A breakthrough in quantum dot single-photon emission suggests that the era of optical circuits is on the horizon.

"Chips" are crucial components that support computers, phones, the internet, and other applications. By 2025, humanity is expected to create 175 zettabytes (175 trillion gigabytes) of new data. How can we ensure the security of such vast amounts of sensitive data? And how can we use this data to address issues ranging from privacy and security to climate change, especially given the limitations of current computing power?

Emerging quantum communication and computing technologies offer a promising alternative.

To achieve this, extensive development of powerful new quantum optical circuits will be required. These circuits can securely process the massive amounts of information we generate daily. Researchers from the Mork Family Department of Chemical Engineering and Materials Science at the University of Southern California (USC) have made a breakthrough that contributes to this technology's realization.

Traditional circuits consist of paths through which electrons in charge flow, while quantum optical circuits use light particles or photons generated on-demand to serve as information-carrying bits (qubits or quantum bits). These light sources are nanoscale semiconductor "quantum dots"—tiny manufactured assemblies of tens of thousands to millions of atoms, stacked within a volume of linear size smaller than a thousandth of the thickness of a typical human hair, embedded in another suitable semiconductor matrix.

To date, quantum dots have proven to be the most versatile on-demand single-photon generators. Optical circuits require arranging these single-photon sources in a regular pattern on a semiconductor chip. Photons from the source must then be released with nearly identical wavelengths in a guided direction. This allows them to be manipulated to interact with other photons and particles, transmitting and processing information.

So far, developing these circuits has faced significant hurdles. For example, with current manufacturing technology, quantum dots vary in size and shape, assembled in random locations on the chip. The variability in their size and shape means the emitted photons do not have uniform wavelengths. This inconsistency and lack of positional order make them unsuitable for optical circuit development.

In recent work, USC researchers demonstrated that it's possible to emit single photons uniformly from quantum dots arranged in a precise pattern. Notably, the method to align quantum dots was first developed nearly 30 years ago by Professor Anupam Madhukar and his team at USC, well before the recent surge in quantum information research and interest in single-photon emitters. In this latest work, the USC team used this approach to create single quantum dots with excellent single-photon emission characteristics. The ability to precisely align uniformly emitting quantum dots is expected to enable the production of optical circuits, potentially leading to new advances in quantum computing and communication technology.

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

Zhang said, "This breakthrough represents the next step in moving from lab demonstrations of single-photon physics to chip-level manufacturing of quantum photonic circuits." "This has potential applications in quantum (secure) communication, imaging, sensing, and quantum simulation and computation."

Madhukar explained that quantum dots must be sorted in a precise way to allow photons released from any two or more dots to interact on the chip. This forms the building blocks for quantum photonic circuits.

"If the source of the photons is randomly placed, it won't work," Madhukar noted. "The technology allowing us to communicate online, such as using platforms like Zoom, is based on silicon-integrated electronic circuits. If transistors on the chip aren't placed at precisely designed locations, integrated circuits wouldn't work," Madhukar said. The requirement for quantum photonic circuits involving sources of light particles like quantum dots is similar.

The research received support from the Air Force Research Office (AFOSR) and the US Army Research Office (ARO).

Evan Runnerstrom, program manager at the Army Research Office, said, "This breakthrough is an example of how solving fundamental materials science challenges, such as creating quantum dots with precise locations and compositions, can have significant downstream impacts on technologies like quantum computing." Runnerstrom's office is part of the US Army Combat Capabilities Development Command Army Research Laboratory. "It illustrates how ARO's targeted investment in fundamental research supports enduring modernization efforts across domains like networks."

To create the precise layout of quantum dots for circuits, the team used a method called SESRE (substrate-encoded size-reduction epitaxy), developed in the early 1990s by the Madhukar group. In their current work, the team fabricated arrays of regularly sized nanoscale mesas on a flat semiconductor substrate composed of gallium arsenide (GaAs), with defined edge orientation, shape (sidewalls), and depth. By adding the appropriate atoms, they created quantum dots at the top of these mesas.

Initially, incoming gallium (Ga) atoms gathered and deposited GaAs on the nanoscale mesa top under surface energy forces. Then, the incoming flux switched to indium (In) atoms, successively depositing indium arsenide (InAs), followed by Ga atoms to form GaAs, creating the final single quantum dot that emits individual photons. For creating optical circuits, the space between the pyramid-shaped mesas must be filled with material to flatten the surface. The final chip with opaque GaAs has a transparent covering layer under which the quantum dots are located.

"This work also sets a new world record for ordered and scalable quantum dots, with a simultaneous single-photon purity of greater than 99.5%, and the wavelength uniformity of emitted photons can be as narrow as 1.8 nm, which is 20 to 40 times better than typical quantum dots," Zhang noted.

Zhang explained that with this level of uniformity, it's feasible to use established methods like local heating or electric fields to fine-tune the photon wavelength of quantum dots to precisely match each other. This is necessary for creating the required interconnections between different quantum dots for circuits.

This means researchers can now use mature semiconductor processing techniques to create scalable quantum photonic chips. The team is currently focusing on understanding how similar photons from the same or different quantum dots are. The degree of indistinguishability is central to quantum effects like interference and entanglement, the foundation for quantum information processing (communication, sensing, imaging, or computing).

Zhang concluded, "We now have a method and a material platform that provides scalable and ordered sources for generating potentially indistinguishable single photons for quantum information applications. This method is universal and can be applied to other suitable material combinations to create quantum dots emitting at various wavelengths." This wide range of wavelengths can suit different applications, such as fiber-based optical communications or mid-infrared schemes for environmental monitoring and medical diagnosis."

Gernot S. Pomrenke, AFOSR's photonics and optics program officer, stated that reliable arrays of on-demand single-photon sources on a chip represent an important step forward.

Pomrenke noted, "Before quantum information research became mainstream, this impressive growth and materials science work required over 30 years of dedication." The initial funding and resources provided by AFOSR and other US Department of Defense agencies were crucial in supporting Madhukar, his students, and collaborators in their challenging work and ideas. This work could potentially revolutionize data centers, medical diagnostics, defense, and other sectors."