In a major milestone for the field of quantum physics, researchers at the Max Planck Institute for the Science of Light (MPL) in Erlangen, Germany, have successfully developed a groundbreaking technique to study individual molecules on a solid surface with unprecedented precision. For the very first time in scientific history, researchers have demonstrated that molecules attached directly to a surface can reach the ultimate quantum limit of optical coherence. This achievement overcomes a formidable, long-standing obstacle that has historically restricted molecular quantum research and severely limited how scientists interact with nanoscale quantum systems.
The findings, which have been published in the prestigious academic journal Science, mark a critical turning point in experimental physics. They open up vast, previously unimaginable possibilities for investigating the fundamental ways in which complex molecules interact with surfaces, while simultaneously laying a robust foundation for the development of advanced, highly scalable quantum technologies.
Why Surfaces Have Been a Major Obstacle in Quantum Physics
Modern quantum technology relies heavily on the manipulation of extremely small physical objects—such as individual atoms and molecules—that possess the unique ability to interact directly with light. These tiny, highly sensitive systems are formally known as quantum emitters. When properly controlled and stimulated, quantum emitters are capable of producing single, isolated particles of light known as photons. Furthermore, they can store fragile quantum information and help establish secure, long-range quantum connections between distant physical systems. These core capabilities are widely considered indispensable for the realization of future applications in ultra-powerful quantum computing and secure global quantum communication networks.
To properly study, analyze, and harness the properties of a single quantum emitter, experimental scientists must keep the system remarkably stable and precisely confined to one fixed location for extended periods. Historically, researchers have accomplished this delicate feat by employing one of two primary methods: either trapping individual atoms or molecules in an ultra-high vacuum using complex laser and magnetic fields, or embedding them safely deep inside solid, transparent materials.
Placing quantum emitters directly on an exposed surface, however, offers an extraordinarily intriguing and powerful advantage. If scientists could successfully position and stabilize molecules on a surface, they could potentially manipulate individual molecules by physically "touching them" with an exquisitely sharp nanoscale probe. Advanced investigative techniques such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) already routinely utilize specialized tips with atomic-level precision to investigate matter at extraordinarily small scales. Combining these powerful microscopic tools with quantum optical emitters would give researchers unprecedented control over individual quantum states.
The central difficulty that has plagued researchers for decades is that surfaces are rarely as clean, uniform, and stable as delicate quantum experiments demand. In the real world, microscopic contaminants from the surrounding environment inevitably accumulate on exposed surfaces, creating invisible disturbances, electromagnetic noise, and chemical irregularities. These environmental factors aggressively interfere with the fragile quantum behavior of nearby atoms and molecules, causing their quantum states to degrade almost instantly. Until now, researchers around the globe had continuously struggled to preserve the delicate quantum optical properties of individual emitters once they were attached to solid surfaces.
Now, however, a dedicated team of scientists working within the Nano-Optics Division at the Max Planck Institute for the Science of Light has successfully found an ingenious way around this persistent physical barrier.
A Clever Crystal Cleaning Method Solves the Problem
The research team, operating under the leadership of Professor Vahid Sandoghdar, a director at MPL and head of the Nano-Optics Division, set out to solve the contamination problem by developing an unusual and highly effective cleaning technique. Their method cleverly takes advantage of a natural, ongoing physical property inherent to specific organic crystals.
At standard room temperature, these crystal materials gradually evaporate through a well-known thermodynamic process called sublimation, in which a solid substance transforms directly into a gaseous state without ever passing through an intermediate liquid phase. The researchers harnessed this natural evaporation process to continuously purge the surface of impurities.
To execute the experiment, the team placed a small, specially prepared organic crystal inside a specialized vacuum chamber equipped for extremely low temperatures, known scientifically as a cryostat. As the uppermost atomic layers of the crystal slowly and continuously evaporated away through sublimation, they physically carried away any unwanted dust, atmospheric residue, and chemical contaminants resting on the surface. This dynamic cleaning action left behind a remarkably clean, atomically pristine surface layer.
Once the surface was thoroughly purified by this self-cleaning sublimation process, the team rapidly cooled the entire crystal down to just a few Kelvin above absolute zero, representing the lowest temperature theoretically possible in the universe. This extreme thermal cooling successfully halted any further sublimation of the crystal, while simultaneously locking the pristine, contamination-free surface into a state of absolute structural stability.
Following this cryogenic stabilization, the scientists utilized a tiny, custom-fabricated micro-oven positioned inside the vacuum system to gently evaporate selected molecules onto the ice-cold crystal. This allowed the incoming molecules to settle softly and precisely onto the impeccably clean surface without introducing foreign debris or thermal disruptions.
Molecules Reach the Ultimate Quantum Limit
With this exceptionally stable and pristine experimental environment successfully established, the MPL researchers were finally ready to investigate whether the delicate molecules could retain their fragile quantum properties while resting on a solid substrate.
Dr. Alexey Shkarin, a researcher in the Nano-Optics Division at MPL who was closely involved in the project, elaborated on the evaluation criteria, explaining that the overall quality of quantum emitters is fundamentally evaluated by measuring their coherence times. Coherence time is a critical metric that indicates precisely how long a quantum system can successfully maintain its internal "quantumness"—meaning the phase relationships required to exhibit quantum interference and superposition.
Coherence time describes how long a quantum system can maintain the precise properties required for advanced quantum behavior. The longer this coherence persists without degradation, the better the emitter can perform in real-world applications that depend on ultra-precise interactions with light.
However, in the realm of quantum mechanics, there is a strict and fundamental physical restriction governing how long coherence can possibly last. Known universally to physicists as the Fourier limit, this absolute boundary is determined entirely by the natural time required for a quantum emitter to release its stored energy back into its surrounding environment.
In a typical, unstable, or "noisy" experimental environment, environmental disturbances and electromagnetic fluctuations interact with the quantum system, causing its coherence to disappear hundreds or even thousands of times faster than this theoretical Fourier limit allows. This rapid decay has long been the primary bottleneck for surface-based quantum research.
To their astonishment, the Max Planck scientists discovered that molecules deposited onto their newly developed, exceptionally clean crystal surface consistently reached the Fourier limit. This definitive 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.
It marks the very first time in the history of physics that researchers have successfully achieved this fundamental limit using individual molecules attached directly to a solid surface.
New Possibilities for Controlling Individual Molecules
Beyond merely proving that the Fourier limit could be reached on a surface, this major technological breakthrough also allowed the research team to investigate the subtle, detailed ways in which a crystal surface actively influences the individual molecules resting upon it.
Further experimental analysis revealed that direct interactions with the underlying surface can cause molecules to align themselves in particular, predictable directions, as well as alter their internal energy levels in measurable ways. Furthermore, the researchers uncovered intriguing indications that the surface might even influence the physical shapes of the molecules or subtly modify the ways in which their chemical bonds vibrate.
These nuanced physical effects could provide scientists with exceptionally valuable clues about how matter behaves at the absolute smallest molecular scales, while simultaneously offering entirely new pathways and opportunities to control complex quantum systems.
Looking toward the future, Professor Vahid Sandoghdar outlined the team’s upcoming research trajectory, noting that their future work will focus on combining this innovative cleaning method with advanced scanning probe technologies. By integrating their ultra-clean surface technique with atomic force microscopy and scanning tunneling microscopy, the research group aims to gain local, nanometer-scale control over individual quantum emitters.
Through the successful combination of their ultra-clean surface preparation technique with these powerful, high-resolution microscopy methods, the researchers hope to manipulate individual quantum emitters with a precision measured in billionths of a meter.
Such advanced capabilities could soon reveal previously inaccessible, deeply fundamental details concerning the complex interactions between molecules and solid surfaces. At the same time, this achievement lays a robust, highly promising groundwork for engineering entirely new quantum states of matter and decisively advancing the next generation of molecular quantum technologies.
