SQUID maps magnetic fields of superconducting qubit circuits

Advanced quantum computing is based on superconducting qubit circuits. To improve the performance of these circuits, researchers need to analyze the magnetic fields they produce.

Marchiori et al. used a magnetic imaging technique called scanning superconducting quantum interference device (SQUID) microscopy on a superconducting qubit circuit consisting of three qubits and three flux-control lines.

Scanning SQUID microscopy produced nanoscale maps of the circuit’s magnetic field, showing the flow of current density within the device. These maps could be used to design superconducting qubit circuits with less qubit decoherence to enhance quantum computing performance.

The maps showed where magnetic flux could get trapped in the circuit and cause qubit decoherence. They also characterized couplings between the qubits and flux-control lines. These couplings can reduce qubit decoherence if they are efficient.

“The images give a precise picture of both the desired and undesired effects of the circuit design, providing important information for further optimization,” said author Martino Poggio. “The precise knowledge of where current flows in a qubit circuit, and the ability to change its design and reinspect the flow, will allow for optimizing designs in ways which have not been attempted.”

The authors created the maps with a SQUID-on-tip probe and a high-vacuum microscope operating at 4.2 K. Next, they hope to employ this technique below 1 K, which is closer to quantum computing operating temperatures, to try to optimize couplings and reduce undesired crosstalk.

“Quantifying and mitigating such crosstalk is particularly important in large circuits involving many qubits, as in state-of-the-art superconducting quantum computers,” Poggio said.

Source: “Magnetic imaging of superconducting qubit devices with scanning SQUID-on-tip,” by E. Marchiori, L. Ceccarelli, N. Rossi, G. Romagnoli, J. Herrmann, J.-C. Besse, S. Krinner, A. Wallraff, and M. Poggio, Applied Physics Letters (2022). The article can be accessed at https://doi.org/10.1063/5.0103597 .