797 research outputs found
The spectro-contextual encoding and retrieval theory of episodic memory.
The spectral fingerprint hypothesis, which posits that different frequencies of oscillations underlie different cognitive operations, provides one account for how interactions between brain regions support perceptual and attentive processes (Siegel etal., 2012). Here, we explore and extend this idea to the domain of human episodic memory encoding and retrieval. Incorporating findings from the synaptic to cognitive levels of organization, we argue that spectrally precise cross-frequency coupling and phase-synchronization promote the formation of hippocampal-neocortical cell assemblies that form the basis for episodic memory. We suggest that both cell assembly firing patterns as well as the global pattern of brain oscillatory activity within hippocampal-neocortical networks represents the contents of a particular memory. Drawing upon the ideas of context reinstatement and multiple trace theory, we argue that memory retrieval is driven by internal and/or external factors which recreate these frequency-specific oscillatory patterns which occur during episodic encoding. These ideas are synthesized into a novel model of episodic memory (the spectro-contextual encoding and retrieval theory, or "SCERT") that provides several testable predictions for future research
Sharp Quantum vs. Classical Query Complexity Separations
We obtain the strongest separation between quantum and classical query
complexity known to date -- specifically, we define a black-box problem that
requires exponentially many queries in the classical bounded-error case, but
can be solved exactly in the quantum case with a single query (and a polynomial
number of auxiliary operations). The problem is simple to define and the
quantum algorithm solving it is also simple when described in terms of certain
quantum Fourier transforms (QFTs) that have natural properties with respect to
the algebraic structures of finite fields. These QFTs may be of independent
interest, and we also investigate generalizations of them to noncommutative
finite rings.Comment: 13 pages, change in title, improvements in presentation, and minor
corrections. To appear in Algorithmic
A universally programmable Quantum Cellular Automaton
We discuss the role of classical control in the context of reversible quantum
cellular automata. Employing the structure theorem for quantum cellular
automata, we give a general construction scheme to turn an arbitrary cellular
automaton with external classical control into an autonomous one, thereby
proving the computational equivalence of these two models. We use this
technique to construct a universally programmable cellular automaton on a
one-dimensional lattice with single cell dimension 12.Comment: 4 pages, 4 figures, minor changes in introduction, fixed typos,
accepted for publication in Physical Review Letter
Characterization of distillability of entanglement in terms of positive maps
A necessary and sufficient condition for 1-distillability is formulated in
terms of decomposable positive maps. As an application we provide insight into
why all states violating the reduction criterion map are distillable and
demonstrate how to construct such maps in a systematic way. We establish a
connection between a number of existing results, which leads to an elementary
proof for the characterisation of distillability in terms of 2-positive maps.Comment: 4 pages, revtex4. Published revised version, title changed, expanded
discussion, main result unchange
Quantum computation via translation-invariant operations on a chain of qubits
A scheme of universal quantum computation on a chain of qubits is described
that does not require local control. All the required operations, an Ising-type
interaction and spatially uniform simultaneous one-qubit gates, are
translation-invariant.Comment: Comment after Eq. (2) inserted, journal versio
Adaptive versus non-adaptive strategies for quantum channel discrimination
We provide a simple example that illustrates the advantage of adaptive over
non-adaptive strategies for quantum channel discrimination. In particular, we
give a pair of entanglement-breaking channels that can be perfectly
discriminated by means of an adaptive strategy that requires just two channel
evaluations, but for which no non-adaptive strategy can give a perfect
discrimination using any finite number of channel evaluations.Comment: 11 page
Measurement in control and discrimination of entangled pairs under self-distortion
Quantum correlations and entanglement are fundamental resources for quantum
information and quantum communication processes. Developments in these fields
normally assume these resources stable and not susceptible of distortion. That
is not always the case, Heisenberg interactions between qubits can produce
distortion on entangled pairs generated for engineering purposes (e. g. for
quantum computation or quantum cryptography). Experimental work shows how to
produce entangled spin qubits in quantum dots and electron gases, so its
identification and control are crucial for later applications. The presence of
parasite magnetic fields modifies the expected properties and behavior for
which the pair was intended. Quantum measurement and control help to
discriminate the original state in order to correct it or, just to try of
reconstruct it using some procedures which do not alter their quantum nature.
Two different kinds of quantum entangled pairs driven by a Heisenberg
Hamiltonian with an additional inhomogeneous magnetic field which becoming
self-distorted, can be reconstructed without previous discrimination by adding
an external magnetic field, with fidelity close to 1 (with respect to the
original state, but without discrimination). After, each state can be more
efficiently discriminated. The aim of this work is to show how combining both
processes, first reconstruction without discrimination and after discrimination
with adequate non-local measurements, it's possible a) improve the
discrimination, and b) reprepare faithfully the original states. The complete
process gives fidelities better than 0.9. In the meanwhile, some results about
a class of equivalence for the required measurements were found. This property
lets us select the adequate measurement in order to ease the repreparation
after of discrimination, without loss of entanglement.Comment: 6 figure
New multiplicativity results for qubit maps
Let be a trace-preserving, positivity-preserving (but not necessarily
completely positive) linear map on the algebra of complex
matrices, and let be any finite-dimensional completely positive map.
For and , we prove that the maximal -norm of the product map
\Phi \ot \Omega is the product of the maximal -norms of and
. Restricting to the class of completely positive maps, this
settles the multiplicativity question for all qubit channels in the range of
values .Comment: 14 pages; original proof simplified by using Gorini and Sudarshan's
classification of extreme affine maps on R^
Robust randomized benchmarking of quantum processes
We describe a simple randomized benchmarking protocol for quantum information
processors and obtain a sequence of models for the observable fidelity decay as
a function of a perturbative expansion of the errors. We are able to prove that
the protocol provides an efficient and reliable estimate of an average
error-rate for a set operations (gates) under a general noise model that allows
for both time and gate-dependent errors. We determine the conditions under
which this estimate remains valid and illustrate the protocol through numerical
examples.Comment: 4+ pages, 1 figure, and 1 tabl
The Dynamical Additivity And The Strong Dynamical Additivity Of Quantum Operations
In the paper, the dynamical additivity of bi-stochastic quantum operations is
characterized and the strong dynamical additivity is obtained under some
restrictions.Comment: 9 pages, LaTeX, change the order of name
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