712 research outputs found
Weak measurements are universal
It is well known that any projective measurement can be decomposed into a
sequence of weak measurements, which cause only small changes to the state.
Similar constructions for generalized measurements, however, have relied on the
use of an ancilla system. We show that any generalized measurement can be
decomposed into a sequence of weak measurements without the use of an ancilla,
and give an explicit construction for these weak measurements. The measurement
procedure has the structure of a random walk along a curve in state space, with
the measurement ending when one of the end points is reached. This shows that
any measurement can be generated by weak measurements, and hence that weak
measurements are universal. This may have important applications to the theory
of entanglement.Comment: 4 pages, RevTeX format, essentially the published version, reference
update
Jahn-Teller Spectral Fingerprint in Molecular Photoemission: C60
The h_u hole spectral intensity for C60 -> C60+ molecular photoemission is
calculated at finite temperature by a parameter-free Lanczos diagonalization of
the electron-vibration Hamiltonian, including the full 8 H_g, 6 G_g, and 2 A_g
mode couplings. The computed spectrum at 800 K is in striking agreement with
gas-phase data. The energy separation of the first main shoulder from the main
photoemission peak, 230 meV in C60, is shown to measure directly and rather
generally the strength of the final-state Jahn-Teller coupling.Comment: 5 pages, 3 figure
Vacuum Polarization and the Electric Charge of the Positron
We show that higher-order vacuum polarization would contribute a measureable
net charge to atoms, if the charges of electrons and positrons do not balance
precisely. We obtain the limit for the sum of
the charges of electron and positron. This also constitutes a new bound on
certain violations of PCT invariance.Comment: 9 pages, 1 figure attached as PostScript file, DUKE-TH-92-38. Revised
versio
Cep63 and Cep152 Cooperate to Ensure Centriole Duplication
Centrosomes consist of two centrioles embedded in pericentriolar material and function as the main microtubule organising centres in dividing animal cells. They ensure proper formation and orientation of the mitotic spindle and are therefore essential for the maintenance of genome stability. Centrosome function is crucial during embryonic development, highlighted by the discovery of mutations in genes encoding centrosome or spindle pole proteins that cause autosomal recessive primary microcephaly, including Cep63 and Cep152. In this study we show that Cep63 functions to ensure that centriole duplication occurs reliably in dividing mammalian cells. We show that the interaction between Cep63 and Cep152 can occur independently of centrosome localisation and that the two proteins are dependent on one another for centrosomal localisation. Further, both mouse and human Cep63 and Cep152 cooperate to ensure efficient centriole duplication by promoting the accumulation of essential centriole duplication factors upstream of SAS-6 recruitment and procentriole formation. These observations describe the requirement for Cep63 in maintaining centriole number in dividing mammalian cells and further establish the order of events in centriole formation
Derivation of the quantum probability law from minimal non-demolition measurement
One more derivation of the quantum probability rule is presented in order to
shed more light on the versatile aspects of this fundamental law. It is shown
that the change of state in minimal quantum non-demolition measurement, also
known as ideal measurement, implies the probability law in a simple way.
Namely, the very requirement of minimal change of state, put in proper
mathematical form, gives the well known Lueders formula, which contains the
probability rule.Comment: 8 page
Microbial methane formation in deep aquifers of a coal-bearing sedimentary basin, Germany
Published version. Also available at http://dx.doi.org/10.3389/fmicb.2015.00200Coal-bearing sediments are major reservoirs of organic matter potentially available for methanogenic subsurface microbial communities. In this study the specific microbial community inside lignite-bearing sedimentary basin in Germany and its contribution to methanogenic hydrocarbon degradation processes was investigated. The stable isotope signature of methane measured in groundwater and coal-rich sediment samples indicated methanogenic activity. Analysis of 16S rRNA gene sequences showed the presence of methanogenic Archaea, predominantly belonging to the orders Methanosarcinales and Methanomicrobiales, capable of acetoclastic or hydrogenotrophic methanogenesis. Furthermore, we identified fermenting, sulfate-, nitrate-, and metal-reducing, or acetogenic Bacteria clustering within the phyla Proteobacteria, complemented by members of the classes Actinobacteria, and Clostridia. The indigenous microbial communities found in the groundwater as well as in the coal-rich sediments are able to degrade coal-derived organic components and to produce methane as the final product. Lignite-bearing sediments may be an important nutrient and energy source influencing larger compartments via groundwater transport
Different Types of Conditional Expectation and the Lueders - von Neumann Quantum Measurement
In operator algebra theory, a conditional expectation is usually assumed to
be a projection map onto a sub-algebra. In the paper, a further type of
conditional expectation and an extension of the Lueders - von Neumann
measurement to observables with continuous spectra are considered; both are
defined for a single operator and become a projection map only if they exist
for all operators. Criteria for the existence of the different types of
conditional expectation and of the extension of the Lueders - von Neumann
measurement are presented, and the question whether they coincide is studied.
All this is done in the general framework of Jordan operator algebras. The
examples considered include the type I and type II operator algebras, the
standard Hilbert space model of quantum mechanics, and a no-go result
concerning the conditional expectation of observables that satisfy the
canonical commutator relation.Comment: 10 pages, the original publication is available at
http://www.springerlink.co
Coherence of a Josephson phase qubit under partial-collapse measurement
We discuss quantum evolution of a decaying state in relation to a recent
experiment of Katz et al. Based on exact analytical and numerical solutions of
a simple model, we identify a regime where qubit retains coherence over a
finite time interval independently of the rates of three competing decoherence
processes. In this regime, the quantum decay process can be continuously
monitored via a ``weak'' measurement without affecting the qubit coherence.Comment: 4p., 2eps figure
State Measurements with Short Laser Pulses and Lower-Efficiency Photon Detectors
It has been proposed by Cook (Phys. Scr. T 21, 49 (1988)) to use a short
probe laser pulse for state measurements of two-level systems. In previous work
we have investigated to what extent this proposal fulfills the projection
postulate if ideal photon detectors are considered. For detectors with overall
efficiency less than 1 complications arise for single systems, and for this
case we present a simple criterion for a laser pulse to act as a state
measurement and to cause an almost complete state reduction.Comment: 13 pages, LaTeX; submitted to J. mod. Op
Low-energy excitations of a linearly Jahn-Teller coupled orbital quintet
The low-energy spectra of the single-mode h x (G+H) linear Jahn-Teller model
is studied by means of exact diagonalization. Both eigenenergies and
photoemission spectral intensities are computed. These spectra are useful to
understand the vibronic dynamics of icosahedral clusters with partly filled
orbital quintet molecular shells, for example C60 positive ions.Comment: 14 pages revte
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