550 research outputs found
Entanglement assisted random access codes
An (n,m,p) Random Access Code (RAC) allows to encode n bits in an m bit
message, in such a way that a receiver of the message can guess any of the
original bits with probability p, greater than 1/2. In Quantum RAC's
(QRACs) one transmits n qubits. The full set of primitive Entanglement Assisted
Random Access Codes (EARACs) is introduced, in which parties are allowed to
share a two-qubit singlet. It is shown that via a concatenation of these, one
can build for any n an (n,1,p) EARAC. QRAC's for n>3 exist only if parties
additionally share classical randomness (SR). We show that EARACs outperform
the best of known QRACs not only in the success probabilities but also in the
amount of communication needed in the preparatory stage of the protocol. Upper
bounds on the performance of EARACs are given, and shown to limit also QRACs.Comment: 4 pages, 1 figure, published versio
Positive cosmological constant in loop quantum cosmology
The k=0 Friedmann Lemaitre Robertson Walker model with a positive
cosmological constant and a massless scalar field is analyzed in detail. If one
uses the scalar field as relational time, new features arise already in the
Hamiltonian framework of classical general relativity: In a finite interval of
relational time, the universe expands out to infinite proper time and zero
matter density. In the deparameterized quantum theory, the true Hamiltonian now
fails to be essentially self-adjoint both in the Wheeler DeWitt (WDW) approach
and in LQC. Irrespective of the choice of the self-adjoint extension, the big
bang singularity persists in the WDW theory while it is resolved and replaced
by a big bounce in loop quantum cosmology (LQC). Furthermore, the quantum
evolution is surprisingly insensitive to the choice of the self-adjoint
extension. This may be a special case of an yet to be discovered general
property of a certain class of symmetric operators that fail to be essentially
self-adjoint.Comment: 36 pages, 6 figures, RevTex
Statistical properties of one dimensional attractive Bose gas
Using classical field approximation we present the first study of statistical
properties of one dimensional Bose gas with attractive interaction. The
canonical probability distribution is generated with the help of a Monte Carlo
method. This way we obtain not only the depletion of the condensate with
growing temperature but also its fluctuations. The most important is our
discovery of a reduced coherence length, the phenomenon observed earlier only
for the repulsive gas, known as quasicondensation.Comment: 4 pages, 4 figure
Why the Tsirelson bound?
Wheeler's question 'why the quantum' has two aspects: why is the world
quantum and not classical, and why is it quantum rather than superquantum,
i.e., why the Tsirelson bound for quantum correlations? I discuss a remarkable
answer to this question proposed by Pawlowski et al (2009), who provide an
information-theoretic derivation of the Tsirelson bound from a principle they
call 'information causality.'Comment: 17 page
Degree of entanglement as a physically ill-posed problem: The case of entanglement with vacuum
We analyze an example of a photon in superposition of different modes, and
ask what is the degree of their entanglement with vacuum. The problem turns out
to be ill-posed since we do not know which representation of the algebra of
canonical commutation relations (CCR) to choose for field quantization. Once we
make a choice, we can solve the question of entanglement unambiguously. So the
difficulty is not with mathematics, but with physics of the problem. In order
to make the discussion explicit we analyze from this perspective a popular
argument based on a photon leaving a beam splitter and interacting with two
two-level atoms. We first solve the problem algebraically in Heisenberg
picture, without any assumption about the form of representation of CCR. Then
we take the -representation and show in two ways that in two-mode
states the modes are maximally entangled with vacuum, but single-mode states
are not entangled. Next we repeat the analysis in terms of the representation
of CCR taken from Berezin's book and show that two-mode states do not involve
the mode-vacuum entanglement. Finally, we switch to a family of reducible
representations of CCR recently investigated in the context of field
quantization, and show that the entanglement with vacuum is present even for
single-mode states. Still, the degree of entanglement is here difficult to
estimate, mainly because there are subsystems, with unspecified and
large.Comment: This paper is basically a reply to quant-ph/0507189 by S. J. van Enk
and to the remarks we got from L. Vaidman after our preliminary
quant-ph/0507151. Version accepted in Phys. Rev.
Closed FRW model in Loop Quantum Cosmology
The basic idea of the LQC applies to every spatially homogeneous cosmological
model, however only the spatially flat (so called ) case has been
understood in detail in the literature thus far. In the closed (so called: k=1)
case certain technical difficulties have been the obstacle that stopped the
development. In this work the difficulties are overcome, and a new LQC model of
the spatially closed, homogeneous, isotropic universe is constructed. The
topology of the spacelike section of the universe is assumed to be that of
SU(2) or SO(3). Surprisingly, according to the results achieved in this work,
the two cases can be distinguished from each other just by the local properties
of the quantum geometry of the universe. The quantum hamiltonian operator of
the gravitational field takes the form of a difference operator, where the
elementary step is the quantum of the 3-volume derived in the flat case by
Ashtekar, Pawlowski and Singh. The mathematical properties of the operator are
studied: it is essentially self-adjoint, bounded from above by 0, the 0 itself
is not an eigenvalue, the eigenvectors form a basis. An estimate on the
dimension of the spectral projection on any finite interval is provided.Comment: 19 pages, latex, no figures, high quality, nea
Cosmic recall and the scattering picture of Loop Quantum Cosmology
The global dynamics of a homogeneous universe in Loop Quantum Cosmology is
viewed as a scattering process of its geometrodynamical equivalent. This
picture is applied to build a flexible (easy to generalize) and not restricted
just to exactly solvable models method of verifying the preservation of the
semiclassicality through the bounce. The devised method is next applied to two
simple examples: (i) the isotropic Friedman Robertson Walker universe, and (ii)
the isotropic sector of the Bianchi I model. For both of them we show, that the
dispersions in the logarithm of the volume ln(v) and scalar field momentum
ln(p_phi) in the distant future and past are related via strong triangle
inequalities. This implies in particular a strict preservation of the
semiclassicality (in considered degrees of freedom) in both the cases (i) and
(ii). Derived inequalities are general: valid for all the physical states
within the considered models.Comment: RevTex4, 19 pages, 3 figure
Organizational issues and major problems of palliative care concerning treatment of end-stage renal disease in Polish residential hospices and hospital- -based palliative medicine wards
Copyright © Via Medica. Background. Patients diagnosed with end-stage renal disease experience a significant level of symptom burden, including pain, nausea and vomiting, inability to urinate, fatigue etc. At this point in disease progression, it is important to establish what types and choices of therapy are most suitable for these patients, in particular, the value of continuing dialysis treatment. Material and methods. A self-administered questionnaire was distributed among Polish residential hospices and hospital based palliative medicine wards. All responses obtained underwent statistical analysis using Pearson's Chi Square test. Results. Permanent palliative care facilities, from which 73 out of 166 registered in Poland, took part in the survey. ESRD patients were identified to be cared by 81% of the aforementioned institutions. The most common treatment approach for these patients was highlighted as conservative treatment (68%), followed by hemodialysis (47%), whereas merely 11% provided peritoneal dialysis. Differences between facilities were identified relating to therapeutic recommendations for terminal ESRD patients with residential hospices more likely to recommend dialysis in conjunction with palliative care, whereas palliative wards advocated a withdrawal from dialysis followed by the initiation of palliative care. Conclusion. All surveyed facilities considered ESRD patients eligible for guaranteed hospice and palliative care services. However, certain changes are needed to improve care for ESRD patients, including: The development of collaborative partnerships between hospices, dialysis centers and nephrologists, development of guidelines for withdrawing dialysis and applying conservative treatment, introducing better renal-based training for medical personnel as well as the introduction of transparency within rules relating to the financing of these services
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