Max Planck Institute Researchers Stabilize Molecules for Quantum Coherence

Researchers at the Max Planck Institute for the Science of Light (MPL) have developed a technique to stabilize individual molecules on a surface so they reach the ultimate quantum limit of optical coherence. The findings, published in Science, address a long-standing obstacle in molecular quantum research where surface contamination and environmental noise disrupt delicate quantum states.

The Tech TL;DR:

• Max Planck Institute researchers used an organic crystal sublimation method inside a cryogenic vacuum chamber to clean surfaces at the atomic level.
• Molecules deposited on these purified surfaces successfully reached the Fourier limit, maintaining quantum coherence for the maximum physically permitted duration.
• The advance paves the way for combining ultra-clean substrates with scanning tunneling microscopy to achieve nanometer-scale control over quantum emitters.

Sublimation Cleaning Inside Low-Temperature Cryostats

Studying single quantum emitters usually requires trapping atoms or molecules in a vacuum or embedding them inside solid materials. Placing these emitters directly on a surface allows scientists to manipulate individual molecules using sharp probes like atomic force microscopy (AFM) and scanning tunneling microscopy (STM). However, environmental contaminants accumulate on standard surfaces, causing disturbances that degrade optical properties hundreds or thousands of times faster than theoretical limits allow.

To clear these disruptions, a research team led by Prof. Vahid Sandoghdar, director at MPL and head of the Nano-Optics Division, utilized the natural sublimation properties of organic crystals. The team placed a small crystal inside a cryostat operating at extremely low temperatures. The topmost layers of the crystal slowly evaporated into a gas, carrying away surface contaminants and exposing a pristine underlying structure. Cooling the chamber to just a few degrees Kelvin above absolute zero halted further sublimation, after which a tiny, custom-fabricated oven evaporated molecules onto the cold crystal surface.

Achieving the Fourier Limit of Coherence

With the molecules settled on the stable surface, the team evaluated their quantum quality using coherence times, which measure how long a system can maintain properties required for quantum behavior. Dr. Alexey Shkarin, a researcher in the Nano-Optics Division at MPL, noted that coherence indicates how long an emitter retains its quantum state. The fundamental physical boundary governing this duration is the Fourier limit, determined by the time an emitter takes to release energy into its surroundings.

Measurements confirmed that molecules deposited on the cleaned crystal surface consistently reached the Fourier limit. This performance proved that the local environment remained sufficiently quiet to preserve quantum coherence for the maximum physically permitted duration. While the experiments revealed that surface interactions can alter molecular energy levels, alignment, and vibrational states, the underlying coherence remained intact.

Next Steps for Nanoscale Quantum Manipulation

Future work by the MPL Nano-Optics Division will focus on integrating this crystal cleaning method with scanning tunneling microscopy and atomic force microscopy. Vahid Sandoghdar stated that combining the ultra-clean surface technique with these microscopy methods aims to provide local nanometer-level control over individual quantum emitters. These capabilities are intended to yield deeper insights into surface-molecule interactions and support the engineering of new quantum states of matter.