IMPROVEMENTS AND STUDIES OF PLANAR TRANSMON QUBITS

Abstract

This dissertation describes three main projects focused on characterizing and improving superconducting transmon qubits operating nominally at temperature of 20 mK. The first topic I discuss is characterization of ground state fidelity of a passively cooled 3D transmon qubit using two techniques. The first technique was counting the number of false counts when performing single-shot read-out measurements of the weak resonator signal using a nearly quantum limited traveling wave parametric amplifier. Over about a million shots, only 772 counts were found with the system in the excited state, corresponding to a residual excited state population of P_e = 0.083%. The second technique used was performing Rabi oscillations between the first and second excited state levels of the transmon qubit. By fitting the data, the residual excited state population was shown to be P_e = 0.088% +- 0.018%. These state of the art low values for the infidelity of the ground state suggest that the effective temperature of the transmon qubit with fundamental transition frequency of 3.6 GHz was T < 25 mK. The second topic I discuss is improvements in the coherence of our planar transmon qubits such that I was able to measure energy relaxation times and coherence times up to tens of microseconds. There were two main improvements that I achieved during this research. First, I identified a significant loss mechanism associated with the way that the planar transmon qubits were packaged. I did this by measuring the internal quality factors (Qi) for a series of thin-film Al quarter-wave resonators with fundamental resonant frequencies varying between 4.9 and 5.8 GHz. By utilizing resonators with different widths and gaps, I sampled different electromagnetic energy volumes that affected Qi. When the backside of the sapphire substrate of the resonator device was adhered to a Cu package with a conducting silver glue, a monotonic decrease in the maximum achievable Qi was found as the electromagnetic sampling volume was increased. Simulations and modeling showed that the observed dissipation was a result of induced currents in large surface resistance regions underneath the substrate. By placing a hole underneath the substrate and using superconducting material for the package, I was able to decrease the Ohmic losses and increase the maximum Qi by an order of magnitude for the larger size resonators. The second improvement I made to achieve improvements to our planar transmon qubits was developing a new fabrication process to improve the quality of the interface between the substrate and the superconducting shunting capacitor of the transmon. For this new process, the large features (> 1 um) of the thin superconducting film are subtractively defined by etching the film. The small Al/AlOx/Al junction is added in a second step by defining the junction in electron-beam lithography, an ion mill step, and standard double-angle evaporation. Finally, I discuss my study of the coherence recovery time after injecting quasiparticles in several transmon qubits. Quasiparticles in the transmon junction were created by applying a large-amplitude microwave pulse resonant with the readout resonator. Immediately after generating the quasiparticles, a significant decrease of the energy relaxation time of the transmon qubit was observed. By performing relaxation time T1 and coherence time T2 measurements over the course of several milliseconds, I tracked the recovery T1 and T2, which I then used as metrics of the quasiparticle density at the junction. I fitted the recovery data with a numerical model involving differential equations and extracted the quasiparticle trapping rates around 1/ms and recombination rates around 1 / 25ns at the sites of a few transmon qubits. I measured transmons that were either galvanically connected to ground or isolated, fabricated with aluminum (with superconducting gap ~ 200 ueV) or tantalum (with ~ 600 ueV ) for the shunting capacitor. Finally, with a larger quasiparticle injection power and by measuring two transmons on the same chip, I observed a phenomenon that was consistent with phonon-assisted quasiparticle poisoning. I discuss my quantitative modelling of such data, and how this effect presents challenges to further improving the coherence times of transmon qubits.