  We demonstrate improved operation of exchange-coupled semiconductor quantum
dots by substantially reducing the sensitivity of exchange operations to charge
noise. The method involves biasing a double-dot symmetrically between the
charge-state anti-crossings, where the derivative of the exchange energy with
respect to gate voltages is minimized. Exchange remains highly tunable by
adjusting the tunnel coupling. We find that this method reduces the dephasing
effect of charge noise by more than a factor of five in comparison to operation
near a charge-state anti-crossing, increasing the number of observable exchange
oscillations in our qubit by a similar factor. Performance also improves with
exchange rate, favoring fast quantum operations.
