Revealing the finite-frequency response of a bosonic quantum impurity

TL;DR

This paper demonstrates a quantum simulator using cQED to measure the finite-frequency response of a bosonic quantum impurity, revealing many-body effects and boundary nonlinearity renormalization, with implications for quantum criticality and entanglement.

Contribution

It introduces a precisely characterized cQED-based quantum simulator for the boundary sine-Gordon model and maps out its finite frequency linear response, bridging experimental results with advanced theoretical calculations.

Findings

Reactive response shows strong nonlinearity renormalization.

Dissipative response exhibits many-body broadening from multi-photon processes.

Quantitative agreement with diagrammatic calculations and identification of regimes beyond current models.

Abstract

Quantum impurities are ubiquitous in condensed matter physics and constitute the most stripped-down realization of many-body problems. While measuring their finite-frequency response could give access to key characteristics such as excitations spectra or dynamical properties, this goal has remained elusive despite over two decades of studies in nanoelectronic quantum dots. Conflicting experimental constraints of very strong coupling and large measurement bandwidths must be met simultaneously. We get around this problem using cQED tools, and build a precisely characterized quantum simulator of the boundary sine-Gordon model, a non-trivial bosonic impurity problem. We succeeded to fully map out the finite frequency linear response of this system. Its reactive part evidences a strong renormalisation of the nonlinearity at the boundary in agreement with non-perturbative calculations. Its dissipative part reveals a dramatic many-body broadening caused by multi-photon conversion. The experimental results are matched quantitatively to a resummed diagrammatic calculation based on a microscopically calibrated model. Furthermore, we push the device into a regime where diagrammatic calculations break down, which calls for more advanced theoretical tools to model many-body quantum circuits. We also critically examine the technological limitations of cQED platforms to reach universal scaling laws. This work opens exciting perspectives for the future such as quantifying quantum entanglement in the vicinity of a quantum critical point or accessing the dynamical properties of non-trivial many-body problems.