Results of calculations and high source-drain transport measurements are presented which demonstrate voltage-tunable entanglement of electron pairs in lateral quantum dots. At a fixed magnetic field, the application of a judiciously-chosen gate voltage alters the ground-state of an electron pair from an entagled spin singlet to a spin triplet.

We present experimental and theoretical results on a new regime in quantum dots in which the filling factor two-singlet state is replaced by new spin polarized phases. We make use of spin blockade spectroscopy to identify the transition to this new regime as a function of the number of electrons. The key experimental observation is a reversal of the phase in the systematic oscillation of the amplitude of Coulomb blockade peaks as the number of electrons is increased above a critical number. It is found theoretically that correlations are crucial to the existence of the new phases.

We study the quantum dynamics of soliton-like domain walls in anisotropic spin-1/2 chains in the presence of magnetic fields. In the absence of fields, domain walls form a Bloch band of delocalized quantum states while a static field applied along the easy axis localizes them into Wannier wave packets and causes them to execute Bloch oscillations, i.e. the domain walls oscillate along the chain with a finite Bloch frequency and amplitude. In the presence of the field, the Bloch band, with a continuum of extended states, breaks up into the Wannier-Zeeman ladder-a discrete set of equally spaced energy levels. We calculate the dynamical structure factor S zz (q, ω) in the one-soliton sector at finite frequency, wave vector, and temperature, and find sharp peaks at frequencies which are integer multiples of the Bloch frequency. We further calculate the uniform magnetic susceptibility and find that it too exhibits peaks at the Bloch frequency. We identify several candidate materials where these Bloch oscillations should be observable, for example, via neutron scattering measurements. For the particular compound CoCl 2 ·2H 2 O we estimate the Bloch amplitude to be on the order of a few lattice constants, and the Bloch frequency on the order of 100 GHz for magnetic fields in the Tesla range and at temperatures of about 18 Kelvin. 75.60.Ch, 75.40.Gb, 75.90.+w, 76.60.Es, 71.70.Ej

Quantum tunneling of domain walls out of an impurity potential in a mesoscopic ferromagnetic sample is investigated. Using improved expressions for the domain-wall mass and for the pinning potential, we find that the crossover temperature between thermal activation and quantum tunneling is of a different functional form than found previously. In materials such as Ni or yttrium iron garnet, the crossover temperatures are around 5 mK. We also find that the WKB exponent is typically two orders of magnitude larger than current estimates. The sources for these discrepancies are discussed, and precise estimates for the transition from threedimensional-to-one-dimensional magnetic behavior of a wire are given. The crossover temperatures from thermal-to-quantum transitions and tunneling rates are calculated for various materials and sample sizes.

We propose a universal quantum computing scheme in which the orthogonal qubit states $|0>$ and $|1>$ are identical in their single-particle spin and charge properties. Each qubit is contained in a single quantum dot and gate operations are induced all-electrically by changes in the confinement potential. Within the computational space, these qubits are robust against environmental influences that couple to the system through single-particle channels. Due to the identical spin and charge properties of the $|0>$, $|1>$ states, the lowest-order relaxation and decoherence rates $1/T_1$ and $1/T_2$, within the Born-Markov approximation, both vanish for a large class of environmental couplings. We give explicit pulse sequences for a universal set of gates (phase, $\pi/8$, Hadamard, \textsc{cnot}) and discuss state preparation, manipulation, and detection.Comment: 6 pages, 3 eps figures, revtex