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Rochus Klesse

Publications and source records attributed to Rochus Klesse.

At least 19 recordsLinked to original sources
arXiv2014

How two spins can thermalize a third spin

We consider thermalization of a microscopic quantum system by interaction with a thermal bath. Our interest is the minimal size the bath can have while still being able to thermalize the system. Within a specific thermalization scheme we show that a single spin-1/2 can be fully thermalized by interaction with a bath that consists of just two other spin-1/2. The two bath spins are initially in a pure, entangled state, and the thermalizing interaction is a Heisenberg exchange-interaction of the system-spin with one of the bath spins. The time dependent coupling strength has to obey a single integral constraint. We also present a simple generalization of this minimal model in which the bath consists of an arbitrary number of spin-1/2 pairs.

cond-mat.stat-mech↗
arXiv2011

Entanglement of remote quantum systems by environmental modes

We investigate the generation of quantum mechanical entanglement of two remote oscillators that are locally coupled to a common bosonic bath. Starting with a Lagrangian formulation of a suitable model, we derive two coupled Quantum Langevin Equations that exactly describe the time evolution of the two local oscillators in presence of the coupling to the bosonic bath. Numerically obtained solutions of the Langevin Equations allow us to study the entanglement generation between the oscillators in terms of the time evolution of the logarithmic negativity. Our results confirm and extend our previously obtained findings, namely that significant entanglement between oscillators embedded in a free bosonic bath can only be achieved if the system are within a microscopic distance. We also consider the case where the bosonic spectral density is substantially modified by imposing boundary conditions on the bath modes. For boundary conditions corresponding to a wave-guide like geometry of the bath we find significantly enlarged entanglement generation. This phenomenon is additionally illustrated within an approximative model that allows for an analytical treatment.

quant-ph↗
arXiv2009

Distance dependence of entanglement generation via a bosonic heat bath

Within a generalized Caldeira-Leggett model we analyze the conditions under which a bosonic heat bath can entangle two microscopic quantum systems at a distance $r$. We find that the attainable entanglement is extremely distance-sensitive. Significant entanglement can only be achieved if the systems are within a {\em microscopic} distance that is of order of the cut-off wavelength $λ$ of the system-bath interaction. At larger distances the maximal entanglement is exponentially suppressed with a decay length of order $λ$. We conclude that entanglement generation via a heat bath is not suitable for entangling remote objects.

quant-ph↗
arXiv2008

Decoherence of encoded quantum registers

In order to eliminate disturbing effects of decoherence, encoding of quantum information in decoherence-free subspaces has been suggested. We analyze the benefits of this concept for a quantum register that is realized in a spin chain in contact with a common bosonic bath. Within a dissipation-less model we provide explicit analytical results for the average fidelity of plain and encoded quantum registers. For the investigation of dissipative spin-boson couplings we employ a master equation of Bloch-Redfield type.

quant-ph↗
arXiv2007

A random-coding based proof for the quantum coding theorem

We present a proof for the quantum channel coding theorem which relies on the fact that a randomly chosen code space typically is highly suitable for quantum error correction. In this sense, the proof is close to Shannon's original treatment of information transmission via a noisy classical channel.

quant-ph↗
arXiv2007

Approximate quantum error correction, random codes, and quantum channel capacity

We work out a theory of approximate quantum error correction that allows us to derive a general lower bound for the entanglement fidelity of a quantum code. The lower bound is given in terms of Kraus operators of the quantum noise. This result is then used to analyze the average error correcting performance of codes that are randomly drawn from unitarily invariant code ensembles. Our results confirm that random codes of sufficiently large block size are highly suitable for quantum error correction. Moreover, employing a lemma of Bennett, Shor, Smolin, and Thapliyal, we prove that random coding attains information rates of the regularized coherent information.

quant-ph↗
arXiv2005

Quantum Error Correction in Spatially Correlated Quantum Noise

We consider quantum error correction of quantum-noise that is created by a local interaction of qubits with a common bosonic bath. The possible exchange of bath bosons between qubits gives rise to spatial and temporal correlations in the noise. We find that these kind of noise correlations have a strong negative impact on quantum error correction.

quant-ph↗
arXiv2004

Coulomb drag between quantum wires with different electron densities

We study the way back-scattering electron--electron interaction generates Coulomb drag between quantum wires with different densities. At low temperature $T$ the system can undergo a commensurate-- incommensurate transition as the potential difference $|W|$ between the two wires passes a critical value $Δ$, and this transition is reflected in a marked change in the dependence of drag resistivity on $W$ and $T$. At high temperature a density difference between the wires suppresses Coulomb drag induced by back scattering, and we use the Tomonaga--Luttinger model to study this suppression in detail.

cond-mat.mes-hall↗
arXiv2003

Residual resistance in two-dimensional, microwave driven Hall systems

We address the origin of the residual resistance observed in microwave irradiated two-dimensional electron gases in a weak magnetic field. We study charge modulations arising from negative photo-conductivity and show that dissipative currents are exponentially suppressed. Relating the exponent to the temperature dependence of microscopic parameters taken from experiment, we find pseudo activated behaviour. In order to obtain these results it is essential to take into account the finite range of the Coulomb interaction.

cond-mat.mes-hall↗
arXiv2002

Electron scattering in multi-wall carbon-nanotubes

We analyze two scattering mechanisms that might cause intrinsic electronic resistivity in multi-wall carbon nanotubes: scattering by dopant impurities, and scattering by inter-tube electron-electron interaction. We find that for typically doped multi-wall tubes backward scattering at dopants is by far the dominating effect.

cond-mat.mtrl-sci↗
arXiv2001

Point-Contact Conductances from Density Correlations

We formulate and prove an exact relation which expresses the moments of the two-point conductance for an open disordered electron system in terms of certain density correlators of the corresponding closed system. As an application of the relation, we demonstrate that the typical two-point conductance for the Chalker-Coddington model at criticality transforms like a two-point function in conformal field theory.

cond-mat.mes-hall↗
arXiv1999

Coulomb Drag between Quantum Wires

We study Coulomb drag in a pair of parallel one-dimensional electron systems within the framework of the Tomanaga-Luttinger model. We find that Coulomb coupling has a much stronger effect on one dimensional wires than on two-dimensional layers: At zero temperature the trans-resistivity {\em diverges}, due to the formation of locked charge density waves. At temperature well above a cross-over temperature $T^*$ the trans-resistivity follows a power law $ρ\propto T^x$, where the interaction-strength dependent exponent $x$ is determined by the Luttinger Liquid parameter $K_{c-}$ of the relative charge mode. At temperature below $T^*$ relative charge displacements are enabled by solitonic excitations, reflected by an exponential temperature dependence. The cross-over temperature $T^*$ depends sensitively on the wire width, inter-wire distance, Fermi wavelength and the effective Bohr radius. For wire distances $\bar{d} \gg k_F^{-1}$ it is exponentially suppressed with $T/E_F \sim \exp[ - \bar{d} k_F / (1-K_{c-}) ]$. The behavior changes drastically if each of the two wires develop spin gaps. In this case we find that the trans-resistivity {\em vanishes} at zero temperature. We discuss our results in view of possible experimental realizations in GaAs-AlGaAs semiconductor structures.

cond-mat.mes-hall↗
arXiv1999

Wave-packet dynamics at the mobility edge in two- and three-dimensional systems

We study the time evolution of wave packets at the mobility edge of disordered non-interacting electrons in two and three spatial dimensions. The results of numerical calculations are found to agree with the predictions of scaling theory. In particular, we find that the $k$-th moment of the probability density $ (t)$ scales like $t^{k/d}$ in $d$ dimensions. The return probability $P(r=0,t)$ scales like $t^{-D_2/d}$, with the generalized dimension of the participation ratio $D_2$. For long times and short distances the probability density of the wave packet shows power law scaling $P(r,t)\propto t^{-D_2/d}r^{D_2-d}$. The numerical calculations were performed on network models defined by a unitary time evolution operator providing an efficient model for the study of the wave packet dynamics.

cond-mat.mes-hall↗
arXiv1999

Modeling Disordered Quantum Systems with Dynamical Networks

It is the purpose of the present article to show that so-called network models, originally designed to describe static properties of disordered electronic systems, can be easily generalized to quantum-{\em dynamical} models, which then allow for an investigation of dynamical and spectral aspects. This concept is exemplified by the Chalker-Coddington model for the Quantum Hall effect and a three-dimensional generalization of it. We simulate phase coherent diffusion of wave packets and consider spatial and spectral correlations of network eigenstates as well as the distribution of (quasi-)energy levels. Apart from that it is demonstrated how network models can be used to determine two-point conductances. Our numerical calculations for the three-dimensional model at the Metal-Insulator transition point delivers among others an anomalous diffusion exponent of $η= 3 - D_2 = 1.7 \pm 0.1$. The methods presented here in detail have been used partially in earlier work.

cond-mat.dis-nn↗
arXiv1997

Spectral Compressibility at the Metal-Insulator Transition of the Quantum Hall Effect

The spectral properties of a disordered electronic system at the metal-insulator transition point are investigated numerically. A recently derived relation between the anomalous diffusion exponent $η$ and the spectral compressibility $χ$ at the mobility edge, $χ=η/2d$, is confirmed for the integer quantum Hall delocalization transition. Our calculations are performed within the framework of an unitary network-model and represent a new method to investigate spectral properties of disordered systems.

cond-mat.mes-hall↗
arXiv1997

Diffusion and multifractality at the metal-insulator transition

We review the time evolution of wavepackets at the metal-insulator transition in two- and three-dimensional disordered systems. The importance of scale invariance and multifractal eigenfunction fluctuations is stressed. The implications of the frequency- and wavevector-dependence of the diffusion coefficient are compared with the results of numerical simulations. We argue that network models are particularly suited for the investigation of the dynamics of disordered systems.

cond-mat.mes-hall↗
arXiv1997

Spatial and Spectral Multifractality of the Local Density of States at the Mobility Edge

We performed numerical calculations of the local density of states (LDOS) at disorder induced localization-delocalization transitions. The LDOS defines a spatial measure for fixed energy and a spectral measure for fixed position. At the mobility edge both measures are multifractal and their generalized dimensions $D(q)$ and $\tilde{D}(q)$ are found to be proportional: $D(q)=d\tilde{D}(q)$, where $d$ is the dimension of the system. This observation is consistent with the identification of the frequency-dependent length scale $L_ω\propto ω^{-1/d}$ as an effective system size. The calculations are performed for two- and three-dimensional dynamical network models with local time evolution operators. The energy dependence of the LDOS is obtained from the time evolution of the local wavefunction amplitude of a wave packet, providing a numerically efficient way to obtain information about the multifractal exponents of the system.

cond-mat.mes-hall↗