Measurements of Decoherence Times in a Josephson Charge Qubit Using Fast Pulses

We study decoherence of a Josephson charge qubit using fast pulses to perform qubit rotations. The gate charge dependence of the decoherence rate indicates that the low frequency fluctuations affecting the qubit are from charges distributed with a 1/f-type spectrum. Assuming the form S-q(omega) = alpha/omega, we find rootalpha = 4 x 10(-3)e, which is slightly higher than the value found from very low frequency noise measurements