Intrinsic Photon Loss at the Interface of Superconducting Devices.

We present a quantum theory of dielectric energy loss arising from the piezoelectric coupling between photons and phonons in superconducting devices. Photon loss is shown to occur predominantly at the interface, where the piezoelectric effect is nonzero even when the materials are perfectly crystalline (epitaxial) and free of two-level system defects. We present explicit numerical calculations for the value of the intrinsic loss tangent at several interfaces to conclude that the T_{1} of superconducting qubits may reach over 10^{4}  μs if the device is made with defect-free interfaces.