PACS. 03.65.Yz -Decoherence; open systems; quantum statistical methods. PACS. 73.21.La -Quantum dots. PACS. 73.23.Hk -Coulomb blockade; single-electron tunneling.Abstract. -We present a fully electronic analogue of coherent population trapping in quantum optics, based on destructive interference of single-electron tunneling between three quantum dots. A large bias voltage plays the role of the laser illumination. The trapped state is a coherent superposition of the electronic charge in two of these quantum dots, so it is destabilized as a result of decoherence by coupling to external charges. The resulting current I through the device depends on the ratio of the decoherence rate Γ φ and the tunneling rates. For Γ φ → 0 one has simply I = eΓ φ . With increasing Γ φ the current peaks at the inverse trapping time. The direct relation between I and Γ φ can serve as a means of measuring the coherence time of a charge qubit in a transport experiment.c EDP Sciences

Destructive interference of single-electron tunneling between three quantum dots can trap an electron in a coherent superposition of charge on two of the dots. Coupling to external charges causes decoherence of this superposition, and in the presence of a large bias voltage each decoherence event transfers a certain number of electrons through the device. We calculate the counting statistics of the transferred charges, finding a crossover from sub-Poissonian to super-Poissonian statistics with increasing ratio of tunnel and decoherence rates.

A tight binding parametrization of local spin density functional band theory is combined with a dynamical mean field treatment of correlations to obtain a theory of the magnetic transition temperature and optical conductivity and T = 0 spinwave stiffness of a minimal model for the pseudocubic metallic CM R manganites such a La1−X SrxM nO3. The results indicate that previous estimates of Tc obtained by one of us (Phys. Rev. B61 10738-49 (2000)) are in error, that in fact the materials are characterized by Hund's coupling J ≈ 1.5eV , and that magnetic-order driven changes in the kinetic energy may not be the cause of the observed 'colossal' magnetoresistive and multiphase behavior in the manganites, raising questions about our present understanding of these materials.PACS numbers:

The entanglement transfer from electrons localized in a pair of quantum dots to circularly polarized photons is governed by optical selection rules, enforced by conservation of angular momentum. We point out that the transfer can not be achieved by means of unitary evolution unless the angular momentum of the two initial qubit states differs by 2 units ofh. In particular, for spin-entangled electrons the difference in angular momentum is 1 unit -so the transfer fails. Nevertheless, the transfer can be successfully completed if the unitary evolution is followed by a measurement of the angular momentum of each quantum dot and post-processing of the photons using the measured values as input.

The voltage probe model is a model of incoherent scattering in quantum transport. Here we use this model to study the effect of spin-flip scattering on electrical conduction through a quantum dot with chaotic dynamics. The spin decay rate ␥ is quantified by the correlation of spin-up and spin-down current fluctuations ͑spin-flip noise͒. The resulting decoherence reduces the ability of the quantum dot to produce spin-entangled electronhole pairs. For ␥ greater than a critical value ␥ c , the entanglement production rate vanishes identically. The statistical distribution P͑␥ c ͒ of the critical decay rate in an ensemble of chaotic quantum dots is calculated using the methods of random-matrix theory. For small ␥ c this distribution is ϰ␥ c −1+␤/2 , depending on the presence ͑␤ =1͒ or absence ͑␤ =2͒ of time-reversal symmetry. To make contact with experimental observables, we derive a one-to-one relationship between the entanglement production rate and the spin-resolved shot noise, under the assumption that the density matrix is isotropic in the spin degrees of freedom. Unlike the Bell inequality, this relationship holds for both pure and mixed states. In the tunneling regime, the electron-hole pairs are entangled if and only if the correlator of parallel spin currents is at least twice larger than the correlator of antiparallel spin currents.