  A particular superconducting quantum interference device (SQUID)qubit,
indicated as double SQUID qubit, can be manipulated by rapidly modifying its
potential with the application of fast flux pulses. In this system we observe
coherent oscillations exhibiting non-exponential decay, indicating a non
trivial decoherence mechanism. Moreover, by tuning the qubit in different
conditions (different oscillation frequencies) by changing the pulse height, we
observe a crossover between two distinct decoherence regimes and the existence
of an "optimal" point where the qubit is only weakly sensitive to intrinsic
noise. We find that this behaviour is in agreement with a model considering the
decoherence caused essentially by low frequency noise contributions, and
discuss the experimental results and possible issues.
