1,557 research outputs found
Excitations of amorphous solid helium
We present neutron scattering measurements of the dynamic structure factor,
, of amorphous solid helium confined in 47 pore diameter
MCM-41 at pressure 48.6 bar. At low temperature, = 0.05 K, we observe
of the confined quantum amorphous solid plus the bulk
polycrystalline solid between the MCM-41 powder grains. No liquid-like
phonon-roton modes, other sharply defined modes at low energy ( 1.0
meV) or modes unique to a quantum amorphous solid that might suggest superflow
are observed. Rather the of confined amorphous and bulk
polycrystalline solid appear to be very similar. At higher temperature ( 1
K), the amorphous solid in the MCM-41 pores melts to a liquid which has a broad
peaked near 0 characteristic of normal liquid
He under pressure. Expressions for the of amorphous and
polycrystalline solid helium are presented and compared. In previous
measurements of liquid He confined in MCM-41 at lower pressure the
intensity in the liquid roton mode decreases with increasing pressure until the
roton vanishes at the solidification pressure (38 bars), consistent with no
roton in the solid observed here
Conformal dimension and random groups
We give a lower and an upper bound for the conformal dimension of the
boundaries of certain small cancellation groups. We apply these bounds to the
few relator and density models for random groups. This gives generic bounds of
the following form, where is the relator length, going to infinity.
(a) 1 + 1/C < \Cdim(\bdry G) < C l / \log(l), for the few relator model,
and
(b) 1 + l / (C\log(l)) < \Cdim(\bdry G) < C l, for the density model, at
densities .
In particular, for the density model at densities , as the relator
length goes to infinity, the random groups will pass through infinitely
many different quasi-isometry classes.Comment: 32 pages, 4 figures. v2: Final version. Main result improved to
density < 1/16. Many minor improvements. To appear in GAF
Absence of strong magnetic fluctuations in the iron phosphide superconductors LaFePO and Sr2ScO3FeP
We report neutron inelastic scattering measurements on polycrystalline LaFePO
and Sr2ScO3FeP, two members of the iron phosphide families of superconductors.
No evidence is found for any magnetic fluctuations in the spectrum of either
material in the energy and wavevector ranges probed. Special attention is paid
to the wavevector at which spin-density-wave-like fluctuations are seen in
other iron-based superconductors. We estimate that the magnetic signal, if
present, is at least a factor of four (Sr2ScO3FeP) or seven (LaFePO) smaller
than in the related iron arsenide and chalcogenide superconductors. These
results suggest that magnetic fluctuations are not as influential on the
electronic properties of the iron phosphide systems as they are in other
iron-based superconductors.Comment: 7 pages, 5 figure
Consistent quantum mechanics admits no mereotopology
It is standardly assumed in discussions of quantum theory that physical
systems can be regarded as having well-defined Hilbert spaces. It is shown here
that a Hilbert space can be consistently partitioned only if its components are
assumed not to interact. The assumption that physical systems have well-defined
Hilbert spaces is, therefore, physically unwarranted.Comment: 10 pages; to appear in Axiomathe
Environment as a Witness: Selective Proliferation of Information and Emergence of Objectivity in a Quantum Universe
We study the role of the information deposited in the environment of an open
quantum system in course of the decoherence process. Redundant spreading of
information -- the fact that some observables of the system can be
independently ``read-off'' from many distinct fragments of the environment --
is investigated as the key to effective objectivity, the essential ingredient
of ``classical reality''. This focus on the environment as a communication
channel through which observers learn about physical systems underscores
importance of quantum Darwinism -- selective proliferation of information about
``the fittest states'' chosen by the dynamics of decoherence at the expense of
their superpositions -- as redundancy imposes the existence of preferred
observables. We demonstrate that the only observables that can leave multiple
imprints in the environment are the familiar pointer observables singled out by
environment-induced superselection (einselection) for their predictability.
Many independent observers monitoring the environment will therefore agree on
properties of the system as they can only learn about preferred observables. In
this operational sense, the selective spreading of information leads to
appearance of an objective ``classical reality'' from within quantum substrate.Comment: New figures, to appear in PR
Observation of magnetic fragmentation in spin ice
Fractionalised excitations that emerge from a many body system have revealed
rich physics and concepts, from composite fermions in two-dimensional electron
systems, revealed through the fractional quantum Hall effect, to spinons in
antiferromagnetic chains and, more recently, fractionalisation of Dirac
electrons in graphene and magnetic monopoles in spin ice. Even more surprising
is the fragmentation of the degrees of freedom themselves, leading to
coexisting and a priori independent ground states. This puzzling phenomenon was
recently put forward in the context of spin ice, in which the magnetic moment
field can fragment, resulting in a dual ground state consisting of a
fluctuating spin liquid, a so-called Coulomb phase, on top of a magnetic
monopole crystal. Here we show, by means of neutron scattering measurements,
that such fragmentation occurs in the spin ice candidate NdZrO. We
observe the spectacular coexistence of an antiferromagnetic order induced by
the monopole crystallisation and a fluctuating state with ferromagnetic
correlations. Experimentally, this fragmentation manifests itself via the
superposition of magnetic Bragg peaks, characteristic of the ordered phase, and
a pinch point pattern, characteristic of the Coulomb phase. These results
highlight the relevance of the fragmentation concept to describe the physics of
systems that are simultaneously ordered and fluctuating.Comment: accepted in Nature Physic
V.2 CoRoT heritage in future missions
This book is dedicated to all the people interested in the CoRoT mission and the beautiful data that were delivered during its six year duration. Either amateurs, professional, young or senior researchers, they will find treasures not only at the time of this publication but also in the future twenty or thirty years. It presents the data in their final version, explains how they have been obtained, how to handle them, describes the tools necessary to understand them, and where to find them. It also highlights the most striking first results obtained up to now. CoRoT has opened several unexpected directions of research and certainly new ones still to be discovered
Exponential speed-up with a single bit of quantum information: Testing the quantum butterfly effect
We present an efficient quantum algorithm to measure the average fidelity
decay of a quantum map under perturbation using a single bit of quantum
information. Our algorithm scales only as the complexity of the map under
investigation, so for those maps admitting an efficient gate decomposition, it
provides an exponential speed up over known classical procedures. Fidelity
decay is important in the study of complex dynamical systems, where it is
conjectured to be a signature of quantum chaos. Our result also illustrates the
role of chaos in the process of decoherence.Comment: 4 pages, 2 eps figure
Optimal and Efficient Decoding of Concatenated Quantum Block Codes
We consider the problem of optimally decoding a quantum error correction code
-- that is to find the optimal recovery procedure given the outcomes of partial
"check" measurements on the system. In general, this problem is NP-hard.
However, we demonstrate that for concatenated block codes, the optimal decoding
can be efficiently computed using a message passing algorithm. We compare the
performance of the message passing algorithm to that of the widespread
blockwise hard decoding technique. Our Monte Carlo results using the 5 qubit
and Steane's code on a depolarizing channel demonstrate significant advantages
of the message passing algorithms in two respects. 1) Optimal decoding
increases by as much as 94% the error threshold below which the error
correction procedure can be used to reliably send information over a noisy
channel. 2) For noise levels below these thresholds, the probability of error
after optimal decoding is suppressed at a significantly higher rate, leading to
a substantial reduction of the error correction overhead.Comment: Published versio
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