Protecting transmon qubits from Purcell decay and other external sources of decoherence*

In quantum information processing, qubit coherence is one of the most important factors limiting the ultimate size of quantum cicuits. A fundamental limitation of superconducting qubit coherence is the Purcell limit of qubit photon decay via coupling to the port of its associated readout cavity. In many quantum circuits, this limitation is removed by using an external Purcell filter to stop photon transmission at the qubit frequency. This limit requires us to separate out and protect against other sources of loss in the qubits too. In this talk, we will present both our efforts to regularly achieve high-coherence transmons in 2D and 3D geometries and our implementation of a technique to incorporate protection from the Purcell effect directly into the sample design and geometry [Sunada, et al. PhysRevApplied (2022)]. By visualizing the transmon fields interacting with a resonator, an optimal and unique port placement can be determined to achieve large cavity bandwidths and longer qubit lifetimes. Placing the strongly coupled resonator port within the null point of the transmon fields innately reduces the photon emissions of the qubit while providing fast readout and is applicable to both 2D and 3D sample geometries. We will show that this built-in Purcell protecton is able to reliably produce qubits with 100 us lifetimes with fast readout in tubes and post cavity geometries while icnreasing the coherence limit due to Purcell decay to at least 1 ms.