Implementation of superconductor-ferromagnet-superconductor pi-shifters in superconducting digital and quantum circuits

摘要

The difference between the phases of superconducting order parameter plays insuperconducting circuits the role similar to that played by the electrostaticpotential difference required to drive a current in conventional circuits. Thisfundamental property can be altered by inserting in a superconducting circuit aparticular type of weak link, the so-called Josephson $\pi$-junction havinginverted current-phase relation and enabling a shift of the phase by $\pi$. Wedemonstrate the operation of three superconducting circuits -- two of them areclassical and one quantum -- which all utilize such $\pi$-phase shiftersrealized using superconductor-ferromagnet-superconductor sandwich technology.The classical circuits are based on single-flux-quantum cells, which are shownto be scalable and compatible with conventional niobium-based superconductingelectronics. The quantum circuit is a $\pi$-phase biased qubit, for which weobserve coherent Rabi oscillations and compare the measured coherence time withthat of conventional superconducting phase qubits.