Developing a packaging scheme that meets all of the requirements for operation of solid-state qubits in a cryogenic environment can be a formidable challenge. In this article, we discusswork being done in our group as well as in the broader community, focusing on the role of 3D integration and packaging in quantum processing with solid-state qubits.
The quantum state of a superconducting transmon qubit inside a three-dimensional cavity is monitored by reflection of a microwave field on the cavity. The information inferred fromthe measurement record is incorporated in a density matrix ρt, which is conditioned on probe results until t, and in an auxiliary matrix Et, which is conditioned on probe results obtained after t. Here, we obtain these matrices from experimental data and we illustrate their application to predict and retrodict the outcome of weak and strong qubit measurements.