306 research outputs found
Exceeding classical capacity limit in quantum optical channel
The amount of information transmissible through a communications channel is
determined by the noise characteristics of the channel and by the quantities of
available transmission resources. In classical information theory, the amount
of transmissible information can be increased twice at most when the
transmission resource (e.g. the code length, the bandwidth, the signal power)
is doubled for fixed noise characteristics. In quantum information theory,
however, the amount of information transmitted can increase even more than
twice. We present a proof-of-principle demonstration of this super-additivity
of classical capacity of a quantum channel by using the ternary symmetric
states of a single photon, and by event selection from a weak coherent light
source. We also show how the super-additive coding gain, even in a small code
length, can boost the communication performance of conventional coding
technique.Comment: 4 pages, 3 figure
Mode structure and photon number correlations in squeezed quantum pulses
The question of efficient multimode description of optical pulses is studied.
We show that a relatively very small number of nonmonochromatic modes can be
sufficient for a complete quantum description of pulses with Gaussian
quadrature statistics. For example, a three-mode description was enough to
reproduce the experimental data of photon number correlations in optical
solitons [S. Spalter et al., Phys. Rev. Lett. 81, 786 (1998)]. This approach is
very useful for a detailed understanding of squeezing properties of soliton
pulses with the main potential for quantum communication with continuous
variables. We show how homodyne detection and/or measurements of photon number
correlations can be used to determine the quantum state of the multi-mode
field. We also discuss a possible way of physical separation of the
nonmonochromatic modes.Comment: 14 pages, 4 figures; minor revisions of the text, new references; to
appear in the Phys. Rev.
Implementation of generalized quantum measurements: superadditive quantum coding, accessible information extraction, and classical capacity limit
Quantum information theory predicts that when the transmission resource is
doubled in quantum channels, the amount of information transmitted can be
increased more than twice by quantum channel coding technique, whereas the
increase is at most twice in classical information theory. This remarkable
feature, the superadditive quantum coding gain, can be implemented by
appropriate choices of code words and corresponding quantum decoding which
requires a collective quantum measurement. Recently, the first experimental
demonstration was reported [Phys. Rev. Lett. 90, 167906 (2003)]. The purpose of
this paper is to describe our experiment in detail. Particularly, a design
strategy of quantum collective decoding in physical quantum circuits is
emphasized. We also address the practical implication of the gain on
communication performance by introducing the quantum-classical hybrid coding
scheme. We show how the superadditive quantum coding gain, even in a small code
length, can boost the communication performance of conventional coding
technique.Comment: 15 pages, 14 figure
Entanglement quantification from incomplete measurements: Applications using photon-number-resolving weak homodyne detectors
The certificate of success for a number of important quantum information
processing protocols, such as entanglement distillation, is based on the
difference in the entanglement content of the quantum states before and after
the protocol. In such cases, effective bounds need to be placed on the
entanglement of non-local states consistent with statistics obtained from local
measurements. In this work, we study numerically the ability of a novel type of
homodyne detector which combines phase sensitivity and photon-number resolution
to set accurate bounds on the entanglement content of two-mode quadrature
squeezed states without the need for full state tomography. We show that it is
possible to set tight lower bounds on the entanglement of a family of two-mode
degaussified states using only a few measurements. This presents a significant
improvement over the resource requirements for the experimental demonstration
of continuous-variable entanglement distillation, which traditionally relies on
full quantum state tomography.Comment: 18 pages, 6 figure
Quantum optics in the phase space - A tutorial on Gaussian states
In this tutorial, we introduce the basic concepts and mathematical tools
needed for phase-space description of a very common class of states, whose
phase properties are described by Gaussian Wigner functions: the Gaussian
states. In particular, we address their manipulation, evolution and
characterization in view of their application to quantum information.Comment: Tutorial. 23 pages, 1 figure. Updated version accepted for
publication in EPJ - ST devoted to the memory of Federico Casagrand
Quantifying decoherence in continuous variable systems
We present a detailed report on the decoherence of quantum states of
continuous variable systems under the action of a quantum optical master
equation resulting from the interaction with general Gaussian uncorrelated
environments. The rate of decoherence is quantified by relating it to the decay
rates of various, complementary measures of the quantum nature of a state, such
as the purity, some nonclassicality indicators in phase space and, for two-mode
states, entanglement measures and total correlations between the modes.
Different sets of physically relevant initial configurations are considered,
including one- and two-mode Gaussian states, number states, and coherent
superpositions. Our analysis shows that, generally, the use of initially
squeezed configurations does not help to preserve the coherence of Gaussian
states, whereas it can be effective in protecting coherent superpositions of
both number states and Gaussian wave packets.Comment: Review article; 36 pages, 19 figures; typos corrected, references
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Field test of quantum key distribution in the Tokyo QKD Network
A novel secure communication network with quantum key distribution in a
metropolitan area is reported. Different QKD schemes are integrated to
demonstrate secure TV conferencing over a distance of 45km, stable long-term
operation, and application to secure mobile phones.Comment: 21 pages, 19 figure
High-rate quantum cryptography in untrusted networks
We extend the field of continuous-variable quantum cryptography to a network
formulation where two honest parties connect to an untrusted relay by insecure
quantum links. To generate secret correlations, they transmit coherent states
to the relay where a continuous-variable Bell detection is performed and the
outcome broadcast. Even though the detection could be fully corrupted and the
links subject to optimal coherent attacks, the honest parties can still extract
a secret key, achieving high rates when the relay is proximal to one party, as
typical in public networks with access points or proxy servers. Our theory is
confirmed by an experiment generating key-rates which are orders of magnitude
higher than those achievable with discrete-variable protocols. Thus, using the
cheapest possible quantum resources, we experimentally show the possibility of
high-rate quantum key distribution in network topologies where direct links are
missing between end-users and intermediate relays cannot be trusted.Comment: Theory and Experiment. Main article (6 pages) plus Supplementary
Information (additional 13 pages
Fundamental limits of repeaterless quantum communications
Quantum communications promises reliable transmission of quantum information, efficient distribution of entanglement and generation of completely secure keys. For all these tasks, we need to determine the optimal point-to-point rates that are achievable by two remote parties at the ends of a quantum channel, without restrictions on their local operations and classical communication, which can be unlimited and two-way. These two-way assisted capacities represent the ultimate rates that are reachable without quantum repeaters. Here, by constructing an upper bound based on the relative entropy of entanglement and devising a dimension-independent technique dubbed ‘teleportation stretching’, we establish these capacities for many fundamental channels, namely bosonic lossy channels, quantum-limited amplifiers, dephasing and erasure channels in arbitrary dimension. In particular, we exactly determine the fundamental rate-loss tradeoff affecting any protocol of quantum key distribution. Our findings set the limits of point-to-point quantum communications and provide precise and general benchmarks for quantum repeaters
Locomotor recovery following contusive spinal cord injury does not require oligodendrocyte remyelination
The contribution of oligodendrocytes to remyelination in functional recovery after spinal cord injury is not fully understood. Here, the authors show that oligodendrocyte progenitor cell differentiation is not required for functional recovery after spinal cord injury in mice
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