Scientists at the Max Planck Institute for the Science of Light have reached the ultimate quantum limit for molecules placed on a crystal surface — a milestone that could transform quantum computing, communication and nanoscale sensing technologies.
A team of physicists at the Max Planck Institute for the Science of Light (MPL) in Germany has accomplished something no research group had managed before: coaxing molecules resting on a surface to reach the so-called Fourier limit, the most fundamental quantum boundary on how long a particle can preserve its quantum properties. Their findings, published June 25 in the journal Science, lay the groundwork for a new generation of molecular quantum technologies.
Why the Fourier Limit Matters
At the heart of modern quantum technologies — from secure quantum communication networks to next-generation computers — are tiny objects called quantum emitters. These are nano-scale particles, such as atoms or molecules, that interact powerfully with light. They generate individual photons, hold quantum information, and enable a phenomenon called entanglement distribution, all of which are essential building blocks of quantum computing and communication.
The quality of these emitters depends critically on how long they can maintain their quantum character, a property measured by coherence time.
“The quality of quantum emitters can be evaluated by their coherence times, which indicates how long they keep their quantumness,” co-author Alexey Shkarin, a researcher in the Nano-Optics Division at MPL, said in a news release.
The Fourier limit defines the absolute ceiling for that duration — it is the point at which an emitter’s coherence time equals exactly the time needed for it to release its energy into the surrounding environment. In practice, noisy or contaminated surroundings can slash coherence times to a tiny fraction of that ideal, sometimes hundreds or thousands of times shorter than the theoretical maximum.
Reaching the Fourier limit on a surface, where molecules are exposed to environmental interference, had been considered an enormous challenge. Surfaces are prone to picking up contaminants from the air and their surroundings, creating unstable conditions that destroy the delicate quantum states researchers need to study and exploit.
A Clever Cleaning Trick
To get around the contamination problem, the MPL team devised an ingenious preparation method. The group, led by Vahid Sandoghdar, an Alexander von Humboldt Professor in the Department of Physics at Friedrich Alexander University, director of the MPL and head of its Nano-Optics Division, exploited a natural property of organic crystals: at room temperature, these materials gradually sublimate, meaning their outermost layers slowly evaporate. By placing a small crystal inside a cryostat under vacuum, the scientists allowed the top layers — along with any surface contaminants — to simply fly away. The crystal was then cooled to just a few degrees above absolute zero, halting further sublimation. At those ultra-low temperatures, the researchers used a custom microfabricated oven to deposit molecules directly onto the freshly cleaned crystal surface.
The result was a surface environment so stable and quiet that the molecules placed on it consistently achieved the Fourier limit — something that had never been demonstrated on a surface before. The team’s measurements showed that this was not a one-off occurrence but a repeatable outcome, which significantly strengthens the finding’s credibility and practical relevance.
What the Surface Does to a Molecule
Beyond simply achieving the quantum limit, the researchers uncovered several intriguing ways the crystal surface shapes the behavior of molecules resting on it. The surface can lock molecules into a particular orientation, shift the energy levels at which they absorb and emit light, and may even alter their physical shape or vibration patterns. These effects, while potentially disruptive in some contexts, also represent powerful new levers for controlling and engineering molecular behavior at the quantum level.
The team’s next steps involve combining this surface-preparation approach with atomic force microscopy and scanning tunneling microscopy techniques to achieve precise, nanometer-scale control over individual quantum emitters, according to Sandoghdar.
“Our future work will focus on combining this method with AFM and STM to gain local nanometer control over individual quantum emitters,” he said.
That kind of control would give scientists an unprecedented window into the physics of surfaces and entirely new ways to engineer quantum states of matter.
Why It Matters for Students and Young Researchers
For students studying physics, chemistry, materials science or computer science, this breakthrough sits squarely at the intersection of multiple cutting-edge fields. Quantum technologies are widely expected to drive the next wave of transformative innovation — from unbreakable encryption to drug discovery simulations that classical computers cannot run. The ability to reliably place quantum emitters on surfaces, rather than trapping them in bulky vacuum chambers or embedding them in solid materials, could eventually make quantum devices far more compact, accessible and compatible with existing nanotechnology tools like AFM and STM.
Moreover, the method itself — leveraging natural sublimation to clean a surface rather than relying on complex chemical processes — illustrates how elegant, low-tech thinking can unlock high-tech breakthroughs. It is a reminder that creative experimental design remains as valuable as computational power or expensive equipment.