  A spin-echo-type technique is applied to an artificial two-level system that
utilizes charge degree of freedom in a small superconducting electrode.
Gate-voltage pulses are used to produce the necessary pulse sequence in order
to eliminate the inhomogeneity effect in the time-ensemble measurement and to
obtain refocused echo signals. Comparison of the decay time of the observed
echo signal with estimated decoherence time suggests that low-frequency
energy-level fluctuations due to the 1/f charge noise dominate the dephasing in
the system.
