318 research outputs found
Quadrupole transitions and quantum gates protected by continuous dynamic decoupling
Dynamical decoupling techniques are a versatile tool for engineering quantum
states with tailored properties. In trapped ions, nested layers of continuous
dynamical decoupling by means of radio-frequency field dressing can cancel
dominant magnetic and electric shifts and therefore provide highly prolonged
coherence times of electronic states. Exploiting this enhancement for frequency
metrology, quantum simulation or quantum computation, poses the challenge to
combine the decoupling with laser-ion interactions for the quantum control of
electronic and motional states of trapped ions. Ultimately, this will require
running quantum gates on qubits from dressed decoupled states. We provide here
a compact representation of nested continuous dynamical decoupling in trapped
ions, and apply it to electronic and states and optical quadrupole
transitions. Our treatment provides all effective transition frequencies and
Rabi rates, as well as the effective selection rules of these transitions. On
this basis, we discuss the possibility of combining continuous dynamical
decoupling and M{\o}lmer-S{\o}rensen gates
Vortex-line percolation in the three-dimensional complex |psi|^4 model
In discussing the phase transition of the three-dimensional complex |psi|^4
theory, we study the geometrically defined vortex-loop network as well as the
magnetic properties of the system in the vicinity of the critical point. Using
high-precision Monte Carlo techniques we investigate if both of them exhibit
the same critical behavior leading to the same critical exponents and hence to
a consistent description of the phase transition. Different percolation
observables are taken into account and compared with each other. We find that
different connectivity definitions for constructing the vortex-loop network
lead to different results in the thermodynamic limit, and the percolation
thresholds do not coincide with the thermodynamic phase transition point.Comment: 11 pages, 9 figure
Assessing bias corrections of oceanic surface conditions for atmospheric models
Future sea surface temperature and sea-ice concentration from coupled
ocean–atmosphere general circulation models such as those from the CMIP5
experiment are often used as boundary forcings for the downscaling of future
climate experiments. Yet, these models show some considerable biases when
compared to the observations over present climate. In this paper, existing
methods such as an absolute anomaly method and a quantile–quantile method
for sea surface temperature (SST) as well as a look-up table and a relative
anomaly method for sea-ice concentration (SIC) are presented. For SIC, we
also propose a new analogue method. Each method is objectively evaluated with
a perfect model test using CMIP5 model experiments and some real-case
applications using observations. We find that with respect to other
previously existing methods, the analogue method is a substantial improvement
for the bias correction of future SIC. Consistency between the constructed
SST and SIC fields is an important constraint to consider, as is consistency
between the prescribed sea-ice concentration and thickness; we show that the
latter can be ensured by using a simple parameterisation of sea-ice thickness
as a function of instantaneous and annual minimum SIC.</p
Numerical optimization of amplitude-modulated pulses in microwave-driven entanglement generation
Microwave control of trapped ions can provide an implementation of high-fidelity two-qubit gates free from errors induced by photon scattering. Furthermore, microwave conductors may be embedded into a scalable trap structure, providing the chip-level integration of control that is desirable for scaling. Recent developments have demonstrated how amplitude modulation of the gate drive can permit a two-qubit entangling operation to become robust against motional mode noise and other experimental imperfections. Here, we discuss a method for the numerical optimization of the microwave pulse envelope to produce gate pulses with improved resilience, faster operation and higher energy efficiency
Quantum transport in ultracold atoms
Ultracold atoms confined by engineered magnetic or optical potentials are
ideal systems for studying phenomena otherwise difficult to realize or probe in
the solid state because their atomic interaction strength, number of species,
density, and geometry can be independently controlled. This review focuses on
quantum transport phenomena in atomic gases that mirror and oftentimes either
better elucidate or show fundamental differences with those observed in
mesoscopic and nanoscopic systems. We discuss significant progress in
performing transport experiments in atomic gases, contrast similarities and
differences between transport in cold atoms and in condensed matter systems,
and survey inspiring theoretical predictions that are difficult to verify in
conventional setups. These results further demonstrate the versatility offered
by atomic systems in the study of nonequilibrium phenomena and their promise
for novel applications.Comment: 24 pages, 7 figures. A revie
Chapter 12 - Long-term climate change: Projections, commitments and irreversibility
This chapter assesses long-term projections of climate change for the end of the 21st century and beyond, where the forced signal depends on the scenario and is typically larger than the internal variability of the climate system. Changes are expressed with respect to a baseline period of 1986-2005, unless otherwise stated
Century-scale simulations of the response of the West Antarctic Ice Sheet to a warming climate
We use the BISICLES adaptive mesh ice sheet model to carry out one, two, and three century simulations of the fast-flowing ice streams of the West Antarctic Ice Sheet, deploying sub-kilometer resolution around the grounding line since coarser resolution results in substantial underestimation of the response. Each of the simulations begins with a geometry and velocity close to present-day observations, and evolves according to variation in meteoric ice accumulation rates and oceanic ice shelf melt rates. Future changes in accumulation and melt rates range from no change, through anomalies computed by atmosphere and ocean models driven by the E1 and A1B emissions scenarios, to spatially uniform melt rate anomalies that remove most of the ice shelves over a few centuries. We find that variation in the resulting ice dynamics is dominated by the choice of initial conditions and ice shelf melt rate and mesh resolution, although ice accumulation affects the net change in volume above flotation to a similar degree. Given sufficient melt rates, we compute grounding line retreat over hundreds of kilometers in every major ice stream, but the ocean models do not predict such melt rates outside of the Amundsen Sea Embayment until after 2100. Within the Amundsen Sea Embayment the largest single source of variability is the onset of sustained retreat in Thwaites Glacier, which can triple the rate of eustatic sea level rise
Emergent global patterns of ecosystem structure and function from a mechanistic general ecosystem model
Anthropogenic activities are causing widespread degradation of ecosystems worldwide, threatening the ecosystem services upon which all human life depends. Improved understanding of this degradation is urgently needed to improve avoidance and mitigation measures. One tool to assist these efforts is predictive models of ecosystem structure and function that are mechanistic: based on fundamental ecological principles. Here we present the first mechanistic General Ecosystem Model (GEM) of ecosystem structure and function that is both global and applies in all terrestrial and marine environments. Functional forms and parameter values were derived from the theoretical and empirical literature where possible. Simulations of the fate of all organisms with body masses between 10 µg and 150,000 kg (a range of 14 orders of magnitude) across the globe led to emergent properties at individual (e.g., growth rate), community (e.g., biomass turnover rates), ecosystem (e.g., trophic pyramids), and macroecological scales (e.g., global patterns of trophic structure) that are in general agreement with current data and theory. These properties emerged from our encoding of the biology of, and interactions among, individual organisms without any direct constraints on the properties themselves. Our results indicate that ecologists have gathered sufficient information to begin to build realistic, global, and mechanistic models of ecosystems, capable of predicting a diverse range of ecosystem properties and their response to human pressures
A model for large glacial–interglacial climate-induced changes in dust and sea salt concentrations in deep ice cores (central Antarctica): palaeoclimatic implications and prospects for refining ice core chronologies
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