  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.
