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USC Team Makes Breakthrough In Quantum Photonics

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New method shows how single photons can be emitted in a uniform way from quantum dots arranged in a precise pattern

Researchers at the University of Southern California have made a breakthrough towards building quantum optical circuits from quantum dot (QD) light sources.

QDs seem to be the most versatile on-demand single photon generators. But an optical circuit requires single photon sources to be arranged in a regular pattern. Photons with nearly identical wavelength from the sources must then be released in a guided direction. This allows them to be manipulated to form interactions with other photons and particles to transmit and process information.

Until now, there has been a significant barrier to the development of such circuits. For example, in current manufacturing techniques quantum dots have different sizes and shapes and assemble on the chip in random locations. The fact that 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.

Researchers at USC have now shown that single photons can indeed be emitted in a uniform way from quantum dots arranged in a precise pattern. This method of aligning quantum dots was first developed at USC by the Anupam Madhukar and his team nearly thirty years ago.

In this recent work, the USC team has used such methods to create single-quantum dots.

To create the precise layout of quantum dots for the circuits, the researchers used a method called SESRE (substrate-encoded size-reducing epitaxy) developed in the Madhukar group in the early 1990s. In the current work, the team fabricated regular arrays of nanometer-sized mesas with a defined edge orientation, shape (sidewalls) and depth on a flat semiconductor substrate, composed of GaAs. Quantum dots are then created on top of the mesas by adding appropriate atoms using the following technique.

First, incoming gallium atoms gather on the top of the nanoscale mesas attracted by surface energy forces, where they deposit GaAs. Then, the incoming flux is switched to indium (In) atoms, to in turn deposit InAs followed back by Ga atoms to form GaAs and hence create the desired individual quantum dots that end up releasing single photons. To be useful for creating optical circuits, the space between the pyramid-shaped nano-mesas needs to be filled by material that flattens the surface. The final chip where opaque GaAs is depicted as a translucent overlayer under which the quantum dots are located.

The work, published in APL Photonics, was led by Jiefei Zhang.

"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," Zhang said. "This has potential applications in quantum (secure) communication, imaging, sensing and quantum simulations and computation."

Anupam Madhukar said that 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 with each other 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." Madhukar 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 percent, 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," Zhang said.

Zhang concluded: "We now have an approach and a material platform to provide scalable 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," Zhang said.

The research is supported by the Air Force Office of Scientific Research (AFOSR) and the US Army Research Office (ARO).

'Planarized spatially-regular arrays of spectrally uniform single quantum dots as on-chip single photon sources for quantum optical circuits' by Jiefei Zhang et al; APL Photonics 5, 116106 (2020)



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