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 , 3 , 4 , 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.