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

Interrupting a superconducting loop with a thin ferromagnetic film creates a so-called π-Josephson junction that shifts the phase of a current flowing in the loop by 180°. A demonstration of the use of π-junctions in a variety of device structures suggests they could enable the development of a new class of superconducting logic 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 Φ 0 . Inserting a π-type Josephson junction 1 , 2 in the cell is equivalent to applying flux Φ 0 /2 and thus makes it possible to solve this problem 3 . Moreover, using π-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 π-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 π-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 π-junction.