589 research outputs found
Applications of nonequilibrium Kubo formula to the detection of quantum noise
The Kubo fluctuation-dissipation theorem relates the current fluctuations of
a system in an equilibrium state with the linear AC-conductance. This theorem
holds also out of equilibrium provided that the system is in a stationary state
and that the linear conductance is replaced by the (dynamic) conductance with
respect to the non equilibrium state. We provide a simple proof for that
statement and then apply it in two cases. We first show that in an excess noise
measurement at zero temperature, in which the impedance matching is maintained
while driving a mesoscopic sample out of equilibrium, it is the nonsymmetrized
noise power spectrum which is measured, even if the bare measurement, i.e.
without extracting the excess part of the noise, obtains the symmetrized noise.
As a second application we derive a commutation relation for the two components
of fermionic or bosonic currents which holds in every stationary state and
which is a generalization of the one valid only for bosonic currents. As is
usually the case, such a commutation relation can be used e.g. to derive
Heisenberg uncertainty relationships among these current components.Comment: 10 pages, Invited talk to be given by Y. I. at the SPIE Noise
Conference, Grand Canary, June 2004. Added reference and 2 footnotes,
corrected typo in Eq.
Generalized constraints on quantum amplification
We derive quantum constraints on the minimal amount of noise added in linear
amplification involving input or output signals whose component operators do
not necessarily have c-number commutators, as is the case for fermion currents.
This is a generalization of constraints derived for the amplification of
bosonic fields whose components posses c-number commutators.Comment: 4 pages, 1 figure, submitted to Physical Review Letter
Hard Limits on the Growth of the Internet and Computing Capacity
The last few years have seen an explosion in the deployment and use of the Internet, networking and telecommunication technologies. This was followed by significant increases in the speed and capacity of computing, for example Petaflop supercomputers are becoming common. We will examine some of the developments; explain their importance and potential impact. Many forecasts and predictions have been made about the impact of the increases of computing capacity and the growth of the Internet and the world wide web. In this talk we will introduce some of the favorite predictions and will analyze the possibilities for their realization in the long run. The analysis shows that there exist hard limits on the growth of the Internet and the increase in computing capacity. They prove that it is unlikely that some of the predictions will hold in the long run. The restrictions are based on basic physical and economic limitations, which generate tight bounds on the realization of such predictions. The bounds will occur much faster than expected by the simple forecasters
Output spectrum of a measuring device at arbitrary voltage and temperature
We calculate the noise spectrum of the electrical current in a quantum point
contact which is used for continuous measurements of a two-level system
(qubit). We generalize the previous results obtained for the regime of high
transport voltages (when is much larger than the qubit's energy level
splitting (we put )) to the case of arbitrary voltages and
temperatures. When the background output spectrum is essentially
asymmetric in frequency, i.e., it is no longer classical. Yet, the spectrum of
the amplified signal, i.e., the two coherent peaks at is still
symmetric. In the emission (negative frequency) part of the spectrum the
coherent peak can be 8 times higher than the background pedestal.
Alternatively, this ratio can be seen in the directly measureable {\it excess}
noise. For and T=0 the coherent peaks do not appear at all. We relate
these results to the properties of linear amplifiers.Comment: 7 pages, 5 figures, the results generalized for arbitrary angle
between the magnetic field and the observed component of the spin, minor
corrections and typo
Degeneracies in the length spectra of metric graphs
The spectral theory of quantum graphs is related via an exact trace formula
with the spectrum of the lengths of periodic orbits (cycles) on the graphs. The
latter is a degenerate spectrum, and understanding its structure (i.e.,finding
out how many different lengths exist for periodic orbits with a given period
and the average number of periodic orbits with the same length) is necessary
for the systematic study of spectral fluctuations using the trace formula. This
is a combinatorial problem which we solve exactly for complete (fully
connected) graphs with arbitrary number of vertices.Comment: 13 pages, 7 figure
Quantum noise in current biased Josephson junction
Quantum fluctuations in a current biased Josephson junction, described in
terms of the RCSJ-model, are considered. The fluctuations of the voltage and
phase across the junction are assumed to be initiated by equilibrium current
fluctuations in the shunting resistor. This corresponds to low enough
temperatures, when fluctuations of the normal current in the junction itself
can be neglected. We used the quantum Langevin equation in terms of random
variables related to the limit cycle of the nonlinear Josephson oscillator.
This allows to go beyond the perturbation theory and calculate the widths of
the Josephson radiation lines
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