947 research outputs found
On the irreversibility of entanglement distillation
We investigate the irreversibility of entanglement distillation for a
symmetric d-1 parameter family of mixed bipartite quantum states acting on
Hilbert spaces of arbitrary dimension d x d. We prove that in this family the
entanglement cost is generically strictly larger than the distillable
entanglement, such that the set of states for which the distillation process is
asymptotically reversible is of measure zero. This remains true even if the
distillation process is catalytically assisted by pure state entanglement and
every operation is allowed, which preserves the positivity of the partial
transpose. It is shown, that reversibility occurs only in cases where the state
is quasi-pure in the sense that all its pure state entanglement can be revealed
by a simple operation on a single copy. The reversible cases are shown to be
completely characterized by minimal uncertainty vectors for entropic
uncertainty relations.Comment: 5 pages, revtex
Role of mitochondria in Parkinson disease
The cause of the selective degeneration of nigrostriatal neurons in Parkinson disease (PD) has remained largely unknown. Exceptions include rare missense mutations in the alpha-synuclein gene on chromosome 4, a potentially pathogenic mutation affecting the ubiquitin pathway, and mutations in the parkin gene on chromosome 6. However, unlike classical PD, the latter syndrome is not associated with the formation of typical Lewy bodies. In contrast, a biochemical defect of complex I of the mitochondrial respiratory chain has been described in a relatively large group of confirmed PD cases. Recent cybrid studies indicate that the complex I defect in PD has a genetic cause and that it may arise from mutations in the mitochondrial DNA, Sequence analysis of the mitochondrial genome supports the view that mitochondrial point mutations are involved in PD pathogenesis. However, although mitochondria function as regulators in several known forms of cell death, their exact involvement in PD has remained unresolved. This is of relevance because classical apoptosis does not appear to play a major role in the degeneration of the parkinsonian nigra
An All-But-One Entropic Uncertainty Relation, and Application to Password-based Identification
Entropic uncertainty relations are quantitative characterizations of
Heisenberg's uncertainty principle, which make use of an entropy measure to
quantify uncertainty. In quantum cryptography, they are often used as
convenient tools in security proofs. We propose a new entropic uncertainty
relation. It is the first such uncertainty relation that lower bounds the
uncertainty in the measurement outcome for all but one choice for the
measurement from an arbitrarily large (but specifically chosen) set of possible
measurements, and, at the same time, uses the min-entropy as entropy measure,
rather than the Shannon entropy. This makes it especially suited for quantum
cryptography. As application, we propose a new quantum identification scheme in
the bounded quantum storage model. It makes use of our new uncertainty relation
at the core of its security proof. In contrast to the original quantum
identification scheme proposed by Damg{\aa}rd et al., our new scheme also
offers some security in case the bounded quantum storage assumption fails hold.
Specifically, our scheme remains secure against an adversary that has unbounded
storage capabilities but is restricted to non-adaptive single-qubit operations.
The scheme by Damg{\aa}rd et al., on the other hand, completely breaks down
under such an attack.Comment: 33 pages, v
University Autonomy and Organizational Change Dynamics
In this paper university autonomy is discussed from four different analytical
perspectives. First, a discussion is presented of autonomy as conceptualized in
the academic literature covering public sector governance in general. Second,
the concept of autonomy is deconstructed through discussing its underlying
assumptions and by examining the relationship between state authorities and
universities. In so doing the paper proposes an institutional approach to the
study of autonomy. Third, the way in which autonomy affects organizational
design according to centralization, formalization, standardization,
legitimization and flexibility is addressed. Fourth, relating to our
interpretation of the living autonomy we will discuss how reforms that are
aimed at enhancing university autonomy have affected the internal
governance structure. The empirical setting consists of a study on flagship
universities in eight continental European countries. First findings show
tensions as a consequence of the ways in which enhanced institutional
autonomy is interpreted, operationalized and used within flagship
universities. These tensions are manifested by the nature of the interactions
between the traditional academic domain and the emerging executive
structure inside these institutions
Hamiltonian for coupled flux qubits
An effective Hamiltonian is derived for two coupled three-Josephson-junction
(3JJ) qubits. This is not quite trivial, for the customary "free" 3JJ
Hamiltonian is written in the limit of zero inductance L. Neglecting the
self-flux is already dubious for one qubit when it comes to readout, and
becomes untenable when discussing inductive coupling. First, inductance effects
are analyzed for a single qubit. For small L, the self-flux is a "fast
variable" which can be eliminated adiabatically. However, the commonly used
junction phases are_not_ appropriate "slow variables", and instead one
introduces degrees of freedom which are decoupled from the loop current to
leading order. In the quantum case, the zero-point fluctuations (LC
oscillations) in the loop current diverge as L->0. Fortunately, they merely
renormalize the Josephson couplings of the effective (two-phase) theory.
In the coupled case, the strong zero-point fluctuations render the full
(six-phase) wave function significantly entangled in leading order. However, in
going to the four-phase theory, this uncontrollable entanglement is integrated
out completely, leaving a computationally usable mutual-inductance term of the
expected form as the effective interaction.Comment: REVTeX4, 16pp., one figure. N.B.: "Alec" is my first, and "Maassen
van den Brink" my family name. Informal note. v2: completely rewritten;
correction of final result and major expansion. v3: added numerical
verification plus a discussion of Ref. [2
Linear scaling between momentum and spin scattering in graphene
Spin transport in graphene carries the potential of a long spin diffusion
length at room temperature. However, extrinsic relaxation processes limit the
current experimental values to 1-2 um. We present Hanle spin precession
measurements in gated lateral spin valve devices in the low to high (up to
10^13 cm^-2) carrier density range of graphene. A linear scaling between the
spin diffusion length and the diffusion coefficient is observed. We measure
nearly identical spin- and charge diffusion coefficients indicating that
electron-electron interactions are relatively weak and transport is limited by
impurity potential scattering. When extrapolated to the maximum carrier
mobilities of 2x10^5 cm^2/Vs, our results predict that a considerable increase
in the spin diffusion length should be possible
Quantum Manipulations of Small Josephson Junctions
Low-capacitance Josephson junction arrays in the parameter range where single
charges can be controlled are suggested as possible physical realizations of
the elements which have been considered in the context of quantum computers. We
discuss single and multiple quantum bit systems. The systems are controlled by
applied gate voltages, which also allow the necessary manipulation of the
quantum states. We estimate that the phase coherence time is sufficiently long
for experimental demonstration of the principles of quantum computation.Comment: RevTex, 15 pages,4 postscript figures, uuencoded, submitted to Phys.
Rev. Lett., estimates of the experimental parameters correcte
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