348 research outputs found
Hydrophobic and biospecific chromatography in the purification of maltodextrin phosphorylase from E. Coli
Discussing the Study of High Tc Properties of Superconductors with Electron Paramagnetic Resonance (EPR) Spectrometers
Fermi Velocity Spectrum and Incipient Magnetism in TiBe2
We address the origin of the incipient magnetism in TiBe through precise
first principles calculations, which overestimate the ferromagnetic tendency
and therefore require correction to account for spin fluctuations. TiBe has
sharp fine structure in its electronic density of states, with a van Hove
singularity only 3 meV above the Fermi level. Similarly to the isovalent weak
ferromagnet ZrZn, it is flat bands along the K-W-U lines of hexagonal face
of the fcc Brillouin zone make the system prone to magnetism, and more so if
electrons are added. We find that the Moriya coefficient (multiplying
in the fluctuation susceptibility )
is divergent when the velocity vanishes at a point on the Fermi surface, which
is very close (3 meV) to occurring in TiBe. In exploring how the FM
instability (the =0 Stoner enhancement is ) might be suppressed
by fluctuations in TiBe, we calculate that the Moriya A coefficient (of
) is negative, so =0 is not the primary instability. Explicit
calculation of shows that its maximum occurs at the X point
; TiBe is thus an incipient {\it anti}ferromagnet
rather than ferromagnet as has been supposed. We further show that simple
temperature smearing of the peak accounts for most of the temperature
dependence of the susceptibility, which previously had been attributed to local
moments (via a Curie-Weiss fit), and that energy dependence of the density of
states also strongly affects the magnetic field variation of
Security of practical private randomness generation
Measurements on entangled quantum systems necessarily yield outcomes that are
intrinsically unpredictable if they violate a Bell inequality. This property
can be used to generate certified randomness in a device-independent way, i.e.,
without making detailed assumptions about the internal working of the quantum
devices used to generate the random numbers. Furthermore these numbers are also
private, i.e., they appear random not only to the user, but also to any
adversary that might possess a perfect description of the devices. Since this
process requires a small initial random seed, one usually speaks of
device-independent randomness expansion.
The purpose of this paper is twofold. First, we point out that in most real,
practical situations, where the concept of device-independence is used as a
protection against unintentional flaws or failures of the quantum apparatuses,
it is sufficient to show that the generated string is random with respect to an
adversary that holds only classical-side information, i.e., proving randomness
against quantum-side information is not necessary. Furthermore, the initial
random seed does not need to be private with respect to the adversary, provided
that it is generated in a way that is independent from the measured systems.
The devices, though, will generate cryptographically-secure randomness that
cannot be predicted by the adversary and thus one can, given access to free
public randomness, talk about private randomness generation.
The theoretical tools to quantify the generated randomness according to these
criteria were already introduced in [S. Pironio et al, Nature 464, 1021
(2010)], but the final results were improperly formulated. The second aim of
this paper is to correct this inaccurate formulation and therefore lay out a
precise theoretical framework for practical device-independent randomness
expansion.Comment: 18 pages. v3: important changes: the present version focuses on
security against classical side-information and a discussion about the
significance of these results has been added. v4: minor changes. v5: small
typos correcte
From Low-Distortion Norm Embeddings to Explicit Uncertainty Relations and Efficient Information Locking
The existence of quantum uncertainty relations is the essential reason that
some classically impossible cryptographic primitives become possible when
quantum communication is allowed. One direct operational manifestation of these
uncertainty relations is a purely quantum effect referred to as information
locking. A locking scheme can be viewed as a cryptographic protocol in which a
uniformly random n-bit message is encoded in a quantum system using a classical
key of size much smaller than n. Without the key, no measurement of this
quantum state can extract more than a negligible amount of information about
the message, in which case the message is said to be "locked". Furthermore,
knowing the key, it is possible to recover, that is "unlock", the message. In
this paper, we make the following contributions by exploiting a connection
between uncertainty relations and low-distortion embeddings of L2 into L1. We
introduce the notion of metric uncertainty relations and connect it to
low-distortion embeddings of L2 into L1. A metric uncertainty relation also
implies an entropic uncertainty relation. We prove that random bases satisfy
uncertainty relations with a stronger definition and better parameters than
previously known. Our proof is also considerably simpler than earlier proofs.
We apply this result to show the existence of locking schemes with key size
independent of the message length. We give efficient constructions of metric
uncertainty relations. The bases defining these metric uncertainty relations
are computable by quantum circuits of almost linear size. This leads to the
first explicit construction of a strong information locking scheme. Moreover,
we present a locking scheme that is close to being implementable with current
technology. We apply our metric uncertainty relations to exhibit communication
protocols that perform quantum equality testing.Comment: 60 pages, 5 figures. v4: published versio
Constructive Dimension and Turing Degrees
This paper examines the constructive Hausdorff and packing dimensions of
Turing degrees. The main result is that every infinite sequence S with
constructive Hausdorff dimension dim_H(S) and constructive packing dimension
dim_P(S) is Turing equivalent to a sequence R with dim_H(R) <= (dim_H(S) /
dim_P(S)) - epsilon, for arbitrary epsilon > 0. Furthermore, if dim_P(S) > 0,
then dim_P(R) >= 1 - epsilon. The reduction thus serves as a *randomness
extractor* that increases the algorithmic randomness of S, as measured by
constructive dimension.
A number of applications of this result shed new light on the constructive
dimensions of Turing degrees. A lower bound of dim_H(S) / dim_P(S) is shown to
hold for the Turing degree of any sequence S. A new proof is given of a
previously-known zero-one law for the constructive packing dimension of Turing
degrees. It is also shown that, for any regular sequence S (that is, dim_H(S) =
dim_P(S)) such that dim_H(S) > 0, the Turing degree of S has constructive
Hausdorff and packing dimension equal to 1.
Finally, it is shown that no single Turing reduction can be a universal
constructive Hausdorff dimension extractor, and that bounded Turing reductions
cannot extract constructive Hausdorff dimension. We also exhibit sequences on
which weak truth-table and bounded Turing reductions differ in their ability to
extract dimension.Comment: The version of this paper appearing in Theory of Computing Systems,
45(4):740-755, 2009, had an error in the proof of Theorem 2.4, due to
insufficient care with the choice of delta. This version modifies that proof
to fix the error
Перспективи розвитку експортоорієнтованої стратегії підприємств
Рассмотрен вопрос стратегического развития экспортноориентрованной политики предприятий. Раскрыты перспективы развития международных торговых отношений Украины.Розглянуто питання стратегічного розвитку експортноорієнтовної політики підприємств. Розкрито перспективи розвитку міжнародних торгівельних відносин України
Field cooling memory effect in Bi2212 and Bi2223 single crystals
A memory effect in the Josephson vortex system created by magnetic field in
the highly anisotropic superconductors Bi2212 and Bi2223 is demonstrated using
microwave power absorption. This surprising effect appears despite a very low
viscosity of Josephson vortices compared to Abrikosov vortices. The
superconductor is field cooled in DC magnetic field H_{m} oriented parallel to
the CuO planes through the critical temperature T_{c} down to 4K, with
subsequent reduction of the field to zero and again above H_{m}. Large
microwave power absorption signal is observed at a magnetic field just above
the cooling field clearly indicating a memory effect. The dependence of the
signal on deviation of magnetic field from H_{m} is the same for a wide range
of H_{m} from 0.15T to 1.7T
Endogenous cleavage of the Arg-379-Ala-380 bond in vitronectin results in a distinct conformational change which ‘buries’ Ser-378, its site of phosphorylation by protein kinase A
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