SNOM Terahertz Microscope: New tool to improve critical quantum computing circuit

Visualize the microscope tip for material exposed to terahertz light. The colors on the material represent light scattering data, and the red and blue lines represent terahertz waves. Image supplier: US Department of Energy Ames . National Laboratory

Scientists used SNOM terahertz microscopes to detect flaws in Quantum Computation circuits, especially in Nano Josephson Junction. Addressing these defects is essential to optimizing the faster processing capabilities 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’s (DOE) Ames National Laboratory collaborated with the Center for Superconducting Quantum Materials and Systems (SQMS), the National Center for Quantum Information Science and Research. The DOE team, led by Fermilab, used the SNOM terahertz microscope, which was originally developed. at Ames Laboratories, to study the interface and connectivity of the Josephson Nano Junction (JJ).

JJ, the key component in superconducting quantum computers, was built by Rigetti Computing, a partner of SQMS. JJ effectively creates a two-stage system at very low cryogenic temperatures to generate quantum bits. The images they obtained using terahertz microscopy show that the defect boundary in the nanojunction causes the conductivity disruption and serves as a challenge in generating the long coherence times required for computation. quantum.

Understanding Qubit

Quantum computers consist of quantum bits or qubits. Qubits work similarly to bits in a digital computer. Bit is the smallest unit of data that a computer can process and store. Bits are binary, which means that only two possible states are 0 or 1. However, Qubits exist simultaneously in both 0 and 1 in the quantum state, which allows quantum computers to process process more information faster than computers are commonly used today.

The terahertz SNOM image shows electric field concentrations (lighter colors) and asymmetry (light and dark colors on either side), indicating a connection problem. The transmission electron microscope image below confirms the disconnection in the junction (space gap). Source: US Department of Energy’s Ames National Laboratory

The better qubit in quantum computers lies in understanding the function of Nano Josephson Junction (JJ), which the team tested. Jigang Wang, a scientist from Ames Laboratories and lead researcher, explains that this JJ facilitates the flow of supercurrents through the circuit at cryogenic temperatures, allowing qubits to remain in a state their quantum. It is important that this flow remains uniform and does not dissipate to keep the system coherent.

Challenges and breakthroughs

“Complex structural components in quantum circuits often lead to local electric field concentration, which causes scattering and dissipation of energy and ultimately decoherence,” explains Wang. “So the question for the quantum computing business today is how to minimize decoherence.”

Wang and his team used a terahertz scanning near-field optical microscope (SNOM) previously developed at Ames Laboratories to image JJs under electromagnetic field coupling. This microscope uses a special tip that enhances the resolution of the microscope nanoscale, which barely touches or affects the junction component in any way. Using this microscope, the team recorded images of JJ. If the splice component is fabricated properly, the resulting image will show a consistent electric field across the entire component. What the team found, however, was a disconnect between the two parts of the junction (see image above).

Wang explains that this finding is important for two reasons. First, it identified a problem in the JJ fabrication process that Rigetti can currently solve, thereby improving the quality of their quantum circuitry. Second, it demonstrates that the terahertz microscope developed at Ames Laboratories is a useful tool for high-throughput screening of quantum circuit components.

“This study demonstrates that this terahertz SNOM is an ideal tool that we can use to visualize non-uniform electric field distributions,” said Wang. “And this allows for the non-destructive and non-contact determination of effective boundaries in this nanojunction. It is extremely precise at the nanometer scale.”

Microscope capabilities and future goals

Quantum circuits typically operate at these extremely low, frozen temperatures. Wang’s group has previously demonstrated that SNOM terahertz microscopy can operate at extremely low temperatures, “So the ultimate goal of this study is to further advance this ultra-cold SNOM terahertz machine that can. get to that extremely low temperature to be able to track the supercurrent tunneling in real time and in real space of an active qubit,” he said.

Wang emphasized that progress on this project would not have been possible had Ames Lab not been a member of the SQMS community. “It has been a privilege to work with them and contribute as a community to move things forward. It takes an entire village to really solve this kind of very complex science and technology problem. And having this flexible team is really important,” said Wang. “I am also pleased that as part of the Ames Lab we are making an important contribution to the SQMS hub and the national quantum initiative.”

Reference: “Visualizing heterogeneous dipole fields by coupling terahertz light in individual nanojunctions” by Richard HJ Kim, Joo 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, June 22, 2023, Communication Physics.
DOI: 10.1038/s42005-023-01259-0