Scientists Achieve Record Quantum Coherence in Molecules Attached to Surfaces

Scientists at the Max Planck Institute for the Science of Light (MPL) have developed a method for studying individual molecules on surfaces with extraordinary precision. In a breakthrough for quantum physics, the researchers demonstrated for the first time that molecules attached to a surface can reach the fundamental quantum limit of optical coherence.

The advance could open new possibilities for investigating how molecules interact with surfaces and for developing future quantum technologies.

Why surfaces are difficult for quantum experiments

Many emerging quantum technologies depend on extremely small systems, including individual atoms and molecules, interacting with light. These systems, known as quantum emitters, can produce individual particles of light called photons, store quantum information and help create quantum connections between distant systems. These capabilities are important for future quantum computing and secure quantum communications.

To study a single quantum emitter, scientists must keep it stable and confined in one place for extended periods. Traditionally, researchers have done this by trapping atoms and molecules in a vacuum or embedding them in solid materials.

Placing quantum emitters directly on a surface offers important advantages. Scientists could potentially manipulate individual molecules by “touching” them with extremely sharp probes. Technologies such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) already use atomically precise tips to examine matter at incredibly small scales.

However, surfaces are rarely clean and stable enough for demanding quantum experiments. Contaminants from the surrounding environment can accumulate on them and disrupt the delicate quantum behavior of nearby atoms and molecules. Until now, researchers had struggled to preserve the optical quantum properties of individual emitters attached to a surface.

Scientists from MPL’s Nano-Optics Division have now found a way to overcome this challenge.

A crystal-cleaning method creates an ultra-stable surface

A research team led by Professor Vahid Sandoghdar, Director of MPL and Head of Nano-Optics, developed an unusual cleaning technique that takes advantage of the natural properties of organic crystals.

At room temperature, the crystalline material gradually evaporates through a process called sublimation. During sublimation, a solid changes directly into a gas without first becoming a liquid.

The researchers placed tiny crystals inside a vacuum chamber called a cryostat, which is designed to reach extremely low temperatures. As the upper layer of the crystals slowly sublimated, unwanted contaminants were removed, leaving an exceptionally clean surface.

The team then cooled the crystal to just a few Kelvins above absolute zero, the lowest temperature theoretically possible. This extreme cooling stopped further sublimation and preserved the newly cleaned surface.

Finally, the scientists used a specially constructed oven to evaporate molecules onto the cold crystals. The molecules settled onto the clean surface, creating a highly stable environment for quantum measurements.

Molecules reach the fundamental Fourier limit

With the stable environment in place, the researchers investigated whether the molecules could retain their fragile quantum properties.

“The quality of a quantum emitter can be assessed by its coherence time, which indicates how long its quantum nature remains,” explained Dr. Alexei Shkarin, a researcher in MPL’s Nano-Optics Department.

Coherence time measures how long a quantum system can maintain the properties required for quantum operations. The longer coherence persists, the better the emitter can perform in applications that depend on precise interactions with light.

There are fundamental limits to how long coherence can last. This boundary, known as the Fourier limit, is determined by the time required for a quantum emitter to release its energy into its surroundings.

In unstable or “noisy” environments, disturbances can destroy coherence hundreds or even thousands of times faster than this theoretical limit.

The researchers found that molecules deposited on exceptionally clean crystal surfaces consistently reached the Fourier limit. This result shows that the surrounding environment was sufficiently quiet and stable to preserve molecular quantum coherence for the longest period physically possible.

It is the first time researchers have demonstrated that molecules attached to a surface can reach this fundamental quantum limit.

Surfaces could enable precise control of individual molecules

The breakthrough also enabled the team to examine how the crystal surface affects the molecules above it.

Further experiments revealed that interactions with surfaces can align molecules in specific directions and alter their energy levels. The researchers also found indications that surfaces can influence the shape of molecules and the way they vibrate.

These effects could provide valuable insight into how matter behaves at the molecular scale while creating new ways to control quantum systems.

“Our future research will focus on combining this method with AFM and STM to enable local nanometer control of individual quantum emitters,” said Vahid Sandoghdar.

By combining ultra-clean surface preparation with advanced microscopy, the researchers hope to manipulate individual quantum emitters with precision on the scale of a billionth of a meter.

Such capabilities could reveal previously inaccessible details about interactions between molecules and surfaces. They may also help scientists engineer new quantum states of matter and advance future molecular quantum technologies.