2,604 research outputs found
Quantum discord and local demons
Quantum discord was proposed as a measure of the "quantumness" of
correlations. There are at least three different discord-like quantities, two
of which determine the difference between the efficiencies of a Szilard's
engine under different sets of restrictions. The three discord measures vanish
simulataneosly. We introduce an easy way to test for zero discord, relate it to
the Cerf-Adami conditional entropy and show that there is no relation between
the discord and the local disitnguishability.Comment: 7 pages, RevTeX. Some minor changes after comments from colleagues,
some references added. Similar to published versio
Is a multiple excitation of a single atom equivalent to a single excitation of an ensemble of atoms?
Recent technological advances have enabled to isolate, control and measure
the properties of a single atom, leading to the possibility to perform
statistics on the behavior of single quantum systems. These experiments have
enabled to check a question which was out of reach previously: Is the
statistics of a repeatedly excitation of an atom N times equivalent to a single
excitation of an ensemble of N atoms? We present a new method to analyze
quantum measurements which leads to the postulation that the answer is most
probably no. We discuss the merits of the analysis and its conclusion.Comment: 3 pages, 3 figure
Natural Metric for Quantum Information Theory
We study in detail a very natural metric for quantum states. This new
proposal has two basic ingredients: entropy and purification. The metric for
two mixed states is defined as the square root of the entropy of the average of
representative purifications of those states. Some basic properties are
analyzed and its relation with other distances is investigated. As an
illustrative application, the proposed metric is evaluated for 1-qubit mixed
states.Comment: v2: enlarged; presented at ISIT 2008 (Toronto
Quantum entanglement theory in the presence of superselection rules
Superselection rules severly constrain the operations which can be
implemented on a distributed quantum system. While the restriction to local
operations and classical communication gives rise to entanglement as a nonlocal
resource, particle number conservation additionally confines the possible
operations and should give rise to a new resource. In [Phys. Rev. Lett. 92,
087904 (2004), quant-ph/0310124] we showed that this resource can be quantified
by a single additional number, the superselection induced variance (SiV)
without changing the concept of entanglement. In this paper, we give the
results on pure states in greater detail; additionally, we provide a discussion
of mixed state nonlocality with superselection rules where we consider both
formation and distillation. Finally, we demonstrate that SiV is indeed a
resource, i.e., that it captures how well a state can be used to overcome the
restrictions imposed by the superselection rule.Comment: 16 pages, 5 figure
Parameter estimation in pair hidden Markov models
This paper deals with parameter estimation in pair hidden Markov models
(pair-HMMs). We first provide a rigorous formalism for these models and discuss
possible definitions of likelihoods. The model being biologically motivated,
some restrictions with respect to the full parameter space naturally occur.
Existence of two different Information divergence rates is established and
divergence property (namely positivity at values different from the true one)
is shown under additional assumptions. This yields consistency for the
parameter in parametrization schemes for which the divergence property holds.
Simulations illustrate different cases which are not covered by our results.Comment: corrected typo
Second Law of Thermodynamics with Discrete Quantum Feedback Control
A new thermodynamic inequality is derived which leads to the maximum work
that can be extracted from multi-heat baths with the assistance of discrete
quantum feedback control. The maximum work is determined by the free-energy
difference and a generalized mutual information content between the
thermodynamic system and the feedback controller. This maximum work can exceed
that in conventional thermodynamics and, in the case of a heat cycle with two
heat baths, the heat efficiency can be greater than that of the Carnot cycle.
The consistency of our results with the second law of thermodynamics is ensured
by the fact that work is needed for information processing of the feedback
controller
Information-theoretic temporal Bell inequality and quantum computation
An information-theoretic temporal Bell inequality is formulated to contrast
classical and quantum computations. Any classical algorithm satisfies the
inequality, while quantum ones can violate it. Therefore, the violation of the
inequality is an immediate consequence of the quantumness in the computation.
Furthermore, this approach suggests a notion of temporal nonlocality in quantum
computation.Comment: v2: 5 pages, refereces added, discussion slightly revised, main
result unchanged. v3: typos correcte
Security of Quantum Key Distribution with Coherent States and Homodyne Detection
We assess the security of a quantum key distribution protocol relying on the
transmission of Gaussian-modulated coherent states and homodyne detection. This
protocol is shown to be equivalent to a squeezed state protocol based on a CSS
code construction, and is thus provably secure against any eavesdropping
strategy. We also briefly show how this protocol can be generalized in order to
improve the net key rate.Comment: 7 page
Gaussian capacity of the quantum bosonic channel with additive correlated Gaussian noise
We present an algorithm for calculation of the Gaussian classical capacity of
a quantum bosonic memory channel with additive Gaussian noise. The algorithm,
restricted to Gaussian input states, is applicable to all channels with noise
correlations obeying certain conditions and works in the full input energy
domain, beyond previous treatments of this problem. As an illustration, we
study the optimal input states and capacity of a quantum memory channel with
Gauss-Markov noise [J. Sch\"afer, Phys. Rev. A 80, 062313 (2009)]. We evaluate
the enhancement of the transmission rate when using these optimal entangled
input states by comparison with a product coherent-state encoding and find out
that such a simple coherent-state encoding achieves not less than 90% of the
capacity.Comment: 12+6 pages, 9 figures. Errors corrected, figures were made clearer,
appendix improved and extende
- …