Scientists Follow the Dots to Quantum Optical Circuits

Researchers at the University of Southern California (USC) developed a method that emits uniform single photons from precisely arranged quantum dots. The advancement benefits from an existing method for intentionally arranging quantum dots that USC professor Anupam Madhukar, corresponding author on the latest study, and his team introduced nearly 30 years ago.

The researchers said the development is expected to enable the production of optical circuits and, as a result, advancements in quantum computing and communications technologies. Quantum optical circuits use light sources that generate individual photons, acting as qubits, on demand and one at a time.

The research was led by Jiefei Zhang, a research assistant professor in the Mork Family Department of Chemical Engineering and Materials Science, with corresponding author Madhukar, the Kenneth T. Norris Professor in Engineering and professor of chemical engineering, electrical engineering, materials science, and physics.

In some ways, photonic circuits function similarly or analogously to electronic circuits. Whereas electronic circuits guide electrons carrying data, photonic circuits use light sources generating individual photons carrying data.

The new technology, Zhang said, paves the way toward moving from lab demonstrations of single-photon physics to chip-scale fabrication of quantum photonic circuits.

“The current technology that is allowing us to communicate online, for instance using a technological platform such as Zoom, is based on the silicon integrated electronic chip. If the transistors on that chip are not placed in exact designed locations, there would be no integrated electrical circuit,” Madhukar said. “It is the same requirement for photon sources such as quantum dots to create quantum optical circuits.”

Similarly, the quantum dots must be of a uniform shape and size, something that current manufacturing techniques are unable to achieve. Without a uniform shape and size, the photons the dots release do not have uniform wavelengths.

To make a uniform arrangement of quantum dots, the team used Madhukar’s method developed in the early 1990s called SESRE (substrate-encoded size-reducing epitaxy). The team fabricated regular arrays of nanoscale mesas with a defined edge-orientation, shape, and depth on a flat semiconductor substrate composed of gallium arsenide (GaAs). Then, the quantum dots are created on top of the mesas by adding appropriate atoms.

In optical circuits, nano-size semiconductor quantum dots function as light sources.

First, in the new system, incoming gallium (Ga) atoms gather on top of the nanoscale mesas attracted by surface energy forces where they deposit GaAs. Then the incoming flux is switched to indium (In) atoms, which in turn deposit indium arsenide (InAs), followed back by Ga atoms to form GaAs, and then creating the desired individual quantum dots that release single photons.

To be useful in optical circuits, the space between the pyramid-shape nano-mesas must filled by material that flattens the surface.

In the final chip, opaque GaAs is depicted as a translucent overlayer under which the quantum dots are located.

“This work also sets a new world record of ordered and scalable quantum dots in terms of the simultaneous purity of single-photon emission greater than 99.5%, and in terms of the uniformity of the wavelength of the emitted photons, which can be as narrow as 1.8 nm, which is a factor of 20 to 40 better than typical quantum dots,” Zhang said.

According to Zhang, this uniformity makes it possible to use established methods, such as local heating or electric fields, to fine-tune the wavelengths of the photons emitted by the quantum dots in order to create exact matches. That process is necessary to develop the required interconnections between different quantum dots for circuits.

With the advancements from this research, it becomes possible to apply well-established semiconductor processing techniques to create scalable photonic chips. With that in mind, the researchers are now focused on determining exactly how identical the emitted photons are from the same, and different, quantum dots.

“We now have an approach and a material platform to provide scalable and ordered sources generating potentially indistinguishable single photons for quantum information applications,” Zhang said. “The approach is general and can be used for other suitable material combinations to create quantum dots emitting over a wide range of wavelengths preferred for different applications — for example fiber-based communication or the mid-infrared regime, suited for environmental monitoring and medical diagnostics.”

The research was published in APL Photonics (www.doi.org/10.1063/5.0018422).