3,601 research outputs found
Robust quantum parameter estimation: coherent magnetometry with feedback
We describe the formalism for optimally estimating and controlling both the
state of a spin ensemble and a scalar magnetic field with information obtained
from a continuous quantum limited measurement of the spin precession due to the
field. The full quantum parameter estimation model is reduced to a simplified
equivalent representation to which classical estimation and control theory is
applied. We consider both the tracking of static and fluctuating fields in the
transient and steady state regimes. By using feedback control, the field
estimation can be made robust to uncertainty about the total spin number
Oxygen Gas Abundances at 0.4<z<1.5: Implications for the Chemical Evolution History of Galaxies
We report VLT-ISAAC and Keck-NIRSPEC near-infrared spectroscopy for a sample
of 30 0.47<z<0.92 CFRS galaxies and five [OII]-selected, M_B,AB<-21.5, z~1.4
galaxies. We have measured Halpha and [NII] line fluxes for the CFRS galaxies
which have [OII], Hbeta and [OIII] line fluxes available from optical
spectroscopy. For the z~1.4 objects we measured Hbeta and [OIII] emission line
fluxes from J-band spectra, and Halpha line fluxes plus upper limits for [NII]
fluxes from H-band spectra. We derive the extinction and oxygen abundances for
the sample using a method based on a set of ionisation parameter and oxygen
abundance diagnostics, simultaneously fitting the [OII], Hbeta, [OIII], Halpha
and [NII] line fluxes. Our most salient conclusions are: a) the source of gas
ionisation in the 30 CFRS and in all z~1.4 galaxies is not due to AGN activity;
b) about one third of the 0.47<z<0.92 CFRS galaxies in our sample have
substantially lower metallicities than local galaxies with similar luminosities
and star formation rates; c) comparison with a chemical evolution model
indicates that these low metallicity galaxies are unlikely to be the
progenitors of metal-poor dwarf galaxies at z~0, but more likely the
progenitors of massive spirals; d) the z~1.4 galaxies are characterized by the
high [OIII]/[OII] line ratios, low extinction and low metallicity that are
typical of lower luminosity CADIS galaxies at 0.4<z<0.7, and of more luminous
Lyman Break Galaxies at z~3.1, but not seen in CFRS galaxies at 0.4<z<1.0; e)
the properties of the z~1.4 galaxies suggest that the period of rapid chemical
evolution takes place progressively in lower mass systems as the universe ages,
and thus provides further support for a downsizing picture of galaxy formation,
at least from z~1.4 to today.Comment: Proceedings contribution for "The Fabulous Destiny of Galaxies;
Bridging Past and Present", Marseille, 200
Risk of cancer following primary total hip replacement or primary resurfacing arthroplasty of the hip : A retrospective cohort study in Scotland
Acknowledgements: We are grateful to Lee Barnsdale, Doug Clark, and Richard Dobbie for advice and assistance with data preparation before analysis, and to the three anonymous referees for their helpful comments and suggestions.Peer reviewedPublisher PD
Oxygen Gas Abundances at z~1.4: Implications for the Chemical Evolution History of Galaxies
The 1<z<2 redshift window hosts the peak of the star formation and metal
production rates. Studies of the metal content of the star forming galaxies at
these epochs are however sparse. We report VLT-ISAAC near-infrared spectroscopy
for a sample of five [OII]-selected, M_B,AB<-21.5, z~1.4 galaxies, by which we
measured Hbeta and [OIII]5007 emission line fluxes from J-band spectra, and
Halpha line fluxes plus upper limits for [NII]6584 fluxes from H-band spectra.
The z~1.4 galaxies are characterized by the high [OIII]/[OII] line ratios, low
extinction and low metallicity that are typical of lower luminosity CADIS
galaxies at 0.4<z<0.7, and of more luminous Lyman Break Galaxies at z~3, but
not seen in CFRS galaxies at 0.4<z<0.9. This type of spectrum (e.g., high
[OIII]/[OII]) is seen in progressively more luminous galaxies as the redshift
increases. These spectra are caused by a combination of high ionisation
parameter q and lower [O/H]. Pegase2 chemical evolution models are used to
relate the observed metallicities and luminosities of z~1.4 galaxies to galaxy
samples at lower and higher redshift. Not surpringsingly, we see a relationship
between redshift and inferred chemical age. We suppose that the metal-enriched
reservoirs of star forming gas that we are probing at intermediate redshifts
are being mostly consumed to build up both the disk and the bulge components of
spiral galaxies. Finally, our analysis of the metallicity-luminosity relation
at 0<z<1.5 suggests that the period of rapid chemical evolution may take place
progressively in lower mass systems as the universe ages. These results are
consistent with a ``downsizing'' type picture in the sense that particular
signatures (e.g., high [OIII]/[OII] or low [O/H]) are seen in progressively
more luminous (massive) systems at higher redshifts.Comment: Accepted for publication in Ap
GMRT detection of HI 21 cm associated absorption towards the z=1.2 red quasar 3C 190
We report the GMRT detection of associated HI 21 cm-line absorption in the
z=1.1946 red quasar 3C 190. Most of the absorption is blue-shifted with respect
to the systemic redshift. The absorption, at 647.7 MHz, is broad and
complex, spanning a velocity width of 600 \kms. Since the core is
self-absorbed at this frequency, the absorption is most likely towards the
hotspots. Comparison of the radio and deep optical images reveal linear
filaments in the optical which overlap with the brighter radio jet towards the
south-west. We therefore suggest that most of the HI 21 cm-line absorption
could be occurring in the atomic gas shocked by the south-west jet.Comment: 8 pages, 1 fugure. To appear in Journal of Astrophysics and Astronom
Strategies for Real-Time Position Control of a Single Atom in Cavity QED
Recent realizations of single-atom trapping and tracking in cavity QED open
the door for feedback schemes which actively stabilize the motion of a single
atom in real time. We present feedback algorithms for cooling the radial
component of motion for a single atom trapped by strong coupling to
single-photon fields in an optical cavity. Performance of various algorithms is
studied through simulations of single-atom trajectories, with full dynamical
and measurement noise included. Closed loop feedback algorithms compare
favorably to open-loop "switching" analogs, demonstrating the importance of
applying actual position information in real time. The high optical information
rate in current experiments enables real-time tracking that approaches the
standard quantum limit for broadband position measurements, suggesting that
realistic active feedback schemes may reach a regime where measurement
backaction appreciably alters the motional dynamics.Comment: 12 pages, 10 figures, submitted to J. Opt. B Quant. Semiclass. Op
A Quantum Langevin Formulation of Risk-Sensitive Optimal Control
In this paper we formulate a risk-sensitive optimal control problem for
continuously monitored open quantum systems modelled by quantum Langevin
equations. The optimal controller is expressed in terms of a modified
conditional state, which we call a risk-sensitive state, that represents
measurement knowledge tempered by the control purpose. One of the two
components of the optimal controller is dynamic, a filter that computes the
risk-sensitive state.
The second component is an optimal control feedback function that is found by
solving the dynamic programming equation. The optimal controller can be
implemented using classical electronics.
The ideas are illustrated using an example of feedback control of a two-level
atom
States for phase estimation in quantum interferometry
Ramsey interferometry allows the estimation of the phase of rotation
of the pseudospin vector of an ensemble of two-state quantum systems. For
small, the noise-to-signal ratio scales as the spin-squeezing parameter
, with possible for an entangled ensemble. However states with
minimum are not optimal for single-shot measurements of an arbitrary
phase. We define a phase-squeezing parameter, , which is an appropriate
figure-of-merit for this case. We show that (unlike the states that minimize
), the states that minimize can be created by evolving an
unentangled state (coherent spin state) by the well-known 2-axis
counter-twisting Hamiltonian. We analyse these and other states (for example
the maximally entangled state, analogous to the optical "NOON" state ) using several different properties, including ,
, the coefficients in the pseudo angular momentum basis (in the three
primary directions) and the angular Wigner function . Finally
we discuss the experimental options for creating phase squeezed states and
doing single-shot phase estimation.Comment: 8 pages and 5 figure
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