Demonstration of conditional gate operation using superconducting charge qubits

Following the demonstration of coherent control of the quantum state of a superconducting charge qubit(1), a variety of qubits based on Josephson junctions have been implemented(2-5). Although such solid-state devices are not currently as advanced as microscopic qubits based on nuclear magnetic resonance(6) and ion trap(7) technologies, the potential scalability of the former systems together with progress in their coherence times and read-out schemes - makes them strong candidates for the building block of a quantum computer(8). Recently, coherent oscillations(9) and microwave spectroscopy(10) of capacitively coupled superconducting qubits have been reported; the next challenging step towards quantum computation is the realization of logic gates(11,12). Here we demonstrate conditional gate operation using a pair of coupled superconducting charge qubits. Using a pulse technique, we prepare different input states and show that their amplitude can be transformed by controlled-NOT (C-NOT) gate operation, although the phase evolution during the gate operation remains to be clarified.