Superconducting-Semiconducting Voltage-Tunable Qubits in the Third Dimension

We propose superconducting-semiconducting (super-semi) qubit and coupler designs based on high-quality compact through-silicon vias (TSVs) to overcome challenges introduced by materials and processing required to fabricate high-quality super-semi Josephson junctions. In our designs, an interposer "probe" wafer containing TSVs is used to contact a sample wafer with, e.g., a superconductor-proximitized epitaxially grown germanium quantum well. By utilizing the capacitance of the probe-wafer TSVs, the majority of the electric field in the qubits is pulled away from lossy regions in the semiconducting wafer, such as the graded buffer layers sometimes required for epitaxial growth. Through simulations, we find that the probe wafer can reduce the electric field participation of the qubit in the sample wafer by an order of magnitude for thin substrates and remains small even when the epitaxial-layer thickness approaches 100 mu m. We also show how the qubit-coherence improvements achieved via this scheme are extensible to multiqubit systems that have tunable qubit-qubit couplings without magnetic fields. This approach additionally shrinks the on-chip footprint of voltage-tunable superconducting qubits while homogenizing critical wiring structures independent of semiconducting materials, which could aid in accelerating the understanding of super-semi heterostructures in a variety of systems.