62,032 research outputs found
A fast and robust approach to long-distance quantum communication with atomic ensembles
Quantum repeaters create long-distance entanglement between quantum systems
while overcoming difficulties such as the attenuation of single photons in a
fiber. Recently, an implementation of a repeater protocol based on single
qubits in atomic ensembles and linear optics has been proposed [Nature 414, 413
(2001)]. Motivated by rapid experimental progress towards implementing that
protocol, here we develop a more efficient scheme compatible with active
purification of arbitrary errors. Using similar resources as the earlier
protocol, our approach intrinsically purifies leakage out of the logical
subspace and all errors within the logical subspace, leading to greatly
improved performance in the presence of experimental inefficiencies. Our
analysis indicates that our scheme could generate approximately one pair per 3
minutes over 1280 km distance with fidelity (F>78%) sufficient to violate
Bell's inequality.Comment: 10 pages, 4 figures, 5 tables (Two appendixes are added to justify
two claims used in the maintext.
Sputtered Gold as an Effective Schottky Gate for Strained Si/SiGe Nanostructures
Metallization of Schottky surface gates by sputtering Au on strained Si/SiGe
heterojunctions enables the depletion of the two dimensional electron gas
(2DEG) at a relatively small voltage while maintaining an extremely low level
of leakage current. A fabrication process has been developed to enable the
formation of sub-micron Au electrodes sputtered onto Si/SiGe without the need
of a wetting layer.Comment: 3 pages, 3 figure
Factors of sums and alternating sums involving binomial coefficients and powers of integers
We study divisibility properties of certain sums and alternating sums
involving binomial coefficients and powers of integers. For example, we prove
that for all positive integers , , and any
nonnegative integer , there holds {align*} \sum_{k=0}^{n_1}\epsilon^k
(2k+1)^{2r+1}\prod_{i=1}^{m} {n_i+n_{i+1}+1\choose n_i-k} \equiv 0 \mod
(n_1+n_m+1){n_1+n_m\choose n_1}, {align*} and conjecture that for any
nonnegative integer and positive integer such that is odd, where .Comment: 14 pages, to appear in Int. J. Number Theor
Cavity QED with atomic mirrors
A promising approach to merge atomic systems with scalable photonics has
emerged recently, which consists of trapping cold atoms near tapered
nanofibers. Here, we describe a novel technique to achieve strong, coherent
coupling between a single atom and photon in such a system. Our approach makes
use of collective enhancement effects, which allow a lattice of atoms to form a
high-finesse cavity within the fiber. We show that a specially designated
"impurity" atom within the cavity can experience strongly enhanced interactions
with single photons in the fiber. Under realistic conditions, a "strong
coupling" regime can be reached, wherein it becomes feasible to observe vacuum
Rabi oscillations between the excited impurity atom and a single cavity
quantum. This technique can form the basis for a scalable quantum information
network using atom-nanofiber systems.Comment: 20 pages, 4 figure
Rank-ordered Multifractal Spectrum for Intermittent Fluctuations
We describe a new method that is both physically explicable and
quantitatively accurate in describing the multifractal characteristics of
intermittent events based on groupings of rank-ordered fluctuations. The
generic nature of such rank-ordered spectrum leads it to a natural connection
with the concept of one-parameter scaling for monofractals. We demonstrate this
technique using results obtained from a 2D MHD simulation. The calculated
spectrum suggests a crossover from the near Gaussian characteristics of small
amplitude fluctuations to the extreme intermittent state of large rare events.Comment: 4 pages, 5 figure
Limits to solar cycle predictability: Cross-equatorial flux plumes
Within the Babcock-Leighton framework for the solar dynamo, the strength of a
cycle is expected to depend on the strength of the dipole moment or net
hemispheric flux during the preceding minimum, which depends on how much flux
was present in each hemisphere at the start of the previous cycle and how much
net magnetic flux was transported across the equator during the cycle. Some of
this transport is associated with the random walk of magnetic flux tubes
subject to granular and supergranular buffeting, some of it is due to the
advection caused by systematic cross-equatorial flows such as those associated
with the inflows into active regions, and some crosses the equator during the
emergence process.
We aim to determine how much of the cross-equatorial transport is due to
small-scale disorganized motions (treated as diffusion) compared with other
processes such as emergence flux across the equator. We measure the
cross-equatorial flux transport using Kitt Peak synoptic magnetograms,
estimating both the total and diffusive fluxes. Occasionally a large sunspot
group, with a large tilt angle emerges crossing the equator, with flux from the
two polarities in opposite hemispheres. The largest of these events carry a
substantial amount of flux across the equator (compared to the magnetic flux
near the poles). We call such events cross-equatorial flux plumes. There are
very few such large events during a cycle, which introduces an uncertainty into
the determination of the amount of magnetic flux transported across the equator
in any particular cycle. As the amount of flux which crosses the equator
determines the amount of net flux in each hemisphere, it follows that the
cross-equatorial plumes introduce an uncertainty in the prediction of the net
flux in each hemisphere. This leads to an uncertainty in predictions of the
strength of the following cycle.Comment: A&A, accepte
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