Molecules on a Crystal Surface Reach the Ultimate Quantum Limit for the First Time

Researchers at the Max Planck Institute for the Science of Light (MPL) in Germany have found a way to keep individual molecules on a solid surface stable enough to preserve their quantum properties at the fundamental physical limit, something not previously achieved outside a vacuum or a bulk material. The findings, led by Prof. Vahid Sandoghdar, head of MPL's Nano-Optics Division, were published in the journal Science.

Quantum emitters, such as single atoms and molecules that interact strongly with light, can generate single photons, store quantum information and carry entanglement, capabilities used in quantum communication and computing. Studying one at a time normally means trapping it in a vacuum or embedding it in a bulk material. Keeping one on an accessible surface would make it easier to probe and manipulate, but contamination on real-world surfaces has always created a noisy environment that quickly degrades its delicate quantum state.

The team exploited the fact that a particular organic crystal slowly evaporates at room temperature, carrying surface contaminants away with it. They placed a small crystal in a vacuum-sealed cryostat, let its outer layers evaporate, then cooled it to a few degrees above absolute zero to stop further evaporation before depositing molecules onto the freshly cleaned surface using a microfabricated oven.

The molecules consistently reached the Fourier limit, the theoretical maximum coherence time for a quantum emitter, meaning they experienced an unusually quiet and stable environment. "The quality of quantum emitters can be evaluated by their coherence times, which indicate how long they keep their quantumness," said Dr. Alexey Shkarin of MPL's Nano-Optics Division. In a noisy environment, that coherence can become hundreds or even thousands of times shorter. The surface also caused the molecules to adopt a particular orientation and shifted their energies.

The researchers say future work will combine the technique with atomic force microscopy and scanning tunneling microscopy, tools that could give nanometer-scale control over individual quantum emitters on the surface.