Visualizing heterogeneous dipole fields by terahertz light coupling in individual nano-junctions used in transmon qubits

Abstract The fundamental challenge underlying superconducting quantum computing is to characterize heterogeneity and disorder that lead to local electric field concentration, charge scattering, dissipation and ultimately decoherence. It is particularly challenging to probe deep sub-wavelength electric field distribution under electromagnetic wave coupling at individual nano-junctions and correlate them with structural imperfections from interface and boundary, ubiquitous in Josephson junctions used in transmon qubits. A major obstacle lies in the fact that conventional microscopy tools are incapable of measuring {\em simultaneous} at nanometer and terahertz, ``nano-THz” scales, which often associate with frequency-dependent charge scattering in nano-junctions. Here we directly visualize interface nano-dipole near-field distribution of individual Al/AlO$_{x}$/Al junctions used in transmon qubits. Our THz nanoscope images show a remarkable asymmetry across the junction in electromagnetic wave-junction coupling response that manifests as ``hot" vs ``cold" cusp spatial electrical field structures and correlates with defected boundaries from the multi-angle deposition processes in JJ fabrication inside qubit devices. The asymmetric nano-dipole electric field contrast also correlates with distinguishing, ``overshoot" frequency dependence that characterizes the charge scattering and dissipation at nanoscale, hidden in responses from topographic, structural imaging and spatially-averaged techniques. The scheme can be extended to guide qubit nano-fabrication for ultimately optimizing qubit coherence times.