Non-Gaussian noise spectroscopy with a superconducting qubit sensor

Abstract

Accurate characterization of the noise influencing a quantum system of interest has far-reaching implications across quantum science, ranging from microscopic modeling of decoherence dynamics to noise-optimized quantum control. While the assumption that noise obeys Gaussian statistics is commonly employed, noise is generically non-Gaussian in nature. In particular, the Gaussian approximation breaks down whenever a qubit is strongly coupled to discrete noise sources or has a non-linear response to the environmental degrees of freedom. Thus, in order to both scrutinize the applicability of the Gaussian assumption and capture distinctive non-Gaussian signatures, a tool for characterizing non-Gaussian noise is essential. Here, we experimentally validate a quantum control protocol which, in addition to the spectrum, reconstructs the leading higher-order spectrum of engineered non-Gaussian dephasing noise using a superconducting qubit as a sensor. This first experimental demonstration of non-Gaussian noise spectroscopy represents a major step toward demonstrating a complete spectral estimation toolbox for quantum devices.

Extending comb-based spectral estimation to multiaxis quantum noise

“…On the experimental side, we expect that our results will open new avenues for high quality control of quantum systems, as they give access to all the noise information relevant to the dynamics of a qubit. Indeed, similar experiments to the ones performed in platforms where dephasing noise is dominant [22,24,29], should now also possible in platforms where both T 1 and T 2 processes are significant. As one can see from the above equations, the expectation value of any given observable, given an arbitrary initial state, manifestly depends on both G + and G − filters.…”

“…To date, experimental application of QNS has enabled successful reconstructions of dephasing noise spectra in physical settings as diverse as nuclear magnetic resonance [3,19], superconducting qubits [2,21], spin qubits in semiconductors [22][23][24], trapped ions [25,26], and NV centers in diamond [27,28]. QNS protocols for high-order dephasing spectra resulting from non-Gaussian statistics have also been validated experimentally, using engineered noise on a superconducting qubit sensor operated outside of a linear-response regime [29].…”

“…This may lead to a large inversion problem, making the approach vulnerable to ill-conditioning and to the numerical errors that follow. Even if numerical stability may be improved by employing suitable regularization [29], use of long evolution times for increased sampling resolution may be incompatible with a puredephasing approximation for many systems of interest.…”

Relationship between subjecting the qubit to dynamical decoupling and to a sequence of projective measurements

“…In the case of pure dephasing of the qubit, and the environment being either a source of external classical noise, or (possibly quantum) Gaussian noise, the relation between the dynamical decoupling signals and the properties of the environmental dynamics is well-established [12,13,45]. By an appropriate choice [38,55] of DD sequences one can reconstruct the power spectral density of Gaussian noise, and characterization of polyspectra of non-Gaussian noise is also possible, albeit more challenging [45,56]. Building on earlier results [27], we have recently established a close connection between the DD-based and multiple measurement-based noise spectroscopy in the case of pure dephasing due to external classical noise [29].…”

Quantum Spectrometry for Arbitrary Noise

“…(For an example of a spectrometer which could probe non-Gaussian noise, see Refs. [21,22].) In Appendix 1 of the Supplemental Material we detail an alternative derivation [23].…”