Electrical Control of a Kondo Spin Screening Cloud

In metals and semiconductors, an impurity spin is quantum entangled with andthereby screened by surrounding conduction electrons at low temperatures,called the Kondo screening cloud. Quantum confinement of the Kondo screeningcloud in a region, called a Kondo box, with a length smaller than the originalcloud extension length strongly deforms the screening cloud and provides a wayof controlling the entanglement. Here we realize such a Kondo box and developan approach to controlling and monitoring the entanglement. It is based on aspin localized in a semiconductor quantum dot, which is screened by conductionelectrons along a quasi-one-dimensional channel. The box is formed between thedot and a quantum point contact placed on a channel. As the quantum pointcontact is tuned to make the confinement stronger, electron conductance throughthe dot as a function of temperature starts to deviate from the known universalfunction of the single energy scale, the Kondo temperature. Nevertheless, theentanglement is monitored by the measured conductance according to ourtheoretical development. The dependence of the monitored entanglement on theconfinement strength and temperature implies that the Kondo screening iscontrolled by tuning the quantum point contact. Namely, the Kondo cloud isdeformed by the Kondo box in the region across the original cloud length. Ourfindings offer a way of manipulating and detecting spatially extended quantummany-body entanglement in solids by electrical means.