337 research outputs found
A comparison of spike time prediction and receptive field mapping with point process generalized linear models, Wiener-Voltera kernels, and spike-triggered averaging methods
Poster presentation: Characterizing neuronal encoding is essential for understanding information processing in the brain. Three methods are commonly used to characterize the relationship between neural spiking activity and the features of putative stimuli. These methods include: Wiener-Volterra kernel methods (WVK), the spike-triggered average (STA), and more recently, the point process generalized linear model (GLM). We compared the performance of these three approaches in estimating receptive field properties and orientation tuning of 251 V1 neurons recorded from 2 monkeys during a fixation period in response to a moving bar. The GLM consisted of two formulations of the conditional intensity function for a point process characterization of the spiking activity: one with a stimulus only component and one with the stimulus and spike history. We fit the GLMs by maximum likelihood using GLMfit in Matlab. Goodness-of-fit was assessed using cross-validation with Kolmogorov-Smirnov (KS) tests based on the time-rescaling theorem to evaluate the accuracy with which each model predicts the spiking activity of individual neurons and for each movement direction (4016 models in total, for 251 neurons and 16 different directions). The GLMs that considered spike history of up to 35 ms, accurately predicted neuronal spiking activity (95% confidence intervals for KS test) with a performance of 97.0% (3895/4016) for the training data, and 96.5% (3876/4016) for the test data. If spike history was not considered, performance dropped to 73,1% in the training and 71.3% in the testing data. In contrast, the WVF and the STA predicted spiking accurately for 24.2% and 44.5% of the test data examples respectively. The receptive field size estimates obtained from the GLM (with and without history), WVF and STA were comparable. Relative to the GLM orientation tuning was underestimated on average by a factor of 0.45 by the WVF and the STA. The main reason for using the STA and WVF approaches is their apparent simplicity. However, our analyses suggest that more accurate spike prediction as well as more credible estimates of receptive field size and orientation tuning can be computed easily using GLMs implemented in Matlab with standard functions such as GLMfit
Optimal universal quantum cloning and state estimation
We derive a tight upper bound for the fidelity of a universal N to M qubit
cloner, valid for any M \geq N, where the output of the cloner is required to
be supported on the symmetric subspace. Our proof is based on the concatenation
of two cloners and the connection between quantum cloning and quantum state
estimation. We generalise the operation of a quantum cloner to mixed and/or
entangled input qubits described by a density matrix supported on the symmetric
subspace of the constituent qubits. We also extend the validity of optimal
state estimation methods to inputs of this kind.Comment: 4 pages (RevTeX
Optimal N-to-M Cloning of Quantum Coherent States
The cloning of continuous quantum variables is analyzed based on the concept
of Gaussian cloning machines, i.e., transformations that yield copies that are
Gaussian mixtures centered on the state to be copied. The optimality of
Gaussian cloning machines that transform N identical input states into M output
states is investigated, and bounds on the fidelity of the process are derived
via a connection with quantum estimation theory. In particular, the optimal
N-to-M cloning fidelity for coherent states is found to be equal to
MN/(MN+M-N).Comment: 3 pages, RevTe
Approximate quantum cloning and the impossibility of superluminal information transfer
We show that nonlocality of quantum mechanics cannot lead to superluminal
transmission of information, even if most general local operations are allowed,
as long as they are linear and trace preserving. In particular, any quantum
mechanical approximate cloning transformation does not allow signalling. On the
other hand, the no-signalling constraint on its own is not sufficient to
prevent a transformation from surpassing the known cloning bounds. We
illustrate these concepts on the basis of some examples.Comment: 4 pages, 1eps figur
Local‐Regional Similarity in Drylands Increases During Multiyear Wet and Dry Periods and in Response to Extreme Events
Climate change is predicted to impact ecosystems through altered precipitation (PPT) regimes. In the Chihuahuan Desert, multiyear wet and dry periods and extreme PPT pulses are the most influential climatic events for vegetation. Vegetation responses are most frequently studied locally, and regional responses are often unclear. We present an approach to quantify correlation of PPT and vegetation responses (as Normalized Difference Vegetation Index [NDVI]) at the Jornada ARS‐LTER site (JRN; 550 km2 area) and the surrounding dryland region (from 0 to 500 km distance; 400,000 km2 study area) as a way to understand regional similarity to locally observed patterns. We focused on fluctuating wet and dry years, multiyear wet or dry periods of 3–4 yr, and multiyear wet periods that contained one or more extreme high PPT pulses or extreme low rainfall. In all but extreme high PPT years, JRN PPT was highly correlated... (See article for full abstract)
Optimal estimation of multiple phases
We study the issue of simultaneous estimation of several phase shifts induced
by commuting operators on a quantum state. We derive the optimal positive
operator-valued measure corresponding to the multiple-phase estimation. In
particular, we discuss the explicit case of the optimal detection of double
phase for a system of identical qutrits and generalise these results to optimal
multiple phase detection for d-dimensional quantum states.Comment: 6 page
Bounds for state-dependent quantum cloning
Due to the no-cloning theorem, the unknown quantum state can only be cloned
approximately or exactly with some probability. There are two types of cloners:
universal and state-dependent cloner. The optimal universal cloner has been
found and could be viewed as a special state-dependent quantum cloner which has
no information about the states. In this paper, we investigate the
state-dependent cloning when the state-set contains more than two states. We
get some bounds of the global fidelity for these processes. This method is not
dependent on the number of the states contained in the state-set. It is also
independent of the numbers of copying.Comment: 13 pages, 1 figure, to appear in Phys. Rev.
Quantum Cloning of Mixed States in Symmetric Subspace
Quantum cloning machine for arbitrary mixed states in symmetric subspace is
proposed. This quantum cloning machine can be used to copy part of the output
state of another quantum cloning machine and is useful in quantum computation
and quantum information. The shrinking factor of this quantum cloning achieves
the well-known upper bound. When the input is identical pure states, two
different fidelities of this cloning machine are optimal.Comment: Revtex, 4 page
Cloning a real d-dimensional quantum state on the edge of the no-signaling condition
We investigate a new class of quantum cloning machines that equally duplicate
all real states in a Hilbert space of arbitrary dimension. By using the
no-signaling condition, namely that cloning cannot make superluminal
communication possible, we derive an upper bound on the fidelity of this class
of quantum cloning machines. Then, for each dimension d, we construct an
optimal symmetric cloner whose fidelity saturates this bound. Similar
calculations can also be performed in order to recover the fidelity of the
optimal universal cloner in d dimensions.Comment: 6 pages RevTex, 1 encapuslated Postscript figur
Depolarization channels with zero-bandwidth noises
A simple model describing depolarization channels with zero-bandwidth
environment is presented and exactly solved. The environment is modelled by
Lorentzian, telegraphic and Gaussian zero-bandwidth noises. Such channels can
go beyond the standard Markov dynamics and therefore can illustrate the
influence of memory effects of the noisy communication channel on the
transmitted information. To quantify the disturbance of quantum states the
entanglement fidelity between arbitrary input and output states is
investigated.Comment: 15 pages, 3 figure
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