Experimental investigation of quantum coherence in Josephson circuits with superconductor-ferromagnet-superconductor pi-junctions

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Using of superconductor-ferromagnet-superconductor (SFS) π-junctions having inverted currentphase relation, can open new possibilities for superconducting micro- and nanoelectronic circuits. In the first phase of this project we have shown that the Josephson π-junctions based on Nb (S) and CuNi (F) materials can be naturally integrated with available Nb technology, enabling both digital and quantum circuits. In particular, no degradation of quantum coherence, related to presence of a π-junction, was observed at least on the time scale of several nanoseconds, typical for Nb phase qubits of conventional design. Motivated by this success, in this proposal we would like to explore the decoherence limits of SFS-based π-junction phase shifters by measuring the energy relaxation and dephasing induced by these devices at microwave frequencies. We will elaborate on recent improvement of coherence in the Nb qubits with symmetric potential and new ideas on optimal design of the Josephson qubits. Our final goal will be to integrate and characterize the SFS π-junctions with Josephson ux- and quartic phase qubits. These two types of qubit are able to operate and readout in a symmetry (magic) point and, therefore, they are potentially more immune to various decoherence factors. A longer coherence time of these qubits should allow concluding about possible effect of the π-junctions ensuring in these circuits necessary phase bias. These results will be compared with those obtained in a model of noise in these junctions, physical parameters for which will be found experimentally. We hope that our experiments will open the way towards broad use of SFS π-junctions in quantum information circuits.