Demonstration of conditional gate operation using superconducting charge qubits

Abstract

Following the demonstration of coherent control of the quantum state of a superconducting charge qubit¹, 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 resonance⁶ and ion trap⁷ 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 computer⁸. Recently, coherent oscillations⁹ and microwave spectroscopy¹⁰ 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.