Macroscopic quantum tunneling in high-T_c superconductors: the role of collective charge oscillations

We present a theoretical study of an escape rate for switching from the superconducting state to a resistive one in series arrays of strongly interacting Josephson junctions. At low temperatures such a switching is determined by macroscopic quantum tunneling (MQT) of a spatially extended Josephson phase. An increase of the crossover temperature from the thermal to quantum regimes, and a giant enhancement of MQT escape rate is obtained. Such an effect is explained by excitation of collective charge oscillations forming a {\it charge instanton}. Using a model of screened Coulomb interaction we found that our analysis is in a good accord with recently published experimental results on an enhancement of the MQT in single crystals of high-$T_c$ superconductors.