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T. Vorrath

Publications and source records attributed to T. Vorrath.

7 recordsLinked to original sources
arXiv2003

Dynamics of a large spin with weak dissipation

We investigate the generalization of the spin-boson model to arbitrary spin size. The Born-Markov approximation is employed to derive a master equation in the regime of small coupling strengths to the environment. For spin one half, the master equation transforms into a set of Bloch equations, the solution of which is in good agreement with results of the spin-boson model for weak ohmic dissipation. For larger spins, we find a superradiance-like behavior known from the Dicke model. The influence of the nonresonant bosons of the dissipative environment can lead to the formation of a beat pattern in the dynamics of the $z$-component of the spin. The beat frequency is approximately proportional to the cutoff $ω_c$ of the spectral function.

quant-ph↗
arXiv2003

Dicke Effect in the Tunnel Current through two Double Quantum Dots

We calculate the stationary current through two double quantum dots which are interacting via a common phonon environment. Numerical and analytical solutions of a master equation in the stationary limit show that the current can be increased as well as decreased due to a dissipation mediated interaction. This effect is closely related to collective, spontaneous emission of phonons (Dicke super- and subradiance effect), and the generation of a `cross-coherence' with entanglement of charges in singlet or triplet states between the dots. Furthermore, we discuss an inelastic `current switch' mechanism by which one double dot controls the current of the other.

cond-mat.mes-hall↗
arXiv2002

Phonon Cavity Models for Quantum Dot Based Qubits

Phonon cavities are believed to be the next step towards a control of dephasing in semiconductor quantum dot `qubits'. In this paper, we discuss two models for phonon cavities - a surface acoustic wave (SAW) inter-digitated transducer on an infinite half-space, and an elastic thin slab. The inelastic current through double quantum dots in non-perfect SAW cavities exhibits a gap at small energies and is completely suppressed in a perfect, infinite system. In the free-standing slab model, van Hove singularities evolve in the phonon spectral density. We find that these singularities cause additional side peaks in the inelastic current.

cond-mat.mes-hall↗
arXiv2002

Adiabatic Transfer of Electrons in Coupled Quantum Dots

We investigate the influence of dissipation on one- and two-qubit rotations in coupled semiconductor quantum dots, using a (pseudo) spin-boson model with adiabatically varying parameters. For weak dissipation, we solve a master equation, compare with direct perturbation theory, and derive an expression for the `fidelity loss' during a simple operation that adiabatically moves an electron between two coupled dots. We discuss the possibility of visualizing coherent quantum oscillations in electron `pump' currents, combining quantum adiabaticity and Coulomb blockade. In two-qubit spin-swap operations where the role of intermediate charge states has been discussed recently, we apply our formalism to calculate the fidelity loss due to charge tunneling between two dots.

cond-mat.mes-hall↗
arXiv2001

Dynamics of a large-spin-boson system in the strong coupling regime

We investigate collective effects of an ensemble of biased two level systems interacting with a bosonic bath in the strong coupling regime. The two level systems are described by a large pseudo-spin J. An equation for the expectation value M(t) of the z-component of the pseudo spin is derived and solved numerically for an ohmic bath at T=0. In case of a large cut-off frequency of the spectral function, a Markov approximation is justified and an analytical solution is presented. We find that M(t) relaxes towards a highly correlated state with maximum value $\pm J$ for large times. However, this relaxation is extremely slow for most parameter values so as if the system was "frozen in" by interaction with the bosonic bath.

cond-mat.mes-hall↗
arXiv2000

Phonons and Phonon Confinement in Transport through Double Quantum Dots

We calculate the electron-phonon interaction coefficients for surface acoustic waves and for phonons in free standing quantum wells. These are used to derive the inelastic current through a double quantum dot caused by spontaneous emission of phonons. For the case of the free standing structure (phonon cavity), we predict a staircase-like inelastic current superimposed by van Hove singularities. Therefore, the phonon confinement can be detected by electron transport measurements.

cond-mat.mes-hall↗