QUROPE Quantum Information Processing and Communication in Europe

Superconducting circuits based on Josephson junctions are among the most promising solid state candidates for implementing quantum computing algorithms. Although no efficient superconducting quantum processor has yet been operated since the first demonstration in 1999 of quantum coherence in the Cooper pair box circuit, significant progress has been achieved in terms of quantum coherence, readout fidelity, gate operation and circuit complexity. We have developed a high fidelity readout method for the transmon version of the Cooper pair box, and investigated its backaction on the measured qubit. We have recently fabricated and operated a two qubit quantum processor fitted with a universal quantum gate Iswap coupling the qubits, and with independent single shot readout of each one. Entangled Bell-like states were produced and characterized by state tomography and by entanglement witnesses. The violation of Bell inequality was demonstrated, but without closing the so-called detection loophole. The fidelity of the Iswap gate was determined by performing its process tomography, and by comparing it to the ideal operation. The quantum search Grover’s algorithm for finding one object out of four was implemented with a success probability of about 55% in a single algorithm step. The perspectives offered by superconducting qubits will be discussed.