Terahertz SNOM Microscope: New Tool Helps Improve Quantum Computing Circuits

Viewing the tip of a microscope exposing a material to terahertz light. The colors in the material indicate light scattering data, and the red and blue lines represent terahertz waves. Credit: US Department of Energy Ames National Lab

The scientists used a terahertz SNOM microscope to spot the flaws quantum computing circuits, especially the nano Josephson Junction. Addressing these shortcomings is critical to increasing the speed of quantum computing.

Researchers have used a new tool to help improve a key component in commercially produced quantum computing circuits. A team of scientists from the US Department of Energy (DOE) Ames National Laboratory in collaboration with the Superconducting Quantum Materials and Systems Center (SQMS), a DOE National Science Research Center led by Fermilab, used the terahertz SNOM microscope, which was originally developed . at Ames Lab, to investigate interfaces and nano Josephson Junction (JJ) interactions.

JJ, the leading component in superconducting quantum computers, is made by Rigetti Computing, a partner of SQMS. JJ effectively produces a two-stage process at very low cryogenic temperatures that produces a quantum bit. The images they obtained with a terahertz microscope showed a flaw in the nano junction that causes interference in the conduction system and serves as a challenge to produce the long-term consistency required for quantum computing.

Understanding Qubits

Quantum computers consist of quantum bits or qubits. Qubits work similarly to the bits in a digital computer. Bits are the smallest unit of data that can be processed and stored by a computer. Bits are binary, meaning there are only two possible states they can be in, either 0 or 1. Qubits, however, exist as 0s and 1s simultaneously in their quantum state, which allows quantum computers to run even further. information faster than the computers used today.

The above terahertz SNOM image shows the intensity of the electric field (bright color) and asymmetry (bright color vs. dark color on both sides), indicating a contact issue. The transmission electron microscopy image below confirms the discontinuity at the junction (gap). Credit: US Department of Energy Ames National Laboratory

The best qubits for quantum computing lie in understanding the function of the nano Josephson Junction (JJ), a component explored by the team. Jigang Wang, a scientist at Ames Lab and the leader of the research team, explained that this JJ facilitates the flow of supercurrent in the circuit at cryogenic temperature, which makes it possible for the qubits to exist in their quantum state. It is important that this flow remains the same and does not disperse to keep the process consistent.

Challenges and Successes

“Parts of the complex structure in quantum circuits often lead to the concentration of local electricity, which causes the distribution and dissipation of energy and finally integration,” explains Wang. “So the question for today’s quantum computing industry is how to reduce inaccuracy.”

Wang and his team used a terahertz scanning near-field optical microscope (SNOM) previously developed at Ames Lab to take images of JJ under electromagnetic field coupling. This microscope uses a special tip that improves the resolution of the microscope nanoscale, there is almost no arrest or in any way affecting the cross section. Using this microscope, the team recorded images of JJ. If the junction section is made correctly, the output images will show the corresponding electric field across the section. However, what the team found was a break between the two parts of the junction (see image above).

Wang explained that this finding was important for two reasons. First, it identified an issue with JJ manufacturing, which Rigetti can now solve thereby improving the quality of their quantum circuit. Second, it proves that the terahertz microscope developed at Ames Lab is a useful tool for high-throughput inspection of quantum circuit components.

“This research shows that this terahertz SNOM is a suitable tool to use to observe the heterogeneous electric field distribution,” said Wang. “And this allows for non-destructive and non-contact identification of the active boundaries of this nano-composite. It is very precise on the nanometer scale.”

Microscope potential and future goals

Quantum circuits often operate at these very low, cryogenic temperatures. Wang’s team previously demonstrated that terahertz SNOM microscopes can operate at very low temperatures, “So the main goal of this research is to continue to push this cryogenic terahertz SNOM machine to be able to reach that very low temperature so that they can follow up.” more tunneling in real time and in the real space of an active qubit,” he said.

Wang emphasized that moving forward with this project would not have been possible if Ames Lab had not been a member of the SQMS community. “It has been a real honor to work with them and contribute as a community in moving things forward. It took a village to really solve this kind of difficult problem in technology and science. And it was very important to have a cross-functional team, Wang said. I am also very pleased that as part of Ames Lab we are contributing to the SQMS Institute and the national measurement initiative in a significant way.

Reference: “Detecting heterogeneous dipole fields with terahertz light in light junctions in nano-junctions” by Richard HJ Kim, Joong M. Park, Samuel Haeuser, Chuankun Huang, Di Cheng, Thomas Koschny, Jinsu Oh, Cameron Kopas, Hilal Cansizoglu , Kameshwar Yadavalli , Josh Mutus, Lin Zhou, Liang Luo, Matthew J. Kramer & Jigang Wang, 22 June 2023, Communication Physics.
DOI: 10.1038/s42005-023-01259-0