Some of the most intriguing problems in solid state physics arise when the motion of one electron dramatically affects the motion of surrounding electrons. Traditionally, such highly-correlated electron systems have been studied mainly in materials with complex transition metal chemistry. Over the past decade, researchers have learned to confine one or a few electrons within a nanoscale semiconductor "artificial atom", and to understand and control this simple system in exquisite detail. I will discuss how we can combine such individually well-understood components to create a novel highly-correlated electron system within a nano-engineered semiconductor structure [1,2]. We tune the system in situ through a quantum phase transition between two distinct states, each a version of the Kondo state in which a bound electron interacts with surrounding mobile electrons. The boundary between these competing Kondo states is a quantum critical point: the exotic and previously elusive two-channel Kondo state, in which electrons in two reservoirs are entangled through their interaction with a single localized spin.

1. Y. Oreg and D. Goldhaber-Gordon, PRL 90 133602 (2003).
2. R.M. Potok et al., Nature, March 7, 2007.