Stabilizing and Improving Qubit Coherence by Engineering the Noise Spectrum of Two-Level Systems
Xinyuan You, Ziwen Huang, Uğur Alyanak, Alexander Romanenko, Anna Grassellino, Shaojiang Zhu · 2022 · 16 citationsRead the paper
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.
What the full text shows
Theory or simulation- Increasing the dephasing rate of two-level systems (TLSs) suppresses the temporal fluctuation of qubit lifetime. (Based on the spectral diffusion model where TLS-TLF interactions cause frequency shifts.)
- The variance of qubit depolarization rate over TLS realizations is inversely proportional to the TLS dephasing rate. ($ ext{Var}_{ ext{ens}}(\Gamma_1) \propto 1/\gamma_\phi$)
- The variance of qubit lifetime due to spectral diffusion (temporal fluctuation) has an inverse cubic dependence on the TLS dephasing rate. ($\langle\text{Var}_{ ext{spd}}(\Gamma_1)\rangle_{ ext{ens}} \propto 1/\gamma_\phi^3$)
- Applying transverse noise to depolarize TLSs increases the qubit pure dephasing time. (Based on the negative correlation where larger TLS depolarization rate $\gamma_1$ results in smaller qubit dephasing rate $\Gamma_\phi$.)
- Setup
- Theoretical analysis using joint probability distributions for TLS parameters $P(\epsilon, \Delta)$ and numerical integration to verify analytical expressions across qubit frequencies (2-10 GHz).
- How
- The authors use a theoretical framework based on the standard tunneling model and Lindblad master equations to analyze the noise spectral densities of TLSs. They propose two protocols: applying longitudinal noise to dephase TLSs (to stabilize $T_1$) and transverse noise to depolarize TLSs (to improve $T_\phi$).
- Limitations
- The study is theoretical/predictive and does not provide experimental data from a physical device. · Assumes a constant pure dephasing rate $\gamma_\phi = 10$ MHz for simplicity in some derivations.
Summarized by Gemma 4 from the open-access full text · 10 Oct 2026. Read it