140 research outputs found
A double-slit `which-way' experiment on the complementarity--uncertainty debate
A which-way measurement in Young's double-slit will destroy the interference
pattern. Bohr claimed this complementarity between wave- and particle behaviour
is enforced by Heisenberg's uncertainty principle: distinguishing two positions
a distance s apart transfers a random momentum q \sim \hbar/s to the particle.
This claim has been subject to debate: Scully et al. asserted that in some
situations interference can be destroyed with no momentum transfer, while
Storey et al. asserted that Bohr's stance is always valid. We address this
issue using the experimental technique of weak measurement. We measure a
distribution for q that spreads well beyond [-\hbar/s, \hbar/s], but
nevertheless has a variance consistent with zero. This weakvalued
momentum-transfer distribution P_{wv}(q) thus reflects both sides of the
debate.Comment: 13 pages, 4 figure
In vitro umbilical cord blood expansion resulting in unique CD34Bright cell population that engrafts in NOD/SCID mice
Measuring measurement--disturbance relationships with weak values
Using formal definitions for measurement precision {\epsilon} and disturbance
(measurement backaction) {\eta}, Ozawa [Phys. Rev. A 67, 042105 (2003)] has
shown that Heisenberg's claimed relation between these quantities is false in
general. Here we show that the quantities introduced by Ozawa can be determined
experimentally, using no prior knowledge of the measurement under investigation
--- both quantities correspond to the root-mean-squared difference given by a
weak-valued probability distribution. We propose a simple three-qubit
experiment which would illustrate the failure of Heisenberg's
measurement--disturbance relation, and the validity of an alternative relation
proposed by Ozawa
The Cosmic Microwave Background and Helical Magnetic Fields: the tensor mode
We study the effect of a possible helicity component of a primordial magnetic
field on the tensor part of the cosmic microwave background temperature
anisotropies and polarization. We give analytical approximations for the tensor
contributions induced by helicity, discussing their amplitude and spectral
index in dependence of the power spectrum of the primordial magnetic field. We
find that an helical magnetic field creates a parity odd component of gravity
waves inducing parity odd polarization signals. However, only if the magnetic
field is close to scale invariant and if its helical part is close to maximal,
the effect is sufficiently large to be observable. We also discuss the
implications of causality on the magnetic field spectrum.Comment: We have corrected a normalisation error which was pointed out to us
by Antony Lewis. It enhances our limits on the magnetic fields by
(2\pi)^{3/4} ~
Grounding Bohmian Mechanics in Weak Values and Bayesianism
Bohmian mechanics (BM) is a popular interpretation of quantum mechanics in
which particles have real positions. The velocity of a point x in configuration
space is defined as the standard probability current j(x) divided by the
probability density P(x). However, this ``standard'' j is in fact only one of
infinitely many that transform correctly and satisfy \dot P + \del . j=0. In
this article I show that there is a unique j that can be determined
experimentally as a weak value using techniques that would make sense to a
classical physicist. Moreover, this operationally defined j equals the standard
j, so, assuming \dot x = j/P, the possible Bohmian paths can also be determined
experimentally from a large enough ensemble. Furthermore, this approach to
deriving BM singles out x as the hidden variable, because (for example) the
operationally defined momentum current is in general incompatible with the
evolution of the momentum distribution. Finally I discuss how, in this setting,
the usual quantum probabilities can be derived from a Bayesian standpoint, via
the principle of indifference.Comment: 11 page
Electromagnetic Origin of the CMB Anisotropy in String Cosmology
In the inflationary scenarios suggested by string theory, the vacuum
fluctuations of the electromagnetic field can be amplified by the
time-evolution of the dilaton background, and can grow large enough to explain
both the origin of the cosmic magnetic fields and of the observed CMB
anisotropy. The normalization of the perturbation spectrum is fixed, and
implies a relation between the perturbation amplitude at the COBE scale and the
spectral index . Working within a generic two-parameter family of
backgrounds, a large scale anisotropy is found to
correspond to a spectral index in the range .Comment: 11 pages, LATE
The Uncertainty Relation in "Which-Way" Experiments: How to Observe Directly the Momentum Transfer using Weak Values
A which-way measurement destroys the twin-slit interference pattern. Bohr
argued that distinguishing between two slits a distance s apart gives the
particle a random momentum transfer \wp of order h/s. This was accepted for
more than 60 years, until Scully, Englert and Walther (SEW) proposed a
which-way scheme that, they claimed, entailed no momentum transfer. Storey,
Tan, Collett and Walls (STCW) in turn proved a theorem that, they claimed,
showed that Bohr was right. This work reviews and extends a recent proposal
[Wiseman, Phys. Lett. A 311, 285 (2003)] to resolve the issue using a
weak-valued probability distribution for momentum transfer, P_wv(\wp). We show
that P_wv(\wp) must be wider than h/6s. However, its moments can still be zero
because P_wv(\wp) is not necessarily positive definite. Nevertheless, it is
measurable in a way understandable to a classical physicist. We introduce a new
measure of spread for P_wv(\wp): half of the unit-confidence interval, and
conjecture that it is never less than h/4s. For an idealized example with
infinitely narrow slits, the moments of P_wv(\wp) and of the momentum
distributions are undefined unless a process of apodization is used. We show
that by considering successively smoother initial wave functions, successively
more moments of both P_wv(\wp) and the momentum distributions become defined.
For this example the moments of P_wv(\wp) are zero, and these are equal to the
changes in the moments of the momentum distribution. We prove that this
relation holds for schemes in which the moments of P_wv(\wp) are non-zero, but
only for the first two moments. We also compare these moments to those of two
other momentum-transfer distributions and \hat{p}_f-\hat{p}_i. We find
agreement between all of these, but again only for the first two moments.Comment: 14 pages, 6 figures, submitted to J. Opt.
The Role of Mesoscale Plasma Sheet Dynamics in Ring Current Formation
During geomagnetically active periods ions are transported from the magnetotail into the inner magnetosphere and accelerated to energies of tens to hundreds of keV. These energetic ions, of mixed composition with the most important species being H+ and O+, become the dominant source of plasma pressure in the inner magnetosphere. Ion transport and acceleration can occur at different spatial and temporal scales ranging from global quasi-steady convection to localized impulsive injection events and may depend on the ion gyroradius. In this study we ascertain the relative importance of mesoscale flow structures and the effects of ion non-adiabaticity on the produced ring current. For this we use: global magnetohydrodynamic (MHD) simulations to generate self-consistent electromagnetic fields under typical driving conditions which exhibit bursty bulk flows (BBFs); and injected test particles, initialized to match the plasma moments of the MHD simulation, and subsequently evolved according to the kinetic equations of motion. We show that the BBFs produced by our simulation reproduce thermodynamic and magnetic statistics from in situ measurements and are numerically robust. Mining the simulation data we create a data set, over a billion points, connecting particle transport to characteristics of the MHD flow. From this we show that mesoscale bubbles, localized depleted entropy regions, and particle gradient drifts are critical for ion transport. Finally we show, using identical particle ensembles with varying mass, that O+ non-adiabaticity creates qualitative differences in energization and spatial distribution while H+ non-adiabaticity has non-negligible implications for loss timescales
Re-Inventing Adherence: Toward a Patient-Centered Model of Care for Drug-Resistant Tuberculosis and HIV
BACKGROUND—Despite renewed focus on molecular tuberculosis (TB) diagnostics and new antimycobacterial agents, treatment outcomes for patients co-infected with drug-resistant TB and human immunodeficiency virus (HIV) remain dismal, in part due to lack of focus on medication adherence as part of a patient-centered continuum of care.
OBJECTIVE—To review current barriers to drug-resistant TB-HIV treatment and propose an alternative model to conventional approaches to treatment support.
DISCUSSION—Current national TB control programs rely heavily on directly observed therapy (DOT) as the centerpiece of treatment delivery and adherence support. Medication adherence and care for drug-resistant TB-HIV could be improved by fully implementing team-based patient-centered care, empowering patients through counseling and support, maintaining a rights-based approach while acknowledging the responsibility of health care systems in providing comprehensive care, and prioritizing critical research gaps.
CONCLUSION—It is time to re-invent our understanding of adherence in drug-resistant TB and HIV by focusing attention on the complex clinical, behavioral, social, and structural needs of affected patients and communities
Large-scale magnetic fields from inflation in dilaton electromagnetism
The generation of large-scale magnetic fields is studied in dilaton
electromagnetism in inflationary cosmology, taking into account the dilaton's
evolution throughout inflation and reheating until it is stabilized with
possible entropy production. It is shown that large-scale magnetic fields with
observationally interesting strength at the present time could be generated if
the conformal invariance of the Maxwell theory is broken through the coupling
between the dilaton and electromagnetic fields in such a way that the resultant
quantum fluctuations in the magnetic field has a nearly scale-invariant
spectrum. If this condition is met, the amplitude of the generated magnetic
field could be sufficiently large even in the case huge amount of entropy is
produced with the dilution factor as the dilaton decays.Comment: 28 pages, 5 figures, the version accepted for publication in Phys.
Rev. D; some references are adde
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