Scientists Push Molecules on a Surface to the Ultimate Quantum Limit

An ultra-clean crystal surface allowed individual molecules to preserve quantum coherence at the fundamental Fourier limit.

A molecule placed on a surface should be easier to probe and manipulate than one hidden inside a solid or suspended in vacuum. In practice, however, surface contamination creates an unstable, noisy environment that can quickly degrade the molecule’s delicate quantum properties.

Researchers at the Max Planck Institute for the Science of Light (MPL) have now overcome that barrier. Their technique allows molecules on a surface to be examined with spectroscopic precision while consistently reaching the ultimate quantum limit for coherence, something not previously achieved on a surface. The findings, published in Science, could expand studies of molecule-surface interactions and molecular quantum technologies.

Many optical quantum technologies depend on nanoscale objects such as atoms and molecules that interact strongly with light. These quantum emitters can generate single photons, store quantum information, and distribute entanglement, capabilities used in quantum communication and computation.

Studying one emitter at a time requires keeping it fixed in place for long periods. Researchers generally accomplish this by trapping emitters in vacuum or embedding them inside a bulk material.

A surface offers another possibility because an individual atom or molecule could potentially be manipulated directly with an atomically sharp tip, such as those used in scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Until now, however, preserving the quantum-optical properties of surface-bound emitters has been difficult because contaminants readily collect on surfaces and create fluctuating surroundings.

A clean surface preserved quantum coherence

The group led by Prof. Vahid Sandoghdar, director at MPL and head of the “Nano-Optics” Division, addressed the contamination problem by exploiting a property of an organic crystal: it slowly evaporates at room temperature.

The researchers placed a small crystal inside a cryostat under vacuum. As its uppermost layers naturally evaporated, they carried surface contaminants away with them. The crystal was then cooled to only a few degrees Kelvin above absolute zero, stopping further sublimation. At those low temperatures, the researchers deposited molecules onto the freshly cleaned surface using a microfabricated oven.

The result was an unusually stable environment for the quantum emitters.

Dr. Alexey Shkarin, 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 cannot exceed what is known as the Fourier limit, which is determined by how long an emitter takes to transfer its energy to its surroundings. In a noisy environment, that coherence can become hundreds or even thousands of times shorter. By placing molecules on a clean crystal surface with a suitable molecular structure, the researchers found that the molecules consistently reached the Fourier limit. This was the first time the fundamental limit had been achieved on a surface, indicating that the molecules experienced an extremely quiet and stable environment.

Surfaces also changed molecular behavior

The experiments also showed that the surface does more than simply hold the molecules in place. It caused adsorbed molecules to adopt a particular orientation and shifted their energies, while potentially altering their shape or how they vibrate.

“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.

Combining the clean surface technique with those microscopy tools could allow researchers to investigate individual quantum emitters with nanometer-scale control, providing deeper insight into surface properties and new ways to engineer quantum states of matter.

Reference: “Nano–electron volt Fourier-limited transition of a single surface-adsorbed molecule” by Masoud Mirzaei, Alexey Shkarin, Burak Gurlek, Johannes Zirkelbach, Ashley J. Shin, Irena Deperasińska, Boleslaw Kozankiewicz, Tobias Utikal, Stephan Götzinger and Vahid Sandoghdar, 25 June 2026, Science.
DOI: 10.1126/science.aeg5014

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