QUROPE Quantum Information Processing and Communication in Europe

Program

The transient response of a quantum dot with strong Coulomb interaction to a fast change in the gate potential, as well as the stationary ac response to a slow harmonic variation in the gate potential are computed by means of a real-time diagrammatic expansion in the tunnel-coupling strength. We find that after a fast switching, the exponential relaxation behavior of charge and spin are governed by a single time constant each, which differ from each other due to Coulomb repulsion. We compare the response to a step potential with the RC time extracted from the ac response.

In order to investigate the effects of interference and interaction in adiabatic pumping, we consider an Aharonov-Bohm (AB) interferometer with a quantum dot embedded either in one or in both arms. We employ a real-time formalism and we perform an expansion both in the tunnel-coupling strengths between dot and leads and in the pumping frequency, taking into account the Coulomb interaction nonperturbatively. We find that pumping in a single-dot AB interferometer has a peristaltic but phase-coherent character.

All-electric devices for the generation and filtering of spin currents are of crucial importance for spintronics experiments and applications. Here we consider a quantum dot with spin-orbit interaction coupled to two metallic leads. After analyzing, the conditions for having nonvanishing spin currents in an adiabatically driven two-terminal device, we focus on a dot with two resonant orbitals and we show by specific examples that both spin filtering and pure spin current generation can be achieved. Finally, we discuss the effect of the Coulomb interaction.

We study the effect of Markovian environmental noise on the dynamics of a two-level quantum system which is steered adiabatically by an external driving field. We express the master equation taking consistently into account all the contributions to the lowest nonvanishing order in the coupling to the Markovian environment. We study the master equation numerically and analytically and we find that, in the adiabatic limit, a zero-temperature environment does not affect the ground-state evolution.

One of the challenges of adiabatic control theory is the proper inclusion of the effects of dissipation. Here we study the adiabatic dynamics of an open two-level quantum system deriving a generalized master equation to consistently account for the combined action of the driving and dissipation. We demonstrate that in the zero-temperature limit the ground state dynamics is not affected by environment. As an example, we apply our theory to Cooper pair pumping, which demonstrates the robustness of ground state adiabatic evolution.

We present a superconducting circuit in which non-Abelian geometric transformations can be realized using an adiabatic parameter cycle. In contrast to previous proposals, we employ quantum evolution in the ground state. We propose an experiment in which the transition from non-Abelian to Abelian cycles can be observed by measuring the pumped charge as a function of the period of the cycle. Alternatively, the non-Abelian phase can be detected using a single-electron transistor working as a charge sensor.

The physics of quantum dots is depicted succinctly by the universal Hamiltonian, where only zero-mode interactions are included. In the case in which the latter involve charging and isotropic spin-exchange terms, this would lead to a non-Abelian action. Here we address an Ising spin-exchange interaction, which leads to an Abelian action. The analysis of this simplified yet nontrivial model shed light on a more general case of charge and spin entanglement. We present a calculation of the tunneling density of states and dynamic magnetic susceptibility.

Nonequilibrium bosonization technique is used to study current fluctuations of interacting electrons in a single-channel quantum wire representing a Luttinger liquid (LL) conductor. An exact expression for the time resolved full counting statistics of the transmitted charge is derived. It is given by the Fredholm determinant of the counting operator with a time-dependent scattering phase. The result has a form of counting statistics of noninteracting particles with fractional charges, induced by scattering off the boundaries between the LL wire and the noninteracting leads.

The inclusion of charging and spin-exchange interactions within the Universal Hamiltonian description of
quantum dots is challenging as it leads to a non-Abelian action. Here we present an exact analytical solution
of the probem, in particular, in the vicinity of the Stoner instabilty point. We calculate several observables,
including the tunneling density of states (TDOS) and the spin susceptibility. Near the instability point the
TDOS exhibits a non-monotonous behavior as function of the tunneling energy, even at temperatures higher