  By performing experiments with thin-film resonators of NbSi, we elucidate a
decoherence mechanism at work in disordered superconductors. This decoherence
is caused by charged Two Level Systems (TLS) which couple to the conduction
electrons in the BCS ground state; it does not involve any out-of-equilibrium
quasiparticles, vortices, etc. Standard theories of mesoscopic disordered
conductors enable making predictions regarding this mechanism, notably that
decoherence should increase as the superconductor cross section decreases.
Given the omnipresence of charged TLS in solid-state systems, this decoherence
mechanism affects, to some degree, all experiments involving disordered
superconductors. In particular, we show it easily explains the poor coherence
observed in quantum phase slip experiments and may contribute to lowering the
quality factors in some disordered superconductor resonators.
