April 13, 2024

A new quantum light source

Physical 17, 51

A new device consisting of a semiconductor ring produces entangled pairs of photons that could be used in a photonic quantum processor.

Q. Zhou/University of Electronic Science and Technology of China

Dragon optics. Zhou and his colleagues created this fantastic image in honor of the Chinese Year of the Dragon (2024). The dragons symbolize the gallium nitride ring. One dragon absorbs photons from a laser (left), while the second dragon emits entangled photons (right).

Quantum light sources produce entangled pairs of photons that can be used in quantum computing and cryptography. A new experiment has demonstrated a quantum light source made from the semiconductor gallium nitride. This material provides a versatile platform for device fabrication, having previously been used for lasers, detectors, and on-chip waveguides. Combined with these other optical components, the new quantum light source opens up the potential to build a complex quantum circuit, like a photonic quantum processor, on a single chip.

Quantum optics is a rapidly advancing field, with many experiments using photons to carry quantum information and perform quantum calculations. However, for optical systems to compete with other quantum information technologies, quantum optical devices will have to be reduced from the size of a table to the size of a microchip. An important step in this transformation is the development of quantum light generation on a semiconductor chip. Several research teams have achieved this feat using materials such as aluminum gallium arsenide, indium phosphide, and silicon carbide. And yet, a fully integrated photonic circuit will require a host of components beyond quantum light sources.

With the goal of eventually building such a complete circuit, Qiang Zhou of the University of Electronic Science and Technology of China and his colleagues have set their sights on gallium nitride. This material is well known for its use in the first blue LEDs – a development that was recognized with the 2014 Nobel Prize in Physics (see Editors’ Notes: Blue was the hardest color). Recent work has shown that sapphire-grown gallium nitride can be used for a range of quantum optical functions, such as lasers, optical filtering, and single-photon detection. “The gallium nitride platform offers promising prospects for advancing photonic quantum chips in the near future,” says Zhou.

Miniphoton source. A laser (left) sends light through a waveguide coupled to a gallium nitride microring resonator (blue circle in green square). The light exiting from the right is analyzed by a series of devices, revealing an interference pattern for pairs of photons with wavelengths corresponding to the ring’s resonances.

To create a gallium nitride quantum light source, Zhou and colleagues grew a film of the material on a sapphire substrate and then etched a 120-µm-diameter ring into the film. In this structure, photons can travel around the ring, similar to the way sound waves travel along the curved walls of a whispering gallery (see Viewpoint: Small resonators generate a large optical spectrum). Next to the ring, the researchers etched a waveguide to transmit infrared laser light. A coupling between the two optical elements allows some laser photons to pass from the waveguide to the ring.

In the experiments, a detector recorded the spectrum of light output from the waveguide, revealing discrete dips at various wavelengths. These dips corresponded to resonances in the ring – when the wavelength of a specific photon adjusts an integer number of times within the circumference of the ring. Resonant photons in the waveguide can enter the ring and become trapped inside it.

However, thanks to an effect called four-wave mixing (see Synopsis: Photonic Matchmaking), pairs of resonant photons entering the ring can sometimes annihilate each other, causing a new pair of resonant photons (at different wavelengths) to form. be created and exit through the waveguide. . The two photons in each existing pair are expected to be entangled with each other. To verify this entanglement, the team carried out measurements on pairs of coinciding photons, showing that they generate an interference pattern – bands of light and dark fringes – when photographed. (On the other hand, unentangled pairs would produce a broad, bright spot.)

The interference level – characterized by the amount of contrast between the light and dark fringes – is a measure of the degree of entanglement of the photons. The degree of entanglement produced by the gallium nitride ring was comparable to the level measured for other quantum light sources, says Zhou. “We demonstrated that gallium nitride is a good quantum material platform for photonic quantum information, in which quantum light generation is crucial,” she says.

“Quantum optics has evolved at a tremendous rate in recent years,” says quantum optics expert Thomas Walther of the Technical University of Darmstadt in Germany. He says moving forward will require components that are small, robust, efficient and relatively easy to manufacture. To this end, Zhou and his colleagues demonstrated that gallium nitride is a promising material for making the pump source, quantum light source, and single-photon detectors. Having a platform for all these devices “would constitute a huge advance, as it could reduce the cost of manufacturing these systems, as well as making them much more compact and robust than they are today,” he says.

–Michael Schirber

Michael Schirber is corresponding editor at Physics Magazine based in Lyon, France.


  1. H. Zeng and others.“GaN microring-based quantum light generation toward an all-on-chip source,” Physical. Rev. 132133603 (2024).

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