Transmon Qubit Chip on Single-Crystal Tantalum (110) film

Recent reports on using Ta films to replace Nb films as the platform for superconducting qubits show longer relaxation time [1][2] and potential for applications in quantum circuit dynamics. However, how the crystallinity of Ta films affect the qubit relaxation time remains relatively unexplored. Here, we study the transmon qubits and coplanar waveguide resonators fabricated on single-crystalline and polycrystalline Ta(110) films grown on sapphire substrates. The fabrication processes were optimized to minimize loss. The fabricated chips were characterized in a dilution refrigerator at 10 mK. Quarter-wavelength resonators were coupled to a common feedline. The internal quality factors (Q_i) of the resonators were extracted by fitting the measured transmission (S₂₁) across the feedline. The relaxation time (T₁) and coherence time (T₂) were obtained by time-domain microwave measurements. Our results indicate that single-crystalline Ta(110) films exhibit lower two-level system (TLS) loss compared to polycrystalline Ta films. The resonator internal quality factor at low photon number limit (Q_LP) suggests that there are more TLS defects in the polycrystalline Ta film. Transmon qubits fabricated on single-crystalline Ta films show ~50% longer T₁ time than those fabricated on polycrystalline Ta films. Our work unveils the impacts of grain boundaries (through which TLS may form) on the overall loss tangents. The higher quality factors and longer T₁ time demonstrate the promise of using single-crystal Ta films for developing high-quality quantum circuits.