  The main feature of amorphous materials is the presence of excess vibrational
modes at low energies, giving rise to the so called "boson peak" in neutron and
optical spectroscopy. These same modes manifest themselves as two level systems
(TLSs) causing noise and decoherence in qubits and other sensitive devices.
Here we present an experiment that uses the spin relaxation of dangling bonds
at the Si/(amorphous)SiO$_2$ interface as a probe of TLSs. We introduce a model
that is able to explain the observed non-exponential electron spin inversion
recovery and provides a measure of the degree of spatial localization and
concentration of the TLSs close to the interface, their maximum energy and its
temperature dependence.
