New tool to enhance quantum computing circuits

Researchers from the U.S. Department of Energy’s Ames National Laboratory, in partnership with the Superconducting Quantum Materials and Systems Centre (SQMS), have used a new tool to help improve a key component in commercially produced quantum computing circuits. The researchers used a terahertz SNOM microscope to investigate the interface and connectivity of a nano Josephson Junction (JJ). The JJ, a component in superconducting quantum computers, was fabricated by Rigetti Computing, an SQMS partner. The JJ generates a two-level system at very low cryogenic temperature that produces a quantum bit. The images obtained with the terahertz microscope revealed a defective boundary in the nano junction that caused a disruption in the conductivity and created a challenge to produce long coherence times needed for the quantum computation.

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

Better qubits in a quantum computer require understanding the function of a nano Josephson Junction (JJ), the component the team examined. Jigang Wang, a scientist from Ames Lab, said this JJ facilitates the supercurrent flow through the circuit at cryogenic temperature, which makes it possible for qubits to exist in their quantum state. It is important that this flow remain uniform and non-dissipative to keep the system coherent. Wang said the complex structural components in the quantum circuits often lead to local electrical field concentration, which causes scattering and energy dissipation and decoherence. “So the question for the current quantum computing business is how to mitigate the decoherence,” Wang said.

The researchers used a terahertz scanning near-field optical microscope (SNOM) to take images of the JJ under electromagnetic field coupling. This microscope uses a special tip that enhances the microscope’s resolution to the nanoscale, with nearly no touching or affecting the junction component. Using this microscope, the team recorded images of the JJ. If the junction component is fabricated properly, the resulting images would show a consistent electrical field across the component. However, the researchers found a disconnection between two parts of the junction.

This finding identified an issue with the JJ fabrication, which Rigetti can now resolve, thus improving their quantum circuit quality. It also proves that the terahertz microscope developed at Ames Lab is a useful tool for high throughput screening of quantum circuit components. “This research demonstrates that this terahertz SNOM is an ideal tool that we can use to visualise the heterogeneous electrical field distribution, and this enables a non-destructive and contactless identification of the effective boundaries in this nano junction. It’s extremely precise at the nanometre scale,” Wang said.

Quantum circuits normally operate at these extremely low, cryogenic temperatures. The researchers aim to push this extreme cryogenic terahertz SNOM machine to be able to reach that ultra-low temperature to follow the supercurrent tunnelling in real time and in real space of a functioning qubit. The research findings have been published in Communications Physics.

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