316 research outputs found
Reference frames, superselection rules, and quantum information
Recently, there has been much interest in a new kind of ``unspeakable''
quantum information that stands to regular quantum information in the same way
that a direction in space or a moment in time stands to a classical bit string:
the former can only be encoded using particular degrees of freedom while the
latter are indifferent to the physical nature of the information carriers. The
problem of correlating distant reference frames, of which aligning Cartesian
axes and synchronizing clocks are important instances, is an example of a task
that requires the exchange of unspeakable information and for which it is
interesting to determine the fundamental quantum limit of efficiency. There
have also been many investigations into the information theory that is
appropriate for parties that lack reference frames or that lack correlation
between their reference frames, restrictions that result in global and local
superselection rules. In the presence of these, quantum unspeakable information
becomes a new kind of resource that can be manipulated, depleted, quantified,
etcetera. Methods have also been developed to contend with these restrictions
using relational encodings, particularly in the context of computation,
cryptography, communication, and the manipulation of entanglement. This article
reviews the role of reference frames and superselection rules in the theory of
quantum information processing.Comment: 55 pages, published versio
Multipartite entanglement in the Fenna-Matthews-Olson (FMO) pigment-protein complex
We investigate multipartite states in the Fenna-Matthews-Olson (FMO)
pigment-protein complex of the green sulfur bacteria using a Lorentzian
spectral density of the phonon reservoir fitted with typical parameter
estimates of the species, P. aestuarii. The evolution of the entanglement
measure of the excitonic W qubit states is evaluated in the picosecond time
range, showing increased revivals in the non-Markovian regime. Similar trends
are observed in the evolution dynamics of the Meyer-Wallach measure of the
N-exciton multipartite state, with results showing that multipartite
entanglement can last from 0.5 to 1 ps, between the Bchls of the FMO complex.
The teleportation and quantum information splitting fidelities associated with
the GHZ and W_A resource states of the excitonic qubit channels of the FMO
complex show that revivals in fidelities increase with the degree of
non-Markovian strength of the decoherent environment. Results indicate that
quantum information processing tasks involving teleportation followed by the
decodification process involving W_A states of the FMO complex, may play a
critical role during coherent oscillations at physiological temperatures.Comment: 16 pages, new figs, typo
Performance of Quantum Key Distribution Protocol with Dual-Rail Displaced Photon States
We propose a scheme for quantum key distribution (QKD) protocol with
dual-rail displaced photon states. Displaced single photon states carry bit
value of code which may be extracted while coherent states carry nothing and
they only provide inconclusive outcome. Developed QKD protocol works with
experimental attendant noise to observe presence of malicious Eve. Pulses with
large amplitudes unlike conventional QKD relying on faint laser pulses are used
that may approximate it to standard telecommunication communication and may
show resistance to eavesdropping even in settings with high attenuation.
Information leakage to the eavesdropper is determined from comparison of output
distribution of the outcomes with ideal one that is defined by two additional
inaccessible to nobody, saving for who sends the pulses, parameters. Robustness
to some possible eavesdropping attacks is shown.Comment: 20 pages, 2 figure
Photonic Entanglement for Fundamental Tests and Quantum Communication
Entanglement is at the heart of fundamental tests of quantum mechanics like
tests of Bell-inequalities and, as discovered lately, of quantum computation
and communication. Their technological advance made entangled photons play an
outstanding role in entanglement physics. We give a generalized concept of
qubit entanglement and review the state of the art of photonic experiments.Comment: 54 pages, 33 figures. Review article submitted to QIC (Rinton
The classical-quantum boundary for correlations: discord and related measures
One of the best signatures of nonclassicality in a quantum system is the
existence of correlations that have no classical counterpart. Different methods
for quantifying the quantum and classical parts of correlations are amongst the
more actively-studied topics of quantum information theory over the past
decade. Entanglement is the most prominent of these correlations, but in many
cases unentangled states exhibit nonclassical behavior too. Thus distinguishing
quantum correlations other than entanglement provides a better division between
the quantum and classical worlds, especially when considering mixed states.
Here we review different notions of classical and quantum correlations
quantified by quantum discord and other related measures. In the first half, we
review the mathematical properties of the measures of quantum correlations,
relate them to each other, and discuss the classical-quantum division that is
common among them. In the second half, we show that the measures identify and
quantify the deviation from classicality in various
quantum-information-processing tasks, quantum thermodynamics, open-system
dynamics, and many-body physics. We show that in many cases quantum
correlations indicate an advantage of quantum methods over classical ones.Comment: Close to the published versio
Quantum channel of continuous variable teleportation and nonclassicality of quantum states
Noisy teleportation of nonclassical quantum states via a two-mode
squeezed-vacuum state is studied with the completely positive map and the
Glauber-Sudarshan -function. Using the nonclassical depth as a measure of
transmission performance, we compare the teleportation scheme with the direct
transmission through a noisy channel. The noise model is based on the coupling
to the vacuum field. It is shown that the teleportation channel has better
transmission performance than the direct transmission channel in a certain
region. The bounds for such region and for obtaining the nonvanished
nonclassicality of the teleported quantum states are also discussed. Our model
shows a reasonable agreement with the observed teleportation fidelity in the
experiment by Furusawa et al. [Science {\bf 282}, 706 (1998)]. We finally
mention the required conditions for transmitting nonclassical features in real
experiments.Comment: 16 pages, 4 figure
Quantum information with continuous variables
Quantum information is a rapidly advancing area of interdisciplinary
research. It may lead to real-world applications for communication and
computation unavailable without the exploitation of quantum properties such as
nonorthogonality or entanglement. We review the progress in quantum information
based on continuous quantum variables, with emphasis on quantum optical
implementations in terms of the quadrature amplitudes of the electromagnetic
field.Comment: accepted for publication in Reviews of Modern Physic
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