  A crucial requirement for quantum information processing is the realization
of multiple-qubit quantum gates. Here, we demonstrate an electron spin based
all-electrical two-qubit gate consisting of single spin rotations and inter-dot
spin exchange in a double quantum dot. A partially entangled output state is
obtained by the application of the two-qubit gate to an initial, uncorrelated
state. We find that the degree of entanglement is controllable by the exchange
operation time. The approach represents a key step towards the realization of
universal multiple qubit gates.
