Measurement of the intrinsic dissipation of a macroscopic system in the quantum regime

We report on the first measurements of the intrinsic dissipation in a macro scopic system cooled at very low temperature (35 mK) and operating in the q uantum regime. The system under study is an rf SQUID with a high quality Jo sephson junction. Below 50 mK the tunneling probability of escape from a me tastable well vs applied flux presents a series of maxima due to energy lev el quantization. From the shape of the tunneling probability we can evaluat e the intrinsic dissipation related to the overall system as well as the co herence time related to the Rabi oscillations in a future macroscopic quant um coherence experiment.