Experimental evaluation of the intrinsic dissipation from energy-level quantization in Josephson devices

The main problem in realizing an experiment of macroscopic quantum coherenc e, namely, an experiment where the nonclassical behavior of a macroscopic s ystem must be detected, is the fulfillment of many experimental constraints , in principle very difficult to achieve. One of the most critical paramete rs is the decoherence time of the system. The Rabi oscillations of a two-le vel 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 de coherence time for a SQUID system, once the temperature is given, depends o nly on the effective resistance. To evaluate the effective resistance of ou r system we have measured the energy-level quantization (ELQ) under station ary conditions at a temperature between 13 and 35 mK, for a Josephson junct ion and for an rf SQUID using the same type of junction. For both systems w e can clearly see ELQ, because of the very low level of the intrinsic dissi pation. From these measurements we can then set a lower limit for the effec tive system dissipation and then infer the decoherence time related to the overall setup of our experiment.