  Since the first demonstration of coherent control of a quantum state of a
superconducting charge qubit a variety of Josephson-junction-based qubits have
been implemented with remarkable progress in coherence time and read-out
schemes. Although the current level of this solid-state device is still not as
advanced as that of the most advanced microscopic-system-based qubits, these
developments, together with the potential scalability, have renewed its
position as a strong candidate as a building block for the quantum computer.
Recently, coherent oscillation and microwave spectroscopy in
capacitively-coupled superconducting qubits have been reported. The next
challenging step toward quantum computation is a realization of logic gates.
Here we demonstrate a conditional gate operation using a pair of coupled
superconducting charge qubits. Using a pulse technique, we prepare different
input states and show that they can be transformed by controlled-NOT (C-NOT)
gate operation in the amplitude of the states. Although the phase evolution
during the gate operation is still to be clarified, the present results are a
major step toward the realization of a universal solid-state quantum gate.
