4,699 research outputs found
ALFA & 3D: integral field spectroscopy with adaptive optics
One of the most important techniques for astrophysics with adaptive optics is
the ability to do spectroscopy at diffraction limited scales. The extreme
difficulty of positioning a faint target accurately on a very narrow slit can
be avoided by using an integral field unit, which provides the added benefit of
full spatial coverage. During 1998, working with ALFA and the 3D integral field
spectrometer, we demonstrated the validity of this technique by extracting and
distinguishing spectra from binary stars separated by only 0.26". The
combination of ALFA & 3D is also ideally suited to imaging distant galaxies or
the nuclei of nearby ones, as its field of view can be changed between
1.2"x1.2" and 4"x4", depending on the pixel scale chosen. In this contribution
we present new results both on galactic targets, namely young stellar objects,
as well as extra-galactic objects including a Seyfert and a starburst nucleus.Comment: SPIE meeting 4007 on Adaptive Optical Systems Technology, March 200
Detection of nano scale thin films with polarized neutron reflectometry at the presence of smooth and rough interfaces
By knowing the phase and modules of the reflection coefficient in neutron
reflectometry problems, a unique result for the scattering length density (SLD)
of a thin film can be determined which will lead to the exact determination of
type and thickness of the film. In the past decade, several methods have been
worked out to resolve the phase problem such as dwell time method, reference
layer method and variation of surroundings, among which the reference method
and variation of surroundings by using a magnetic substrate and polarized
neutrons is of the most applicability. All of these methods are based on the
solution of Schrodinger equation for a discontinuous and step-like potential at
each interface. As in real sample there are some smearing and roughness at
boundaries, consideration of smoothness and roughness of interfaces would
affect the final output result. In this paper, we have investigated the effects
of smoothness of interfaces on determination of the phase of reflection as well
as the retrieval process of the SLD, by using a smooth varying function (Eckart
potential). The effects of roughness of interfaces on the same parameters, have
also been investigated by random variation of the interface around it mean
position
ALFA: First Operational Experience of the MPE/MPIA Laser Guide Star System for Adaptive Optics
The sodium laser guide star adaptive optics system ALFA has been constructed
at the Calar Alto 3.5-m telescope. Following the first detection of the laser
beacon on the wavefront sensor in 1997 the system is now being optimized for
best performance. In this contribution we discuss the current status of the
launch beam and the planned improvements and upgrades. We report on the
performance level achieved when it is used with the adaptive optics system, and
relate various aspects of our experience during operation of the system. We
have begun to produce scientific results and mention two of these.Comment: 9 pages, 6 figures, LaTeX (spie.sty). SPIE conf proc 3353, Adaptive
Optical System Technologies, March 199
Performance and structure of single-mode bosonic codes
The early Gottesman, Kitaev, and Preskill (GKP) proposal for encoding a qubit
in an oscillator has recently been followed by cat- and binomial-code
proposals. Numerically optimized codes have also been proposed, and we
introduce new codes of this type here. These codes have yet to be compared
using the same error model; we provide such a comparison by determining the
entanglement fidelity of all codes with respect to the bosonic pure-loss
channel (i.e., photon loss) after the optimal recovery operation. We then
compare achievable communication rates of the combined encoding-error-recovery
channel by calculating the channel's hashing bound for each code. Cat and
binomial codes perform similarly, with binomial codes outperforming cat codes
at small loss rates. Despite not being designed to protect against the
pure-loss channel, GKP codes significantly outperform all other codes for most
values of the loss rate. We show that the performance of GKP and some binomial
codes increases monotonically with increasing average photon number of the
codes. In order to corroborate our numerical evidence of the cat/binomial/GKP
order of performance occurring at small loss rates, we analytically evaluate
the quantum error-correction conditions of those codes. For GKP codes, we find
an essential singularity in the entanglement fidelity in the limit of vanishing
loss rate. In addition to comparing the codes, we draw parallels between
binomial codes and discrete-variable systems. First, we characterize one- and
two-mode binomial as well as multi-qubit permutation-invariant codes in terms
of spin-coherent states. Such a characterization allows us to introduce check
operators and error-correction procedures for binomial codes. Second, we
introduce a generalization of spin-coherent states, extending our
characterization to qudit binomial codes and yielding a new multi-qudit code.Comment: 34 pages, 11 figures, 4 tables. v3: published version. See related
talk at https://absuploads.aps.org/presentation.cfm?pid=1351
High-Field Optical Cesium Magnetometer for Magnetic Resonance Imaging
We present a novel high-field optical quantum magnetometer based on saturated
absorption spectroscopy on the extreme angular-momentum states of the cesium D2
line. With key features including continuous readout, high sampling rate, and
sensitivity and accuracy in the ppm-range, it represents a competitive
alternative to conventional techniques for measuring magnetic fields of several
teslas. The prototype has four small separate field probes, and all support
electronics and optics are fitted into a single 19-inch rack to make it
compact, mobile, and robust. The field probes are fiber coupled and made from
non-metallic components, allowing them to be easily and safely positioned
inside a 7 T MRI scanner. We demonstrate the capabilities of this magnetometer
by measuring two different MRI sequences, and we show how it can be used to
reveal imperfections in the gradient coil system, to highlight the potential
applications in medical MRI. We propose the term EXAAQ (EXtreme
Angular-momentum Absorption-spectroscopy Quantum) magnetometry, for this novel
method.Comment: Corrected a minor mistake in affiliation
- …