641 research outputs found
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.Comment: 7 pages, 2 figure
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.Comment: 4 pages including 2 figure
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.Comment: 12 pages, 8 figure
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.Comment: 4 pages, 1 figur
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.Comment: 7 pages with 3 figures, special style file included; To appear in a
special issue on "Quantum Computation at the Atomic Scale" in Turkish Journal
of Physic
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.Comment: 9 pages, 6 figure
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.Comment: 5 pages including 3 figure
Quantum teleportation by particle-hole annihilation in the Fermi sea
A tunnel barrier in a degenerate electron gas was recently discovered as a
source of entangled particle-hole excitations. The entanglement is produced by
elastic tunneling events, without requiring electron-electron interactions.
Here we investigate the inverse process, the annihilation of an electron and a
hole by elastic scattering. We find that this process leads to teleportation of
the (unknown) state of the annihilated electron to a second, distant electron
-- if the latter was previously entangled with the annihilated hole. We propose
an experiment, involving low-frequency noise measurements on a two-dimensional
electron gas in a high magnetic field, to detect teleportation of electrons and
holes in the two lowest Landau levels.Comment: 5 pages including 2 figures; [2017: fixed broken postscript figures
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