127 research outputs found
Directed percolation effects emerging from superadditivity of quantum networks
Entanglement indcued non--additivity of classical communication capacity in
networks consisting of quantum channels is considered. Communication lattices
consisiting of butterfly-type entanglement breaking channels augmented, with
some probability, by identity channels are analyzed. The capacity
superadditivity in the network is manifested in directed correlated bond
percolation which we consider in two flavours: simply directed and randomly
oriented. The obtained percolation properties show that high capacity
information transfer sets in much faster in the regime of superadditive
communication capacity than otherwise possible. As a byproduct, this sheds
light on a new type of entanglement based quantum capacity percolation
phenomenon.Comment: 6 pages, 4 figure
Quantum superadditivity in linear optics networks: sending bits via multiple access Gaussian channels
We study classical capacity regions of quantum Gaussian multiple access
channels (MAC). In classical variants of such channels, whilst some capacity
superadditivity-type effects such as the so called {\it water filling effect}
may be achieved, a fundamental classical additivity law can still be
identified, {\it viz.} adding resources to one sender is never advantageous to
other senders in sending their respective information to the receiver. Here, we
show that quantum resources allows violation of this law, by providing two
illustrative schemes of experimentally feasible Gaussian MACs.Comment: 4 pages, 2 figure
Optimal entanglement witnesses from generalized reduction and Robertson maps
We provide a generalization of the reduction and Robertson positive maps in
matrix algebras. They give rise to a new class of optimal entanglement
witnesses. Their structural physical approximation is analyzed. As a byproduct
we provide a new examples of PPT (Positive Partial Transpose) entangled states.Comment: 14 page
On structural physical approximations and entanglement breaking maps
Very recently a conjecture saying that the so-called structural physical
approximations (SPAa) to optimal positive maps (optimal entanglement witnesses)
give entanglement breaking (EB) maps (separable states) has been posed [J. K.
Korbicz {\it et al.}, Phys. Rev. A {\bf 78}, 062105 (2008)]. The main purpose
of this contribution is to explore this subject. First, we extend the set of
entanglement witnesses (EWs) supporting the conjecture. Then, we ask if SPAs
constructed from other than the depolarizing channel maps also lead to EB maps
and show that in general this is not the case. On the other hand, we prove an
interesting fact that for any positive map there exists an EB channel
such that the SPA of constructed with the aid of is
again an EB channel. Finally, we ask similar questions in the case of
continuous variable systems. We provide a simple way of construction of SPA and
prove that in the case of the transposition map it gives EB channel.Comment: 22 pages, improved version, accepted by Journal of Physics
Schemes of transmission of classical information via quantum channels with many senders: discrete and continuous variables cases
Superadditivity effects in the classical capacity of discrete multi-access
channels (MACs) and continuous variable (CV) Gaussian MACs are analysed. New
examples of the manifestation of superadditivity in the discrete case are
provided including, in particular, a channel which is fully symmetric with
respect to all senders. Furthermore, we consider a class of channels for which
{\it input entanglement across more than two copies of the channels is
necessary} to saturate the asymptotic rate of transmission from one of the
senders to the receiver. The 5-input entanglement of Shor error correction
codewords surpass the capacity attainable by using arbitrary two-input
entanglement for these channels. In the CV case, we consider the properties of
the two channels (a beam-splitter channel and a "non-demolition" XP gate
channel) analyzed in [Czekaj {\it et al.}, Phys. Rev. A {\bf 82}, 020302 (R)
(2010)] in greater detail and also consider the sensitivity of capacity
superadditivity effects to thermal noise. We observe that the estimates of
amount of two-mode squeezing required to achieve capacity superadditivity are
more optimistic than previously reported.Comment: 19 pages, 11 figure
Quantum internet using code division multiple access
A crucial open problem in large-scale quantum networks is how to efficiently
transmit quantum data among many pairs of users via a common data-transmission
medium. We propose a solution by developing a quantum code division multiple
access (q-CDMA) approach in which quantum information is chaotically encoded to
spread its spectral content, and then decoded via chaos synchronization to
separate different sender-receiver pairs. In comparison to other existing
approaches, such as frequency division multiple access (FDMA), the proposed
q-CDMA can greatly increase the information rates per channel used, especially
for very noisy quantum channels.Comment: 29 pages, 6 figure
The drivers of change for the contribution of small farms to regional food security in Europe
The capacity of the food system to respond to the economic, demographic and environmental challenges ahead has become a topic of increasing interest, with particular attention to the roles and responsibilities of the different actors to ensure more sustainable food systems that can guarantee food and nutrition security for all. In this paper we approach the need to better understand the factors that can condition the potential contribution of small farms to regional food and nutrition security in Europe, acknowledging the role that small farms play in Europe at present. The analysis is based on a survey to 94 experts from 17 regions (NUTS3 level) in 11 different European countries, which identified the drivers of change according to the regional experts. These drivers were then categorized and their relative relevance assessed. The results indicate that some relevant drivers in the European context are linked to the capacity to adopt technologies and practices allowing adaptation to climate change, and the capacity to connect to food markets, with emphasis in the need for cooperation and collective action. The weight of other more European-specific drivers such as ‘consumer values and habits’ reveal that the future role of small farms will be very dependent on a societal change, with equity becoming a relevant component of consumers’ choice
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