Sources of decoherence in fixed frequency transmon qubits.

Significant advances in the coherence of superconducting qubits were made possible by clever microwave engineering. For the transmon qubit this was mainly the reduction of its charge dispersion by capacitively shunting the Josephson junction, the increased size of its capacitor pads, filtering of the microwave controls and operation at noise insensitive points. More recently, efforts were made to disentangle the contributions of various material interfaces by engineering participation ratios of the electromagnetic field with different geometries of resonators and qubits in order to pinpoint the sources of noise.
We report on our efforts to measure and understand the noise sources that effect high coherence fixed frequency transmon qubits in a bottom loading dilution refrigerator. We study the effect of various shielding and filtering strategies using fast noise spectroscopy techniques. Furthermore, we present coherence data from devices that were cooled in a UHV package with a controlled atmosphere and discuss the effect of surface adsorbates on qubit performance.