Integrated on-chip quantum light sources on a van der Waals platform

The dream of building a quantum computer that can solve problems beyond the reach of today's machines often hinges on a single, stubborn challenge: how to get tiny particles of light, called photons, to talk to each other on a single, tiny chip. In the world of quantum information, these photons act as messengers, carrying delicate data across circuits. To make this work on a large scale, scientists need a platform where the light sources that create the photons, the channels that guide them, and the detectors that catch them are all made from compatible materials and built together in one place. For years, researchers have tried to assemble these pieces using different materials, like silicon for the channels and special semiconductors for the light sources, but forcing these different materials to work together is like trying to glue glass to rubber; the process is fragile, complex, and difficult to scale up. A more promising path lies in using a family of materials known as van der Waals materials, which are made of atomically thin sheets that can be stacked like Lego blocks without the need for harsh chemical bonding, offering a cleaner way to build these integrated circuits.

In a new study, researchers at the Technical University of Denmark have taken a significant step toward this goal by successfully building a working quantum light source entirely out of these thin, stackable materials. They created a device where a specific type of atomically thin crystal, made of tungsten diselenide, acts as a factory for single photons. This crystal was placed directly on top of a waveguide, which is a microscopic channel made of a different crystal called tungsten disulfide, designed to catch the light and guide it along a path. To get the light out of this tiny channel and into a measuring instrument, they etched a series of tiny, concentric rings into the channel, acting like a grating that scatters the light upward. The result is a fully integrated system where the light is generated, guided, and extracted all on a single chip made from these two-dimensional materials.

The researchers focused on a specific trick to make the light source brighter and cleaner. Instead of hitting the crystal with a broad spectrum of light, they used a very precise laser tuned to a specific energy level just below the point where the material would normally absorb light. This method, known as discrete-state excitation, allowed them to trigger the emission of single photons while avoiding the noisy background glow that usually comes with other methods. When they tested the light coming out of the device, they found it was incredibly pure. In the world of quantum physics, purity is measured by how often a detector sees two photons arriving at the exact same time; for a true single-photon source, this should almost never happen. The team measured this value to be extremely close to zero, indicating that the device was successfully emitting one photon at a time with very high reliability.

To prove that the light was indeed traveling through the waveguide and not just leaking out randomly, the researchers set up a clever test. They collected the light from both ends of the microscopic channel simultaneously. By comparing the streams of photons arriving at each end, they could confirm that the light was being guided efficiently through the chip. This two-sided measurement showed that the photons were being split between the two ends of the channel, with a significant number of photons making it to the detectors. They calculated that the device was capable of sending out hundreds of thousands of single photons every second, a rate that, when adjusted for the losses in the system, suggests the source is generating millions of photons per second inside the waveguide. This level of brightness and efficiency is a major improvement over previous attempts using similar materials.

A key part of their success was the way they stacked the materials. They placed a thin layer of a material called hexagonal boron nitride between the light source and the waveguide. This layer acted as a protective spacer, preventing the delicate light-emitting crystal from cracking when it was placed on the waveguide, while still allowing the light to couple efficiently into the channel. Without this spacer, the crystals tended to fracture, ruining the device. The team also discovered that the light coming out was highly polarized, meaning the waves of light were vibrating in a specific direction, which is a useful feature for controlling how the light behaves in future circuits.

The study does not claim to have solved all the problems of quantum computing, but it does demonstrate that a complete, working quantum light source can be built using only these versatile, thin materials. The researchers showed that by carefully engineering the layers and the shape of the waveguide, they could overcome the usual difficulties of getting light to move from a source into a channel. They also ruled out the idea that the light was simply scattering off the surface; the measurements confirmed that the photons were traveling through the waveguide modes. While the current device is a prototype, the results suggest a clear path forward. By adding more components, such as detectors and filters made from the same family of materials, scientists could eventually build complex quantum circuits that are easier to manufacture and more reliable than current hybrid systems. This work establishes that the van der Waals platform is a viable and powerful foundation for the next generation of quantum technologies.