Observation of the frozen charge of a Kondo resonance

In a quantum dot in the Kondo regime, electrical charges are effectively frozen, but the quantum dot remains electrically conducting owing to strong electron–electron correlations. Conduction from a frozen charge A central theme in condensed matter physics is the understanding of many-body electron–electron interactions, and nanoscale devices enable us to study the underlying principles at the single-electron level. Matthieu Desjardins et al . reveal a remarkable electron–electron interaction effect by examining a carbon nanotube quantum dot placed within a microwave circuit. They tune the quantum dot to the Kondo regime—an archetype of strong electronic correlations—and use combined electronic and microwave measurements to show that, even though electrical charges are effectively frozen because tunnelling of electrons into the dot is not possible, the dot remains electronically conducting. This is due to the strong Kondo correlations. The authors suggest that their measurement platform could be a useful tool for probing charge dynamics in a range of other correlated systems. The ability to control electronic states at the nanoscale has contributed to our modern understanding of condensed matter. In particular, quantum dot circuits represent model systems for the study of strong electronic correlations, epitomized by the Kondo effect 1 , 2 , 3 . We use circuit quantum electrodynamics architectures to study the internal degrees of freedom of this many-body phenomenon. Specifically, we couple a quantum dot to a high-quality-factor microwave cavity to measure with exceptional sensitivity the dot’s electronic compressibility, that is, its ability to accommodate charges. Because electronic compressibility corresponds solely to the charge response of the electronic system, it is not equivalent to the conductance, which generally involves other degrees of freedom such as spin. Here, by performing dual conductance and compressibility measurements in the Kondo regime, we uncover directly the charge dynamics of this peculiar mechanism of electron transfer. The Kondo resonance, visible in transport measurements, is found to be ‘transparent’ to microwave photons trapped in the high-quality cavity, thereby revealing that (in such a many-body resonance) finite conduction is achieved from a charge frozen by Coulomb interaction. This freezing of charge dynamics 4 , 5 , 6 is in contrast to the physics of a free electron gas. We anticipate that the tools of cavity quantum e