7,109 research outputs found
Noise assisted Ramsey interferometry
I analyze a metrological strategy for improving the precision of frequency
estimation via Ramsey interferometry with strings of atoms in the presence of
correlated dephasing. This strategy does not employ entangled states, but
rather a product state which evolves into a stationary state under the
influence of correlated dephasing. It is shown that by using this state an
improvement in precision compared to standard Ramsey interferometry can be
gained. This improvement is not an improvement in scaling, i.e. the estimation
precision has the same scaling with the number of atoms as the standard quantum
limit, but an improvement proportional to the free evolution time in the Ramsey
interferometer. Since a stationary state is used, this evolution time can be
substantially larger than in standard Ramsey interferometry which is limited by
the coherence time of the atoms.Comment: 8+1 pages; 5 figure
Correlations in Quantum Physics
We provide an historical perspective of how the notion of correlations has
evolved within quantum physics. We begin by reviewing Shannon's information
theory and its first application in quantum physics, due to Everett, in
explaining the information conveyed during a quantum measurement. This
naturally leads us to Lindblad's information theoretic analysis of quantum
measurements and his emphasis of the difference between the classical and
quantum mutual information. After briefly summarising the quantification of
entanglement using these and related ideas, we arrive at the concept of quantum
discord that naturally captures the boundary between entanglement and classical
correlations. Finally we discuss possible links between discord and the
generation of correlations in thermodynamic transformations of coupled harmonic
oscillators.Comment: 10 pages, 1 figure. Submitted to Int. J. Mod. Phys. B, special issue
"Classical Vs Quantum correlations in composite systems" edited by L. Amico,
S. Bose, V. Korepin and V. Vedra
Joint analysis of TeV blazar light curves with FACT and HAWC
Probing the high energy emission processes of blazars through their
variability relies crucially on long-term monitoring. We present unprecedented
light curves from unbiased observations of very high energy fluxes from the
blazars Mrk 421 and Mrk 501 based on a joint analysis of data from the First
G-APD Cherenkov Telescope (FACT) and the High Altitude Water Cherenkov (HAWC)
Observatory. Thanks to an offset of 5.3 hours of the geographic locations, a
complementary coverage of up to 12 hours of observation per day allows us to
track variability on time scales of hours to days in more detail than with
single-instrument analyses. Complementary features, such as better sensitivity
thanks to a lower energy threshold with FACT and more regular coverage
throughout the year with HAWC, provide valuable cross checks and extensions to
the individual analyses. Daily flux comparisons for both Mrk 421 and Mrk 501
show largely correlated variations with a few significant exceptions. These
deviations between measurements can be explained through fast variability
within a few hours and will be discussed in detail.Comment: Presented at the 35th International Cosmic Ray Conference (ICRC2017),
Bexco, Busan, Korea. See arXiv:1708.02572 for all HAWC contribution
Long Distance Entanglement Generation in 2D Networks
We consider 2D networks composed of nodes initially linked by two-qubit mixed
states. In these networks we develop a global error correction scheme that can
generate distance-independent entanglement from arbitrary network geometries
using rank two states. By using this method and combining it with the concept
of percolation we also show that the generation of long distance entanglement
is possible with rank three states. Entanglement percolation and global error
correction have different advantages depending on the given situation. To
reveal the trade-off between them we consider their application on networks
containing pure states. In doing so we find a range of pure-state schemes, each
of which has applications in particular circumstances: For instance, we can
identify a protocol for creating perfect entanglement between two distant
nodes. However, this protocol can not generate a singlet between any two nodes.
On the other hand, we can also construct schemes for creating entanglement
between any nodes, but the corresponding entanglement fidelity is lower.Comment: 10 pages, 9 figures, 1 tabl
The Optical Excitation of Zigzag Carbon Nanotubes with Photons Guided in Nanofibers
We consider the excitation of electrons in semiconducting carbon nanotubes by
photons from the evanescent field created by a subwavelength-diameter optical
fiber. The strongly changing evanescent field of such nanofibers requires
dropping the dipole approximation. We show that this leads to novel effects,
especially a high dependence of the photon absorption on the relative
orientation and geometry of the nanotube-nanofiber setup in the optical and
near infrared domain. In particular, we calculate photon absorption
probabilities for a straight nanotube and nanofiber depending on their relative
angle. Nanotubes orthogonal to the fiber are found to perform much better than
parallel nanotubes when they are short. As the nanotube gets longer the
absorption of parallel nanotubes is found to exceed the orthogonal nanotubes
and approach 100% for extremely long nanotubes. In addition, we show that if
the nanotube is wrapped around the fiber in an appropriate way the absorption
is enhanced. We find that optical and near infrared photons could be converted
to excitations with efficiencies that may exceed 90%. This may provide
opportunities for future photodetectors and we discuss possible setups.Comment: 14 pages, 14 figure
Singlet Generation in Mixed State Quantum Networks
We study the generation of singlets in quantum networks with nodes initially
sharing a finite number of partially entangled bipartite mixed states. We prove
that singlets between arbitrary nodes in such networks can be created if and
only if the initial states connecting the nodes have a particular form. We then
generalize the method of entanglement percolation, previously developed for
pure states, to mixed states of this form. As part of this, we find and compare
different distillation protocols necessary to convert groups of mixed states
shared between neighboring nodes of the network into singlets. In addition, we
discuss protocols that only rely on local rules for the efficient connection of
two remote nodes in the network via entanglement swapping. Further improvements
of the success probability of singlet generation are developed by using
particular forms of `quantum preprocessing' on the network. This includes
generalized forms of entanglement swapping and we show how such strategies can
be embedded in regular and hierarchical quantum networks.Comment: 17 pages, 21 figure
Experimental investigations on sodium-filled heat pipes
The possibilities of producing heat pipes and, especially, the necessary capillary structures are discussed. Several types of heat pipes were made from stainless steel and tested at temperatures between 400 and 1055 deg C. The thermal power was determined by a calorimeter. Results indicate: bubble-free evaporation of sodium from rectangular open chennels is possible with a heat flux of more than 1,940 W/sq cm at 1055 C. The temperature drop along the tube could be measured only at low temperatures. A subdivided heat pipe worked against the gravitational field. A heat pipe with a capillary structure made of a rolled screen was supported by rings and bars operated at 250 W/sq cm heat flux in the evaporating region
The drive system of the Major Atmospheric Gamma-ray Imaging Cherenkov Telescope
The MAGIC telescope is an imaging atmospheric Cherenkov telescope, designed
to observe very high energy gamma-rays while achieving a low energy threshold.
One of the key science goals is fast follow-up of the enigmatic and short lived
gamma-ray bursts. The drive system for the telescope has to meet two basic
demands: (1) During normal observations, the 72-ton telescope has to be
positioned accurately, and has to track a given sky position with high
precision at a typical rotational speed in the order of one revolution per day.
(2) For successfully observing GRB prompt emission and afterglows, it has to be
powerful enough to position to an arbitrary point on the sky within a few ten
seconds and commence normal tracking immediately thereafter. To meet these
requirements, the implementation and realization of the drive system relies
strongly on standard industry components to ensure robustness and reliability.
In this paper, we describe the mechanical setup, the drive control and the
calibration of the pointing, as well as present measurements of the accuracy of
the system. We show that the drive system is mechanically able to operate the
motors with an accuracy even better than the feedback values from the axes. In
the context of future projects, envisaging telescope arrays comprising about
100 individual instruments, the robustness and scalability of the concept is
emphasized.Comment: 15 pages, 12 (10) figures, submitted to Astroparticle Physics, a high
resolution version of the paper (particularly fig. 1) is available at
http://publications.mppmu.mpg.de/2008/MPP-2008-101/FullText.pd
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