Two-qubit spectroscopy of spatiotemporally correlated quantum noise in superconducting qubits

TL;DR

This paper introduces a protocol for two-qubit noise spectroscopy that characterizes spatiotemporal correlations in quantum noise, validated experimentally on superconducting qubits, and applicable to various qubit systems.

Contribution

It presents a novel, experimentally validated method for simultaneous spectral reconstruction of single- and two-qubit noise correlations using only single-qubit controls and measurements.

Findings

Successfully reconstructed all single- and two-qubit cross-correlation spectra.

Demonstrated the protocol's effectiveness on superconducting qubits with engineered noise.

Method is adaptable to different dephasing qubit architectures.

Abstract

Noise that exhibits significant temporal and spatial correlations across multiple qubits can be especially harmful to both fault-tolerant quantum computation and quantum-enhanced metrology. However, a complete spectral characterization of the noise environment of even a two-qubit system has not been reported thus far. We propose and experimentally validate a protocol for two-qubit dephasing noise spectroscopy based on continuous control modulation. By combining ideas from spin-locking relaxometry with a statistically motivated robust estimation approach, our protocol allows for the simultaneous reconstruction of all the single-qubit and two-qubit cross-correlation spectra, including access to their distinctive non-classical features. Only single-qubit control manipulations and state-tomography measurements are employed, with no need for entangled-state preparation or readout of two-qubit observables. While our experimental validation uses two superconducting qubits coupled to a shared engineered noise source, our methodology is portable to a variety of dephasing-dominated qubit architectures. By pushing quantum noise spectroscopy beyond the single-qubit setting, our work paves the way to characterizing spatiotemporal correlations in both engineered and naturally occurring noise environments.