Toward triggered generation of indistinguishable single-photons from MoTe$_2$ quantum emitters

The future of ultra-secure communication and powerful computing relies on a very specific kind of light: single photons. These are individual particles of light that can carry information without the noise and interference that plagues traditional signals. For these systems to work over long distances, the photons must be indistinguishable, meaning they are identical in every way, like two coins minted from the exact same mold at the exact same moment. If they differ even slightly, the delicate quantum effects needed for these technologies break down. While scientists have found ways to create these single photons, doing so at the wavelengths used by global telecommunications networks has proven difficult. Most existing sources operate at visible light frequencies, which are absorbed quickly by fiber optic cables, making them unsuitable for long-haul travel. Researchers have been searching for a material that can generate these perfect, identical photons at the near-infrared wavelengths used by the internet, but finding a source that is both reliable and high-quality has remained a significant hurdle.

A team of researchers at the Technical University of Denmark has now demonstrated a new way to create these elusive light particles using a material called molybdenum ditelluride. This material is a two-dimensional crystal, meaning it is only a few atoms thick, and it naturally emits light in the near-infrared range, which is much closer to the wavelengths used in telecommunications than other materials. The challenge has been that while this material can emit light, the photons it produces have typically been messy, with varying colors and timing that make them impossible to use for advanced quantum tasks. The researchers set out to fix this by creating a controlled environment where they could force the material to emit perfect, identical photons on demand.

To achieve this, the team built a specialized stage for their crystals. They started with a mirror-like surface designed to bounce light back and forth, maximizing the amount of light that could be collected. On top of this, they placed tiny, star-shaped pillars made of a hard resin. When they transferred a thin flake of the molybdenum ditelluride onto these pillars, the material naturally bent and wrinkled around the sharp points. These wrinkles created areas of intense physical stress within the crystal. The researchers then used a focused beam of electrons to create tiny, specific defects in these stressed areas. This combination of physical stress and atomic-scale damage acted as a trap, confining the energy within the crystal to a tiny spot where it could release a single photon.

The results were a significant improvement over previous attempts with this material. The team found that these engineered emitters produced light in a very specific range of wavelengths, between 1090 and 1200 nanometers, which is in the near-infrared part of the spectrum. The light they emitted was highly polarized, meaning the waves of light vibrated in a single, consistent direction, a sign of a well-controlled source. Most importantly, the photons were emitted very quickly, with a lifetime of just 130 to 450 picoseconds, which is a trillionth of a second. This speed is crucial because it reduces the time the photon has to interact with its environment and become "messy."

To prove that these were truly single photons and not just dim light, the researchers measured how the photons behaved when they arrived at a detector. They found that the photons arrived one by one, with a very low chance of two arriving at the same time. By carefully tuning the way they excited the material with a laser, they could reduce the background noise to almost nothing, resulting in a purity level where the chance of a second photon appearing was less than one percent. They also discovered that by applying a small electrical voltage to the device, they could shift the color of the emitted light by a small but useful amount. This tuning capability is essential for matching the colors of different emitters so they can work together in a larger network.

The most critical test for these sources was to see if the photons were indistinguishable. The researchers set up an experiment where two photons from the source were sent to meet at a beam splitter. If the photons are identical, they will interfere with each other in a specific way, causing them to exit together rather than separately. The team observed this interference, confirming that the photons were indeed indistinguishable. While the effect was not perfect, it was the strongest ever measured for this type of material. By filtering the data to look only at the cleanest moments, they achieved a level of indistinguishability that had never been reached before with molybdenum ditelluride.

This work marks a turning point for the use of two-dimensional crystals in quantum technology. It shows that by combining physical strain, precise defect creation, and electrical control, scientists can turn a difficult material into a reliable source of single photons. The ability to tune the emission and achieve high purity suggests that these sources could eventually be integrated into the existing fiber optic networks that power the internet. While the current devices do not yet reach the exact wavelengths used for long-distance telecom, the path forward is clear. By adjusting the thickness of the material and refining the fabrication process, these emitters could soon become a standard component in the next generation of quantum communication systems, bringing the promise of unhackable networks and powerful quantum computers closer to reality.