Molecules on a surface reach the ultimate quantum limit

Scientists at the Max Planck Institute for the Science of Light (MPL) have achieved a milestone in quantum optics by reaching the ultimate quantum limit for molecules positioned on a surface. This development, detailed in research published in Science, enables the precise manipulation of individual quantum emitters—a capability previously hindered by the instability and environmental noise that typically plague surface environments.

Optical quantum technologies, which are foundational for generating single photons, storing quantum information, and distributing entanglement, require stable emitters. While researchers have historically relied on vacuum traps or bulk materials to isolate these emitters, surface-bound molecules have proven difficult to manage. Surfaces act as magnets for environmental contaminants, creating noisy surroundings that degrade quantum-optical properties, specifically coherence times—the duration for which a particle maintains its quantum state.

Overcoming the Surface Barrier

To reach the fundamental Fourier limit, the research group led by Prof. Vahid Sandoghdar, director at MPL and head of the Nano-Optics Division, developed a new cleaning process. The technique exploits the properties of organic crystals, which naturally sublimate at room temperature. By placing a small crystal in a cryostat under vacuum, the team allowed the top layers of the crystal to evaporate, carrying environmental contaminants away with them. Once the surface was cleaned, the crystal was cooled to a few degrees Kelvin above absolute zero to halt further sublimation. Researchers then deposited molecules onto this pristine surface using a microfabricated oven.

Dr. Alexey Shkarin, a researcher in the Nano-Optics Division, noted that the quality of quantum emitters is evaluated by their coherence times, which are bounded by the time an emitter takes to transfer energy to its environment. In unstable conditions, these times can be reduced by factors of hundreds or thousands. By placing molecules on the clean surface of a crystal with a suitable molecular structure, the researchers observed that the molecules reached the Fourier limit. This result indicates that the surroundings were kept exceptionally quiet and stable, marking the first time this fundamental limit has been achieved on a surface.

Surface Influence and Future Integration

Beyond simply hosting the molecules, the study revealed that the surface actively shapes their behavior. The environment can influence the orientation of the adsorbed molecules, shift their energy levels, and alter their vibrational patterns. These findings suggest that surfaces might be engineered to tailor quantum states rather than acting as passive substrates. The research is part of a broader scientific effort to map the limits of nanoscale measurement, with related theoretical work from the American Physical Society exploring the limits of estimating molecular orientation and rotational “wobble” using quantum estimation theory.

Looking ahead, Prof. Vahid Sandoghdar stated that future research will focus on integrating this cleaning method with Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM). This combination aims to provide local nanometer-scale control over individual quantum emitters. Such capabilities are expected to provide insight into surface properties and open new avenues for engineering quantum states of matter, moving from the isolated environments of the past toward functional, controllable interfaces required for next-generation quantum computation and communication.