arXiv subjects

M. Kindermann

Publications and source records attributed to M. Kindermann.

At least 37 records · Page 2Linked to original sources

arXiv2007

Tunneling exponents sensitive to impurity scattering in quantum wires

We show that the scaling exponent for tunneling into a quantum wire in the "Coulomb Tonks gas" regime of impenetrable, but otherwise free, electrons is affected by impurity scattering in the wire. The exponent for tunneling into such a wire thus depends on the conductance through the wire. This striking effect originates from a many-body scattering resonance reminiscent of the Kondo effect. The predicted anomalous scaling is stable against weak perturbations of the ideal Tonks gas limit at sufficiently high energies, similar to the phenomenology of a quantum critical point.

cond-mat.mes-hall↗

arXiv2006

Proposal of an experimentally accessible measure of many-fermion entanglement

We propose a measure of interaction-induced ground state entanglement in many-fermion systems that is experimentally accessible. It is formulated in terms of cross-correlations of currents through resonant fermion levels weakly coupled to the probed system. The proposed entanglement measure vanishes in the absence of many-body interactions and it is related to measures of occupation number entanglement. We evaluate it for two examples of interacting electronic nanostructures.

quant-ph↗

arXiv2006

Multiple crossovers in interacting quantum wires

We study tunneling of electrons into and between interacting wires in the spin-incoherent regime subject to a magnetic field. The tunneling currents follow power laws of the applied voltage with exponents that depend on whether the electron spins at the relevant length scales are polarized or disordered. The crossover length (or energy) scale is exponential in the applied field. In a finite size wire multiple crossovers can occur.

cond-mat.mes-hall↗

arXiv2006

Interference as a Probe of Spin Incoherence in Strongly Interacting Quantum Wires

We show that interference experiments can be used to identify the spin-incoherent regime of strongly interacting one-dimensional conductors. Two qualitative signatures of spin-incoherence are found: a strong magnetic field dependence of the interference contrast and an anomalous scaling of the interference contrast with the applied voltage, with a temperature and magnetic field dependent scaling exponent. The experiments distinguish the spin-incoherent from the spin-polarized regime, and so may be useful in deciding between alternative explanations proposed for the anomalous conductance quantization observed in quantum point contacts and quantum wires at low density.

cond-mat.mes-hall↗

arXiv2005

Shot noise free conductance reduction in quantum wires

We show that a shot noise free current at conductance below 2 e^2/h is possible in short interacting quantum wires without spin-polarization. Our calculation is done for two exactly solvable limits of the ``Coulomb Tonks gas'', a one-dimensional gas of impenetrable electrons that can be realized in ultra-thin quantum wires. In both cases we find that charge transport through such a wire is noiseless at zero temperature while the conductance is reduced to e^2/h.

cond-mat.mes-hall↗

arXiv2005

Hydrodynamic approach to coherent nuclear spin transport

We develop a linear response formalism for nuclear spin diffusion in a dipolar coupled solid. The theory applies to the high-temperature, long-wavelength regime studied in the recent experiments of Boutis et al. [Phys. Rev. Lett. 92, 137201 (2004)], which provided direct measurement of interspin energy diffusion in such a system. A systematic expansion of Kubo's formula in the flip-flop term of the Hamiltonian is used to calculate the diffusion coefficients. We show that this approach is equivalent to the method of Lowe and Gade [Phys. Rev. 156, 817 (1967)] and Kaplan [Phys. Rev. B 2, 4578 (1970)], but has several calculational and conceptual advantages. Although the lowest orders in this expansion agree with the experimental results for magnetization diffusion, this is not the case for energy diffusion. Possible reasons for this disparity are suggested.

cond-mat.mes-hall↗

arXiv2005

Frequency-dependent transport through a quantum dot in the Kondo regime

We study the AC conductance and equilibrium current fluctuations of a Coulomb blockaded quantum dot. A relation between the equilibrium spectral function and the linear AC conductance is derived which is valid for frequencies well below the charging energy of the quantum dot. Frequency-dependent transport measurements can thus give experimental access to the Kondo peak in the equilibrium spectral function of a quantum dot. We illustrate this in detail for typical experimental parameters using the numerical renormalization group method in combination with the Kubo formalism.

cond-mat.str-el↗

arXiv2004

Read out of a Nuclear Spin Qubit

We propose a detector to read out the state of a single nuclear spin, with potential application in future scalable NMR quantum computers. It is based on a ``spin valve'' between bulk nuclear spin systems that is highly sensitive to the state of the measured spin. We suggest a concrete realization of that detector in a Silicon lattice. Transport of spin through the proposed spin valve is analogous to that of charge through an electronic nanostructure, but exhibits distinctive new features.

quant-ph↗

arXiv2004

Correlations of spin currents through a quantum dot induced by the Kondo effect

We study correlations of spin currents flowing through a Coulomb blockaded quantum dot. While vanishing for elastic co-tunneling, these correlations develop as the quantum dot enters the Kondo regime. They are a manifestation of Kondo physics in quantum dots. We demonstrate that the spin current correlator is non-perturbative in the Kondo coupling.

cond-mat.mes-hall↗

arXiv2004

Charge detection enables free-electron quantum computation

It is known that a quantum computer operating on electron-spin qubits with single-electron Hamiltonians and assisted by single-spin measurements can be simulated efficiently on a classical computer. We show that the exponential speed-up of quantum algorithms is restored if single-charge measurements are added. These enable the construction of a CNOT (controlled NOT) gate for free fermions, using only beam splitters and spin rotations. The gate is nearly deterministic if the charge detector counts the number of electrons in a mode, and fully deterministic if it only measures the parity of that number.

quant-ph↗

arXiv2003

Statistics of Heat Transfer in Mesoscopic Circuits

A method to calculate the statistics of energy exchange between quantum systems is presented. The generating function of this statistics is expressed through a Keldysh path integral. The method is first applied to the problem of heat dissipation from a biased mesoscopic conductor into the adjacent reservoirs. We then consider energy dissipation in an electrical circuit around a mesoscopic conductor. We derive the conditions under which measurements of the fluctuations of heat dissipation can be used to investigate higher order cumulants of the charge counting statistics of a mesoscopic conductor.

cond-mat.mes-hall↗

arXiv2003

Entanglement production in a chaotic quantum dot

It has recently been shown theoretically that elastic scattering in the Fermi sea produces quantum mechanically entangled states. The mechanism is similar to entanglement by a beam splitter in optics, but a key distinction is that the electronic mechanism works even if the source is in local thermal equilibrium. An experimental realization was proposed using tunneling between two edge channels in a strong magnetic field. Here we investigate a low-magnetic field alternative, using multiple scattering in a quantum dot. Two pairs of single-channel point contacts define a pair of qubits. If the scattering is chaotic, a universal statistical description of the entanglement production (quantified by the concurrence) is possible. The mean concurrence turns out to be almost independent on whether time-reversal symmetry is broken or not. We show how the concurrence can be extracted from a Bell inequality using low-frequency noise measurements, without requiring the tunneling assumption of earlier work.

cond-mat.mes-hall↗

arXiv2003

Dephasing of entangled electron-hole pairs in a degenerate electron gas

A tunnel barrier in a degenerate electron gas was recently discovered as a source of entangled electron-hole pairs. Here, we investigate the loss of entanglement by dephasing. We calculate both the maximal violation E_max of the Bell inequality and the degree of entanglement (concurrence) C. If the initially maximally entangled electron-hole pair is in a Bell state, then the Bell inequality is violated for arbitrary strong dephasing. The same relation E_max=2\sqrt{1+C^{2}} then holds as in the absence of dephasing. More generally, for a maximally entangled superposition of Bell states, the Bell inequality is satisfied for a finite dephasing strength and the entanglement vanishes for somewhat stronger (but still finite) dephasing strength. There is then no one-to-one relation between E_max and C.

cond-mat↗

arXiv2003

Feedback of the electromagnetic environment on current and voltage fluctuations out of equilibrium

A theory is presented for low-frequency current and voltage correlators of a mesoscopic conductor embedded in a macroscopic electromagnetic environment. This Keldysh field theory evaluated at its saddle-point provides the microscopic justification for our earlier phenomenological calculation (using the cascaded Langevin approach). The nonlinear feedback from the environment mixes correlators of different orders, which explains the unexpected temperature dependence of the third moment of tunneling noise observed in a recent experiment. At non-zero temperature, current and voltage correlators of order three and higher are no longer linearly related. We show that a Hall bar measures voltage correlators in the longitudinal voltage and current correlators in the Hall voltage. We go beyond the saddle-point approximation to consider the environmental Coulomb blockade. We derive that the leading order Coulomb blockade correction to the n-th cumulant of current fluctuations is proportional to the voltage derivative of the (n+1)-th cumulant, generalizing to any n the earlier results for n=1,2.

cond-mat.mes-hall↗

arXiv2003

Production and detection of entangled electron-hole pairs in a degenerate electron gas

We demonstrate theoretically that the shot noise produced by a tunnel barrier in a two-channel conductor violates a Bell inequality. The non-locality is shown to originate from entangled electron-hole pairs created by tunneling events -- without requiring electron-electron interactions. The degree of entanglement (concurrence) equals 2(T_1 T_2)^1/2 (T_1+T_2)^-1, with T_1,T_2 << 1 the transmission eigenvalues. A pair of edge channels in the quantum Hall effect is proposed as experimental realization.

cond-mat.mes-hall↗

arXiv2003

Interaction effects on counting statistics and the transmission distribution

We investigate the effect of weak interactions on the full counting statistics of charge transfer through an arbitrary mesoscopic conductor. We show that the main effect can be incorporated into an energy dependence of the transmission eigenvalues and study this dependence in a non-perturbative approach. An unexpected result is that all mesoscopic conductors behave at low energies like either a single or a double tunnel junction, which divides them into two broad classes.

cond-mat.mes-hall↗

arXiv2003

Temperature dependent third cumulant of tunneling noise

Poisson statistics predicts that the shot noise in a tunnel junction has a temperature independent third cumulant e^2\I, determined solely by the mean current I. Experimental data, however, show a puzzling temperature dependence. We demonstrate theoretically that the third cumulant becomes strongly temperature dependent and may even change sign as a result of feedback from the electromagnetic environment. In the limit of a noninvasive (zero-impedance) measurement circuit in thermal equilibrium with the junction, we find that the third cumulant crosses over from e^2/I at low temperatures to -e^2/I at high temperatures.

cond-mat.mes-hall↗

arXiv2002

Distribution of voltage fluctuations in a current-biased conductor

We calculate the fluctuating voltage V(t) over a conductor driven out of equilibrium by a current source. This is the dual of the shot noise problem of current fluctuations I(t) in a voltage-biased circuit. In the single-channel case the distribution of the accumulated phase Phi=(e/hbar)\int Vdt is the Pascal (or binomial waiting-time) distribution -- distinct from the binomial distribution of transferred charge Q=\int Idt. The weak-coupling limit of a Poissonian P(Phi) is reached in the limit of a ballistic conductor, while in the tunneling limit P(Phi) has the chi-square form.

cond-mat.mes-hall↗