1,086 research outputs found
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.Comment: 5 pages, 3 figures, published versio
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
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
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
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