Absence of Charge Offset Drift in a Transmon Qubit

The electric environment of mesoscopic devices is noisy. The dynamics of that noise and its impact on a Cooper pair box is entirely captured by a single parameter: the equilibrium number of Cooper pairs on the island, called the charge offset. The transmon qubit, a Cooper pair box with a large capacitance, is designed to be insensitive to charge offset noise. Yet the qubit readout speed depends on this charge offset and the noise also limits the coherence of protected qubits based on junction arrays.

Many experiments have used Cooper pair boxes to probe the dynamics of the charge offset, and all have observed that it drifts with timescales of minutes to hours. I will present an experiment on a transmon qubit, where we observed the charge offset to be zero in all the measurements we performed over the course of 85 days and a thermal cycle.

We attribute this cancellation of the charge offset to the presence of a highly resistive thin film of tantalum in parallel to the Josephson junction, that we observe with X-ray Photoelectron Spectroscopy and Energy Dispersive X-Ray Spectroscopy. Remarkably, this film is so resistive that it does not limit the transmon relaxation and coherence times, nor the tunneling of quasiparticles in and out of the transmon island.