arXiv subjects

C. Bruder

Publications and source records attributed to C. Bruder.

At least 55 records · Page 3Linked to original sources

arXiv2005

Quantum state transfer in arrays of flux qubits

In this work, we describe a possible experimental realization of Bose's idea to use spin chains for short distance quantum communication [S. Bose, {\it Phys. Rev. Lett.} {\bf 91} 207901]. Josephson arrays have been proposed and analyzed as transmission channels for systems of superconducting charge qubits. Here, we consider a chain of persistent current qubits, that is appropriate for state transfer with high fidelity in systems containing flux qubits. We calculate the fidelity of state transfer for this system. In general, the Hamiltonian of this system is not of XXZ-type, and we analyze the magnitude and the effect of the terms that don't conserve the z-component of the total spin.

cond-mat.mes-hall↗

arXiv2005

Spin filter using a semiconductor quantum ring side-coupled to a quantum wire

We introduce a new spin filter based on spin-resolved Fano resonances due to spin-split levels in a quantum ring (QR) side-coupled to a quantum wire (QW). Spin-orbit coupling inside the QR, together with external magnetic fields, induces spin splitting, and the Fano resonances due to the spin-split levels result in perfect or considerable suppression of the transport of either spin direction. Using the numerical renormalization group method, we find that the Coulomb interaction in the QR enhances the spin filter operation by widening the separation between dips in conductances for different spins and by allowing perfect blocking for one spin direction and perfect transmission for the other. The spin-filter effect persists as long as the temperature is less than the broadening of QR levels due to the QW-QR coupling. We discuss realistic conditions for the QR-based spin filter and its advantages to other similar devices.

cond-mat.mes-hall↗

arXiv2004

Effects of a nonlinear bath at low temperatures

We use the numerical flow-equation renormalization method to study a nonlinear bath at low temperatures. The model of our nonlinear bath consists of a single two-level system coupled to a linear oscillator bath. The effects of this nonlinear bath are analyzed by coupling it to a spin, whose relaxational dynamics under the action of the bath is studied by calculating spin-spin correlation functions. As a first result, we derive flow equations for a general four-level system coupled to an oscillator bath, valid at low temperatures. We then treat the two-level system coupled to our nonlinear bath as a special case of the dissipative four-level system. We compare the effects of the nonlinear bath with those obtained using an effective linear bath, and study the differences between the two cases at low temperatures.

cond-mat.other↗

arXiv2004

Solid-State Quantum Communication With Josephson Arrays

Josephson junction arrays can be used as quantum channels to transfer quantum information between distant sites. In this work we discuss simple protocols to realize state transfer with high fidelity. The channels do not require complicate gating but use the natural dynamics of a properly designed array. We investigate the influence of static disorder both in the Josephson energies and in the coupling to the background gate charges, as well as the effect of dynamical noise. We also analyze the readout process, and its backaction on the state transfer.

quant-ph↗

arXiv2004

Perturbative corrections to the Gutzwiller mean-field solution of the Mott-Hubbard model

We study the Mott-insulator transition of bosonic atoms in optical lattices. Using perturbation theory, we analyze the deviations from the mean-field Gutzwiller ansatz, which become appreciable for intermediate values of the ratio between hopping amplitude and interaction energy. We discuss corrections to number fluctuations, order parameter, and compressibility. In particular, we improve the description of the short-range correlations in the one-particle density matrix. These corrections are important for experimentally observed expansion patterns, both for bulk lattices and in a confining trap potential.

cond-mat.soft↗

arXiv2004

Positive cross-correlations due to Dynamical Channel-Blockade in a three-terminal quantum dot

We investigate current fluctuations in a three-terminal quantum dot in the sequential tunneling regime. In the voltage-bias configuration chosen here, the circuit is operated like a beam splitter, i.e. one lead is used as an input and the other two as outputs. In the limit where a double occupancy of the dot is not possible, a super-Poissonian Fano factor of the current in the input lead and positive cross-correlations between the current fluctuations in the two output leads can be obtained, due to dynamical channel-blockade. When a single orbital of the dot transports current, this effect can be obtained by lifting the spin-degeneracy of the circuit with ferromagnetic leads or with a magnetic field. When several orbitals participate in the electronic conduction, lifting spin-degeneracy is not necessary. In all cases, we show that a super-Poissonian Fano factor for the input current is not equivalent to positive cross-correlations between the outputs. We identify the conditions for obtaining these two effects and discuss possible experimental realizations.

cond-mat.mes-hall↗

arXiv2004

Positive cross-correlations in a three-terminal quantum dot with ferromagnetic contacts

We study current fluctuations in an interacting three-terminal quantum dot with ferromagnetic leads. For appropriately polarized contacts, the transport through the dot is governed by a novel dynamical spin blockade, i.e., a spin-dependent bunching of tunneling events not present in the paramagnetic case. This leads for instance to positive zero-frequency cross-correlations of the currents in the output leads even in the absence of spin accumulation on the dot. We include the influence of spin-flip scattering and identify favorable conditions for the experimental observation of this effect with respect to polarization of the contacts and tunneling rates.

cond-mat.mes-hall↗

arXiv2004

Effects of dephasing on shot-noise in an electronic Mach-Zehnder interferometer

We present a theoretical study of the influence of dephasing on shot noise in an electronic Mach-Zehnder interferometer. In contrast to phenomenological approaches, we employ a microscopic model where dephasing is induced by the fluctuations of a classical potential. This enables us to treat the influence of the environment's fluctuation spectrum on the shot noise. We compare against the results obtained from a simple classical model of incoherent transport, as well as those derived from the phenomenological dephasing terminal approach, arguing that the latter runs into a problem when applied to shot noise calculations for interferometer geometries. From our model, we find two different limiting regimes: If the fluctuations are slow as compared to the time-scales set by voltage and temperature, the usual partition noise expression T(1-T) is averaged over the fluctuating phase difference. For the case of ``fast'' fluctuations, it is replaced by a more complicated expression involving an average over transmission amplitudes. The full current noise also contains other contributions, and we provide a general formula, as well as explicit expressions and plots for specific examples.

cond-mat.mes-hall↗

arXiv2004

Charge transport through a SET with a mechanically oscillating island

We consider a single-electron transistor (SET) whose central island is a nanomechanical oscillator. The gate capacitance of the SET depends on the mechanical displacement, thus, the vibrations of the island vibrations may strongly influence the current-voltage characteristics, current noise, and higher cumulants of the current. Harmonic oscillations of the island and oscillations with random amplitude (e.g., due to the thermal activation) change the transport characteristics in a different way. The noise spectrum has a peak at the frequency of the island oscillations; when the island oscillates harmonically, the peak reduces to a $δ$-peak. We show that knowledge of the SET transport properties helps to determine in what way the island oscillates, to estimate the amplitude, and the frequency of the oscillations.

cond-mat.mes-hall↗

arXiv2003

Quantum Dissipative Dynamics of the Magnetic Resonance Force Microscope in the Single-Spin Detection Limit

We study a model of a magnetic resonance force microscope (MRFM) based on the cyclic adiabatic inversion technique as a high-resolution tool to detect single electron spins. We investigate the quantum dynamics of spin and cantilever in the presence of coupling to an environment. To obtain the reduced dynamics of the combined system of spin and cantilever, we use the Feynman-Vernon influence functional and get results valid at any temperature as well as at arbitrary system-bath coupling strength. We propose that the MRFM can be used as a quantum measurement device, i.e., not only to detect the modulus of the spin but also its direction.

cond-mat↗

arXiv2003

Variable electrostatic transformer: controllable coupling of two charge qubits

We propose and investigate a novel method for the controlled coupling of two Josephson charge qubits by means of a variable electrostatic transformer. The value of the coupling capacitance is given by the discretized curvature of the lowest energy band of a Josephson junction, which can be positive, negative, or zero. We calculate the charging diagram of the two-qubit system that reflects the transition from positive to negative through vanishing coupling. We also discuss how to construct a phase gate making use of the controllable coupling.

cond-mat.mes-hall↗

arXiv2003

Decoherence of cold atomic gases in magnetic micro-traps

We derive a model to describe decoherence of atomic clouds in atom-chip traps taking the excited states of the trapping potential into account. We use this model to investigate decoherence for a single trapping well and for a pair of trapping wells that form the two arms of an atom interferometer. Including the discrete spectrum of the trapping potential gives rise to a decoherence mechanism with a decoherence rate $Γ$ that scales like $Γ\sim 1/r_0^4$ with the distance $r_0$ from the trap minimum to the wire.

quant-ph↗

arXiv2003

Influence of dephasing on shot noise in a Mach-Zehnder interferometer: Dephasing terminal model

This paper has been retracted. The authors have discovered a possible problem in applications of the dephasing terminal approach to shot noise calculations in interference situations: Eq. (7) correctly re-introduces the anticorrelations between the currents in the two arms, but due to its semiclassical nature probably fails to describe the consequential diminishing of exchange effects at the second beamsplitter.

cond-mat.mes-hall↗

arXiv2003

Multiple Andreev Reflections in a Carbon Nanotube Quantum Dot

We report resonant multiple Andreev relections in a multiwall carbon nanotube quantum dot coupled to superconducting leads. The position and magnitude of the subharmonic gap structure is found to depend strongly on the level positions of the single-electron states which are adjusted with a gate electrode. We discuss a theoretical model of the device and compare the calculated differential conductance with the experimental data. (pdf including figures, see: www.unibas.ch/phys-meso/Research/Papers/2003/MAR-MWNT.pdf)

cond-mat.mes-hall↗

arXiv2003

Dephasing in sequential tunneling through a double-dot interferometer

We analyze dephasing in a model system where electrons tunnel sequentially through a symmetric interference setup consisting of two single-level quantum dots. Depending on the phase difference between the two tunneling paths, this may result in perfect destructive interference. However, if the dots are coupled to a bath, it may act as a which-way detector, leading to partial suppression of the phase-coherence and the reappearance of a finite tunneling current. In our approach, the tunneling is treated in leading order whereas coupling to the bath is kept to all orders (using P(E) theory). We discuss the influence of different bath spectra on the visibility of the interference pattern, including the distinction between "mere renormalization effects" and "true dephasing".

cond-mat.mes-hall↗

arXiv2002

Shot Noise and Proximity Effect in Superconductor--Normal-Metal Heterostructures

Current noise provides useful information on correlations important for transport properties of mesoscopic systems. Of particular interest, both experimentally and theoretically, are heterostructures of normal metals and superconductors. We use an extended Keldysh-Green's function approach to calculate the current correlations fully accounting for the superconducting proximity effect. The shot noise in diffusive wires shows a reentrant behaviour, similar to the reentrance effect of the conductance. At intermediate energies the effective charge (the ratio of the differential noise and differential conductance) is suppressed below the value of the Cooper pair charge 2e due to higher correlations. In superconducting heterostructures with more than two normal leads, we address the question of the sign of correlations between currents in different terminals. We show that positive crosscorrelations are a generic feature in these structures, which should be easily observable experimentally.

cond-mat.supr-con↗

arXiv2002

Josephson effect in S$_{\rm F}$XS$_{\rm F}$ junctions

We investigate the Josephson effect in S$_{\rm F}$XS$_{\rm F}$ junctions, where S$_{\rm F}$ is a superconducting material with a ferromagnetic exchange field, and X a weak link. The critical current $I_c$ increases with the (antiparallel) exchange fields if the distribution of transmission eigenvalues of the X-layer has its maximum weight at small values. This exchange field enhancement of the supercurrent does not exist if X is a diffusive normal metal. At low temperatures, there is a correspondence between the critical current in an S$_{\rm F}$IS$_{\rm F}$ junction with collinear orientations of the two exchange fields, and the AC supercurrent amplitude in an SIS tunnel junction. The difference of the exchange fields $h_1-h_2$ in an S$_{\rm F}$IS$_{\rm F}$ junction corresponds to the potential difference $V_1-V_2$ in an SIS junction; i.e., the singularity in $I_c$ [in an S$_{\rm F}$IS$_{\rm F}$ junction] at $|h_1-h_2|=Δ_1+Δ_2$ is the analogue of the Riedel peak. We also discuss the AC Josephson effect in S$_{\rm F}$IS$_{\rm F}$ junctions.

cond-mat.mes-hall↗

arXiv2002

Full Counting Statistics of a Superconducting Beam Splitter

We study the statistics of charge transport in a mesoscopic three-terminal device with one superconducting terminal and two normal-metal terminals. We calculate the full distribution of transmitted charges into the two symmetrically biased normal terminals. In a wide parameter range, we find large positive crosscorrelations between the currents in the two normal arms. We also determine the third cumulant that provides additional information on the statistics not contained in the current noise.

cond-mat.supr-con↗