  We introduce an always-on, exchange-only qubit made up of three localized
semiconductor spins that offers a true "sweet spot" to fluctuations of the
quantum dot energy levels. Both single- and two-qubit gate operations can be
performed using only exchange pulses while maintaining this sweet spot. We show
how to interconvert this qubit to other three-spin encoded qubits as a new
resource for quantum computation and communication.
