Super-resolution Control of Two-dimensional Quantum Emitters

In the realm of modern physics, scientists are increasingly turning their attention to materials that are only a few atoms thick. These two-dimensional sheets, often made of layers of different crystals stacked like a sandwich, can host strange and exotic states of matter. Within these thin layers, electrons and the "holes" they leave behind can bind together to form particles called excitons. When these excitons get trapped in tiny, invisible pockets of energy, they act as quantum emitters, releasing single particles of light. Because these trapped particles are so small and sensitive, they can serve as microscopic sensors, detecting the electric charges and magnetic spins of their neighbors. However, a major hurdle has always been their size. These emitters are smaller than the wavelength of visible light, meaning that even the most powerful standard microscopes cannot see them as distinct points; they blur together into a single, indistinct glow. This limitation has made it nearly impossible to study how individual emitters interact with one another or to control their specific behavior, leaving a gap in our ability to map the complex quantum landscapes hidden within these materials.

To bridge this gap, a team of researchers at the University of Geneva and collaborators in Japan has developed a new experimental approach that combines the precision of a scanning probe with the sensitivity of optical spectroscopy. They created a specialized microscope setup that uses a sharp, metallic tip, similar to the one found on an atomic force microscope, to act as a local, movable gate. By bringing this tip very close to a sample made of stacked tungsten diselenide and molybdenum diselenide, the researchers could generate a highly localized electric field that changes depending on exactly where the tip is positioned. This setup allowed them to manipulate the energy of the trapped excitons with nanometer-scale precision, far beyond what is possible with standard lenses. The researchers demonstrated that by moving this tip, they could not only pinpoint the exact location of individual emitters separated by small distances, but they could also selectively change the electrical charge of a single emitter without affecting its neighbors.

The core of their discovery lies in how these trapped excitons respond to the electric field generated by the tip. As the tip approaches a specific emitter, the electric field pulls on the particle, causing its light emission to shift in color. The researchers found that by carefully measuring this shift, they could map the position of the emitter with a resolution of about 20 nanometers, effectively seeing details that are normally hidden by the diffraction limit of light. They observed that some of these emitters possess a specific orientation, with their internal charge separation pointing in a particular direction within the plane of the material. This allowed the team to distinguish between emitters that were very close together, resolving pairs of dots separated by small distances, a feat that would be impossible with conventional optical methods.

Beyond simply locating these particles, the team showed they could control their electrical state with surgical precision. In a standard setup, changing the voltage across the entire sample would alter the charge of every emitter at once. However, by using the local tip, the researchers could tune the voltage for just one specific dot. They demonstrated this by shifting the point at which a single emitter would capture an extra electron to become a negatively charged particle, known as a trion, while leaving nearby emitters in their neutral state. This selective control is a critical step toward creating networks of quantum dots that can communicate with one another, a requirement for building future quantum technologies.

The study also revealed a surprising interaction between the metallic tip and the way these emitters release light. As the tip moved closer to an emitter, the researchers noticed that the brightness of the light dropped and the time it took for the light to be emitted increased. This happened because the metal tip altered the local environment, effectively making it harder for the emitter to release its energy as light. This effect was far more significant than what would be expected from the electric field alone, suggesting that the physical presence of the tip was reshaping the electromagnetic vacuum around the particle. This finding opens a new avenue for controlling how quantum emitters behave, not just by changing their energy, but by modifying the very space they occupy.

The researchers achieved these results using a sample where layers of the two different crystals were stacked with a specific alignment, creating a natural trap for the excitons. They cooled the sample to extremely low temperatures to ensure the particles remained stable and their light emission was sharp. By scanning the tip across the surface while measuring the light, they built a detailed map of the quantum landscape, identifying over 65 distinct emitters in a single view. They confirmed that the shifts they observed were not random noise but were directly linked to the position and voltage of the tip. The ability to isolate and manipulate individual quantum dots in this manner suggests a powerful new tool for studying complex quantum phases, such as those involving fractional charges or exotic particles, which are difficult to probe with traditional electrical measurements.

This work represents a significant advance in the ability to observe and control quantum matter at the nanoscale. By combining optical spectroscopy with a scanning probe, the team has created a platform that offers both high spatial resolution and the ability to tune the properties of individual quantum emitters. The findings suggest that these localized excitons can serve as sensitive probes for the surrounding electronic environment, potentially allowing scientists to detect and study the dynamics of exotic quantum states that have so far remained out of reach. The demonstration of super-resolution localization and deterministic charge control provides a clear path forward for investigating the fundamental physics of two-dimensional materials and developing new quantum devices.