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

TL;DR

This paper demonstrates superconductor-ferromagnet-superconductor pi-shifters integrated into classical and quantum superconducting circuits, showing their scalability, compatibility, and impact on qubit coherence.

Contribution

It introduces and experimentally validates pi-shifters using S-F-S technology in both classical and quantum superconducting circuits, advancing circuit design.

Findings

Pi-shifters enable phase control in superconducting circuits.

Classical circuits with pi-shifters are scalable and compatible with existing technology.

Quantum pi-shifted qubits exhibit coherent Rabi oscillations and comparable coherence times.

Abstract

The difference between the phases of superconducting order parameter plays in superconducting circuits the role similar to that played by the electrostatic potential difference required to drive a current in conventional circuits. This fundamental property can be altered by inserting in a superconducting circuit a particular type of weak link, the so-called Josephson $\pi$-junction having inverted current-phase relation and enabling a shift of the phase by $\pi$. We demonstrate the operation of three superconducting circuits -- two of them are classical and one quantum -- which all utilize such $\pi$-phase shifters realized using superconductor-ferromagnet-superconductor sandwich technology. 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 $\pi$-phase biased qubit, for which we observe coherent Rabi oscillations and compare the measured coherence time with that of conventional superconducting phase qubits.