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

High operation speed and low energy consumption may allow the superconducting digital single-flux-quantum circuits to outperform traditional complementary metal-oxide-semiconductor logic. The remaining major obstacle towards high element densities on-chip is a relatively large cell size necessary to hold a magnetic flux quantum Phi(0). Inserting a pi-type Josephson junction(1,2) in the cell is equivalent to applying flux Phi(0)/2 and thus makes it possible to solve this problem(3). Moreover, using pi-junctions in superconducting qubits may help to protect them from noise(4,5). Here we demonstrate the operation of three superconducting circuits-two of them are classical and one quantum-that all utilize such pi-phase shifters realized using superconductor/ferromagnet/superconductor sandwich technology(6). The classical circuits are based on single-flux-quantum cells, which are shown to be scalable and compatible with conventional niobium-based superconducting electronics. The quantum circuit is a pi-biased phase qubit, for which we observe coherent Rabi oscillations. We find no degradation of the measured coherence time compared to that of a reference qubit without a pi-junction.