Stabilizing and improving qubit coherence by engineering noise spectrum of two-level systems

TL;DR

This paper proposes methods to engineer the noise spectrum of two-level systems in superconducting circuits, significantly stabilizing qubit coherence times by reducing low- and high-frequency noise effects.

Contribution

It introduces protocols to modify TLS noise spectra, enhancing qubit coherence by smoothing high-frequency noise and suppressing low-frequency fluctuations.

Findings

Predicted stabilization of qubit lifetime.

Significant increase in qubit dephasing time.

Feasible experimental implementation without adverse effects.

Abstract

Superconducting circuits are a leading platform for quantum computing. However, their coherence times are still limited and exhibit temporal fluctuations. Those phenomena are often attributed to the coupling between qubits and material defects that can be well described as an ensemble of two-level systems (TLSs). Among them, charge fluctuators inside amorphous oxide layers contribute to both low-frequency $1/f$ charge noise and high-frequency dielectric loss, causing fast qubit dephasing and relaxation. Moreover, spectral diffusion from mutual TLS interactions varies the noise amplitude over time, fluctuating the qubit lifetime. Here, we propose to mitigate those harmful effects by engineering the relevant TLS noise spectral densities. Specifically, our protocols smooth the high-frequency noise spectrum and suppress the low-frequency noise amplitude via depolarizing and dephasing the TLSs, respectively. As a result, we predict a drastic stabilization in qubit lifetime and an increase in qubit pure dephasing time. Our detailed analysis of feasible experimental implementations shows that the improvement is not compromised by spurious coupling from the applied noise to the qubit.