12,286 research outputs found
Measuring the degree of unitarity for any quantum process
Quantum processes can be divided into two categories: unitary and non-unitary
ones. For a given quantum process, we can define a \textit{degree of the
unitarity (DU)} of this process to be the fidelity between it and its closest
unitary one. The DU, as an intrinsic property of a given quantum process, is
able to quantify the distance between the process and the group of unitary
ones, and is closely related to the noise of this quantum process. We derive
analytical results of DU for qubit unital channels, and obtain the lower and
upper bounds in general. The lower bound is tight for most of quantum
processes, and is particularly tight when the corresponding DU is sufficiently
large. The upper bound is found to be an indicator for the tightness of the
lower bound. Moreover, we study the distribution of DU in random quantum
processes with different environments. In particular, The relationship between
the DU of any quantum process and the non-markovian behavior of it is also
addressed.Comment: 7 pages, 2 figure
Abundance of moderate-redshift clusters in the Cold + Hot dark matter model
Using a set of \pppm simulation which accurately treats the density
evolution of two components of dark matter, we study the evolution of clusters
in the Cold + Hot dark matter (CHDM) model. The mass function, the velocity
dispersion function and the temperature function of clusters are calculated for
four different epochs of . We also use the simulation data to test
the Press-Schechter expression of the halo abundance as a function of the
velocity dispersion . The model predictions are in good agreement
with the observational data of local cluster abundances (). We also
tentatively compare the model with the Gunn and his collaborators' observation
of rich clusters at and with the x-ray luminous clusters at
of the {\it Einstein} Extended Medium Sensitivity Survey. The
important feature of the model is the rapid formation of clusters in the near
past: the abundances of clusters of \sigma_v\ge 700\kms and of \sigma_v\ge
1200 \kms at are only 1/4 and 1/10 respectively of the present values
(). Ongoing ROSAT and AXAF surveys of distant clusters will provide
sensitive tests to the model. The abundance of clusters at would
also be a good discriminator between the CHDM model and a low-density flat CDM
model both of which show very similar clustering properties at .Comment: 21 pages + 6 figures (uuencoded version of the PS files), Steward
Preprints No. 118
Transmission statistics and focusing in single disordered samples
We show in microwave experiments and random matrix calculations that in
samples with a large number of channels the statistics of transmission for
different incident channels relative to the average transmission is determined
by a single parameter, the participation number of the eigenvalues of the
transmission matrix, M. Its inverse, M-1, is equal to the variance of relative
total transmission of the sample, while the contrast in maximal focusing is
equal to M. The distribution of relative total transmission changes from
Gaussian to negative exponential over the range in which M-1 changes from 0 to
1. This provides a framework for transmission and imaging in single samples.Comment: 9 pages, 4 figure
Tunnelling Effect and Hawking Radiation from a Vaidya Black Hole
In this paper, we extend Parikh' work to the non-stationary black hole. As an
example of the non-stationary black hole, we study the tunnelling effect and
Hawking radiation from a Vaidya black hole whose Bondi mass is identical to its
mass parameter. We view Hawking radiation as a tunnelling process across the
event horizon and calculate the tunnelling probability. We find that the result
is different from Parikh's work because is the function of
Bondi mass m(v)
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