Molecular coating cleans up noisy quantum light

Quantum technologies demand perfection: one photon at a time, every time, all with the same energy. Even tiny deviations in the number or energy of photons can derail devices, threatening the performance of quantum computers that someday could make up a quantum internet.

While this level of precision is difficult to achieve, Northwestern University engineers have developed a novel strategy that makes quantum light sources, which dispense single photons, more consistent, precise and reliable.

In a new study, the team coated an atomically thin semiconductor (tungsten diselenide) with a sheetlike organic molecule called PTCDA. The coating transformed the tungsten diselenide’s behavior—turning noisy signals into clean bursts of single photons. Not only did the coating increase the photons’ spectral purity by 87%, but it also shifted the color of photons in a controlled way and lowered the photon activation energy—all without altering the material’s underlying semiconducting properties.

The work appears in Science Advances.

The simple, scalable method could pave the way for 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 C. 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.”

Hersam is the chair of the Department of Materials Science and Engineering and Walter P. Murphy Professor of Materials Science and Engineering at Northwestern’s McCormick School of Engineering. He also is director of the Materials Research Science and Engineering Center and a member of the executive committee for the Institute for Quantum Information Research and Engineering.

Like a particle vending machine, quantum light sources release one—and only one—photon at a time. If a source emits multiple photons at the same time or photons of differing energies, the consequences can be serious. In quantum communication, for example, extra photons limit cybersecurity. In quantum sensing, photons of differing energies can reduce precision.

As these seemingly futuristic technologies come closer to reality, researchers have struggled to develop photon sources that are both bright and pure—delivering one identical photon, on demand, every time.

In the new study, Hersam and his team focused on two-dimensional semiconductor tungsten diselenide, which can host atomic-scale defects that emit individual photons. Because tungsten diselenide is atomically thin, its defects and emitters are right on the surface, leaving them 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.

To overcome these issues, Hersam’s team coated both sides of tungsten diselenide with PTCDA (perylenetetracarboxylic dianhydride), an organic molecule often found in pigments and dyes. 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.”

Next, Hersam’s group plans 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 emission, which will facilitate 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.”