  We study quantum coherence in a semiconductor charge qubit formed from a GaAs
double quantum dot containing a single electron. Voltage pulses are applied to
depletion gates to drive qubit rotations and non-invasive state readout is
achieved using a quantum point contact charge detector. We measure a maximum
coherence time of ~7 ns at the charge degeneracy point, where the qubit level
splitting is first-order-insensitive to gate voltage fluctuations. We compare
measurements of the coherence time as a function of detuning with predictions
from a 1/f noise model.
