Letter

A quantum point contact (QPC) is a narrow constriction between two wider electron reservoirs, and is the standard building block of sub-micrometre devices such as quantum dots and qubits (the proposed basic elements of quantum computers). The conductance through a QPC changes as a function of its width in integer steps of G₀ = 2e²/h (where e is the charge on an electron, and h is Planck's constant), signalling the quantization of its transverse modes^1,2. But measurements of these conductance steps also reveal an additional shoulder at a value around 0.7G₀ (refs 1–4), an observation that has remained a puzzle for more than a decade. It has recently been suggested^5,6 that this phenomenon can be explained by the existence of a magnetic 'impurity' in the QPC at low electron densities. Here we present extensive numerical density-functional calculations that reveal the formation of an electronic state with a spin-1/2 magnetic moment in the channel under very general conditions. In addition, we show that such an impurity will also form at large magnetic fields, for a specific value of the field, and sometimes even at the opening of the second transverse mode in the QPC. Beyond explaining the source of the '0.7 anomaly', these results may have far-reaching implications for spin-filling of electronic states in quantum dots and for the dephasing of quantum information stored in semiconductor qubits.

1. Department of Physics,
2. The Ilse Katz Center for Meso- and Nano-Scale Science and Technology, Ben Gurion University, Beer Sheva 84105, Israel

Correspondence to: Toma Rejec¹ Correspondence and requests for materials should be addressed to T.R. (Email: tomaz.rejec@ijs.si).

Received 10 April 2006 |Accepted 5 July 2006