What is the 3D cavity qubit architecture and how does it achieve longer coherence times?

The 3D cavity qubit architecture achieves longer coherence times by placing a superconducting transmon qubit inside a three-dimensional microwave cavity (a machined aluminum or copper box resonator) instead of coupling it to a planar (2D) coplanar waveguide resonator on a chip. The 3D cavity provides longer coherence because: reduced surface loss (in a 3D cavity: the electromagnetic field is distributed throughout the large cavity volume (typically 10-100 cm^3), and only a tiny fraction of the field interacts with the lossy surfaces (metal walls and dielectric interfaces). In a 2D planar resonator: the field is concentrated at the metal-dielectric interface (a few micrometers of substrate surface), where TLS defects reside. The reduced surface participation ratio in a 3D cavity directly translates to lower loss and higher Q_i (Q_i greater than 10^8 demonstrated in aluminum 3D cavities, vs. approximately 10^6 for planar resonators)), better electromagnetic isolation (the 3D cavity is a completely enclosed metallic box, providing natural shielding from external electromagnetic interference; the qubit inside the cavity is protected from stray radiation, noise, and crosstalk), and higher mode purity (the 3D cavity's modes are well-defined and widely spaced in frequency; unwanted modes can be suppressed by appropriate cavity geometry design; this reduces the Purcell decay through unwanted channels).