5,785 research outputs found
Stochastic resonance in Gaussian quantum channels
We determine conditions for the presence of stochastic resonance in a lossy
bosonic channel with a nonlinear, threshold decoding. The stochastic resonance
effect occurs if and only if the detection threshold is outside of a "forbidden
interval". We show that it takes place in different settings: when transmitting
classical messages through a lossy bosonic channel, when transmitting over an
entanglement-assisted lossy bosonic channel, and when discriminating channels
with different loss parameters. Moreover, we consider a setting in which
stochastic resonance occurs in the transmission of a qubit over a lossy bosonic
channel with a particular encoding and decoding. In all cases, we assume the
addition of Gaussian noise to the signal and show that it does not matter who,
between sender and receiver, introduces such a noise. Remarkably, different
results are obtained when considering a setting for private communication. In
this case the symmetry between sender and receiver is broken and the "forbidden
interval" may vanish, leading to the occurrence of stochastic resonance effects
for any value of the detection threshold.Comment: 17 pages, 6 figures. Manuscript improved in many ways. New results on
private communication adde
Great expectations: the significance of concepts of normality, care, and social support in cultural discourses of disabled motherhood
This paper addresses the topic of the pressures facing disabled mothers, bearing these cultural values in mind. It investigates the ways that different concerns affecting disabled women interact and shape choices about parenthood, especially those affecting the decision to remain childless or terminate a pregnancy, due to the mother’s impairment
Encoding One Logical Qubit Into Six Physical Qubits
We discuss two methods to encode one qubit into six physical qubits. Each of
our two examples corrects an arbitrary single-qubit error. Our first example is
a degenerate six-qubit quantum error-correcting code. We explicitly provide the
stabilizer generators, encoding circuit, codewords, logical Pauli operators,
and logical CNOT operator for this code. We also show how to convert this code
into a non-trivial subsystem code that saturates the subsystem Singleton bound.
We then prove that a six-qubit code without entanglement assistance cannot
simultaneously possess a Calderbank-Shor-Steane (CSS) stabilizer and correct an
arbitrary single-qubit error. A corollary of this result is that the Steane
seven-qubit code is the smallest single-error correcting CSS code. Our second
example is the construction of a non-degenerate six-qubit CSS
entanglement-assisted code. This code uses one bit of entanglement (an ebit)
shared between the sender and the receiver and corrects an arbitrary
single-qubit error. The code we obtain is globally equivalent to the Steane
seven-qubit code and thus corrects an arbitrary error on the receiver's half of
the ebit as well. We prove that this code is the smallest code with a CSS
structure that uses only one ebit and corrects an arbitrary single-qubit error
on the sender's side. We discuss the advantages and disadvantages for each of
the two codes.Comment: 13 pages, 3 figures, 4 table
Joint source-channel coding for a quantum multiple access channel
Suppose that two senders each obtain one share of the output of a classical,
bivariate, correlated information source. They would like to transmit the
correlated source to a receiver using a quantum multiple access channel. In
prior work, Cover, El Gamal, and Salehi provided a combined source-channel
coding strategy for a classical multiple access channel which outperforms the
simpler "separation" strategy where separate codebooks are used for the source
coding and the channel coding tasks. In the present paper, we prove that a
coding strategy similar to the Cover-El Gamal-Salehi strategy and a
corresponding quantum simultaneous decoder allow for the reliable transmission
of a source over a quantum multiple access channel, as long as a set of
information inequalities involving the Holevo quantity hold.Comment: 21 pages, v2: minor changes, accepted into Journal of Physics
Positronium collisions with rare-gas atoms
We calculate elastic scattering of positronium (Ps) by the Xe atom using the
recently developed pseudopotential method [I. I. Fabrikant and G. F. Gribakin,
Phys. Rev. A 90, 052717 (2014)] and review general features of Ps scattering
from heavier rare-gas atoms: Ar, Kr, and Xe. The total scattering cross section
is dominated by two contributions: elastic scattering and Ps ionization
(breakup). To calculate the Ps ionization cross sections we use the
binary-encounter method for Ps collisions with an atomic target. Our results
for the ionization cross section agree well with previous calculations carried
out in the impulse approximation. Our total Ps-Xe cross section, when plotted
as a function of the projectile velocity, exhibits similarity with the
electron-Xe cross section for the collision velocities higher than 0.8 a.u.,
and agrees very well with the measurements at Ps velocities above 0.5 a.u.Comment: 7 pages, 7 figures, submitted to J. Phys.
The squashed entanglement of the noiseless quantum Gaussian attenuator and amplifier
We determine the maximum squashed entanglement achievable between sender and
receiver of the noiseless quantum Gaussian attenuators and amplifiers and we
prove that it is achieved sending half of an infinitely squeezed two-mode
vacuum state. The key ingredient of the proof is a lower bound to the squashed
entanglement of the quantum Gaussian states obtained applying a two-mode
squeezing operation to a quantum thermal Gaussian state tensored with the
vacuum state. This is the first lower bound to the squashed entanglement of a
quantum Gaussian state and opens the way to determine the squashed entanglement
of all quantum Gaussian channels. Moreover, we determine the classical squashed
entanglement of the quantum Gaussian states above and show that it is strictly
larger than their squashed entanglement. This is the first time that the
classical squashed entanglement of a mixed quantum Gaussian state is
determined
Exploring the effectiveness of media in communicating public health messages to people with learning disabilities during the pandemic
The article aims to explore mass and social media’s role in communicating public health messages in Britain during the COVID-19 pandemic. The article presents findings from a realist mixed methods study analysing data collected from 137 participants who have a learning disability and/or autism. Our study discovered that participants reported that social media only led to confusion because of contradictory messages being presented on COVID-19. Although people with learning disabilities and/or autism preferred gaining information from TV news, they also reported that this information was often confusing and inaccessible. Participants drew on family members, and social care professionals, to explain and help them negotiate the complexities of public health messages during the global pandemic. The study concludes by suggesting the need for accessible information and health communications to effectively contend with any future global pandemic or health emergency to reduce the health risks for people with learning disabilities and/or autism
Minimal-memory realization of pearl-necklace encoders of general quantum convolutional codes
Quantum convolutional codes, like their classical counterparts, promise to
offer higher error correction performance than block codes of equivalent
encoding complexity, and are expected to find important applications in
reliable quantum communication where a continuous stream of qubits is
transmitted. Grassl and Roetteler devised an algorithm to encode a quantum
convolutional code with a "pearl-necklace encoder." Despite their theoretical
significance as a neat way of representing quantum convolutional codes, they
are not well-suited to practical realization. In fact, there is no
straightforward way to implement any given pearl-necklace structure. This paper
closes the gap between theoretical representation and practical implementation.
In our previous work, we presented an efficient algorithm for finding a
minimal-memory realization of a pearl-necklace encoder for
Calderbank-Shor-Steane (CSS) convolutional codes. This work extends our
previous work and presents an algorithm for turning a pearl-necklace encoder
for a general (non-CSS) quantum convolutional code into a realizable quantum
convolutional encoder. We show that a minimal-memory realization depends on the
commutativity relations between the gate strings in the pearl-necklace encoder.
We find a realization by means of a weighted graph which details the
non-commutative paths through the pearl-necklace. The weight of the longest
path in this graph is equal to the minimal amount of memory needed to implement
the encoder. The algorithm has a polynomial-time complexity in the number of
gate strings in the pearl-necklace encoder.Comment: 16 pages, 5 figures; extends paper arXiv:1004.5179v
A Feynman-Kac Formula for Anticommuting Brownian Motion
Motivated by application to quantum physics, anticommuting analogues of
Wiener measure and Brownian motion are constructed. The corresponding Ito
integrals are defined and the existence and uniqueness of solutions to a class
of stochastic differential equations is established. This machinery is used to
provide a Feynman-Kac formula for a class of Hamiltonians. Several specific
examples are considered.Comment: 21 page
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