31,105 research outputs found
Fundamental Limits to Coherent Photon Generation with Solid-State Atomlike Transitions
Coherent generation of indistinguishable single photons is crucial for many
quantum communication and processing protocols. Solid-state realizations of
two-level atomic transitions or three-level spin- systems offer
significant advantages over their atomic counterparts for this purpose, albeit
decoherence can arise due to environmental couplings. One popular approach to
mitigate dephasing is to operate in the weak excitation limit, where excited
state population is minimal and coherently scattered photons dominate over
incoherent emission. Here we probe the coherence of photons produced using
two-level and spin- solid-state systems. We observe that the coupling
of the atomic-like transitions to the vibronic transitions of the crystal
lattice is independent of driving strength and detuning. We apply a polaron
master equation to capture the non-Markovian dynamics of the ground state
vibrational manifolds. These results provide insight into the fundamental
limitations for photon coherence from solid-state quantum emitters, with the
consequence that deterministic single-shot quantum protocols are impossible and
inherently probabilistic approaches must be embraced.Comment: 16 pages [with supplementary information], 8 figure
Epcast: Controlled Dissemination in Human-based Wireless Networks by means of Epidemic Spreading Models
Epidemics-inspired techniques have received huge attention in recent years
from the distributed systems and networking communities. These algorithms and
protocols rely on probabilistic message replication and redundancy to ensure
reliable communication. Moreover, they have been successfully exploited to
support group communication in distributed systems, broadcasting, multicasting
and information dissemination in fixed and mobile networks. However, in most of
the existing work, the probability of infection is determined heuristically,
without relying on any analytical model. This often leads to unnecessarily high
transmission overheads.
In this paper we show that models of epidemic spreading in complex networks
can be applied to the problem of tuning and controlling the dissemination of
information in wireless ad hoc networks composed of devices carried by
individuals, i.e., human-based networks. The novelty of our idea resides in the
evaluation and exploitation of the structure of the underlying human network
for the automatic tuning of the dissemination process in order to improve the
protocol performance. We evaluate the results using synthetic mobility models
and real human contacts traces
On the Convergence of Population Protocols When Population Goes to Infinity
Population protocols have been introduced as a model of sensor networks
consisting of very limited mobile agents with no control over their own
movement. A population protocol corresponds to a collection of anonymous
agents, modeled by finite automata, that interact with one another to carry out
computations, by updating their states, using some rules. Their computational
power has been investigated under several hypotheses but always when restricted
to finite size populations. In particular, predicates stably computable in the
original model have been characterized as those definable in Presburger
arithmetic. We study mathematically the convergence of population protocols
when the size of the population goes to infinity. We do so by giving general
results, that we illustrate through the example of a particular population
protocol for which we even obtain an asymptotic development. This example shows
in particular that these protocols seem to have a rather different
computational power when a huge population hypothesis is considered.Comment: Submitted to Applied Mathematics and Computation. 200
Synaptic mechanisms of interference in working memory
Information from preceding trials of cognitive tasks can bias performance in
the current trial, a phenomenon referred to as interference. Subjects
performing visual working memory tasks exhibit interference in their
trial-to-trial response correlations: the recalled target location in the
current trial is biased in the direction of the target presented on the
previous trial. We present modeling work that (a) develops a probabilistic
inference model of this history-dependent bias, and (b) links our probabilistic
model to computations of a recurrent network wherein short-term facilitation
accounts for the dynamics of the observed bias. Network connectivity is
reshaped dynamically during each trial, providing a mechanism for generating
predictions from prior trial observations. Applying timescale separation
methods, we can obtain a low-dimensional description of the trial-to-trial bias
based on the history of target locations. The model has response statistics
whose mean is centered at the true target location across many trials, typical
of such visual working memory tasks. Furthermore, we demonstrate task protocols
for which the plastic model performs better than a model with static
connectivity: repetitively presented targets are better retained in working
memory than targets drawn from uncorrelated sequences.Comment: 28 pages, 7 figure
Correlation-based model of artificially induced plasticity in motor cortex by a bidirectional brain-computer interface
Experiments show that spike-triggered stimulation performed with
Bidirectional Brain-Computer-Interfaces (BBCI) can artificially strengthen
connections between separate neural sites in motor cortex (MC). What are the
neuronal mechanisms responsible for these changes and how does targeted
stimulation by a BBCI shape population-level synaptic connectivity? The present
work describes a recurrent neural network model with probabilistic spiking
mechanisms and plastic synapses capable of capturing both neural and synaptic
activity statistics relevant to BBCI conditioning protocols. When spikes from a
neuron recorded at one MC site trigger stimuli at a second target site after a
fixed delay, the connections between sites are strengthened for spike-stimulus
delays consistent with experimentally derived spike time dependent plasticity
(STDP) rules. However, the relationship between STDP mechanisms at the level of
networks, and their modification with neural implants remains poorly
understood. Using our model, we successfully reproduces key experimental
results and use analytical derivations, along with novel experimental data. We
then derive optimal operational regimes for BBCIs, and formulate predictions
concerning the efficacy of spike-triggered stimulation in different regimes of
cortical activity.Comment: 35 pages, 9 figure
Immunization strategies for epidemic processes in time-varying contact networks
Spreading processes represent a very efficient tool to investigate the
structural properties of networks and the relative importance of their
constituents, and have been widely used to this aim in static networks. Here we
consider simple disease spreading processes on empirical time-varying networks
of contacts between individuals, and compare the effect of several immunization
strategies on these processes. An immunization strategy is defined as the
choice of a set of nodes (individuals) who cannot catch nor transmit the
disease. This choice is performed according to a certain ranking of the nodes
of the contact network. We consider various ranking strategies, focusing in
particular on the role of the training window during which the nodes'
properties are measured in the time-varying network: longer training windows
correspond to a larger amount of information collected and could be expected to
result in better performances of the immunization strategies. We find instead
an unexpected saturation in the efficiency of strategies based on nodes'
characteristics when the length of the training window is increased, showing
that a limited amount of information on the contact patterns is sufficient to
design efficient immunization strategies. This finding is balanced by the large
variations of the contact patterns, which strongly alter the importance of
nodes from one period to the next and therefore significantly limit the
efficiency of any strategy based on an importance ranking of nodes. We also
observe that the efficiency of strategies that include an element of randomness
and are based on temporally local information do not perform as well but are
largely independent on the amount of information available
The Dynamics and Stability of Probabilistic Population Processes
We study here the dynamics and stability of Probabilistic Population Processes, via the differential equations approach. We provide a quite general model following the work of Kurtz [15] for approximating discrete processes with continuous differential equations. We show that it includes the model of Angluin et al. [1], in the case of very large populations. We require that the long-term behavior of the family of increasingly large discrete processes is a good approximation to the long-term behavior of the continuous process, i.e., we exclude population protocols that are extremely unstable such as parity-dependent decision processes. For the general model, we give a sufficient condition for stability that can be checked in polynomial time. We also study two interesting sub cases: (a) Protocols whose specifications (in our terms) are configuration independent. We show that they are always stable and that their eventual subpopulation percentages are actually a Markov Chain stationary distribution. (b) Protocols that have dynamics resembling virus spread. We show that their dynamics are actually similar to the well-known Replicator Dynamics of Evolutionary Games. We also provide a sufficient condition for stability in this case
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