  A crucial requirement for scalable quantum-information processing is the
realization of multiple-qubit quantum gates. Universal multiple-qubit gates can
be implemented by a set of universal single qubit gates and any one kind of
two-qubit gate, such as a controlled-NOT (CNOT) gate. Semiconductor quantum dot
qubits are a leading approach for the physical implementation of quantum
computation, due to their potential for large-scale integration. Two-qubit gate
operations have been so far only demonstrated in individual electron spin-based
quantum dot systems. Due to the relatively short de-coherence time, charge
qubits in quantum dots are generally considered to be inferior for going beyond
single qubit level. Here, we demonstrate the benchmarking CNOT gate in two
capacitively coupled charge qubits, each consisting of an electron confined in
a GaAs/AlGaAs double quantum dot. Owing to the strong inter-qubit coupling
strength, gate operations with a clock speed up to 5GHz has been realized. A
processing tomography shows encouragingly that the universal two-qubit gate
operations have comparable fidelities to that of spin-based two-qubit gates.
Our results suggest that semiconductor charge qubits have a considerable
potential for scalable quantum computing and may stimulate the use of
long-range Coulomb interaction for coherent quantum control in other devices.
