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

利用超导体-铁磁-超导体具有反向电流-相位关系的π结，可以为超导微纳电子电路开辟新的可能性。在这个项目的第一阶段，我们已经证明了基于Nb(S)和CuNi(F)材料的约瑟夫森π结可以与现有的Nb技术自然地集成在一起，从而实现数字和量子电路。特别是，没有观察到与π结的存在有关的量子相干性退化，至少在几纳秒的时间尺度上观察到，这对于传统设计的Nb相位量子比特来说是典型的。受这一成功的启发，在本方案中，我们希望通过测量基于自聚焦光纤的π结移相器在微波频率下引起的能量松弛和退相来探索这些移相器的退相干极限。我们将详细阐述对称势NB量子比特相干性的最新改进和约瑟夫森量子比特优化设计的新思路。我们的最终目标将是集成和表征自聚焦π结与约瑟夫森UX和四次相位量子比特。这两种类型的量子比特能够在一个对称(魔术)点上运行和读出，因此，它们可能更不受各种消相干因素的影响。这些量子比特的较长相干时间应该允许得出π结的可能影响的结论，从而确保在这些电路中存在必要的相位偏差。这些结果将与在这些结中的噪声模型中获得的结果进行比较，该模型的物理参数将通过实验获得。我们希望我们的实验将为自聚焦π结在量子信息电路中的广泛应用开辟道路。

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.