Assessing Spatiotemporally Correlated Noise in Superconducting Qubits via Pulse-Based Quantum Noise Spectroscopy

Abstract

Spatiotemporally correlated errors are widespread in quantum devices and are particularly adversarial to error correcting schemes. To characterize these errors, we propose and validate a nonparametric quantum noise spectroscopy (QNS) protocol to estimate both spectra and static errors associated with spatiotemporally correlated dephasing noise and fluctuating quantum crosstalk on two qubits. Our scheme reconstructs the real and imaginary components of the two-qubit cross-spectrum by using fixed total time pulse sequences and single qubit and joint two-qubit measurements to separately resolve spatially correlated noise processes. We benchmark our protocol by reconstructing the spectra of spatiotemporally correlated noise processes engineered via the Schrödinger Wave Autoregressive Moving Average technique, emulating dephasing errors. Furthermore, we show that the protocol can outperform existing comb-based QNS protocols. Our results demonstrate the utility of our protocol in characterizing spatiotemporally correlated noise and quantum crosstalk in a multi-qubit device for potential use in noise-adapted control or error protection schemes.

Figures (9)

• Figure 1: (a) The filter function $G_{n,n}(\omega,T)$ for a fttps with $k=5$. (inset) The cosine function (dashed) for the pulse sequence (solid) yielding the filter function in (a). The (b) real and (c) imaginary parts of the filter function $G_{0,1}(\omega,T)$ generated by the cos-cos and cos-sin switching functions, respectively. The imaginary part of the cos-cos and real part of the cos-sin filter functions are zero and not shown here.
• Figure 2: Simulation of the noise spectral estimates for correlated noise with a delay of $T/20$ and correlated crosstalk. The estimated spectra (marker) are in good agreement with the engineered spectra. (a-b) The single-qubit noise power spectrum for $q_{0}$ and $q_{1}$, respectively. (c) The crosstalk noise spectrum with Lorentzian spectrum and bandpass feature. (d) The real and imaginary components of the cross-spectrum.
• Figure 3: The (a) real and (b) imaginary cross-spectra at four delay times $\{T/4, T/8, T/16\}$, where $T$ is the total time of the fttps. The oscillation frequency increases with longer delay between the two noise processes on the two qubits. The dashed lines show the engineered spectra.
• Figure 4: Reconstructed two-qubit noise spectra for engineered Lorentzian noise, averaging over 50 noise trajectories, with the native-noise baseline subtracted. The dashed lines are the engineered noise spectra, solid lines with markers are the reconstructed spectra, and shaded regions are the CI determined with bootstrapping. Reconstructed (a) single-qubit spectra and (b) cross-spectra for Lorentzian noise with no time delay between the two qubits' noise trajectories. The crosstalk spectrum (inset) shows noise at lower frequencies. The (c) real and (d) imaginary cross-spectra for different delays between the two noise trajectories injected on both qubits.
• Figure 5: Difference in the reconstructed noise spectra using the measurement error mitigated counts pokharel2024scalable and raw counts for (a) $q_1$ and (b) $q_2$ self-spectrum, (c) real- and (d) imaginary cross-spectra, and (e) crosstalk spectrum. The different in the reconstructed noise spectra is shown in (f).