s Department of Materials Science and Engineering and corresponding author of the study. “But these points of single-photon emission are exquisitely sensitive to any contaminants from the atmosphere. Even oxygen in air can interact with these quantum emitters and change their ability to produce identical single photons. Any variability in the number or energy of the emitted photons limits the performance of quantum technologies. By adding a highly uniform molecular layer, we protect the single-photon emitters from unwanted contaminants.”
The approach uses PTCDA molecules deposited one layer at a time under vacuum conditions to ensure uniformity across both sides of tungsten diselenide. This process protects surface-level quantum emitting defects from environmental contamination while preserving their electronic structure.
“It’s a molecularly perfect coating, which presents a uniform environment for the single-photon-emitting sites,” Hersam explained. “In other words, the coating protects the sensitive quantum emitters from being corrupted by atmospheric contaminants.”
With this protection, not only did photon quality improve but emissions became more predictable—a critical need for practical applications in secure communications and high-precision sensors.
“While the coating does interact with the quantum emitting defects, it shifts the photon energy in a uniform way,” Hersam added. “In contrast, when you have a random contaminant interacting with a quantum emitter, it shifts the energy in an unpredictable manner. Uniformity is the key to getting reproducibility in quantum devices.”
Looking ahead, Hersam’s group intends to test additional materials and coatings for further improvements and plans to investigate electrically driven emission methods that could help network multiple quantum computers into what they envision as a future “quantum internet.”
“The big idea is that we want to go from individual quantum computers to quantum networks and, ultimately, a quantum internet,” Hersam said. “Quantum communication will occur using single photons. Our technology will help build single-photon sources that are stable, tunable and scalable —the essential components for making that vision a reality.”
The project received primary funding from the U.S. Department of Energy (award number DE-SC0021314), with additional support from both the National Science Foundation (award number DMR-2308691) and Army Research Office (award number W911NF-25-2-0018).