An article published in Nature by the HQC team : observation of the frozen charge of a Kondo resonance

The Hybrid Quantum Circuits team of the Laboratory Pierre Aigrain published in May an article in Nature about the observation of the frozen charge of a Kondo resonance. By coupling high finesse microwave cavities to quantum dot circuits, they have, for the first time, highlighted the freezing of charge dynamics of a Kondo system.

This work opens a new path for the study of strongly correlated electronic systems with the tools of atomic physics and the quantum simulation of fermion-boson systems.

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The study of electronic transport in metallic alloys hosting magnetic impurities brought to light in the sixties a generic problem in condensed matter, the Kondo effect. This effect, which is a paradigm for strongly correlated electron systems, can be nowadays simulated experimentally at the elementary level, by using quantum dots circuits, acting as a magnetic impurity. By coupling such a circuit to a high quality-factor-microwave cavity, the mesoscopics team in collaboration with the theory team of the Laboratoire Pierre Aigrain has succeeded to study the internal dynamics of a strongly correlated electron gas.

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More precisely, the microwave cavity allowed them to measure with an exceptional sensitivity the compressibility of the electron gas, that is, its ability to accommodate charge. The compressibility accounts solely for the charge response of the electronic system, unlike the conductance that generally involves other degrees of freedom such as spin.

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The experimental device allows them to measure simultaneously the conductance and the compressibility, on the contrary to other experiments that rely on electronic transport measurements. The advantage of using mesoscopic circuits with a quantum dot, realized here by confining electrons in a carbon nanotube, is the ability to tune in situ the gas parameters, enabling thus the transition from a free electron gas to a strongly correlated gas, by tuning simply the tunnel coupling between the quantum dot and the nearby electrodes.

In this experiment published in Nature, the physicists have for the first time observed that the Kondo resonance visible in transport measurements was absent from the microwave cavity signal, revealing thus the freezing of the internal charge dynamics of the strongly correlated gas. Unlike the free electron gas which always shifts the cavity resonance frequency, the electron transfer in the Kondo effect relies on a charge ‘frozen’ by Coulomb interactions and on resulting correlations in the electronic cloud. This result has been reproduced by numerical simulations by two theoreticians from the Kyung Hee and Korea universities. This finite conductance from a ’frozen’ charge is a cornerstone of the Kondo model but had not be observed yet. The sensitivity of the high-quality-factor microwave cavities and the tools of cavity quantum electrodynamics could be used in other types of mesoscopic circuits with many-body correlations. Such architecture coupling mesoscopic circuits to microwave cavities provide a model system in which to perform quantum simulation of fermion-boson problems.

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