Scientists at the Max Planck Institute for the Science of Light (MPL) in Germany have achieved a significant breakthrough in the field of quantum physics, developing an innovative technique to study individual molecules on a surface with extraordinary precision. For the very first time, researchers have successfully demonstrated that molecules attached directly to a solid surface can reach the ultimate quantum limit of optical coherence, effectively overcoming a major technical obstacle that has restricted the boundaries of molecular quantum research for decades.
The findings, which have been published in the prestigious scientific journal Science, mark a major milestone in experimental physics. This development could open up entirely new pathways for investigating how individual molecules interact with various surfaces, while simultaneously laying crucial groundwork for the future engineering and advancement of sophisticated quantum technologies.
Why Surfaces Have Been a Major Obstacle in Quantum Physics
In the rapidly evolving domain of quantum technology, many emerging systems rely fundamentally on extremely small objects—such as individual atoms and molecules—that are capable of interacting directly with light. These fundamental building blocks, widely known in the scientific community as quantum emitters, possess remarkable capabilities. They can produce single, discrete particles of light called photons, securely store complex quantum information, and help establish reliable quantum connections between otherwise distant systems. These advanced capabilities are considered indispensable requirements for the realization of future applications in fields like ultra-powerful quantum computing and secure quantum communication networks.
To study a single quantum emitter effectively, researchers must first ensure it remains exceptionally stable and confined securely to a single, well-defined location for extended periods. Traditionally, experimental physicists have accomplished this stringent requirement by trapping individual atoms or molecules in a high vacuum or by embedding them deeply inside solid host materials.
Placing quantum emitters directly on an exposed surface, however, offers a uniquely intriguing advantage for researchers. By operating on a surface, scientists could potentially manipulate individual molecules directly by physically "touching" them with an extremely sharp probe. Advanced scientific techniques such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) already utilize specialized tips with absolute atomic precision to investigate matter at astonishingly small scales.
The core difficulty, however, has always been that physical surfaces are rarely as clean and stable as delicate quantum experiments demand. Contaminants and stray molecules from the surrounding laboratory or environmental atmosphere can easily accumulate on these surfaces over time. This creates persistent disturbances and environmental noise that drastically interfere with the fragile quantum behavior of nearby atoms and molecules. Until now, researchers around the world had continually struggled to preserve the sensitive quantum optical properties of individual emitters once they were attached to open surfaces.
Scientists working within MPL’s Nano-Optics Division have now successfully found a clever way around this persistent and frustrating problem, opening new doors for surface-based quantum investigations.
A Clever Crystal Cleaning Method Solves the Problem
The dedicated research team, led by Professor Vahid Sandoghdar, who serves as a director at MPL and head of the Institute’s Nano-Optics Division, developed an unusual and highly effective cleaning method that cleverly takes advantage of a natural chemical and physical property inherent to organic crystals.
At room temperature, the specific crystal material utilized by the team gradually evaporates through a well-known physical process called sublimation. During sublimation, a solid substance transforms directly into a gas without ever passing through an intermediate liquid phase.
To harness this phenomenon, the researchers placed a small, carefully prepared organic crystal inside a specialized vacuum chamber that was fully equipped for handling extremely low temperatures, known as a cryostat. As the uppermost layers of the crystal slowly and continuously evaporated away inside the vacuum environment, they carried away unwanted surface contaminants with them. This natural peeling effect left behind a remarkably clean, atomically pristine surface underneath.
Following this cleaning phase, the team rapidly cooled the crystal down to just a few degrees Kelvin—which is only slightly above absolute zero, the lowest theoretically possible temperature in the universe. This extreme and rapid cooling completely halted any further sublimation of the crystal and helped maintain the newly achieved, immaculate surface state.
Finally, the scientists utilized a tiny, specially fabricated microscopic oven to carefully evaporate selected molecules onto the extremely cold crystal, allowing them to gently settle and rest upon the perfectly clean surface in a controlled manner.
Molecules Reach the Ultimate Quantum Limit
With this exceptionally stable and pristine experimental environment successfully established, the researchers could finally investigate whether the deposited molecules successfully retained their fragile and easily disrupted quantum properties.
Dr. Alexey Shkarin, a researcher in the Nano-Optics Division at MPL, elaborated on the evaluation process, explaining that the overall quality of quantum emitters can be effectively evaluated by measuring their coherence times. Coherence times essentially indicate precisely how long these microscopic systems can maintain their essential "quantumness."
Coherence time describes the exact duration that a quantum system can maintain the specific properties required for sustained quantum behavior. The longer this coherence persists without degradation, the better the quantum emitter can perform in demanding applications that rely on precise, unperturbed interactions with light.
However, fundamental physical restrictions dictate just how long this coherence can possibly last. Known to physicists as the Fourier limit, this strict boundary is fundamentally determined by the precise amount of time required for a quantum emitter to release its stored energy out into its surrounding environment.
In an unstable, noisy, or contaminated environment, external disturbances typically cause quantum coherence to disappear hundreds or even thousands of times faster than this theoretical limit ever allows, ruining the experiment.
To their great excitement, the scientists discovered that the molecules deposited on their exceptionally clean crystal surface consistently reached the Fourier limit. This clear result demonstrated that the surrounding environment was sufficiently quiet, clean, and stable to preserve the molecules’ delicate quantum coherence for the maximum physically permitted duration.
This achievement marks the very first time that researchers have successfully reached this fundamental physical limit using molecules attached directly to an exposed surface.
New Possibilities for Controlling Individual Molecules
The breakthrough achievement also allowed the research team to investigate in detail how an underlying crystal surface actively influences the individual molecules resting upon it.
Further experiments conducted by the team revealed that interactions with the surface can cause molecules to align themselves in particular, predictable directions and alter their internal energy levels. Furthermore, the researchers found intriguing indications that the surface might even influence the physical shapes of the molecules or subtly modify the specific ways in which they vibrate.
These profound effects could provide researchers with valuable clues about how matter fundamentally behaves at the molecular scale, while simultaneously offering brand new opportunities to control complex quantum systems with high accuracy.
Looking toward the future, Professor Vahid Sandoghdar outlined the next steps for the research group. "Our future work will focus on combining this method with AFM and STM to gain local nanometer control over individual quantum emitters," he stated.
By combining their newly developed ultra-clean surface technique with powerful, high-resolution microscopy methods, the researchers hope to manipulate individual quantum emitters with a precision measured strictly in billionths of a meter.
Such advanced capabilities could soon reveal previously inaccessible details concerning the fundamental interactions between molecules and solid surfaces, while simultaneously laying a solid groundwork for engineering entirely new quantum states of matter and advancing future molecular quantum technologies.
