  The proper functioning of some micro-fabricated novel quantum devices, such
as superconducting resonators and qubits, is severely affected by the presence
of parasitic structural material defects known as tunneling two-level-systems
(TLS). Recent experiments have reported unambiguous evidence of the strong
interaction between individual (coherent) TLS using strain-assisted
spectroscopy. This work provides an alternative and simple theoretical insight
that illustrates how to obtain the spectral response of such strongly
interacting defects residing inside the amorphous tunnel barrier of a qubit's
Josephson junction. Moreover, the corresponding spectral signatures obtained
here may serve to quickly and efficiently elucidate the actual state of these
interacting TLS in experiments based on strain- or electric-field spectroscopy.
