7,878 research outputs found
Sudden death of effective entanglement
Sudden death of entanglement is a well-known effect resulting from the finite
volume of separable states. We study the case when the observer has a limited
measurement capability and analyse the effective entanglement, i.e.
entanglement minimized over the output data. We show that in the well defined
system of two quantum dots monitored by single electron transistors, one may
observe a sudden death of effective entanglement when real, physical
entanglement is still alive. For certain measurement setups, this occurs even
for initial states for which sudden death of physical entanglement is not
possible at all. The principles of the analysis may be applied to other
analogous scenarios, such as etimation of the parameters arising from quantum
process tomography.Comment: final version, 5 pages, 3 figure
Semiconductor superlattice photodetectors
A superlattice photomultiplier and a photodetector based on the real space transfer mechanism were studied. The wavelength for the first device is of the order of a micron or flexible corresponding to the bandgap absorption in a semiconductor. The wavelength for the second device is in the micron range (about 2 to 12 microns) corresponding to the energy of the conduction band edge discontinuity between an Al/(sub x)Ga(sub 1-x)As and GaAs interface. Both devices are described
Semiconductor superlattice photodetectors
Two novel types of superlattice photodetectors were studied. The first was a superlattice photomultiplier and the second a photodetector based on the real space transfer mechanism. A summary of the results is presented
Semiconductor superlattice photodetectors
Superlattice photodetectors were investigated. A few major physical processes in the quantum-well heterostructures related to the photon detection and electron conduction mechanisms, the field effect on the wave functions and the energy levels of the electrons, and the optical absorption with and without the photon assistance were studied
Full characterization of a three-photon GHZ state using quantum state tomography
We have performed the first experimental tomographic reconstruction of a
three-photon polarization state. Quantum state tomography is a powerful tool
for fully describing the density matrix of a quantum system. We measured 64
three-photon polarization correlations and used a "maximum-likelihood"
reconstruction method to reconstruct the GHZ state. The entanglement class has
been characterized using an entanglement witness operator and the maximum
predicted values for the Mermin inequality was extracted.Comment: 3 pages, 3 figure
Decoherence Free Subspaces for Quantum Computation
Decoherence in quantum computers is formulated within the Semigroup approach.
The error generators are identified with the generators of a Lie algebra. This
allows for a comprehensive description which includes as a special case the
frequently assumed spin-boson model. A generic condition is presented for
error-less quantum computation: decoherence-free subspaces are spanned by those
states which are annihilated by all the generators. It is shown that these
subspaces are stable to perturbations and moreover, that universal quantum
computation is possible within them.Comment: 4 pages, no figures. Conditions for decoherence-free subspaces made
more explicit, updated references. To appear in PR
On clocks and clouds
Cumulus clouds exhibit a life cycle that consists of (a) the growth phase
(increasing size, most notably in the vertical direction); (b) the mature
phase (growth ceases; any precipitation that develops is strongest during
this period); and (c) the dissipation phase (cloud dissipates because of
precipitation and/or entrainment; no more dynamical support). Although radar
can track clouds over time and give some sense of the age of a cloud, most
aircraft in situ measurements lack temporal context. We use large eddy
simulations of trade wind cumulus cloud fields from cases during the Barbados
Oceanographic and Meteorological Experiment (BOMEX) and Rain In Cumulus over
the Ocean (RICO) campaigns to demonstrate a potential cumulus cloud
"clock." We find that the volume-averaged total water mixing ratio rt
is a useful cloud clock for the 12 clouds studied. A cloud's initial rt
is set by the subcloud mixed-layer mean rt and decreases monotonically
from the initial value due primarily to entrainment. The clock is insensitive
to aerosol loading, environmental sounding and extrinsic cloud properties
such as lifetime and volume. In some cases (more commonly for larger clouds),
multiple pulses of buoyancy occur, which complicate the cumulus clock by
replenishing rt. The clock is most effectively used to classify clouds
by life phase
Realization of logically labeled effective pure states for bulk quantum computation
We report the first use of "logical labeling" to perform a quantum
computation with a room-temperature bulk system. This method entails the
selection of a subsystem which behaves as if it were at zero temperature -
except for a decrease in signal strength - conditioned upon the state of the
remaining system. No averaging over differently prepared molecules is required.
In order to test this concept, we execute a quantum search algorithm in a
subspace of two nuclear spins, labeled by a third spin, using solution nuclear
magnetic resonance (NMR), and employing a novel choice of reference frame to
uncouple nuclei.Comment: PRL 83, 3085 (1999). Small changes made to improve readability and
remove ambiguitie
Noise in Grover's Quantum Search Algorithm
Grover's quantum algorithm improves any classical search algorithm. We show
how random Gaussian noise at each step of the algorithm can be modelled easily
because of the exact recursion formulas available for computing the quantum
amplitude in Grover's algorithm. We study the algorithm's intrinsic robustness
when no quantum correction codes are used, and evaluate how much noise the
algorithm can bear with, in terms of the size of the phone book and a desired
probability of finding the correct result. The algorithm loses efficiency when
noise is added, but does not slow down. We also study the maximal noise under
which the iterated quantum algorithm is just as slow as the classical
algorithm. In all cases, the width of the allowed noise scales with the size of
the phone book as N^-2/3.Comment: 17 pages, 2 eps figures. Revised version. To be published in PRA,
December 199
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