NIST smooths path from lab to fab for quantum dots

27 Mar 2024

Improved positioning of QDs on photonic chips brings new quantum applications within reach.

A project at the US National Institute of Standards and Technology (NIST) has studied how the positioning of quantum dots on photonic structures can be improved.

This small detail can have big consequences for the operation and efficiency of systems using quantum dots as emitters, in present and future applications such as quantum information technologies and biological imaging.

The NIST team's new approach to standards and calibrations allows quantum dots to be aligned with the center of a photonic component to within an error of 10 to 20 nanometers, when positioned using an optical microscope. This helps to meet the stringent requirements for chip-scale devices storing and transmitting quantum information.

"Devices that capture the brilliant light from millions of quantum dots, including chip-scale lasers and optical amplifiers, have made the transition from laboratory experiments to commercial products," commented NIST.

"But newer types of quantum dot devices have been slower to come to market, because they require extraordinarily accurate alignment between individual dots and the miniature optics that extract and guide the emitted radiation. We introduce a methodology to solve this widespread but poorly understood problem."

This positioning difficulty is inherently related to both the microscopic instruments used to observe and control the localization of the quantum dots, and the standards against which the degree of success is measured.

NIST noted that optical microscopes used to examine a dot's position can be inaccurate due to magnitication errors and image distortion, with the use of cryogenic temperatures to cool sample and objective lenses in such operations making the problems worse. These optical imperfections duly cause the apparent position of a quantum dot in a microscope image to differ from the true location.

Helping quantum dot devices perform as predicted

The project's solution, published in Optica Quantum, was to manufacture new positioning standards by fabricating a pattern of sub-micron pillars on a silicon substrate. This can then act as a reliable measurement reference material, since the contraction and physical behavior of silicon at different temperatures is well understood, and more data is available relating to silicon than other possible photonic integration materials.

Once fabricated, the spacing between pillars can be monitored under the microscope during a manufacturing cycle to provide a basis of comparison for quantum dot positioning, with locations in the microscope coordinate system mapped onto the ultimate lithographic design.

In trials NIST found that the same principle could also be extended to room-temperature standards, using an array of nanoscale holes in a metal film.

The project team believes that positioning errors have to date prevented researchers from fabricating quantum dot devices that perform as predicted, and calculates that use of its new standards during production could improve matters a hundred-fold.

"A researcher might be happy if one out of a hundred devices works for their first experiment, but a manufacturer might need ninety nine out of a hundred devices to work," noted NIST's Samual Stavis. "Our work is a leap ahead in this lab-to-fab transition."