Long-lifetime NbN-based self-shunted flux qubit with large anharmonicity

Superconducting flux qubits exhibit larger anharmonicity than that of transmons, enabling high-speed qubit gates. In the flux qubits, the capacitively shunted design has achieved the long coherence by increasing the total capacitance with a discrete shunt capacitor, as in transmons. However, the shunt capacitor greatly increases the device footprint and becomes a bottleneck for the large-scale integration. To achieve both the large anharmonicity and small footprint, we demonstrate a ultracompact self-shunted flux qubit (SSFQ) in which the shunt capacitance is provided by the large-area Josephson junctions, eliminating the discrete capacitors and reducing the device footprint to ~ 6 μm × 6 μm. Devices were fabricated with the epitaxial NbN/AlN/NbN junctions on an Si(100) substrate. By frequency-domain spectroscopy, we observed the large anharmonicity of 790 MHz, which was around twice that of the typical transmons. Furthermore, by the time-domain measurements, we found the long lifetime (energy-relaxation time) up to 24.7 μs, which is the longest value reported for three- or four-junction flux qubits without the shunt capacitors, while maintaining the large anharmonicity.