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 nanometre-scale semiconductor 'artificial atom', and to understand and control this simple system in detail(3). Here we combine artificial atoms to create a highly correlated electron system within a nano-engineered semiconductor structure. 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-namely, 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.

We demonstrate the operation of a quantum spin pump based on cyclic radiofrequency excitation of a GaAs quantum dot, including the ability to pump pure spin without pumping charge. The device takes advantage of bidirectional mesoscopic fluctuations of pumped current, made spin-dependent by the application of an in-plane Zeeman field. Spin currents are measured by placing the pump in a focusing geometry with a spin-selective collector.Using electron spin to encode information in semiconductors holds promise for integrating computation and storage [1] and, in coherent systems, is expected to provide significantly increased immunity from environmental decoherence compared with conventional charge-based electronics [1, 2]. Among the needed elements for any spin-based electronic system is a device that generates a spin current, the analog of a battery in conventional electronics. Candidates for such devices include injection schemes based on magnetic semiconductors [3,4] and ferromagnetic metals [5,6], ferromagnetic resonance devices [7], and a variety of spin-dependent pumps [8][9][10][11][12][13][14].In this Letter, we demonstrate the operation of such a quantum-dot-based spin pump-including the ability to pump pure spin without pumping charge-using a gatedefined lateral quantum dot fabricated on a GaAs/AlGaAs heterostructure. Pumping of

Scaling laws and universality play an important role in our understanding of critical phenomena and the Kondo effect. We present measurements of nonequilibrium transport through a single-channel Kondo quantum dot at low temperature and bias. We find that the low-energy Kondo conductance is consistent with universality between temperature and bias and is characterized by a quadratic scaling exponent, as expected for the spin-1/2 Kondo effect. We show that the nonequilibrium Kondo transport measurements are well described by a universal scaling function with two scaling parameters.