Zero-field operation of NbN-based flux qubits with ferromagnetic π-junctions

Superconducting qubits are leading candidates for scalable quantum computing owing to their controllability and compatibility with integrated circuit technology. While transmon qubits are widely used, their low anharmonicity causes frequency crowding in multi-qubit architectures. Flux qubits, which exhibit stronger anharmonicity, can mitigate this issue; however, they require external magnetic fields to achieve the optimal bias, leading to flux noise and complex wiring that hinder large-scale integration.

To overcome these limitations, a flux qubit incorporating a ferromagnetic π-junction was proposed [1]. The π-junction provides an intrinsic π-phase shift without the need for external magnetic fields, enabling spontaneous optimal-point operation. By integrating NbN-based superconducting technology [2] with PdNi π-junction fabrication [3], a flux qubit capable of zero-field operation was realized.

Experimental measurements confirmed optimal operation at zero magnetic field with a coherence time of approximately 1 μs [4], representing a 360-fold improvement over previous π-junction phase qubits [5]. Although additional improvement is needed to reach the coherence levels of conventional flux qubits, this achievement demonstrates, for the first time, a magnetic-field-free flux qubit operating in the microsecond regime. These results can simplify circuit design, reduce flux noise and energy consumption, and provide a promising path toward scalable superconducting quantum processors.

Recent progress on enhancing the coherence of NbN-based qubits, including transmons, through material optimization and device engineering will also be discussed.