All Van der Waals Josephson Junctions

Abstract

Two-level-systems (TLSs) are identified as a major coherence-limiting factor that exist both within and at the interface of amorphous oxides found in current superconducting quantum circuits. Design improvements that reduce the spurious coupling to TLSs have led to significant improvements of superconducting qubit coherence over the past two decades. However, material and fabrication advancement that reduce the sources of TLSs has been relatively limited. Van der Waals (vdW) materials and their heterostructures are known for their extraordinary crystalline quality, versatile electronic properties, and flexible assembly that can be achieved with epitaxially-precise interfaces. In this thesis, we aim to explore and incorporate the advantages of vdW materials into the circuit quantum electrodynamics (cQED) platform of relevance to advancing quantum technologies. In particular, we fabricate and characterize high-quality, all-vdW Josephson junctions, a key component in superconducting quantum circuits. VdW heterostructures of 2-4 layers of hBN sandwiched between NbSe₂ superconductors demonstrate Josephson effect and the critical current increases exponentially with decreasing hBN layer number. Additionally, we observed a superconducting gap close to the bulk NbSe₂ gap, [delta] = 1.1meV in all the vdW junctions, evidencing little degradation of superconductivity. We expect these efforts will lead to high-coherence all-vdW qubit devices with small footprints.