Over the last decade, we have witnessed a spectacular improvement in the understanding and mitigation of dissipation and dephasing in quantum microwave oscillators based on superconducting circuits. This evolution fueled the optimism of academic and industrial communities alike. Nevertheless, for fault-tolerant quantum computing machines we still require orders of magnitude of improvement in quantum coherence. I will present a few emerging research directions which I believe to be promising in providing the next breakthroughs in quantum hardware, and I will also put forward a few open questions. In particular, I will present the lessons learned over the last few years from using superconducting granular Aluminum circuits [1,2,3]. In these circuits one of the leading sources of dissipation and noise are non-equilibrium quasiparticles, resulting from the interaction with high energy radiation. I will discuss mitigation strategies [4], including deep underground shielding; a research effort bridging to the kinetic inductance detector community.
Refrences:
[1] Circuit Quantum Electrodynamics of Granular Aluminum Resonators, N. Maleeva, et al. Nature Comm. 3889 (2018);
[2] Loss Mechanisms and Quasiparticle Dynamics in Superconducting Microwave Resonators Made of Thin-Film Granular Aluminum, L. Grunhaupt, et al. Phys. Rev. Lett. 121, 117001 (2018);
[3] Granular aluminium as a superconducting material for high-impedance quantum circuits, L. Grunhaupt, M. Spiecker, et al. Nature Materials 18, 816-819 (2019);
[4] Phonon traps reduce the quasiparticle density in superconducting circuits, F. Henriques, F. Valenti, et al. Appl. Phys. Lett. 115, 212601 (2019);