Superconducting flux qubit with ferromagnetic Josephson pi junction operating at zero magnetic field

The operation of a conventional superconducting flux qubit requires the application of a precisely tuned magnetic field to set the operation point at half a flux quantum through the qubit loop, which makes the scaling of quantum circuits based on this type of qubits difficult. It has been proposed that, by inducing a pi phase shift in the superconducting order parameter using a precisely controlled nanoscale-thickness superconductor/ferromagnet/superconductor Josephson junction, commonly referred to as pi-junction, it is possible to realize a flux qubit operating at zero magnetic flux. We report the realization of a zero-flux-biased flux qubit based on three NbN/AlN/NbN Josephson junctions and a NbN/PdNi/NbN ferromagnetic pi-junction. The qubit lifetime is in the microsecond range, which we argue is limited by quasiparticle excitations in the metallic ferromagnet layer. With further improvements in the materials of the ferromagnetic junction, the zero-flux-biased flux qubits can become a promising platform for quantum computing.