Scientists at the Max Planck Institute for the Science of Light (MPL) have achieved a breakthrough in quantum physics, developing a way to study individual molecules on a surface with extraordinary precision. For the first time, researchers have demonstrated that molecules attached to a surface can reach the ultimate quantum limit of optical coherence, overcoming a major obstacle that has long restricted molecular quantum research.
The findings, published in Science, could open new possibilities for investigating how molecules interact with surfaces and developing advanced quantum technologies.
Why Surfaces Have Been a Major Obstacle in Quantum Physics
Many emerging quantum technologies depend on extremely small objects, such as individual atoms and molecules, that interact with light. Known as quantum emitters, these tiny systems can produce single particles of light called photons, store quantum information, and help establish quantum connections between distant systems. These capabilities are important for future applications in quantum computing and secure quantum communication.
To study a single quantum emitter, scientists must keep it stable and confined to one location for extended periods. Traditionally, researchers have accomplished this by trapping atoms or molecules in a vacuum or embedding them inside solid materials.
Placing quantum emitters directly on a surface, however, offers an intriguing advantage. Scientists could potentially manipulate individual molecules by “touching them” with an extremely sharp probe. Techniques such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) already use tips with atomic precision to investigate matter at incredibly small scales.
The difficulty is that surfaces are rarely as clean and stable as quantum experiments require. Contaminants from the surrounding environment can accumulate on them, creating disturbances that interfere with the delicate quantum behavior of nearby atoms and molecules. Until now, researchers had struggled to preserve the quantum optical properties of individual emitters attached to surfaces.
Scientists in MPL’s Nano-Optics Division have now found a way around this problem.
A Clever Crystal Cleaning Method Solves the Problem
The research team, led by Prof. Vahid Sandoghdar, director at MPL and head of the “Nano-Optics” Division, developed an unusual cleaning method that takes advantage of a natural property of organic crystals.
At room temperature, the crystal material gradually evaporates through a process known as sublimation, in which a solid transforms directly into a gas without first becoming a liquid.
The researchers placed a small crystal inside a vacuum chamber equipped for extremely low temperatures, called a cryostat. As the crystal’s uppermost layers slowly evaporated, they carried away unwanted contaminants, leaving behind a remarkably clean surface.
The team then cooled the crystal to just a few degrees Kelvin above absolute zero, the lowest theoretically possible temperature. This extreme cooling halted further sublimation and helped maintain the pristine surface.
Finally, the scientists used a tiny, specially fabricated oven to evaporate molecules onto the cold crystal, allowing them to settle on the clean surface.
Molecules Reach the Ultimate Quantum Limit
With this exceptionally stable environment in place, the researchers could investigate whether the molecules retained their fragile quantum properties.
Dr. Alexey Shkarin, a researcher in the Nano-Optics Division at MPL, explained: “The quality of quantum emitters can be evaluated by their coherence times, which indicate how long they keep their quantumness.”
Coherence time describes how long a quantum system can maintain the properties needed for quantum behavior. The longer this coherence persists, the better the emitter can perform in applications that depend on precise interactions with light.
However, there is a fundamental physical restriction on how long coherence can last. Known as the Fourier limit, this boundary is determined by the time required for a quantum emitter to release its energy into its surroundings.
In an unstable or ‘noisy’ environment, disturbances can cause coherence to disappear hundreds or even thousands of times faster than this theoretical limit allows.
The scientists discovered that molecules deposited on their exceptionally clean crystal surface consistently reached the Fourier limit. This demonstrated that the surrounding environment was sufficiently quiet and stable to preserve the molecules’ quantum coherence for the maximum physically permitted duration.
It is the first time researchers have achieved this fundamental limit with molecules attached to a surface.
New Possibilities for Controlling Individual Molecules
The breakthrough also allowed the team to investigate how a crystal surface influences the molecules resting on it.
Further experiments revealed that interactions with the surface can cause molecules to align in particular directions and alter their energy levels. The researchers also found indications that the surface might influence the molecules’ shapes or the ways they vibrate.
These effects could provide valuable clues about how matter behaves at the molecular scale, while also offering new opportunities to control quantum systems.
“Our future work will focus on combining this method with AFM and STM to gain local nanometer control over individual quantum emitters,” says Vahid Sandoghdar.
By combining their ultra-clean surface technique with powerful microscopy methods, the researchers hope to manipulate individual quantum emitters with precision measured in billionths of a meter.
Such capabilities could reveal previously inaccessible details about interactions between molecules and surfaces, while laying the groundwork for engineering new quantum states of matter and advancing future molecular quantum technologies.
