A world-first method to enable quantum optical circuits that use photons

Breakthrough in quantum photonics promises a new era in optical circuits.


Till 2025, the collective sum of the world’s data will grow from 33 zettabytes this year to a 175ZB by 2025. The security and privacy of such sensitive data remain a big concern.

Emerging quantum communication and the latest computation technologies offer a promising solution. However, it requires powerful quantum optical circuits that can securely process the massive amounts of information we generate every day.

To help enable this technology, scientists in USC’s Mork Family Department of Chemical Engineering and Materials Science have made a breakthrough in quantum photonics.

A quantum optical circuit uses light sources to generate photons on-demand in real-time. The photons act as information-carrying bits (qubits).

These light sources are nano-sized semiconductor “quantum dots”–tiny manufactured collections of tens of thousands to a million atoms packed within a volume of linear size less than a thousandth of the thickness of typical human hair buried in a matrix of another suitable semiconductor.

They have so far been demonstrated to be the most flexible on-demand single-photon generators. The optical circuit requires these single-photon sources to be masterminded on a semiconductor chip. Photons with an almost identical wavelength from the sources should then be delivered a guided way. This permits them to be controlled to shape collaborations with different photons and particles to transmit and process information.

Until now, there has been a significant barrier to the development of such circuits. The dots have different sizes, and shapes mean that the photons they release do not have uniform wavelengths. This and the lack of positional order make them unsuitable for use in the development of optical circuits.

In this study, scientists showed that single photons could be emitted uniformly from quantum dots arranged precisely. Scientists used the method of aligning quantum dots to create single-quantum dot, with their remarkable single-photon emission characteristics.

It is expected that the ability to align uniformly-emitting quantum dots precisely will enable the production of optical circuits, potentially leading to novel advancements in quantum computing and communications technologies.

Jiefei Zhang, currently a research assistant professor in the Mork Family Department of Chemical Engineering and Materials Science, said, “The breakthrough paves the way to the next steps required to move from lab demonstration of single-photon physics to chip-scale fabrication of quantum photonic circuits. This has potential applications in quantum (secure) communication, imaging, sensing, and quantum simulations and computation.”

The corresponding author Anupam Madhukar said, “it is essential that quantum dots be ordered in a precise way so that photons released from any two or more dots can be manipulated to connect on the chip. This will form the basis of building unit for quantum optical circuits.”

“If the source where the photons come from is randomly located, this can’t be made to happen.”

“The current technology that allows 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. It is the same requirement for photon sources such as quantum dots to create quantum optical circuits.”

Evan Runnerstrom, program manager, Army Research Office, an element of the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory, said, “This advance is an important example of how fundamental solving materials science challenges, like how to create quantum dots with precise position and composition, can have big downstream implications for technologies like quantum computing. This shows how ARO’s targeted investments in basic research support the Army’s enduring modernization efforts in areas like networking.”

Using a method called SESRE (substrate-encoded size-reducing epitaxy), scientists created a precise layout of quantum dots for the circuits. They then fabricated regular arrays of nanometer-sized mesas with a defined edge orientation, shape, and depth on a flat semiconductor substrate composed of gallium arsenide (GaAs). Quantum dots are then created on top of the mesas by adding appropriate atoms using the following technique.

Zhang said“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 1.8nm, which is a factor of 20 to 40 better than typical quantum dots.”

“That with this uniformity, it becomes feasible to apply established methods such as local heating or electric fields to fine-tune the photon wavelengths of the quantum dots to exactly match each other, which is necessary for creating the required interconnections between different quantum dots for circuits.”

“We now have an approach and a material platform to provide scalably and ordered sources generating potentially indistinguishable single-photons for quantum information applications. 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 optical communication or the mid-infrared regime, suited for environmental monitoring and medical diagnostics.”

Journal Reference:
  1. Jiefei Zhang, Qi Huang, Lucas Jordao, Swarnabha Chattaraj, Siyuan Lu, Anupam Madhukar. Planarized spatially-regular arrays of spectrally uniform single quantum dots as on-chip single-photon sources for quantum optical circuits. APL Photonics, 2020; 5 (11): 116106 DOI: 10.1063/5.0018422
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