  The sensitivity of superconducting qubits allows for spectroscopy and
coherence measurements on individual two-level systems present in the
disordered tunnel barrier of an $\mathrm{Al/AlO_x/Al}$ Josephson junction. We
report experimental evidence for the decoherence of two-level systems by
Bogoliubov quasiparticles leaking into the insulating $\mathrm{AlO_x}$ barrier.
We control the density of quasiparticles in the junction electrodes either by
the sample temperature or by injecting them using an on-chip dc-SQUID driven to
its resistive state. The decoherence rates were measured by observing the
two-level system's quantum state evolving under application of resonant
microwave pulses and were found to increase linearly with quasiparticle
density, in agreement with theory. This interaction with electronic states
provides a noise and decoherence mechanism that is relevant for various
microfabricated devices such as qubits, single-electron transistors, and
field-effect transistors. The presented experiments also offer a possibility to
determine the location of the probed two-level systems across the tunnel
barrier, providing clues about the fabrication step in which they emerge.
