  Understanding the nature of two-level tunneling defects is important for
minimizing their disruptive effects in various nano-devices. By exploiting the
resonant coupling of these defects to a superconducting qubit, one can probe
and coherently manipulate them individually. In this work we utilize a phase
qubit to induce Rabi oscillations of single tunneling defects and measure their
dephasing rates as a function of the defect's asymmetry energy, which is tuned
by an applied strain. The dephasing rates scale quadratically with the external
strain and are inversely proportional to the Rabi frequency. These results are
analyzed and explained within a model of interacting standard defects, in which
pure dephasing of coherent high-frequency (GHz) defects is caused by
interaction with incoherent low-frequency thermally excited defects.
