This world-first method for enabling quantum optical circuits using photons–light particles — is critical to a secure future in quantum communication and quantum computing.
Modern technology is powered by the electrical circuitry of a chip, which is the semiconductor chip that underpins computers, cell phones, and other applications. Humans will create 175 zettabytes (175 trillion megabytes) of new information by 2025. How do we protect sensitive data in such high volumes? Given our limited computing power, how can we tackle grand-challenge-like issues, such as privacy, security, and climate change, and leverage this data?
Emerging quantum communication and computation technology is a promising alternative. However, this will require the widespread development and deployment of quantum optical circuits capable of processing the vast amounts of information generated daily. The University of Southern California was founded in 1880 and is one of the world’s most prestigious private research universities.
A traditional electrical circuit is a path along which electrons of an electric charge flow. However, a quantum optical system uses light sources to generate individual light particles or photons on demand. These light particles act as information-carrying bits (quantum bits, qubits). These “quantum dots,” tiny semiconductors, are small-sized collections of tens to a million atoms. They are buried in a matrix made of another suitable semiconductor.
They are the most versatile single-photon generators available on demand. These single photon sources must consistently be placed on a semiconductor chip to make an optical circuit. The photons must be released from sources with almost identical wavelengths in a controlled direction. This allows them to interact with other particles and photons to transmit and process information.
This has been a significant barrier to developing such circuits. Quantum dots, currently manufactured in different sizes and shapes, are assembled on the chip randomly. Because the beads are other in size and shape, the emitted photons do not have uniform wavelengths. Because of this and their lack of positional order, they need to be more suitable for developing optical circuits.
Researchers at USC recently published work that showed single photons can be produced from quantum dots in a specific pattern. The method of aligning quantum dots was developed by Professor Anupam Madhukar and his team at USC almost thirty years ago. This is well before the current explosion in quantum information research and interest in single-photon sources on-chip. The USC team used these methods to create single-quantum dot structures with remarkable single-photon emission properties. The ability to align uniformly emitting quantum dots precisely will allow for producing optical circuits. This could lead to new advancements in quantum computing.
Jiefei Zhu, currently a research assistant professor at the Mork Family Department for Chemical Engineering and Materials Science with Anupam Madhukar (Kennett T. Norris Professor of Engineering and Professor of Chemical Engineering and Electrical Engineering, Materials Science and Physics), led the work.
Zhang stated that the breakthrough opens the door to the following steps: lab demonstrations of single-photon physics and chip-scale fabrications of quantum photonic circuits. This has potential quantum applications (secure) communication and imaging, sensing, and quantum simulations, computation.
Madhukar stated that it was essential for quantum dots to be ordered in a specific way so that photons from any two or three drops could be controlled to connect on the chip. This will be the foundation of quantum optical circuits.
Madhukar stated, “If the source from which the photons originate is randomized, this cannot occur.”
The silicon-integrated electronic chip is the basis of current technology, which allows us to communicate online. Madhukar stated that if the transistors on this chip were not placed in precisely designed locations, there wouldn’t be an integrated electrical circuit. It is the requirement that photon sources, such as quantum dots, be placed in precise locations to create quantum optical systems.
“This is an important example of how solving fundamental material science challenges, such as how to create quantum dots in precise position and composition, can have huge downstream implications for technologies, like quantum computing,” said Evan Runnerstrom (program manager, Army Research Office), an element of U.S Army Combat Capabilities Development Command’s Army Research Laboratory. This is a clear example of how ARO’s targeted investments into basic research support the Army’s enduring modernization efforts, such as networking.
The team used a technique called SESRE (substrate-encoded size-reducing epitaxy), which the Madhukar group developed in the early 1990s to create the exact layout of quantum dots in circuits. The current work involved the fabrication of regular arrays containing mesas measuring in the nanometers (Fig. 1(a), which has a defined edge orientation (sidewalls), shape, and depth on a flat semiconductor substrate made of gallium arsenide (GaAs). The following method is used to create quantum dots on top of these mesas:
First, the incoming gallium (Ga atoms) gather at the top of nanoscale mesas. (black arrows in Figure 1. (b)) Attracted by surface energy forces, they deposit GaAs. (black outline on the mesa top, Fig. 1(b)). The incoming flux is then switched to In (indium) atoms to deposit Indium arsenide. (red region of Fig. 1(b), then Ga atoms are used to create GaAs, and thus the desired quantum dots (upper image from Fig. 1(b), which end up releasing single photos. To make optical circuits, the space between the pyramid-shaped nano-mesas must be filled with material that flattens its surface. Fig. 1(c) shows the final chip schematically. This is where opaque GaAs act as a sheer overlayer, where the quantum dots can be found.
Zhang said this work sets a new world record for ordered and scalable quantum dots. Zhang also spoke of the unprecedented purity of single-photon emissions greater than 99.5% and the uniformity in the wavelengths of the emitted photons. This is 20 to 40 times better than standard quantum dots.
Zhang stated that this uniformity makes it possible to use established methods like local heating or electric field to adjust the photon wavelengths to match each other. This is essential for creating interconnections between different quantum dots for circuits.
Researchers can now create quantum photonic chips that scale using established semiconductor processing techniques. The team also focuses on how similar the photons emitted from the same and different quantum dots are. Quantum effects of interference or entanglement are fundamental to quantum information processing, such as communication, sensing, and imaging.
Zhang concluded, “We now have an idea and a platform to provide scalable, ordered sources that can generate single-photons potentially indistinguishable for quantum information applications.” Zhang stated that the approach could be applied to other material combinations to create quantum dots emitting at a wide range of wavelengths, which are preferred for different applications such as fiber-based optical communications or mid-infrared, which is suited for medical diagnostics and environmental monitoring.
Gernot S. Pomrenke (AFOSR Program Officer Optoelectronics and Photonics) stated that reliable arrays of on-chip single-photon sources were a significant step forward.
Pomrenke stated that “this impressive growth and material scientific work spans over three decades of dedicated efforts before research activities related to quantum information were mainstream.” Madhukar, his students, and their collaborators have significantly contributed to the success of their visionary work. This work will likely transform the capabilities of data centers, medical diagnostics, and defense technology.