Scientists at the Max Planck Institute for the Science of Light (MPL) have developed a technique for interrogating molecules on surfaces with spectroscopic precision and thereby reaching the ultimate quantum limit for the first time. With their findings, published in Science, the researchers open new opportunities for the study of molecule-surface interactions and molecular quantum technologies.
Many optical quantum technologies rely on nano-scale objects, such as atoms or molecules, which interact strongly with light. These quantum emitters are used for generating single photons, storing quantum information and entanglement distribution, 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 is used in scanning tunneling microscopy (STM) and atomic force microscopy (AFM). However, scientists have not previously been able to gain control over surface-bound atoms and molecules to preserve their quantum-optical properties. The reason for this 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 have now devised a way to overcome this barrier.
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. Afterwards the crystal is cooled down to only a few degrees Kelvin above absolute zero to stop further sublimation. Then the researchers evaporated molecules onto the surface at these low temperatures with a microfabricated oven.
Dr. Alexey Shkarin, researcher in the Nano-Optics Division at MPL, explained: "The quality of quantum emitters can be evaluated by their coherence times, which indicates how long they keep their quantumness." These times can never be longer than the so-called Fourier limit, which is given by the time it takes for the emitter to transfer its energy to its environment. However, in noisy neighborhoods the coherence time can become hundreds or thousands of times shorter. By placing their molecules on a clean surface of a crystal with a suitable molecular structure, the scientists found that their 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.
In further elaborate studies, the group discovered several ways in which the surface affects the behavior of the adsorbed molecules: it turns them in a specific orientation, shifts their energies, and might 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.
