988 research outputs found
Superadditivity of the Classical Capacity with Limited Entanglement Assistance
Finding the optimal encoding strategies can be challenging for communication
using quantum channels, as classical and quantum capacities may be
superadditive. Entanglement assistance can often simplify this task, as the
entanglement-assisted classical capacity for any channel is additive, making
entanglement across channel uses unnecessary. If the entanglement assistance is
limited, the picture is much more unclear. Suppose the classical capacity is
superadditive, then the classical capacity with limited entanglement assistance
could retain superadditivity by continuity arguments. If the classical capacity
is additive, it is unknown if superadditivity can still be developed with
limited entanglement assistance. We show this is possible, by providing an
example. We construct a channel for which, the classical capacity is additive,
but that with limited entanglement assistance can be superadditive. This shows
entanglement plays a weird role in communication and we still understand very
little about it.Comment: 13 page
Superadditivity in trade-off capacities of quantum channels
In this article, we investigate the additivity phenomenon in the dynamic
capacity of a quantum channel for trading classical communication, quantum
communication and entanglement. Understanding such additivity property is
important if we want to optimally use a quantum channel for general
communication purpose. However, in a lot of cases, the channel one will be
using only has an additive single or double resource capacity, and it is
largely unknown if this could lead to an superadditive double or triple
resource capacity. For example, if a channel has an additive classical and
quantum capacity, can the classical-quantum capacity be superadditive? In this
work, we answer such questions affirmatively.
We give proof-of-principle requirements for these channels to exist. In most
cases, we can provide an explicit construction of these quantum channels. The
existence of these superadditive phenomena is surprising in contrast to the
result that the additivity of both classical-entanglement and classical-quantum
capacity regions imply the additivity of the triple capacity region.Comment: 15 pages. v2: typo correcte
Exclusion zone phenomena in water -- a critical review of experimental findings and theories
The existence of the exclusion zone (EZ), a layer of water in which plastic
microspheres are repelled from hydrophilic surfaces, has now been independently
demonstrated by several groups. A better understanding of the mechanisms which
generate EZs would help with understanding the possible importance of EZs in
biology and in engineering applications such as filtration and microfluidics.
Here we review the experimental evidence for EZ phenomena in water and the
major theories that have been proposed. We review experimental results from
birefringence, neutron radiography, nuclear magnetic resonance, and other
studies. Pollack and others have theorized that water in the EZ exists has a
different structure than bulk water, and that this accounts for the EZ. We
present several alternative explanations for EZs and argue that Schurr's theory
based on diffusiophoresis presents a compelling alternative explanation for the
core EZ phenomenon. Among other things, Schurr's theory makes predictions about
the growth of the EZ with time which have been confirmed by Florea et al. and
others. We also touch on several possible confounding factors that make
experimentation on EZs difficult, such as charged surface groups, dissolved
solutes, and adsorbed nanobubbles.Comment: 14 pg
Additive Classical Capacity of Quantum Channels Assisted by Noisy Entanglement
We give a capacity formula for the classical information transmission over a noisy quantum channel, with separable encoding by the sender and limited resources provided by the receiver’s preshared ancilla. Instead of a pure state, we consider the signal-ancilla pair in a mixed state, purified by a “witness.” Thus, the signal-witness correlation limits the resource available from the signal-ancilla correlation. Our formula characterizes the utility of different forms of resources, including noisy or limited entanglement assistance, for classical communication. With separable encoding, the sender’s signals across multiple channel uses are still allowed to be entangled, yet our capacity formula is additive. In particular, for generalized covariant channels, our capacity formula has a simple closed form. Moreover, our additive capacity formula upper bounds the general coherent attack’s information gain in various two-way quantum key distribution protocols. For Gaussian protocols, the additivity of the formula indicates that the collective Gaussian attack is the most powerful.United States. Air Force. Office of Scientific Research (Grant FA9550-14-1-0052)Massachusetts Institute of Technology. Research Laboratory of Electronics (Claude E. Shannon Fellowship)National Science Foundation (U.S.) (Grant CCF-1525130)National Science Foundation (U.S.) (Center for Science of Information. Grant CCF0-939370
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