4,579 research outputs found
Continuous variable qumodes as non-destructive probes of quantum systems
With the rise of quantum technologies, it is necessary to have practical and
preferably non-destructive methods to measure and read-out from such devices. A
current line of research towards this has focussed on the use of ancilla
systems which couple to the system under investigation, and through their
interaction, enable properties of the primary system to be imprinted onto and
inferred from the ancillae. We propose the use of continuous variable qumodes
as ancillary probes, and show that the interaction Hamiltonian can be fully
characterised and directly sampled from measurements of the qumode alone. We
suggest how such probes may also be used to determine thermodynamical
properties, including reconstruction of the partition function. We show that
the method is robust to realistic experimental imperfections such as
finite-sized measurement bins and squeezing, and discuss how such probes are
already feasible with current experimental setups.Comment: 8 pages, 3 figure
Using quantum theory to reduce the complexity of input-output processes
All natural things process and transform information. They receive
environmental information as input, and transform it into appropriate output
responses. Much of science is dedicated to building models of such systems --
algorithmic abstractions of their input-output behavior that allow us to
simulate how such systems can behave in the future, conditioned on what has
transpired in the past. Here, we show that classical models cannot avoid
inefficiency -- storing past information that is unnecessary for correct future
simulation. We construct quantum models that mitigate this waste, whenever it
is physically possible to do so. This suggests that the complexity of general
input-output processes depends fundamentally on what sort of information theory
we use to describe them.Comment: 10 pages, 5 figure
African Perspectives on Progress and Challenges in Agricultural Transformation
International Development, Downloads July 2008 - June 2009: 9,
Does Subsidizing Fertilizer Increase Yields? Evidence from Malawi
Despite their strain on government and donor budgets, fertilizer subsidies have once again become popular policy tools in several Sub-Saharan Africa countries as a potential way to increase yields in staple crops like maize. Policy makers often assume that farmers who receive the subsidy will achieve yield responses that are similar to those obtained by farmers who pay commercial prices for the input. This notion has not been verified empirically. Our study uses panel data from Malawi, a country that recently implemented a fertilizer subsidy program, to compare maize yield response to fertilizer from farmers who received subsidized fertilizer with yield responses from those who paid commercial prices for the input. Descriptive results indicate that maize plots using commercial fertilizer obtain higher yields per kilogram of fertilizer than maize plots that used subsidized fertilizer. Conversely, the results obtained using a fixed-effects estimator indicate that when other factors are controlled for, maize plots that use subsidized fertilizer obtain a higher yield response than other plots. The results seems to be influenced by a group of farmers who used no fertilizer before the subsidy program began, but used subsidized fertilizer after the program was implemented. This group of farmers obtained significantly higher yields in the year when they receive the subsidy than did the rest of the farmers in the sample during that year. These findings indicate that in order to be effective, government officials should specifically target fertilizer subsidies to farmers who lack access to commercial markets or would not otherwise find it profitable to purchase the input.Malawi, Fertilizer Subsidies, Production Function, Crop Production/Industries,
The Effects of Government Maize Marketing Policies on Maize Market Prices in Kenya
The Government of Kenya pursues maize marketing policy objectives through the National Cereals and Produce Board (NCPB) which procures and sells maize at administratively determined prices, and stores maize as a contingency against future shortages. A private sector marketing channel competes with the NCPB and prices in this channel are set by supply and demand forces. This paper estimates the effects of NCPB activities on the historical path of private sector maize market prices in Kenya between 1989 and 2004. Results provide important insights into the historical effects of the NCPB, and will provide useful input into deliberations on the appropriate role for the NCPB in the future. It was not possible to use a fully structural econometric model to estimate the historical policy effects because of data limitations in Kenya, which are typical of many developing countries. Instead we use a reduced form vector autoregression model (VAR) and show how policy simulation results can be obtained from a fairly parsimonious VAR that can be estimated with sparse data and imposes only minimal identification restrictions. Results show that NCPB activities have stabilized maize market prices in Kenya, reduced price levels in the early 1990s, and raised price levels by roughly 20 percent between 1995 and 2004. Because roughly 60 percent of Kenya's rural households purchase maize while less than 30 percent sell maize, the government's maize marketing board operations have transferred income from urban consumers and most small rural households to maize selling farmers.Kenya, income transfers, maize policy, price stabilization, VAR, International Development, C22, O2, Q13, Q18,
Swimming in Granular Media
We study a simple model of periodic contraction and extension of large
intruders in a granular bed to understand the mechanism for swimming in an
otherwise solid media. Using an event-driven simulation, we find optimal
conditions that idealized swimmers must use to critically fluidize a sand bed
so that it is rigid enough to support a load when needed, but fluid enough to
permit motion with minimal resistance. Swimmers - or other intruders - that
agitate the bed too rapidly produce large voids that prevent traction from
being achieved, while swimmers that move too slowly cannot travel before the
bed re-solidifies around them i.e., the swimmers locally probe the fundamental
time-scale in a granular packing
Radon Measures on Banach Spaces with their Weak Topologies
The main concern of this paper is to present some improvements
to results on the existence or non-existence of countably additive Borel measures
that are not Radon measures on Banach spaces taken with their weak topologies, on
the standard axioms (ZFC) of set-theory. However, to put the results in perspective we
shall need to say something about consistency results concerning measurable cardinals
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