Tunneling two-level systems and 1/ <i>f</i> -noise: A century of nuisance

Tunneling two-level systems (TLS) were proposed as an ad hoc explanation of the ubiquitous low-frequency (1/f) noise in solid-state devices. Now their existence and, in certain cases, quantum coherent behavior are confirmed by direct observation. The latter allowed the use of TLS within superconducting qubits as quantum memory devices.