Quantum computers, computers which leverage the laws of quantum mechanics to solve certain problems far more efficiently than classical computers, has the promise of revolutionising the fields of cybersecurity, optimisation, drug discovery, clean energy, and data management and searching, among others. Unfortunately, realising this promise of quantum computing has been challenging because quantum states in these computers are extremely fragile and can decohere before any practical problem can be solved. For superconducting quantum bits (qubits)—one of the most mature and widely used quantum computing platforms—a dominant source of decoherence arises from microscopic material defects known as two-level system (TLS) defects. Surprisingly little is known about them, including where they reside in the device and how they form. To address this challenge, the Quantum Technologies Department at the UK’s National Physical Laboratory (NPL) i...