1,180 research outputs found
Narrow Linewidth 780 nm Distributed Feedback Lasers for Cold Atom Quantum Technology
Cold atom quantum technology systems have a wide range of potential applications which includes atomic clocks, rotational sensors, inertial sensors, quantum navigators, magnetometers and gravimeters. The UK Quantum Technology Hub in Sensors and Metrology has the aim of developing miniature cold atom systems using an approach similar to that pioneered by the chip scale atomic clock where microfabricated vacuum chambers have atomic transitions excited and probed by lasers. Whilst narrow linewidth Ti:Sa and external cavity diode lasers have been required for cooling and control, such lasers are too large, power hungry and expensive for future miniature cold atom systems.
Here we demonstrate 1 mm long 780.24 nm GaAs/AlGaAs distributed feedback (DFB) lasers aimed at 87Rb cold atom systems operating at 20 ˚C with over 50 mW of power and side-mode suppression ratios of 46 dB using sidewall gratings and no regrowth. Rb spectroscopy is used to demonstrate linewidths below the required 6.07 MHz natural linewidth of the 87Rb D2 optical transition used for cooling. Initial packaged fibre-coupled devices demonstrate lifetimes greater than 200 hours. We also investigate the use of integrated semiconductor amplifiers (SOAs) and longer devices to further reduce the linewidths well below 1 MHz. A range of options to control the populations of electrons in the hyperfine split energy levels spaced by 3.417 GHz are examined. Two integrated lasers, integrated electro-absorption modulators (EAMs) and the direct modulation of a single DFB laser approaches are investigated and we will discuss which is best suited to integrated cold atom systems
Rogue wave modes for a derivative nonlinear Schrödinger model
published_or_final_versio
Aspect-ratio dependence of the spin stiffness of a two-dimensional XY model
We calculate the superfluid stiffness of 2D lattice hard-core bosons at
half-filling (equivalent to the S=1/2 XY-model) using the squared winding
number quantum Monte Carlo estimator. For L_x x L_y lattices with aspect ratio
L_x/L_y=R, and L_x,L_y -> infinity, we confirm the recent prediction [N.
Prokof'ev and B.V. Svistunov, Phys. Rev. B 61, 11282 (1999)] that the
finite-temperature stiffness parameters \rho^W_x and \rho^W_y determined from
the winding number differ from each other and from the true superfluid density
\rho_s. Formally, \rho^W_y -> \rho_s in the limit in which L_x -> infinity
first and then L_y -> infinity. In practice we find that \rho^W_y converges
exponentially to \rho_s for R>1. We also confirm that for 3D systems, \rho^W_x
= \rho^W_y = \rho^W_z = \rho_s for any R. In addition, we determine the
Kosterlitz-Thouless transition temperature to be T_KT/J=0.34303(8) for the 2D
model.Comment: 7 pages, 8 figures, 1 table. Minor changes to published versio
Mixed symmetry superconductivity in two-dimensional Fermi liquids
We consider a 2D isotropic Fermi liquid with attraction in both and
channels and examine the possibility of a superconducting state with mixed
and symmetry of the gap function. We show that both in the weak coupling
limit and at strong coupling, a mixed symmetry state is realized in a
certain range of interaction. Phase transitions between the mixed and the pure
symmetry states are second order. We also show that there is no stable mixed
symmetry state at any coupling.Comment: 3 figures attached in uuencoded gzipped file
Lattice Boltzmann for Binary Fluids with Suspended Colloids
A new description of the binary fluid problem via the lattice Boltzmann
method is presented which highlights the use of the moments in constructing two
equilibrium distribution functions. This offers a number of benefits, including
better isotropy, and a more natural route to the inclusion of multiple
relaxation times for the binary fluid problem. In addition, the implementation
of solid colloidal particles suspended in the binary mixture is addressed,
which extends the solid-fluid boundary conditions for mass and momentum to
include a single conserved compositional order parameter. A number of simple
benchmark problems involving a single particle at or near a fluid-fluid
interface are undertaken and show good agreement with available theoretical or
numerical results.Comment: 10 pages, 4 figures, ICMMES 200
Complex Ashtekar variables and reality conditions for Holst's action
From the Holst action in terms of complex valued Ashtekar variables
additional reality conditions mimicking the linear simplicity constraints of
spin foam gravity are found. In quantum theory with the results of You and
Rovelli we are able to implement these constraints weakly, that is in the sense
of Gupta and Bleuler. The resulting kinematical Hilbert space matches the
original one of loop quantum gravity, that is for real valued Ashtekar
connection. Our result perfectly fit with recent developments of Rovelli and
Speziale concerning Lorentz covariance within spin-form gravity.Comment: 24 pages, 2 picture
Synthesis, characterisation and corrosion behaviour of simulant Chernobyl nuclear meltdown materials
Understanding the physical and chemical properties of materials arising from nuclear meltdowns, such as the Chernobyl and Fukushima accidents, is critical to supporting decommissioning operations and reducing the hazard to personnel and the environment surrounding the stricken reactors. Relatively few samples of meltdown materials are available for study, and their analysis is made challenging due to the radiation hazard associated with handling them. In this study, small-scale batches of low radioactivity (i.e., containing depleted uranium only) simulants for Chernobyl lava-like fuel-containing materials (LFCMs) have been prepared, and were found to closely approximate the microstructure and mineralogy of real LFCM. The addition of excess of ZrO2 to the composition resulted in the first successful synthesis of high uranium–zircon (chernobylite) by crystallisation from a glass melt. Use of these simulant materials allowed further analysis of the thermal characteristics of LFCM and the corrosion kinetics, giving results that are in good agreement with the limited available literature on real samples. It should, therefore, be possible to use these new simulant materials to support decommissioning operations of nuclear reactors post-accident
Doping dependence of the superconducting gap in Bi2Sr2CaCu2O{8 + delta}
Bi2Sr2CaCu2O{8 + \delta} crystals with varying hole concentrations (0.12 < p
< 0.23) were studied to investigate the effects of doping on the symmetry and
magnitude of the superconducting gap. Electronic Raman scattering experiments
that sample regions of the Fermi surface near the diagonal (B_{2g}) and
principal axes (B_{1g}) of the Brillouin Zone have been utilized. The frequency
dependence of the Raman response function at low energies is found to be linear
for B_{2g} and cubic for B_{1g} (T< T_c). The latter observations have led us
to conclude that the doping dependence of the superconducting gap is consistent
with d_{x^2-y^2} symmetry, for slightly underdoped and overdoped crystals.
Studies of the pair-breaking peak found in the B_{1g} spectra demonstrate that
the magnitude of the maximum gap decreases monotonically with increasing hole
doping, for p > 0.12. Based on the magnitude of the B_{1g} renormalization, it
is found that the number of quasiparticles participating in pairing increases
monotonically with increased doping. On the other hand, the B_{2g} spectra show
a weak "pair-breaking peak" that follows a parabolic-like dependence on hole
concentration, for 0.12 < p < 0.23.Comment: 9 pages REvTex document including 8 eps figures; new table II;
changes to Fig. 5 and tex
High resolution Compton scattering as a Probe of the Fermi surface in the Iron-based superconductor
We have carried out first principles all-electron calculations of the
(001)-projected 2D electron momentum density and the directional Compton
profiles along the [100], [001] and [110] directions in the Fe-based
superconductor LaOFeAs within the framework of the local density approximation.
We identify Fermi surface features in the 2D electron momentum density and the
directional Compton profiles, and discuss issues related to the observation of
these features via Compton scattering experiments.Comment: 4 pages, 3 figure
Quantum interference between non-magnetic impurities in d_x2-y2-wave superconductors
We study quantum interference of electronic waves that are scattered by
multiple non-magnetic impurities in a d_x2-y2-wave superconductor. We show that
the number of resonance states in the density-of-states (DOS), as well as their
frequency and spatial dependence change significantly as the distance between
the impurities or their orientation relative to the crystal lattice is varied.
Since the latter effect arises from the momentum dependence of the
superconducting gap, we argue that quantum interference is a novel tool to
identify the symmetry of unconventional superconductors.Comment: 4 pages, 4 figure
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