694 research outputs found
Intracavity weak nonlinear phase shifts with single photon driving
We investigate a doubly resonant optical cavity containing a Kerr nonlinear
medium that couples two modes by a cross phase modulation. One of these modes
is driven by a single photon pulsed field, and the other mode is driven by a
coherent state. We find an intrinsic phase noise mechanism for the cross phase
shift on the coherent beam which can be attributed to the random emission times
of single photons from the cavity. An application to a weak nonlinearity phase
gate is discussed
Towards a novel carbon device for the treatment of sepsis
Sepsis is a systemic inflammatory response to infection in which the balance of pro- andanti-inflammatory mediators, which normally isolate and eliminate infection, is disrupted[1]. Gram negative sepsis is initiated by bacterial endotoxin release which activatesmacrophages and circulating monocytes to release TNF and IL-1β followed by IL-6 andother inflammatory cytokines [2]. As the disease progresses, an unregulatedinflammatory response results in, tissue injury, haematological dysfunction and organdysfunction. Severe sepsis, involving organ hypoperfusion may be further complicatedby hypotension that is unresponsive to adequate fluid replacement, resulting in septicshock and finally death [3].Despite improvements in anti-microbial and supportive therapies, sepsis remains asignificant cause of morbidity and mortality in ICUs worldwide [4]. The complexity ofprocesses mediating the progression of sepsis suggests that an extracorporeal devicecombining blood filtration with adsorption of a wide range of toxins, and inflammatorymediators offers the most comprehensive treatment strategy. However, no such deviceexists at present. A novel, uncoated, polymer pyrolysed synthetic carbon device isproposed which combines the superior adsorption properties of uncoated activatedcarbons with the capacity to manipulate porous structure for controlled adsorption oftarget plasma proteins and polypeptides [5]. Preliminary haemocompatibility andadsorptive capacity was assessed using a carbon matrix prototype
Universal quantum control in irreducible state-space sectors: application to bosonic and spin-boson systems
We analyze the dynamical-algebraic approach to universal quantum control
introduced in P. Zanardi, S. Lloyd, quant-ph/0305013. The quantum state-space
encoding information decomposes into irreducible sectors and
subsystems associated to the group of available evolutions. If this group
coincides with the unitary part of the group-algebra \CC{\cal K} of some
group then universal control is achievable over the -irreducible components of . This general strategy is applied to
different kind of bosonic systems. We first consider massive bosons in a
double-well and show how to achieve universal control over all
finite-dimensional
Fock sectors. We then discuss a multi-mode massless case giving the
conditions for generating the whole infinite-dimensional multi-mode
Heisenberg-Weyl enveloping-algebra. Finally we show how to use an auxiliary
bosonic mode coupled to finite-dimensional systems to generate high-order
non-linearities needed for universal control.Comment: 10 pages, LaTeX, no figure
Sequential bottlenecks drive viral evolution in early acute Hepatitis C virus infection
Hepatitis C is a pandemic human RNA virus, which commonly causes chronic infection and liver disease. The characterization of viral populations that successfully initiate infection, and also those that drive progression to chronicity is instrumental for understanding pathogenesis and vaccine design. A comprehensive and longitudinal analysis of the viral population was conducted in four subjects followed from very early acute infection to resolution of disease outcome. By means of next generation sequencing (NGS) and standard cloning/Sanger sequencing, genetic diversity and viral variants were quantified over the course of the infection at frequencies as low as 0.1%. Phylogenetic analysis of reassembled viral variants revealed acute infection was dominated by two sequential bottleneck events, irrespective of subsequent chronicity or clearance. The first bottleneck was associated with transmission, with one to two viral variants successfully establishing infection. The second occurred approximately 100 days post-infection, and was characterized by a decline in viral diversity. In the two subjects who developed chronic infection, this second bottleneck was followed by the emergence of a new viral population, which evolved from the founder variants via a selective sweep with fixation in a small number of mutated sites. The diversity at sites with non-synonymous mutation was higher in predicted cytotoxic T cell epitopes, suggesting immune-driven evolution. These results provide the first detailed analysis of early within-host evolution of HCV, indicating strong selective forces limit viral evolution in the acute phase of infection
G\"odel Incompleteness and the Black Hole Information Paradox
Semiclassical reasoning suggests that the process by which an object
collapses into a black hole and then evaporates by emitting Hawking radiation
may destroy information, a problem often referred to as the black hole
information paradox. Further, there seems to be no unique prediction of where
the information about the collapsing body is localized. We propose that the
latter aspect of the paradox may be a manifestation of an inconsistent
self-reference in the semiclassical theory of black hole evolution. This
suggests the inadequacy of the semiclassical approach or, at worst, that
standard quantum mechanics and general relavity are fundamentally incompatible.
One option for the resolution for the paradox in the localization is to
identify the G\"odel-like incompleteness that corresponds to an imposition of
consistency, and introduce possibly new physics that supplies this
incompleteness. Another option is to modify the theory in such a way as to
prohibit self-reference. We discuss various possible scenarios to implement
these options, including eternally collapsing objects, black hole remnants,
black hole final states, and simple variants of semiclassical quantum gravity.Comment: 14 pages, 2 figures; revised according to journal requirement
A Behavioural Foundation for Natural Computing and a Programmability Test
What does it mean to claim that a physical or natural system computes? One
answer, endorsed here, is that computing is about programming a system to
behave in different ways. This paper offers an account of what it means for a
physical system to compute based on this notion. It proposes a behavioural
characterisation of computing in terms of a measure of programmability, which
reflects a system's ability to react to external stimuli. The proposed measure
of programmability is useful for classifying computers in terms of the apparent
algorithmic complexity of their evolution in time. I make some specific
proposals in this connection and discuss this approach in the context of other
behavioural approaches, notably Turing's test of machine intelligence. I also
anticipate possible objections and consider the applicability of these
proposals to the task of relating abstract computation to nature-like
computation.Comment: 37 pages, 4 figures. Based on an invited Talk at the Symposium on
Natural/Unconventional Computing and its Philosophical Significance, Alan
Turing World Congress 2012, Birmingham, UK.
http://link.springer.com/article/10.1007/s13347-012-0095-2 Ref. glitch fixed
in 2nd. version; Philosophy & Technology (special issue on History and
Philosophy of Computing), Springer, 201
Computational and Biological Analogies for Understanding Fine-Tuned Parameters in Physics
In this philosophical paper, we explore computational and biological
analogies to address the fine-tuning problem in cosmology. We first clarify
what it means for physical constants or initial conditions to be fine-tuned. We
review important distinctions such as the dimensionless and dimensional
physical constants, and the classification of constants proposed by
Levy-Leblond. Then we explore how two great analogies, computational and
biological, can give new insights into our problem. This paper includes a
preliminary study to examine the two analogies. Importantly, analogies are both
useful and fundamental cognitive tools, but can also be misused or
misinterpreted. The idea that our universe might be modelled as a computational
entity is analysed, and we discuss the distinction between physical laws and
initial conditions using algorithmic information theory. Smolin introduced the
theory of "Cosmological Natural Selection" with a biological analogy in mind.
We examine an extension of this analogy involving intelligent life. We discuss
if and how this extension could be legitimated.
Keywords: origin of the universe, fine-tuning, physical constants, initial
conditions, computational universe, biological universe, role of intelligent
life, cosmological natural selection, cosmological artificial selection,
artificial cosmogenesis.Comment: 25 pages, Foundations of Science, in pres
Environment-Induced Decoherence and the Transition From Quantum to Classical
We study dynamics of quantum open systems, paying special attention to those
aspects of their evolution which are relevant to the transition from quantum to
classical. We begin with a discussion of the conditional dynamics of simple
systems. The resulting models are straightforward but suffice to illustrate
basic physical ideas behind quantum measurements and decoherence. To discuss
decoherence and environment-induced superselection einselection in a more
general setting, we sketch perturbative as well as exact derivations of several
master equations valid for various systems. Using these equations we study
einselection employing the general strategy of the predictability sieve.
Assumptions that are usually made in the discussion of decoherence are
critically reexamined along with the ``standard lore'' to which they lead.
Restoration of quantum-classical correspondence in systems that are classically
chaotic is discussed. The dynamical second law -it is shown- can be traced to
the same phenomena that allow for the restoration of the correspondence
principle in decohering chaotic systems (where it is otherwise lost on a very
short time-scale). Quantum error correction is discussed as an example of an
anti-decoherence strategy. Implications of decoherence and einselection for the
interpretation of quantum theory are briefly pointed out.Comment: 80 pages, 7 figures included, Lectures given by both authors at the
72nd Les Houches Summer School on "Coherent Matter Waves", July-August 199
The Mathematical Universe
I explore physics implications of the External Reality Hypothesis (ERH) that
there exists an external physical reality completely independent of us humans.
I argue that with a sufficiently broad definition of mathematics, it implies
the Mathematical Universe Hypothesis (MUH) that our physical world is an
abstract mathematical structure. I discuss various implications of the ERH and
MUH, ranging from standard physics topics like symmetries, irreducible
representations, units, free parameters, randomness and initial conditions to
broader issues like consciousness, parallel universes and Godel incompleteness.
I hypothesize that only computable and decidable (in Godel's sense) structures
exist, which alleviates the cosmological measure problem and help explain why
our physical laws appear so simple. I also comment on the intimate relation
between mathematical structures, computations, simulations and physical
systems.Comment: Replaced to match accepted Found. Phys. version, 31 pages, 5 figs;
more details at http://space.mit.edu/home/tegmark/toe.htm
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