Kondo-cloud extension around quantum dots – KONEX

Nanofabrication
Scanning gate microscopy
Electronic transport at very low temperature

Data under analysis

Scanning gate microscopy

No publication yet

The Kondo effect in quantum dots is a complex many-body effect that emerges at very low temperature and describes the screening of an unpaired electron spin in the dot by the conduction electrons of the reservoirs. The project focuses on the spatial extension of the screening cloud around the dot, which remains largely unexplored experimentally. According to theory, this so-called Kondo cloud extends over several microns in the reservoirs with a high degree of spin entanglement. We propose to explore these properties by combining transport experiments and scanning gate microscopy at very low temperature on devices containing one or two semiconductor quantum dots coupled to different reservoirs. The expected results include the first measurement of the Kondo cloud extension and the demonstration of its spin entanglement.

The project involves four partners with recognized and complementary expertise in :
- epitaxial growth of GaAs/AlGaAs heterostructures and nanofabrication
(Dominique Mailly and Ulf Gennser at C2N),
- quantum electronics at very low temperature and high frequency
(Louis Jansen and Marc Sanquer at PHELIQS),
- scanning gate microscopy of quantum transport in semiconductor devices
(Hermann Sellier at NEEL, and Benoit Hackens at IMCN in Belgium).

The first objective is to measure the extension of the Kondo cloud around a quantum dot by real space investigations. This task will make use of the scanning gate technique to vary continuously the length of the Kondo reservoir via a movable gate electrode. The measurement of the Kondo length will result from the evolution of the Kondo resonance with the reservoir length, whose density of states will be modulated by finite-size effects. Scanning gate interferometry will also be applied to probe the evolution with distance of the electron transmission phase through the quantum dot, which has a characteristic signature in the Kondo regime.

The second objective is to demonstrate the existence of a Kondo-mediated coupling between two distant quantum dots separated by a common reservoir and placed in the Kondo regime. For a typical Kondo temperature of 300 mK, the predicted Kondo length is about 5 µm, which is very large compared to the size of each quantum dot. The mutual interaction between the Kondo resonances of the two quantum dots will be investigated by transport measurements, using source and drain contacts on each dot. The overlap of the two Kondo clouds should give rise to a global Kondo effect which is sensitive to the spin state of the two quantum dots. The interplay between the RKKY interaction and the Kondo effect will be central in this study.

The third objective is to obtain experimental evidence for the entanglement properties of the Kondo cloud. The approach will be to measure the correlations that the common Kondo cloud should imprint on the spins of the two separate quantum dots coupled to the same Kondo reservoir. Fast single-shot measurements will be performed to determine simultaneously the spin state of the two initially coupled quantum dots after a fast suppression of the Kondo interaction. This experiment will use high-bandwidth charge detectors and the Pauli spin blockade effect in double-dot systems. Observation of non-zero correlations will be a signature of the entangled nature of the Kondo cloud.

KONEX is a fundamental research project in condensed matter physics, corresponding to the evaluation committee CES 30 (section 10, axis 2) of the ANR call. It addresses the general questions of the Kondo-cloud existence, extension, and entanglement. The impact of the conducted research will go beyond the specific field of the physics in quantum dot systems, because the Kondo effect also plays a central role in the challenging field of strongly correlated electron systems.