Physicists Clean a Crystal Surface Well Enough for Molecules to Hit the Fourier Limit

Researchers at the Max Planck Institute for the Science of Light (MPL) report that they have created a surface clean and quiet enough for individual molecules to keep their quantum properties for the longest time physics allows. The findings are published in the journal Science.

The work addresses a long-standing obstacle. Many emerging quantum technologies rely on tiny objects such as single atoms and molecules that interact with light. These quantum emitters can produce single photons, store quantum information and help link distant quantum systems, capabilities that matter for quantum computing and secure quantum communication. To study one emitter, scientists need to hold it steady in one place for long periods. Researchers have traditionally done this by trapping atoms or molecules in a vacuum or embedding them in solid materials.

Placing emitters directly on a surface could offer another advantage: scientists could potentially manipulate individual molecules by touching them with an extremely sharp probe, as techniques such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) already do with atomic precision. The problem is that surfaces are rarely clean or stable enough. Contaminants from the surrounding environment can accumulate and disturb the delicate quantum behavior of nearby molecules, and until now researchers had struggled to preserve the quantum optical properties of individual emitters on surfaces.

The MPL team, led by Prof. Vahid Sandoghdar, head of the institute's Nano-Optics Division, developed a cleaning approach that exploits a natural property of organic crystals. At room temperature, the crystal material slowly evaporates through sublimation, in which a solid turns directly into a gas. The researchers placed a small crystal in a vacuum chamber equipped for extremely low temperatures, known as a cryostat. As the uppermost layers of the crystal evaporated, they carried away unwanted contaminants and left a remarkably clean surface.

The team then cooled the crystal to just a few degrees Kelvin above absolute zero, which halted further sublimation and preserved the pristine surface. Finally, they used a tiny, specially fabricated oven to evaporate molecules onto the cold crystal, where the molecules settled on the clean surface.

To test the quality of the molecules as quantum emitters, the researchers measured coherence time, which indicates how long a quantum system keeps the properties needed for quantum behavior. Dr. Alexey Shkarin, a researcher in the division, explained that the quality of quantum emitters can be evaluated by their coherence times, which indicate how long they keep their quantumness.

Coherence cannot last indefinitely. A boundary called the Fourier limit, set by the time an emitter takes to release its energy into its surroundings, caps how long it can persist. In a noisy environment, coherence can vanish hundreds or even thousands of times faster than this limit. The researchers found that molecules on their cleaned crystal surface consistently reached the Fourier limit, showing that the environment was quiet enough to preserve coherence for the maximum physically permitted duration. According to the researchers, this is the first time the limit has been reached for molecules attached to a surface.

The clean surface also let the team study how the crystal affects the molecules sitting on it. Further experiments showed that interactions with the surface can make molecules align in particular directions and shift their energy levels. The researchers also found indications that the surface might influence the molecules' shapes or the ways they vibrate. Such effects could offer clues about matter at the molecular scale and new ways to control quantum systems.

The source describes the vibration and shape effects only as indications, so they will likely need further study. The main next step is to combine the method with AFM and STM. "Our future work will focus on combining this method with AFM and STM to gain local nanometer control over individual quantum emitters," Sandoghdar said. The researchers hope this could reveal previously inaccessible details of molecule-surface interactions and lay groundwork for engineering new quantum states of matter and future molecular quantum technologies.