Substrate materials for quantum processors

In a general aspect, an integrated microwave circuit is disclosed for processing quantum information. The integrated microwave circuit includes a substrate having a first surface and a second surface opposite the first surface. The substrate is formed of a silicon oxide material having a loss tangent no greater than 1×10−5 at cryogenic temperatures at or below 120 K. The integrated microwave circuit also includes qubit circuitry disposed on the first surface that includes a Josephson junction. A ground plane is disposed on the first surface or the second surface. In some variations, the silicon oxide material is fused silica. In other variations, the silicon oxide material is crystalline quartz.