Tiny deviations in photon quantity or energy can derail devices in quantum technology, affecting the performance of quantum systems such as computers. By coating the atomically thin semiconductor tungsten diselenide with a sheet-like organic molecule, researchers at Northwestern University have developed a solution to this bottleneck. The team's approach targets quantum light sources, to render them more consistent and precise.
In the work, the researchers used the organic molecule PTCDA. Their coating transformed the tungsten diselenide’s behavior, which turned noisy signals into clean bursts of single photons, which are released one at a time by quantum light sources. Additionally, the coating increased the photon's spectral purity by 87%, and it shifted the color of photons in a controlled way. And, it lowered the photon activation energy — without altering the material’s underlying semiconducting properties.
According to the team, the simple method could pave the way for scalable single photon sources and, ultimately, reliable, efficient quantum technologies for secure communications and ultra-precise sensors.
“When there are defects, such as missing atoms, in tungsten diselenide, the material can emit single photons,” said Northwestern’s Mark Hersam, the study’s corresponding author. “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.”
If a quantum light source emits multiple photons at the same time, or photons of differing energies, the consequences can be serious. In quantum communication, extra photons limit cybersecurity. And in quantum sensing, photons of differing energies can reduce precision.
The researchers sought to develop photon sources that are bright and pure, delivering one identical photon, on demand, every time. The team focused on 2D tungsten diselenide, which can host atomic-scale defects that emit individual photons. Since this material is atomically thin, its defects and emitters are right on the material surface, exposed to unwanted interactions with atmospheric contaminants. This susceptibility to variability from random atmospheric species limits the reliability of tungsten diselenide for the precise operations required in quantum devices.
Optical micrograph of the monolayer tungsten diselenide (WSe2) sample, with the right-hand side functionalized with PTCDA. Courtesy of Northwestern University/Mark Hersam.
The team deposited the molecules in a vacuum chamber one molecular layer at a time, which ensured the coating remained uniform. The molecular coating protected the surface of tungsten diselenide and its quantum emitting defects, without changing its core electronic structure.
“It’s a molecularly perfect coating, which presents a uniform environment for the single-photon-emitting sites,” Hersam said. “In other words, the coating protects the sensitive quantum emitters from being corrupted by atmospheric contaminants.”
By protecting the material from environmental disturbances, the coating dramatically improved the photons’ spectral purity. The coating also caused the photons to shift to a lower energy, which is advantageous in quantum communication devices. The result is a more controlled, reproducible and higher-quality single-photon output, which is critical for quantum technologies.
“While the coating does interact with the quantum emitting defects, it shifts the photon energy in a uniform way,” Hersam said. “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.”
The next steps of this project are to investigate other semiconducting materials and to explore additional molecular coatings to achieve further control over single-photon-emitting sites. The team also plans to use an electric current to drive quantum emissions, which will enable networking of quantum computers into a 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.”
This research was published in Science Advances (www.doi.org/10.1126/sciadv.ady7557).<