Photonic Waveguide Connects Quantum Dots

Researchers at the U.S. Naval Research Labora­tory NRL developed a new technique that could enable future advancements in quantum tech­nology. The technique squeezes quantum dots, tiny particles made of thousands of atoms, to emit single photons with precisely the same color and with positions that can be less than a millionth of a meter apart.

Schematic of a photonic crystal waveguide that contains quantum dots that can interact with one another when they are tuned to the same wavelength. (Source: C. S. Kim, NRL)

“This breakthrough could accelerate the development of quantum infor­mation technologies and brain-inspired computing,” said Allan Bracker, a chemist at NRL and one of the researchers on the project. In order for quantum dots to interact, they have to emit light at the same wave­length. The size of a quantum dot determines this emission wave­length. However, just as no two snowflakes are alike, no two quantum dots have exactly the same size and shape – at least when they’re initially created.

This natural varia­bility makes it impossible for researchers to create quantum dots that emit light at precisely the same wave­length, said Joel Grim, the lead researcher on the project. “Instead of making quantum dots perfectly identical to begin with, we change their wave­length afterwards by shrink-wrapping them with laser-crystal­lized hafnium oxide,” Grim said. “The shrink wrap squeezes the quantum dots, which shifts their wavelength in a very control­lable way.”

While other scientists have demons­trated tuning of quantum dot wave­lengths in the past, this is the first time researchers have achieved it precisely in both wave­length and position. “This means that we can do it not just for two or three, but for many quantum dots in an inte­grated circuit, which could be used for optical, rather than elec­trical computing,” Bracker said.

The wide breadth of researcher expertise and science assets at NRL allowed the team to test various approaches to making this quantum dot breakt­hrough in a relatively short amount of time. “NRL has in-house faci­lities for crystal growth, device fabrication, and quantum optical measure­ments,” Grim said. “This means that we could imme­diately coor­dinate our efforts to focus on rapidly improving the material properties.”

According to Grim and Bracker, this milestone in the mani­pulation of quantum dots could lay the groundwork for future strides in a number of areas. “NRL’s new method for tuning the wavelength of quantum dots could enable new tech­nologies that use the strange properties of quantum physics for computing, communication and sensing,” Bracker said. “It may also lead to neuro­morphic or brain-inspired computing based on a network of tiny lasers.” Applications in which space and power-effi­ciency are limiting factors may also benefit from this break­through approach, researchers said. (Source: NRL)

Reference: J. Q. Grim et al.: Scalable in operando strain tuning in nanophotonic waveguides enabling three-quantum-dot superradiance, Nat. Mat., online 8 July 2019; DOI: 10.1038/s41563-019-0418-0

Link: Institute for Nanoscience, US Naval Research Laboratory, Washington, DC, USA

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