Metallic Grains Found to Limit Coherence in Quantum Circuits

Researchers at the National Physical Laboratory have identified a pervasive and previously unrecognized source of decoherence in superconducting quantum circuits: microscopic metallic grains. Utilizing scanning gate microscopy on live circuits, the team discovered that these ubiquitous defects, introduced during standard lithography, limit coherence through Coulomb blockade and microwave-driven charge tunneling. The work demonstrates that metallic grains are as common and as debilitating to device performance as traditionally studied two-level system (TLS) defects, yet stem from a different physical mechanism. Conventional characterisation techniques would misattribute this loss to other, microwave power-independent processes, suggesting this significant source of noise has been overlooked in past analyses and represents a critical barrier to improving…