1,644 research outputs found
Spacetime torsion and parity violation: a gauge invariant formulation
The possibility of parity violation through spacetime torsion has been
explored in a scenario containing fields with different spins. Taking the
Kalb-Ramond field as the source of torsion, an explicitly parity violating
gauge invariant theory has been constructed by extending the KR
field with a Chern-Simons term.Comment: 4 pages, RevTe
Observation simulation experiments with regional prediction models
Research efforts in FY 1990 included studies employing regional scale numerical models as aids in evaluating potential contributions of specific satellite observing systems (current and future) to numerical prediction. One study involves Observing System Simulation Experiments (OSSEs) which mimic operational initialization/forecast cycles but incorporate simulated Advanced Microwave Sounding Unit (AMSU) radiances as input data. The objective of this and related studies is to anticipate the potential value of data from these satellite systems, and develop applications of remotely sensed data for the benefit of short range forecasts. Techniques are also being used that rely on numerical model-based synthetic satellite radiances to interpret the information content of various types of remotely sensed image and sounding products. With this approach, evolution of simulated channel radiance image features can be directly interpreted in terms of the atmospheric dynamical processes depicted by a model. Progress is being made in a study using the internal consistency of a regional prediction model to simplify the assessment of forced diabatic heating and moisture initialization in reducing model spinup times. Techniques for model initialization are being examined, with focus on implications for potential applications of remote microwave observations, including AMSU and Special Sensor Microwave Imager (SSM/I), in shortening model spinup time for regional prediction
Dynamics of single polymers under extreme confinement
We study the dynamics of a single chain polymer confined to a two dimensional
cell. We introduce a kinetically constrained lattice gas model that preserves
the connectivity of the chain, and we use this kinetically constrained model to
study the dynamics of the polymer at varying densities through Monte Carlo
simulations. Even at densities close to the fully-packed configuration, we find
that the monomers comprising the chain manage to diffuse around the box with a
root mean square displacement of the order of the box dimensions over time
scales for which the overall geometry of the polymer is, nevertheless, largely
preserved. To capture this shape persistence, we define the local tangent field
and study the two-time tangent-tangent correlation function, which exhibits a
glass-like behavior. In both closed and open chains, we observe reptational
motion and reshaping through local fingering events which entail global monomer
displacement.Comment: 22 pages, 18 figures, slightly extended version to appear in JSTA
Direct numerical simulation of homogeneous nucleation and growth in a phase-field model using cell dynamics method
Homogeneous nucleation and growth in a simplest two-dimensional phase field
model is numerically studied using the cell dynamics method. Whole process from
nucleation to growth is simulated and is shown to follow closely the
Kolmogorov-Johnson-Mehl-Avrami (KJMA) scenario of phase transformation.
Specifically the time evolution of the volume fraction of new stable phase is
found to follow closely the KJMA formula. By fitting the KJMA formula directly
to the simulation data, not only the Avrami exponent but the magnitude of
nucleation rate and, in particular, of incubation time are quantitatively
studied. The modified Avrami plot is also used to verify the derived KJMA
parameters. It is found that the Avrami exponent is close to the ideal
theoretical value m=3. The temperature dependence of nucleation rate follows
the activation-type behavior expected from the classical nucleation theory. On
the other hand, the temperature dependence of incubation time does not follow
the exponential activation-type behavior. Rather the incubation time is
inversely proportional to the temperature predicted from the theory of
Shneidman and Weinberg [J. Non-Cryst. Solids {\bf 160}, 89 (1993)]. A need to
restrict thermal noise in simulation to deduce correct Avrami exponent is also
discussed.Comment: 9 pages, 8 figures, Journal of Chemical Physics to be publishe
One-second coherence for a single electron spin coupled to a multi-qubit nuclear-spin environment
Single electron spins coupled to multiple nuclear spins provide promising
multi-qubit registers for quantum sensing and quantum networks. The obtainable
level of control is determined by how well the electron spin can be selectively
coupled to, and decoupled from, the surrounding nuclear spins. Here we realize
a coherence time exceeding a second for a single electron spin through
decoupling sequences tailored to its microscopic nuclear-spin environment. We
first use the electron spin to probe the environment, which is accurately
described by seven individual and six pairs of coupled carbon-13 spins. We
develop initialization, control and readout of the carbon-13 pairs in order to
directly reveal their atomic structure. We then exploit this knowledge to store
quantum states for over a second by carefully avoiding unwanted interactions.
These results provide a proof-of-principle for quantum sensing of complex
multi-spin systems and an opportunity for multi-qubit quantum registers with
long coherence times
Robust quantum-network memory using decoherence-protected subspaces of nuclear spins
The realization of a network of quantum registers is an outstanding challenge
in quantum science and technology. We experimentally investigate a network node
that consists of a single nitrogen-vacancy (NV) center electronic spin
hyperfine-coupled to nearby nuclear spins. We demonstrate individual control
and readout of five nuclear spin qubits within one node. We then characterize
the storage of quantum superpositions in individual nuclear spins under
repeated application of a probabilistic optical inter-node entangling protocol.
We find that the storage fidelity is limited by dephasing during the electronic
spin reset after failed attempts. By encoding quantum states into a
decoherence-protected subspace of two nuclear spins we show that quantum
coherence can be maintained for over 1000 repetitions of the remote entangling
protocol. These results and insights pave the way towards remote entanglement
purification and the realisation of a quantum repeater using NV center quantum
network nodes
Deterministic delivery of remote entanglement on a quantum network
Large-scale quantum networks promise to enable secure communication,
distributed quantum computing, enhanced sensing and fundamental tests of
quantum mechanics through the distribution of entanglement across nodes. Moving
beyond current two-node networks requires the rate of entanglement generation
between nodes to exceed their decoherence rates. Beyond this critical
threshold, intrinsically probabilistic entangling protocols can be subsumed
into a powerful building block that deterministically provides remote entangled
links at pre-specified times. Here we surpass this threshold using diamond spin
qubit nodes separated by 2 metres. We realise a fully heralded single-photon
entanglement protocol that achieves entangling rates up to 39 Hz, three orders
of magnitude higher than previously demonstrated two-photon protocols on this
platform. At the same time, we suppress the decoherence rate of remote
entangled states to 5 Hz by dynamical decoupling. By combining these results
with efficient charge-state control and mitigation of spectral diffusion, we
are able to deterministically deliver a fresh remote state with average
entanglement fidelity exceeding 0.5 at every clock cycle of 100 ms
without any pre- or post-selection. These results demonstrate a key building
block for extended quantum networks and open the door to entanglement
distribution across multiple remote nodes.Comment: v2 - updated to include relevant citatio
String Fields and the Standard Model
The Cremmer-Scherk mechanism is generalised in a non-Abelian context. In the
presence of the Higgs scalars of the standard model it is argued that fields
arising from the low energy effective string action may contribute to the mass
generation of the observed vector bosons that mediate the electroweak
interactions and that future analyses of experimental data should consider the
possibility of string induced radiative corrections to the Weinberg angle
coming from physics beyond the standard model.Comment: 4 pages, LATEX, no figure
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