Molecules on a surface reach the ultimate quantum limit

Molecules on a surface reach the ultimate quantum limit

Researchers at the Max Planck Institute for the Science of Light (MPL) have developed a technique to interrogate molecules on surfaces with spectroscopic precision, reaching the ultimate quantum limit for the first time. The findings, published in Science, offer new possibilities for molecular quantum technologies and the study of how molecules interact with surfaces.

Optical quantum technologies frequently rely on nanoscale objects, including atoms or molecules, that interact strongly with light. These quantum emitters are utilized for entanglement distribution, storing quantum information, and generating single photons—processes essential to quantum computation and communication.

To study these emitters individually, they must be held in a fixed position for an extended period. While this is typically done by placing them inside a bulk material or trapping them in a vacuum, surface-bound emitters offer the advantage of being manipulated via "touching." This can be achieved using atomically sharp tips, such as those found in atomic force microscopy (AFM) and scanning tunneling microscopy (STM).

Until now, scientists could not control surface-bound atoms and molecules while preserving their quantum-optical properties. Surfaces often absorb environmental contaminants, creating "noisy" and unstable surroundings that degrade the properties of the quantum emitters.

The Nano-Optics Division at MPL, led by director Prof. Vahid Sandoghdar, overcame this barrier by utilizing the properties of an organic crystal that slowly evaporates at room temperature. By placing a small crystal in a vacuum within a cryostat, the researchers allowed the top layers of the crystal to naturally fly away, carrying contaminants with them. The crystal was then cooled to a few degrees Kelvin above absolute zero to prevent further sublimation. Following this, the team used a microfabricated oven to evaporate molecules onto the surface at these low temperatures.

Dr. Alexey Shkarin, a researcher in the Nano-Optics Division, stated:

"The quality of quantum emitters can be evaluated by their coherence times, which indicates how long they keep their quantumness."

Dr. Alexey Shkarin, researcher in the Nano-Optics Division at MPL, via mpl.mpg.de

These coherence times are capped by the Fourier limit, which is defined by the time required for an emitter to transfer energy to its environment. In noisy environments, coherence times can be hundreds or thousands of times shorter than this limit. By using a crystal with a suitable molecular structure, the MPL scientists found that their molecules consistently reached the Fourier limit, proving the surroundings were stable and quiet.

The researchers also observed that the surface influences the behavior of adsorbed molecules in several ways:

• It shifts their energies.
• It forces them into a specific orientation.
• It may affect their shape or the way they vibrate.

The team plans to combine this method with STM and AFM to achieve local nanometer control over individual quantum emitters. 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"

Vahid Sandoghdar, director at MPL, via mpl.mpg.de

Such advancements are expected to provide new insights into surface properties and create paths for engineering quantum states of matter.