Explaining the environmental interactions of a single molecule on a surface
Theoretical work at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg has helped explain a new experimental milestone: for the first time, a single molecule sitting on a surface have been shown to reach the fundamental quantum limit of optical coherence. The experiments were performed at the Max Planck Institute for the Science of Light (MPL) in Erlangen; MPSD researcher Burak Gurlek provided the theoretical models explaining how temperature and the surface shape the molecules' behavior. The findings, published in Science, open new opportunities for the study of molecule-surface interactions and molecular quantum technologies.
Many optical quantum technologies rely on nanoscale objects, such as atoms or molecules, that interact strongly with light. These quantum emitters are used for generating single photons, storing quantum information and distributing entanglement, processes that find application in quantum communication and computation.
To investigate these emitters individually, researchers need to keep them in one place for a long time. This is usually achieved by either trapping them in vacuum or placing them inside a bulk material. Quantum emitters located on a surface would create new opportunities to manipulate their functionalities by "touching them", for example with an atomically sharp tip, as used in scanning tunneling microscopy (STM) and atomic force microscopy (AFM). However, scientists had not previously been able to gain control over surface-bound atoms and molecules while preserving their quantum-optical properties. The reason is that surfaces can easily absorb contaminants from the environment, creating highly unstable and "noisy" surroundings that compromise the properties of the quantum emitters. Researchers in the Nano-Optics Division of MPL, together with theoretical colleagues at MPSD, have now found a way to overcome this barrier and, just as importantly, to understand it.
To obtain a clean surface, the group led by Prof. Vahid Sandoghdar, director at MPL and head of the Nano-Optics Division, devised a new approach. The scientists took advantage of the fact that an organic crystal slowly evaporates at room temperature. On placing a small crystal in a cryostat under vacuum, the top crystal layers naturally fly away, taking the contaminants with them. The crystal is then cooled to only a few degrees Kelvin above absolute zero to stop further sublimation, after which the researchers evaporate molecules onto the surface at these low temperatures with a microfabricated oven.
The quality of quantum emitters can be evaluated by their coherence times, which indicate how long they keep their "quantumness". These times can never be longer than the so-called Fourier limit, given by the time it takes for the emitter to transfer its energy to its environment. In noisy neighbourhoods, however, the coherence time can become hundreds or thousands of times shorter. By placing their molecules on a clean crystal surface with a suitable molecular structure, the scientists at MPL found that the molecules consistently reached the Fourier limit, indicating that their surroundings are extremely quiet and stable. This marks the first time this fundamental limit has been reached on a surface.
Reaching the limit experimentally is the central achievement; explaining how the surface influences the molecule is a further question. This is where the theoretical work carried out at MPSD comes in. Burak Gurlek developed the theoretical models that explain the temperature dependence of a surface-adsorbed molecule's transition linewidth and frequency, the quantities that reveal how strongly the molecule couples to its environment and thus how well it preserves its quantum coherence. He further explained the large adsorption-induced frequency shift of the molecule, revealing how the local environment influences its optical transition. These calculations link the measured spectroscopy directly to electron-phonon coupling and van der Waals interactions.
“Reaching the Fourier limit on a surface tells us the molecule's environment is remarkably quiet, but to use such a platform we need to understand precisely how the environment influences the molecule's properties.", says Burak Gurlek, researcher at the MPSD. "This experimental platform could provide a new way of studying the fundamental limits of molecular coherence across chemical space, a key question for emerging molecular quantum technologies."
In further detailed studies, the team discovered several ways in which the surface affects the behaviour of the adsorbed molecules: it turns them into a specific orientation, shifts their energies, and may even affect their shape or the way the molecules vibrate.
"Our future work will focus on combining this method with AFM and STM to gain local nanometer control over individual quantum emitters", says Vahid Sandoghdar. Such studies will provide unprecedented insight into the properties of surfaces and open new avenues to engineering quantum states of matter.
This article was written using this original press release by the MPL Erlaggen
