Dephasing in Flux-Tunable Transmon Qubits

Qubit coherence is limited by energy relaxation and dephasing.  In state-of-the art superconducting devices, energy relaxation (T₁) is dominated by dielectric loss from amorphous interfaces, and systematic improvements to fabrication have pushed qubit T₁ towards 1 ms.  On the other hand, qubit dephasing can be limited by many factors, including 1/f flux noise intrinsic to the device, 1/f or white flux noise from the control electronics, quasiparticle poisoning, and cavity photon shot noise.  

In this work, we discuss the characterization of dephasing in flux-tunable transmon qubits.  We find that the 1/f noise and white noise specifications of the control electronics are not sufficient to accurately model our devices, but a measurement of the full voltage noise spectral density is.  The modeling allows us to optimize qubit wiring and filtering and to quantify other residual sources of dephasing.