Quantum dynamics of LC shunted Nb/Al-AlOx/Nb Josephson junctions

Superconducting systems including Josephson junctions (JJs) are good candidates for studying a macroscopic quantum variable if the system is suitably decoupled from its environment. We investigate the behavior of this macroscopic quantum variable φ in circuits containing a JJ, a capacitor and an inductance. It was shown that φ should be confined by a 2-dimensional potential, leading to two energy scales for the quantum levels in the potential wells. We designed and fabricated such LC-JJ systems with different inductance values, investigated their quantum dynamics and compared the results to the theoretical expectations. The samples were fabricated in Karlsruhe by a thoroughly optimized Nb/AlOx/Nb process employing electron beam lithography. Afterwards, we explored the quantum mechanical energy levels and the tunnelling rates of our samples in a carefully shielded dilution refrigerator in Goeteborg. Our results are in very good agreement with the theoretical predictions. We achieve the quantum regime for our samples and can clearly identify the predicted energy level splittings in our experimental spectroscopy data.