Evidence of macroscopic quantum tunneling from both wells in a φ Josephson junction

We study Nb-AlOx-Nb Josephson junctions (JJs) with a phase-discontinuity K created by a pair of current injectors attached to one of the Nb electrodes. For kappa approximate to pi the Josephson potential energy U as a function of the average phase psi across the JJ has the form of a 2 pi-periodic double-well potential. Thus, the device behaves as a phi JJ with degenerate ground state phases psi = +/-phi (the value of phi depends on the system parameters). Experimentally, the existence of two wells of the potential is confirmed by the observation of two different critical currents I-c +/-, corresponding to the escape from different wells. We investigate the escape of the Josephson phase from both wells by collecting statistics of the switching currents. The histogram of switching current exhibits two peaks corresponding to I-c +/-. The dependence of the width sigma(+) and sigma(-) of each peak on the bath temperature T indicates the transition from thermal activation to macroscopic quantum tunneling (MQT) at T-star approximate to 260 mK as T decreases. We argue that the observed saturation value of sigma(+) and sigma(-) below T-star is indeed related to quantum tunneling rather than to parasitic noise in the system, as the histogram width can be reduced by tuning the value of kappa away from pi. The comparison of the experimental escape rate Gamma with theoretical predictions further confirms MQT of the phase psi from both wells.