Experimental Evaluation of the Intrinsic Dissipation from Energy-Level Quantization in Josephson Devices

The main problem in realizing an experiment of macroscopic quantum coherence, namely, an experiment where the nonclassical behavior of a macroscopic system must be detected, is the fulfillment of many experimental constraints, in principle very difficult to achieve. One of the most critical parameters is the decoherence time of the system. The Rabi oscillations of a two-level system, in fact, are canceled if the quality factor associated with the oscillation is less than unity. In particular, it can be shown that the decoherence time for a SQUID system, once the temperature is given, depends only on the effective resistance. To evaluate the effective resistance of our system we have measured the energy-level quantization (ELQ) under stationary conditions at a temperature between 13 and 35 mK, for a Josephson junction and for an rf SQUID using the same type of junction. For both systems we can clearly see ELQ, because of the very low level of the intrinsic dissipation. From these measurements we can then set a lower limit for the effective system dissipation and then infer the decoherence time related to the overall setup of our experiment.