Microwave Losses in YBCO Coplanar Waveguide Resonators at Low Power and Millikelvin Range

We have investigated the temperature dependence of microwave losses in the low-power limit and millikelvin temperature range of a thin-film
$\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7-x}$
(YBCO)
$\lambda/2$
coplanar waveguide resonator patterned on a
$(\hbox{La}_{0.3}\hbox{Sr}_{0.7})$

$(\hbox{Al}_{0.65}\hbox{Ta}_{0.35})\hbox{O}_{3}$
(LSAT) substrate covered by a
$\hbox{CeO}_{2}$
seed layer. The unloaded quality factor is mainly governed by the surface resistance of the YBCO film and the dielectric losses caused by resonant absorption due to a bath of two-level fluctuators in the dielectrics. The value of the unloaded quality factor at 20 mK and low power, i.e.,
$Q_{0}\simeq 2000$
, allows for the realization of YBCO microwave quantum circuits implementing biepitaxial grain-boundary Josephson junctions.