122 research outputs found
Spin noise and Bell inequalities in a realistic superconductor-quantum dot entangler
Charge and spin current correlations are analyzed in a source of
spin-entangled electrons built from a superconductor and two quantum dots in
parallel. In addition to the ideal (crossed Andreev) channel, parasitic
channels (direct Andreev and cotunneling) and spin flip processes are fully
described in a density matrix framework. The way they reduce both the
efficiency and the fidelity of the entangler is quantitatively described by
analyzing the zero-frequency noise correlations of charge current as well as
spin current in the two output branches. Spin current noise is characterized by
a spin Fano factor, equal to 0 (total current noise) and -1 (crossed
correlations) for an ideal entangler. The violation of the Bell inequalities,
as a test of non-locality (entanglement) of split pairs, is formulated in terms
of the correlations of electron charge and spin numbers counted in a specific
time window . The efficiency of the test is analyzed, comparing to
the various time scales in the entangler operation.Comment: 8 pages, 5 figures, references added, to appear in Phys. Rev.
Full counting statistics of super-Poissonian shot noise in multi-level quantum dots
We examine the full counting statistics of quantum dots, which display
super-Poissonian shot noise. By an extension to a generic situation with many
excited states we identify the underlying transport process. The statistics is
a sum of independent Poissonian processes of bunches of different sizes, which
leads to the enhanced noise. The obtained results could be useful to determine
transport characteristics in molecules and large quantum dots, since the noise
(and higher cumulants) allow to identify the internal level structure, which is
not visible in the average current.Comment: 4 pages, 1 figure, submitted to PR
Magnetic flux noise in the three Josephson junctions superconducting ring
We analyze the influence of noise on magnetic properties of a su-
perconducting loop which contains three Josephson junctions. This circuit is a
classical analog of a persistent current (flux) qubit. A loop supercurrent
induced by external magnetic field in the presence of thermal fluctuations is
calculated. In order to get connection with experiment we calculate the
impedance of the low-frequency tank cir- cuit which is inductively coupled with
a loop of interest. We compare obtained results with the results in quantum
mode - when the three junction loop exhibits quantum tunneling of the magnetic
flux. We demonstrate that the tank-loop impedance in the classical and quan-
tum modes have different temperature dependence and can be easily distinguished
experimentally.Comment: 19 pages 9 figure
Advances in point-contact spectroscopy: two-band superconductor MgB2 (A review)
Analysis of the point-contact spectroscopy (PCS) data on the new dramatic
high-T superconductor MgB reveals quite different behavior of two
disconnected and electronic bands, deriving from their
anisotropy, different dimensionality, and electron-phonon interaction. PCS
allows direct registration of both the superconducting gaps and
electron-phonon-interaction spectral function of the two-dimensional
and three-dimensional band, establishing correlation between the gap
value and intensity of the high-T driving force -- the boron
vibration mode. PCS data on some nonsuperconducting transition-metal diborides
are surveyed for comparison.Comment: 17 pages, 30 figs., will be published in Low Temp. Phys. V.30 (2004)
N
Shot noise in superconducting junctions with weak link formed by Anderson impurity
A theory is developed to study shot noise in superconducting (SAS) and hybrid
(SAN) junctions with singly occupied Anderson impurity (A) as a weak link. The
zero-frequency DC component of the shot noise spectral density is calculated at
zero temperature as a function of the bias at different Coulomb repulsion
strengths U, and show a remarkable structure resulting from combination of
electron-electron interaction and Andreev reflections.Comment: 4 two column pages including 4 .eps figure
Shot noise and Coulomb blockade of Andreev reflection
We derive low energy effective action for a short coherent conductor between
normal (N) and superconducting (S) reservoirs. We evaluate interaction
correction to Andreev conductance and demonstrate a close relation
between Coulomb effects and shot noise in NS systems. In the diffusive limit
doubling of both shot noise power and charge of the carriers yields four times bigger than in the normal case. Our predictions can be directly
tested in future experiments.Comment: 4 pages, 2 figure
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.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Kondo effects and shot noise enhancement in a laterally coupled double quantum dot
The spin and orbital Kondo effects and the related shot noise for a laterally
coupled double quantum dot are studied taking account of coherent indirect
coupling via a reservoir. We calculate the linear conductance and shot noise
for various charge states to distinguish between the spin and orbital Kondo
effects. We find that a novel antiferromagnetic exchange coupling can be
generated by the coherent indirect coupling, and it works to suppress the spin
Kondo effect when each quantum dot holds just one electron. We also show that
we can capture the feature of the pseudospin Kondo effect from the shot noise
measurement.Comment: 11 pages, 5 figures, accepted for publication in Physical Review
The effect of symmetry class transitions on the shot noise in chaotic quantum dots
Using the random matrix theory (RMT) approach, we calculated the weak
localization correction to the shot noise power in a chaotic cavity as a
function of magnetic field and spin-orbit coupling. We found a remarkably
simple relation between the weak localization correction to the conductance and
to the shot noise power, that depends only on the channel number asymmetry of
the cavity. In the special case of an orthogonal-unitary crossover, our result
coincides with the prediction of Braun et. al [J. Phys. A: Math. Gen. {\bf 39},
L159-L165 (2006)], illustrating the equivalence of the semiclassical method to
RMT.Comment: 4 pages, 1 figur
Experimental Test of the High-Frequency Quantum Shot Noise Theory in a Quantum Point Contact
We report on direct measurements of the electronic shot noise of a quantum
point contact at frequencies nu in the range 4-8 GHz. The very small energy
scale used ensures energy independent transmissions of the few transmitted
electronic modes and their accurate knowledge. Both the thermal energy and the
quantum point contact drain-source voltage Vds are comparable to the photon
energy hnu leading to observation of the shot noise suppression when
. Our measurements provide the first complete test of the finite
frequency shot noise scattering theory without adjustable parameters.Comment: Version Published in Phys. Rev. Lett. (Phys. Rev. Lett. 99, 236803
(2007)
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