Superconducting flux qubit with ferromagnetic Josephson π-junction operating at zero magnetic field
Communications Materials volume 5, Article number: 216 (2024 ) Cite this article
Conventional superconducting flux qubits require the application of a precisely tuned magnetic field to set the operation point at half a flux quantum through the qubit loop, which complicates the on-chip integration of this type of device. It has been proposed that by inducing a π-phase shift in the superconducting order parameter using a precisely controlled nanoscale-thickness superconductor/ferromagnet/superconductor Josephson junction, commonly referred to as π-junction, it is possible to realize a flux qubit operating at zero magnetic flux. Here, 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 π-junction. The qubit lifetime is in the microsecond range, which we argue is limited by quasiparticle excitations in the metallic ferromagnet layer. Our results pave the way for developing quantum coherent devices, including qubits and sensors, that utilize the interplay between ferromagnetism and superconductivity.
The essential component in superconducting quantum bits (qubits) is the Josephson junction (JJ) composed of a nanoscale tunnel barrier sandwiched between two superconducting layers. These junctions, typically formed by superconductor/insulator/superconductor structures, introduce circuit nonlinearity while preserving its quantum nature, enabling the circuit to behave as a macroscopic artificial atom. The conventional choice for these JJs, ever since the first demonstration of nanosecond-scale quantum coherent oscillations in a charge qubit in 19991, is the aluminum (Al)/aluminum oxide (AlOx)/Al JJ. This choice is preferred due to its simplicity of fabrication using the shadow evaporation technique and its ability to provide a reliable sample quality for achieving long coherence times. Despite significant progress in improving the coherence times of Al-based qubits through advanced qubit designs2,3,4, there remain challenges in terms of material improvements to deal with two-level fluctuators originating from uncontrollable defects in the amorphous AlOx in Al-based JJs5. Consequently, there is a growing need for materials-oriented research and design innovations to enhance the performance of superconducting qubits and reduce noise or the sensitivity of qubits to noise.
