3,249 research outputs found
Limits to clock synchronization induced by completely dephasing communication channels
Clock synchronization procedures are analyzed in the presence of imperfect
communications. In this context we show that there are physical limitations
which prevent one from synchronizing distant clocks when the intervening medium
is completely dephasing, as in the case of a rapidly varying dispersive medium.Comment: 6 Pages. Revised version as published in PR
The role of entanglement in dynamical evolution
Entanglement or entanglement generating interactions permit to achieve the
maximum allowed speed in the dynamical evolution of a composite system, when
the energy resources are distributed among subsystems. The cases of
pre-existing entanglement and of entanglement-building interactions are
separately addressed. The role of classical correlations is also discussed.Comment: 5 pages, 1 figure. Revised versio
Quantum channels and their entropic characteristics
One of the major achievements of the recently emerged quantum information
theory is the introduction and thorough investigation of the notion of quantum
channel which is a basic building block of any data-transmitting or
data-processing system. This development resulted in an elaborated structural
theory and was accompanied by the discovery of a whole spectrum of entropic
quantities, notably the channel capacities, characterizing
information-processing performance of the channels. This paper gives a survey
of the main properties of quantum channels and of their entropic
characterization, with a variety of examples for finite dimensional quantum
systems. We also touch upon the "continuous-variables" case, which provides an
arena for quantum Gaussian systems. Most of the practical realizations of
quantum information processing were implemented in such systems, in particular
based on principles of quantum optics. Several important entropic quantities
are introduced and used to describe the basic channel capacity formulas. The
remarkable role of the specific quantum correlations - entanglement - as a
novel communication resource, is stressed.Comment: review article, 60 pages, 5 figures, 194 references; Rep. Prog. Phys.
(in press
Radiation Pressure Induced Einstein-Podolsky-Rosen Paradox
We demonstrate the appearance of Einstein-Podolsky-Rosen (EPR) paradox when a
radiation field impinges on a movable mirror. The, the possibility of a local
realism test within a pendular Fabry-Perot cavity is shown to be feasible.Comment: 4 pages ReVTeX, 1 eps figur
Bosonic Memory Channels
We discuss a Bosonic channel model with memory effects. It relies on a
multi-mode squeezed (entangled) environment's state. The case of lossy Bosonic
channels is analyzed in detail. We show that in the absence of input energy
constraints the memory channels are equivalent to their memoryless
counterparts. In the case of input energy constraint we provide lower and upper
bounds for the memory channel capacity.Comment: 6 pages, 2 figure
Quantum state majorization at the output of bosonic Gaussian channels
Quantum communication theory explores the implications of quantum mechanics
to the tasks of information transmission. Many physical channels can be
formally described as quantum Gaussian operations acting on bosonic quantum
states. Depending on the input state and on the quality of the channel, the
output suffers certain amount of noise. For a long time it has been
conjectured, but never proved, that output states of Gaussian channels
corresponding to coherent input signals are the less noisy ones (in the sense
of a majorization criterion). In this work we prove this conjecture.
Specifically we show that every output state of a phase insensitive Gaussian
channel is majorized by the output state corresponding to a coherent input. The
proof is based on the optimality of coherent states for the minimization of
strictly concave output functionals. Moreover we show that coherent states are
the unique optimizers.Comment: 7 pages, 1 figure. Published versio
A solution of the Gaussian optimizer conjecture
The long-standing conjectures of the optimality of Gaussian inputs for
Gaussian channel and Gaussian additivity are solved for a broad class of
covariant or contravariant Bosonic Gaussian channels (which includes in
particular thermal, additive classical noise, and amplifier channels)
restricting to the class of states with finite second moments. We show that the
vacuum is the input state which minimizes the entropy at the output of such
channels. This allows us to show also that the classical capacity of these
channels (under the input energy constraint) is additive and is achieved by
Gaussian encodings.Comment: 24 pages, no figures (minor typos corrected
Quantum metrology
We point out a general framework that encompasses most cases in which quantum
effects enable an increase in precision when estimating a parameter (quantum
metrology). The typical quantum precision-enhancement is of the order of the
square root of the number of times the system is sampled. We prove that this is
optimal and we point out the different strategies (classical and quantum) that
permit to attain this bound.Comment: 4 pages, 2 figure
Quantum MERA Channels
Tensor networks representations of many-body quantum systems can be described
in terms of quantum channels. We focus on channels associated with the
Multi-scale Entanglement Renormalization Ansatz (MERA) tensor network that has
been recently introduced to efficiently describe critical systems. Our approach
allows us to compute the MERA correspondent to the thermodynamic limit of a
critical system introducing a transfer matrix formalism, and to relate the
system critical exponents to the convergence rates of the associated channels.Comment: 4 pages, 2 figure
Sub-Heisenberg estimation strategies are ineffective
In interferometry, sub-Heisenberg strategies claim to achieve a phase
estimation error smaller than the inverse of the mean number of photons
employed (Heisenberg bound). Here we show that one can achieve a comparable
precision without performing any measurement, just using the large prior
information that sub-Heisenberg strategies require. For uniform prior (i.e. no
prior information), we prove that these strategies cannot achieve more than a
fixed gain of about 1.73 over Heisenberg-limited interferometry. Analogous
results hold for arbitrary single-mode prior distributions. These results
extend also beyond interferometry: the effective error in estimating any
parameter is lower bounded by a quantity proportional to the inverse
expectation value (above a ground state) of the generator of translations of
the parameter.Comment: 4 pages, 2 figures, revised version that was publishe
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