Surface-bound molecules reach quantum coherence limit

A surface offers a different route because a scanning tunneling microscope, or STM, and an atomic force microscope, or AFM, can approach an individual molecule with an atomically sharp tip. In principle, researchers could move or manipulate that molecule.

Contamination has blocked that approach. Environmental material accumulates on surfaces and disturbs the quantum behavior of nearby emitters, while ordinary surfaces rarely remain clean and stable enough for extended study.

The MPL team addressed both requirements with one material-cleaning strategy. Their method treats the crystal surface as the object to prepare and the molecule as the object to measure.

How did sublimation clean the crystal?

At room temperature, the chosen organic crystal gradually sublimates inside a vacuum chamber. Sublimation converts a solid directly into a gas, allowing the uppermost crystal layers to escape without passing through a liquid.

The researchers performed the preparation in four steps:

• They placed a small crystal inside a cryostat, a vacuum chamber designed to reach extremely low temperatures.
• As the crystal’s upper layers evaporated, they carried contaminants away and exposed a clean surface beneath.
• They cooled the crystal to a few kelvin above absolute zero, or 0 kelvin, the theoretical temperature floor. The cold halted further sublimation and helped preserve the cleaned surface.
• Finally, a tiny fabricated oven evaporated molecules onto the cold crystal, where they settled and attached.

This sequence let the team prepare the surface before adding the molecules that would serve as quantum emitters.

What did the molecules reveal about the limit?

With that environment in place, the team could test whether surface-bound molecules retained coherence. They consistently reached the Fourier limit, marking the first demonstration of that limit for a molecule attached to a surface.

The limit depends on how quickly an optical transition—a change between molecular energy states—releases energy into the environment. Reaching it indicates more than successful attachment: the surrounding crystal must remain sufficiently free of time-varying disturbances.

Surface interactions also changed molecular behavior. The researchers found that those interactions could align molecules in particular directions and alter their energy levels.

They also detected indications that the surface influenced molecular shape or vibration. Those observations may guide efforts to engineer molecular behavior, although the reported experiment did not yet provide deliberate control of those properties.

Can researchers control one molecule at a time?

Prof. Vahid Sandoghdar said: “Our future work will focus on combining this method with AFM and STM to gain local nanometer control over individual quantum emitters.”

That remains a future target rather than a completed step. Combining ultra-clean surfaces with AFM and STM could give researchers local control across distances of a nanometer, one billionth of a meter.

Such control could let researchers test molecular interactions at otherwise inaccessible positions and possibly build new quantum states of matter. The immediate scientific payoff is a stable platform for studying one emitter at the Fourier limit; practical devices still require further work.

The next test is whether researchers can steer one molecule without disturbing the coherence achieved at its physical limit.