16,383 research outputs found
Adaptive Phase Measurements in Linear Optical Quantum Computation
Photon counting induces an effective nonlinear optical phase shift on certain
states derived by linear optics from single photons. Although this no
nlinearity is nondeterministic, it is sufficient in principle to allow scalable
linear optics quantum computation (LOQC). The most obvious way to encode a
qubit optically is as a superposition of the vacuum and a single photon in one
mode -- so-called "single-rail" logic. Until now this approach was thought to
be prohibitively expensive (in resources) compared to "dual-rail" logic where a
qubit is stored by a photon across two modes. Here we attack this problem with
real-time feedback control, which can realize a quantum-limited phase
measurement on a single mode, as has been recently demonstrated experimentally.
We show that with this added measurement resource, the resource requirements
for single-rail LOQC are not substantially different from those of dual-rail
LOQC. In particular, with adaptive phase measurements an arbitrary qubit state
can be prepared deterministically
Conditional Production of Superpositions of Coherent States with Inefficient Photon Detection
It is shown that a linear superposition of two macroscopically
distinguishable optical coherent states can be generated using a single photon
source and simple all-optical operations. Weak squeezing on a single photon,
beam mixing with an auxiliary coherent state, and photon detecting with
imperfect threshold detectors are enough to generate a coherent state
superposition in a free propagating optical field with a large coherent
amplitude () and high fidelity (). In contrast to all
previous schemes to generate such a state, our scheme does not need photon
number resolving measurements nor Kerr-type nonlinear interactions.
Furthermore, it is robust to detection inefficiency and exhibits some
resilience to photon production inefficiency.Comment: Some important new results added, to appear in Phys.Rev.A (Rapid
Communication
Comparison of LOQC C-sign gates with ancilla inefficiency and an improvement to functionality under these conditions
We compare three proposals for non-deterministic C-sign gates implemented
using linear optics and conditional measurements with non-ideal ancilla mode
production and detection. The simplified KLM gate [Ralph et al, Phys.Rev.A {\bf
65}, 012314 (2001)] appears to be the most resilient under these conditions. We
also find that the operation of this gate can be improved by adjusting the
beamsplitter ratios to compensate to some extent for the effects of the
imperfect ancilla.Comment: to appear in PR
Fault-tolerant linear optical quantum computing with small-amplitude coherent states
Quantum computing using two optical coherent states as qubit basis states has
been suggested as an interesting alternative to single photon optical quantum
computing with lower physical resource overheads. These proposals have been
questioned as a practical way of performing quantum computing in the short term
due to the requirement of generating fragile diagonal states with large
coherent amplitudes. Here we show that by using a fault-tolerant error
correction scheme, one need only use relatively small coherent state amplitudes
() to achieve universal quantum computing. We study the effects
of small coherent state amplitude and photon loss on fault tolerance within the
error correction scheme using a Monte Carlo simulation and show the quantity of
resources used for the first level of encoding is orders of magnitude lower
than the best known single photon scheme. %We study this reigem using a Monte
Carlo simulation and incorporate %the effects of photon loss in this
simulation
Photon number discrimination without a photon counter and its application to reconstructing non-Gaussian states
The non-linearity of a conditional photon-counting measurement can be used to
`de-Gaussify' a Gaussian state of light. Here we present and experimentally
demonstrate a technique for photon number resolution using only homodyne
detection. We then apply this technique to inform a conditional measurement;
unambiguously reconstructing the statistics of the non-Gaussian one and two
photon subtracted squeezed vacuum states. Although our photon number
measurement relies on ensemble averages and cannot be used to prepare
non-Gaussian states of light, its high efficiency, photon number resolving
capabilities, and compatibility with the telecommunications band make it
suitable for quantum information tasks relying on the outcomes of mean values.Comment: 4 pages, 3 figures. Theory section expanded in response to referee
comment
Scattering of second sound waves by quantum vorticity
A new method of detection and measurement of quantum vorticity by scattering
second sound off quantized vortices in superfluid Helium is suggested.
Theoretical calculations of the relative amplitude of the scattered second
sound waves from a single quantum vortex, a vortex ring, and bulk vorticity are
presented. The relevant estimates show that an experimental verification of the
method is feasible. Moreover, it can even be used for the detection of a single
quantum vortex.Comment: Latex file, 9 page
Sclerostin does not play a major role in the pathogenesis of skeletal complications in type 2 diabetes mellitus
In contrast to previously reported elevations in serum sclerostin levels in diabetic patients, the present study shows that the impaired bone microarchitecture and cellular turnover associated with type 2 diabetes mellitus (T2DM)-like conditions in ZDF rats are not correlated with changes in serum and bone sclerostin expression. INTRODUCTION: T2DM is associated with impaired skeletal structure and a higher prevalence of bone fractures. Sclerostin, a negative regulator of bone formation, is elevated in serum of diabetic patients. We aimed to relate changes in bone architecture and cellular activities to sclerostin production in the Zucker diabetic fatty (ZDF) rat. METHODS: Bone density and architecture were measured by micro-CT and bone remodelling by histomorphometry in tibiae and femurs of 14-week-old male ZDF rats and lean Zucker controls (n = 6/group). RESULTS: ZDF rats showed lower trabecular bone mineral density and bone mass compared to controls, due to decreases in bone volume and thickness, along with impaired bone connectivity and cortical bone geometry. Bone remodelling was impaired in diabetic rats, demonstrated by decreased bone formation rate and increased percentage of tartrate-resistant acid phosphatase-positive osteoclastic surfaces. Serum sclerostin levels (ELISA) were higher in ZDF compared to lean rats at 9 weeks (+40 %, p < 0.01), but this difference disappeared as their glucose control deteriorated and by week 14, ZDF rats had lower sclerostin levels than control rats (-44 %, p < 0.0001). Bone sclerostin mRNA (qPCR) and protein (immunohistochemistry) were similar in ZDF, and lean rats at 14 weeks and genotype did not affect the number of empty osteocytic lacunae in cortical and trabecular bone. CONCLUSION: T2DM results in impaired skeletal architecture through altered remodelling pathways, but despite altered serum levels, it does not appear that sclerostin contributes to the deleterious effect of T2DM in rat bone
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Blimp-1-dependent and -independent natural antibody production by B-1 and B-1-derived plasma cells.
Natural antibodies contribute to tissue homeostasis and protect against infections. They are secreted constitutively without external antigenic stimulation. The differentiation state and regulatory pathways that enable continuous natural antibody production by B-1 cells, the main cellular source in mice, remain incompletely understood. Here we demonstrate that natural IgM-secreting B-1 cells in the spleen and bone marrow are heterogeneous, consisting of (a) terminally differentiated B-1-derived plasma cells expressing the transcriptional regulator of differentiation, Blimp-1, (b) Blimp-1+, and (c) Blimp-1neg phenotypic B-1 cells. Blimp-1neg IgM-secreting B-1 cells are not simply intermediates of cellular differentiation. Instead, they secrete similar amounts of IgM in wild-type and Blimp-1-deficient (PRDM-1ΔEx1A) mice. Blimp-1neg B-1 cells are also a major source of IgG3. Consequently, deletion of Blimp-1 changes neither serum IgG3 levels nor the amount of IgG3 secreted per cell. Thus, the pool of natural antibody-secreting B-1 cells is heterogeneous and contains a distinct subset of cells that do not use Blimp-1 for initiation or maximal antibody secretion
Maximum entanglement of formation for a two-mode Gaussian state over passive operations
We quantify the maximum amount of entanglement of formation (EoF) that can be
achieved by continuous-variable states under passive operations, which we refer
to as EoF-potential. Focusing, in particular, on two-mode Gaussian states we
derive analytical expressions for the EoF-potential for specific classes of
states. For more general states, we demonstrate that this quantity can be
upper-bounded by the minimum amount of squeezing needed to synthesize the
Gaussian modes, a quantity called squeezing of formation. Our work, thus,
provides a new link between non-classicality of quantum states and the
non-classicality of correlations.Comment: Revised versio
Measuring measurement--disturbance relationships with weak values
Using formal definitions for measurement precision {\epsilon} and disturbance
(measurement backaction) {\eta}, Ozawa [Phys. Rev. A 67, 042105 (2003)] has
shown that Heisenberg's claimed relation between these quantities is false in
general. Here we show that the quantities introduced by Ozawa can be determined
experimentally, using no prior knowledge of the measurement under investigation
--- both quantities correspond to the root-mean-squared difference given by a
weak-valued probability distribution. We propose a simple three-qubit
experiment which would illustrate the failure of Heisenberg's
measurement--disturbance relation, and the validity of an alternative relation
proposed by Ozawa
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