New tool helps improve key quantum computing circuit

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

Scientists used SNOM’s terahertz microscope to detect defects in… Quantitative statistics circles, specifically at the nano-Josephson junction. Addressing these shortcomings is crucial to improving the faster processing capabilities of quantum computing.

The researchers used a new tool to help improve a key component of commercially produced quantum computing circuits. The team of scientists from the U.S. Department of Energy’s (DOE) Ames National Laboratory in partnership with the Center for Superconducting Quantum Materials and Systems (SQMS), the Department of Energy’s National Quantum Information Science Research Center led by Fermilab, used the SNOM terahertz microscope, which was originally developed at Ames. Lab, to study the interface and connectivity of the nano Josephson Junction (JJ).

The JJ, a key component in superconducting quantum computers, is manufactured by SQMS partner Rigetti Computing. JJ effectively generates a two-level system at an extremely low temperature that results in a qubit. The images they obtained using terahertz microscopy revealed defective boundaries in the nanojunction that perturbed conductivity and presented a challenge to produce the long coherence times needed for quantum computation.

Understanding qubits

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

Terahertz SNOM image showing electric field focus and asymmetry

The SNOM terahertz image above shows electric field concentration (brighter color) and asymmetry (bright color vs darker color on both sides), indicating a connectivity problem. The electron microscopy image below confirms the separation at the junction (the spatial gap). Credit: US Department of Energy, Ames National Laboratory

The best qubits in a quantum computer lie in understanding the function of the nano Josephson Junction (JJ), which is the component the team investigated. This JJ facilitates the flow of supercurrent through the circuit at the cooling temperature, which makes it possible for qubits to exist in their quantum state, explained Jigang Wang, a scientist from the Ames lab and lead of the research team. It is important that this flow remains uniform and not dissipated to maintain the coherence of the system.

Challenges and achievements

“The complex structural components in quantum circuits often lead to local electric field concentration, which causes scattering, energy dissipation and eventually decoherence,” Wang explained. “So the question in the current field of quantum computing is how to mitigate decoherence.”

Wang and his team used a near-field optical microscope (SNOM) previously developed at the Ames lab to take images of JJ under electromagnetic field coupling. This microscope uses a special tip that enhances the accuracy of the microscope nanoscalevirtually without touching or in any way affecting the link component. Using this microscope, the team recorded images of JJ. If the junction component has been fabricated correctly, the resulting images will show a constant electric field across the component. However, what the team found was a discontinuity between two parts of the intersection (see image above).

Wang explained that this finding is significant for two reasons. First, it identified a problem in JJ’s fabrication, which Rigetti can now solve and thus improve the quality of its quantum circuits. Second, the terahertz microscope developed in the Ames lab proves to be a useful tool for high-throughput examination of quantum circuit components.

“This research shows that SNOM terahertz is an ideal tool that we can use to visualize inhomogeneous electric field distribution,” said Wang. “This enables non-destructive, non-contact identification of the effective boundaries at this nanoscale junction. It is extremely accurate on the nanometer scale.”

Microscopy capabilities and future goals

Quantum circuits typically operate at extremely low temperatures. Wang’s team had previously demonstrated that SNOM’s terahertz microscope can operate at extremely low temperatures, “so the ultimate goal of this research is to continue to drive the ultra-cooled SNOM terahertz machine to be able to reach the extremely low temperature to be able to proceed.” “Ultracurrent tunneling in real time and in real space to efficient qubits,” he said.

Wang emphasized that progress on 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 to move things forward. It took a village to solve this kind of very complex technological and scientific problem. “It was really important that we had this versatile team,” Wang said. “I am also very pleased that as part of the Ames Lab we are contributing to the SQMS Center and the National Quantum Initiative in an important way.”

Reference: “Visualization of Heterogeneous Dipole Fields by Coupling of Terahertz Light in Single Nanoparticles” by Richard HJ Kim, Jong M Park, Samuel Heuser, Chuankun Huang, De Cheng, Thomas Cushney, Jinsu Oh, Cameron Kopas, Hilal Kansizoglu, Kameshwar Yadavali, Josh Motus, Lin Zhu, Liang Lu, Matthew J. Kramer and Jigang Wang, June 22, 2023, Available here. Communication Physics.
doi: 10.1038/s42005-023-01259-0

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