Coherent and compact van der Waals transmon qubits

Superconducting qubits can benefit from expanding their materials palette to overcome limitations of conventional thin-film superconductors and oxides. Focusing on van der Waals materials as a promising crystalline platform, we demonstrate coherent merged-element transmon qubits with microsecond-scale lifetimes constructed entirely from van der Waals Josephson junctions. These qubits have exceptionally small mode volumes and form factors five orders of magnitude smaller than conventional transmons. From energy-relaxation measurements, we extract junction loss tangents and corroborate these values with independent microwave loss measurements of structurally similar van der Waals capacitors, identifying dielectric loss as the dominant relaxation mechanism. Finally, we discuss intrinsic and extrinsic mechanisms limiting coherence, as well as prospects for higher-temperature operation and scalable fabrication of van der Waals qubits.