10,054 research outputs found
Twisted equivariant K-theory, groupoids and proper actions
In this paper we define twisted equivariant K-theory for actions of Lie
groupoids. For a Bredon-compatible Lie groupoid, this defines a periodic
cohomology theory on the category of finite CW-complexes with equivariant
stable projective bundles. A classification of these bundles is shown. We also
obtain a completion theorem and apply these results to proper actions of
groups.Comment: 26 page
Double wells, scalar fields and quantum phase transitions in ions traps
Since Hund's work on the ammonia molecule, the double well potential has
formed a key paradigm in physics. Its importance is further underlined by the
central role it plays in the Landau theory of phase transitions. Recently, the
study of entanglement properties of many-body systems has added a new angle to
the study of quantum phase transitions of discrete and continuous degrees of
freedom, i.e., spin and harmonic chains. Here we show that control of the
radial degree of freedom of trapped ion chains allows for the simulation of
linear and non-linear Klein-Gordon fields on a lattice, in which the parameters
of the lattice, the non-linearity and mass can be controlled at will. The
system may be driven through a phase transition creating a double well
potential between different configurations of the ion crystal. The dynamics of
the system are controllable, local properties are measurable and tunnelling in
the double well potential would be observable.Comment: 6 pages, 5 figure
QED in external fields from the spin representation
Systematic use of the infinite-dimensional spin representation simplifies and
rigorizes several questions in Quantum Field Theory. This representation
permutes ``Gaussian'' elements in the fermion Fock space, and is necessarily
projective: we compute its cocycle at the group level, and obtain Schwinger
terms and anomalies from infinitesimal versions of this cocycle. Quantization,
in this framework, depends on the choice of the ``right'' complex structure on
the space of solutions of the Dirac equation. We show how the spin
representation allows one to compute exactly the S-matrix for fermions in an
external field; the cocycle yields a causality condition needed to determine
the phase.Comment: 32 pages, Plain TeX, UCR-FM-01-9
Novel designs for Penning ion traps
We present a number of alternative designs for Penning ion traps suitable for
quantum information processing (QIP) applications with atomic ions. The first
trap design is a simple array of long straight wires which allows easy optical
access. A prototype of this trap has been built to trap Ca+ and a simple
electronic detection scheme has been employed to demonstrate the operation of
the trap. Another trap design consists of a conducting plate with a hole in it
situated above a continuous conducting plane. The final trap design is based on
an array of pad electrodes. Although this trap design lacks the open geometry
of the traps described above, the pad design may prove useful in a hybrid
scheme in which information processing and qubit storage take place in
different types of trap. The behaviour of the pad traps is simulated
numerically and techniques for moving ions rapidly between traps are discussed.
Future experiments with these various designs are discussed. All of the designs
lend themselves to the construction of multiple trap arrays, as required for
scalable ion trap QIP.Comment: 11 pages, 10 figure
Hammerhead, an ultrahigh resolution ePix camera for wavelength-dispersive spectrometers
Wavelength-dispersive spectrometers (WDS) are often used in synchrotron and
FEL applications where high energy resolution (in the order of eV) is
important. Increasing WDS energy resolution requires increasing spatial
resolution of the detectors in the dispersion direction. The common approaches
with strip detectors or small pixel detectors are not ideal. We present a novel
approach, with a sensor using rectangular pixels with a high aspect ratio
(between strips and pixels, further called "strixels"), and strixel
redistribution to match the square pixel arrays of typical ASICs while avoiding
the considerable effort of redesigning ASICs. This results in a sensor area of
17.4 mm x 77 mm, with a fine pitch of 25 m in the horizontal direction
resulting in 3072 columns and 176 rows. The sensors use ePix100 readout ASICs,
leveraging their low noise (43 e, or 180 eV rms). We present results
obtained with a Hammerhead ePix100 camera, showing that the small pitch (25
m) in the dispersion direction maximizes performance for both high and low
photon occupancies, resulting in optimal WDS energy resolution. The low noise
level at high photon occupancy allows precise photon counting, while at low
occupancy, both the energy and the subpixel position can be reconstructed for
every photon, allowing an ultrahigh resolution (in the order of 1 m) in
the dispersion direction and rejection of scattered beam and harmonics. Using
strixel sensors with redistribution and flip-chip bonding to standard ePix
readout ASICs results in ultrahigh position resolution (1 m) and low
noise in WDS applications, leveraging the advantages of hybrid pixel detectors
(high production yield, good availability, relatively inexpensive) while
minimizing development complexity through sharing the ASIC, hardware, software
and DAQ development with existing versions of ePix cameras.Comment: 8 pages, 6 figure
Doppler-free laser spectroscopy of buffer gas cooled molecular radicals
We demonstrate Doppler-free saturated absorption spectroscopy of cold
molecular radicals formed by laser ablation inside a cryogenic buffer gas cell.
By lowering the temperature, congested regions of the spectrum can be
simplified, and by using different temperatures for different regions of the
spectrum a wide range of rotational states can be studied optimally. We use the
technique to study the optical spectrum of YbF radicals with a resolution of 30
MHz, measuring the magnetic hyperfine parameters of the electronic ground
state. The method is suitable for high resolution spectroscopy of a great
variety of molecules at controlled temperature and pressure, and is
particularly well-suited to those that are difficult to produce in the gas
phase.Comment: 11 pages, 4 figure
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