721 research outputs found

### Coulomb Charging at Large Conduction

We discuss the suppression of Coulomb charging effects on a small metallic
island coupled to an electrode by a tunnel junction. At high temperatures the
quantum corrections to the classical charging energy $E_c=e^2/2C$, where $C$ is
the island capacitance, are evaluated. At low temperatures the large quantum
fluctuations of the island charge cause a strong reduction of the effective
$E_c$ which is determined explicitly in the limit of a large tunneling
conductance.Comment: 4 page

### Statistics of the polariton condensate

The influence of polariton-polariton scattering on the statistics of the
polariton condensate in a non-resonantly excited semiconductor quantum well
embedded in a CdTe semiconductor microcavity is discussed. Taking advantage of
the existence of a bottleneck in the polariton dispersion curve, the polariton
states are separated into two domains: reservoir polaritons inside the
bottleneck and active polaritons with wave vector q whose energy lies below the
bottleneck. In the framework of the master equation formalism, the
non-equilibrium stationary reduced density matrix is calculated and the
statistics of polaritons in the condensate at q=0 is determined. The anomalous
correlations between the polaritons in the condensate and those with wave
vectors q, -q leads to an enhancement of the noise in the condensate. As a
consequence, the second order correlation function of the condensate does not
show the full coherence that is characteristic of laser emission.Comment: 35 pages, 5 figure

### Accuracy of a mechanical single electron shuttle

Motivated by recent experiments, we calculate both the average current and
the current fluctuations for a metallic island which oscillates between two
symmetric electrodes. Electrons can only tunnel on or off the island when it is
close to one of the electrodes. Using a Master equation we investigate the
accuracy of such an electron shuttle both analytically and numerically. It is
shown that optimum operation is reached when the contact time is much larger
than the RC-time.Comment: RevTeX, 8 pages, 5 figure

### Phase diffusion and charging effects in Josephson junctions

The supercurrent of a Josephson junction is reduced by phase diffusion. For
ultrasmall capacitance junctions the current may be further decreased by
Coulomb blockade effects. We calculate the Cooper pair current by means of
time-dependent perturbation theory to all orders in the Josephson coupling
energy and obtain the current-voltage characteristic in closed form in a range
of parameters of experimental interest. The results comprehend phase diffusion
of the coherent Josephson current in the classical regime as well as the
supercurrent peak due to incoherent Cooper pair tunneling in the strong Coulomb
blockade regime.Comment: 4 pages, 3 figures, RevTe

### Is the dynamics of open quantum systems always linear?

We study the influence of the preparation of an open quantum system on its
reduced time evolution. In contrast to the frequently considered case of an
initial preparation where the total density matrix factorizes into a product of
a system density matrix and a bath density matrix the time evolution generally
is no longer governed by a linear map nor is this map affine. Put differently,
the evolution is truly nonlinear and cannot be cast into the form of a linear
map plus a term that is independent of the initial density matrix of the open
quantum system. As a consequence, the inhomogeneity that emerges in formally
exact generalized master equations is in fact a nonlinear term that vanishes
for a factorizing initial state. The general results are elucidated with the
example of two interacting spins prepared at thermal equilibrium with one spin
subjected to an external field. The second spin represents the environment. The
field allows the preparation of mixed density matrices of the first spin that
can be represented as a convex combination of two limiting pure states, i.e.
the preparable reduced density matrices make up a convex set. Moreover, the map
from these reduced density matrices onto the corresponding density matrices of
the total system is affine only for vanishing coupling between the spins. In
general, the set of the accessible total density matrices is nonconvex.Comment: 19 pages, 3 figures, minor changes to improve readability, discussion
on Mori's linear regime and references adde

### Fractional charge in the noise of Luttinger liquid systems

The current noise of a voltage biased interacting quantum wire adiabatically
connected to metallic leads is computed in presence of an impurity in the wire.
We find that in the weak backscattering limit the Fano factor characterizing
the ratio between shot noise and backscattering current crucially depends on
the noise frequency relative to the ballistic frequency v_F/gL, where v_F is
the Fermi velocity, g the Luttinger liquid interaction parameter, and L the
length of the wire. In contrast to chiral Luttinger liquids, the noise is not
only due to the Poissonian backscattering of fractionally charged
quasiparticles at the impurity, but also depends on Andreev-type reflections of
plasmons at the contacts, so that the frequency dependence of the noise needs
to be analyzed to extract the fractional charge e*=e g of the bulk excitations.
We show that the frequencies needed to see interaction effects in the Fano
factor are within experimental reach.Comment: 9 pages, 4 figures, conference proceedings of Fluctuations and Noise
2005, Austin, Texa

### Direct measurement of the maximum tunnel rate in a radio frequency single electron transistor operated as a microwave mixer

By operating the radio frequency single electron transistor (rf-SET) as a
mixer we present measurements in which the RC roll-off of the tunnel junctions
is observed at high frequencies. Our technique makes use of the non-linear
rf-SET transconductance to mix high frequency gate signals and produce
difference-frequency components that fall within the bandwidth of the rf-SET.
At gate frequencies >15GHz the induced charge on the rf-SET island is altered
on time-scales faster than the inverse tunnel rate, preventing mixer operation.
We suggest the possibility of utilizing this technique to sense high frequency
signals beyond the usual rf-SET bandwidth.Comment: Submitted to Applied Physics Letters. Comments always very welcome,
email:[email protected] (New version contains extra data and new figs

### Appearance of fractional charge in the noise of non-chiral Luttinger liquids

The current noise of a voltage biased interacting quantum wire adiabatically
connected to metallic leads is computed in presence of an impurity in the wire.
We find that in the weak backscattering limit the Fano factor characterizing
the ratio between noise and backscattered current crucially depends on the
noise frequency $\omega$ relative to the ballistic frequency $v_F/gL$, where
$v_F$ is the Fermi velocity, $g$ the Luttinger liquid interaction parameter,
and $L$ the length of the wire. In contrast to chiral Luttinger liquids the
noise is not only due to the Poissonian backscattering of fractionally charged
quasiparticles at the impurity, but also depends on Andreev-type reflections at
the contacts, so that the frequency dependence of the noise needs to be
analyzed to extract the fractional charge $e^*=e g$ of the bulk excitations.Comment: 4 pages, 2 figures, final version, to appear in PR

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