2,343 research outputs found
The effect of quantization on the FCIQMC sign problem
The sign problem in Full Configuration Interaction Quantum Monte Carlo
(FCIQMC) without annihilation can be understood as an instability of the
psi-particle population to the ground state of the matrix obtained by making
all off-diagonal elements of the Hamiltonian negative. Such a matrix, and hence
the sign problem, is basis dependent. In this paper we discuss the properties
of a physically important basis choice: first versus second quantization. For a
given choice of single-particle orbitals, we identify the conditions under
which the fermion sign problem in the second quantized basis of antisymmetric
Slater determinants is identical to the sign problem in the first quantized
basis of unsymmetrized Hartree products. We also show that, when the two
differ, the fermion sign problem is always less severe in the second quantized
basis. This supports the idea that FCIQMC, even in the absence of annihilation,
improves the sign problem relative to first quantized methods. Finally, we
point out some theoretically interesting classes of Hamiltonians where first
and second quantized sign problems differ, and others where they do not.Comment: 4 pages w/ 2 page appendix, 2 figures, 1 tabl
Ramsey numbers and adiabatic quantum computing
The graph-theoretic Ramsey numbers are notoriously difficult to calculate. In
fact, for the two-color Ramsey numbers with , only nine are
currently known. We present a quantum algorithm for the computation of the
Ramsey numbers . We show how the computation of can be mapped
to a combinatorial optimization problem whose solution can be found using
adiabatic quantum evolution. We numerically simulate this adiabatic quantum
algorithm and show that it correctly determines the Ramsey numbers R(3,3) and
R(2,s) for . We then discuss the algorithm's experimental
implementation, and close by showing that Ramsey number computation belongs to
the quantum complexity class QMA.Comment: 4 pages, 1 table, no figures, published versio
The ecology of seamounts: structure, function, and human impacts.
In this review of seamount ecology, we address a number of key scientific issues concerning the structure and function of benthic communities, human impacts, and seamount management and conservation. We consider whether community composition and diversity differ between seamounts and continental slopes, how important dispersal capabilities are in seamount connectivity, what environmental factors drive species composition and diversity, whether seamounts are centers of enhanced biological productivity, and whether they have unique trophic architecture. We discuss how vulnerable seamount communities are to fishing and mining, and how we can balance exploitation of resources and conservation of habitat. Despite considerable advances in recent years, there remain many questions about seamount ecosystems that need closer integration of molecular, oceanographic, and ecological research
No difference in surgical outcomes between Open and Closed exposure of palatally displaced maxillary canines
Purpose: To investigate differences in the surgical outcomes between Open and Closed exposure for palatally displaced maxillary cuspids (PDC).
Methods: A multicenter, RCT involving two parallel groups. The settings were one dental teaching hospital in, and two hospital units near Sheffield, UK. Participants were aged <20 years with a unilateral PDC, who provided informed consent. They were randomly allocated to either receive the Open (O) or the Closed (C) surgical procedure. The outcomes were time spent in the operating room and 10-day post-operative patient questionnaire. Statistical differences between the two techniques were tested using independent t tests for continuous variables and chi-squared tests for frequencies.
Results: The final study sample was composed of 71 participants (64% females). There were no differences in the gender ratios (O: F=27, M=13; C: F=25, M=16) or mean ages of the two groups (O: 14.3 yrs SD 1.3; C: 14.1 yrs SD 1.6) at the start. The mean operating times for the Open and Closed techniques were 34.3 mins (SD 11.2) and 34.3 mins (SD 11.9) respectively (p=.986). There were no statistically significant differences between the two treatment groups for any of the patient-assessed outcomes (p>.05).
Conclusions: There were no differences in the surgical outcomes investigated in this study between Open and Closed exposure for PDC
Rapid Exchange Cooling with Trapped Ions
The trapped-ion quantum charge-coupled device (QCCD) architecture is a
leading candidate for advanced quantum information processing. In current QCCD
implementations, imperfect ion transport and anomalous heating can excite ion
motion during a calculation. To counteract this, intermediate cooling is
necessary to maintain high-fidelity gate performance. Cooling the computational
ions sympathetically with ions of another species, a commonly employed
strategy, creates a significant runtime bottleneck. Here, we demonstrate a
different approach we call exchange cooling. Unlike sympathetic cooling,
exchange cooling does not require trapping two different atomic species. The
protocol introduces a bank of coolant ions which are repeatedly laser
cooled. A computational ion can then be cooled by transporting a coolant ion
into its proximity. We test this concept experimentally with two ions,
executing the necessary transport in 107 , an order of magnitude faster
than typical sympathetic cooling durations. We remove over 96%, and as many as
102(5) quanta, of axial motional energy from the computational ion. We verify
that re-cooling the coolant ion does not decohere the computational ion. This
approach validates the feasibility of a single-species QCCD processor, capable
of fast quantum simulation and computation
Finding the trigger to Iapetus' odd global albedo pattern: Dynamics of dust from Saturn's irregular satellites
The leading face of Saturn's moon Iapetus, Cassini Regio, has an albedo only
one tenth that on its trailing side. The origin of this enigmatic dichotomy has
been debated for over forty years, but with new data, a clearer picture is
emerging. Motivated by Cassini radar and imaging observations, we investigate
Soter's model of dark exogenous dust striking an originally brighter Iapetus by
modeling the dynamics of the dark dust from the ring of the exterior retrograde
satellite Phoebe under the relevant perturbations. In particular, we study the
particles' probabilities of striking Iapetus, as well as their expected spatial
distribution on the Iapetian surface. We find that, of the long-lived particles
(greater than about 5 microns), most particle sizes (greater than about 10
microns) are virtually certain to strike Iapetus, and their calculated
distribution on the surface matches up well with Cassini Regio's extent in its
longitudinal span. The satellite's polar regions are observed to be bright,
presumably because ice is deposited there. Thus, in the latitudinal direction
we estimate polar dust deposition rates to help constrain models of thermal
migration invoked to explain the bright poles (Spencer & Denk 2010). We also
analyze dust originating from other irregular outer moons, determining that a
significant fraction of that material will eventually coat Iapetus--perhaps
explaining why the spectrum of Iapetus' dark material differs somewhat from
that of Phoebe. Finally we track the dust particles that do not strike Iapetus,
and find that most land on Titan, with a smaller fraction hitting Hyperion. As
has been previously conjectured, such exogenous dust, coupled with Hyperion's
chaotic rotation, could produce Hyperion's roughly isotropic, moderate-albedo
surface.Comment: Accepted for publication in Icaru
RAD6 promotes DNA repair and stem cell signaling in ovarian cancer and is a promising therapeutic target to prevent and treat acquired chemoresistance
Ovarian cancer (OC) is the most deadly gynecological cancer and unlike most other neoplasms, survival rates for OC have not significantly improved in recent decades. We show that RAD6, an ubiquitin-conjugating enzyme, is significantly overexpressed in ovarian tumors and its expression increases in response to carboplatin chemotherapy. RAD6 expression correlated strongly with acquired chemoresistance and malignant behavior of OC cells, expression of stem cell genes and poor prognosis of OC patients, suggesting an important role for RAD6 in ovarian tumor progression. Upregulated RAD6 enhances DNA damage tolerance and repair efficiency of OC cells and promotes their survival. Increased RAD6 levels cause histone 2B ubiquitination-mediated epigenetic changes that stimulate transcription of stem cell genes, including ALDH1A1 and SOX2, leading to a cancer stem cell phenotype, which is implicated in disease recurrence and metastasis. Downregulation of RAD6 or its inhibition using a small molecule inhibitor attenuated DNA repair signaling and expression of cancer stem cells markers and sensitized chemoresistant OC cells to carboplatin. Together, these results suggest that RAD6 could be a therapeutic target to prevent and treat acquired chemoresistance and disease recurrence in OC and enhance the efficacy of standard chemotherapy
Charge dynamics at heterojunctions for PbS/ZnO colloidal quantum dot solar cells probed with time-resolved surface photovoltage spectroscopy
Time-resolved laser-pump X-ray-photoemission-probe spectroscopy of a ZnO (101 ⎯ ⎯ 0
101¯0
) substrate with and without PbS quantum dots (QDs) chemically linked to the surface is performed, using laser photon energies resonant with and below the band gap energy of the substrate (λ = 372 and 640 nm, hν = 3.33 and 1.94 eV). Charge injection from the photoexcited QDs to ZnO is demonstrated through the change in the surface photovoltage of the ZnO substrate observed when the heterojunction is illuminated with 1.94 eV radiation. The measured carrier dynamics are limited by the persistent photoconductivity of ZnO, giving dark carrier lifetimes of the order of 200 μs in a depletion layer at the interface. The chemical specificity of soft X-rays is used to separately measure the charge dynamics in the quantum dots and the substrate, yielding evidence that the depletion region at the interface extends into the PbS QD layer
Cube law, condition factor and weight-length relationships: history, meta-analysis and recommendations
This study presents a historical review, a meta-analysis, and recommendations for users about weight–length relationships, condition factors and relative weight equations. The historical review traces the developments of the respective concepts. The meta-analysis explores 3929 weight–length relationships of the type W = aLb for 1773 species of fishes. It shows that 82% of the variance in a plot of log a over b can be explained by allometric versus isometric growth patterns and by different body shapes of the respective species. Across species median b = 3.03 is significantly larger than 3.0, thus indicating a tendency towards slightly positive-allometric growth (increase in relative body thickness or plumpness) in most fishes. The expected range of 2.5 < b < 3.5 is confirmed. Mean estimates of b outside this range are often based on only one or two weight–length relationships per species. However, true cases of strong allometric growth do exist and three examples are given. Within species, a plot of log a vs b can be used to detect outliers in weight–length relationships. An equation to calculate mean condition factors from weight–length relationships is given as Kmean = 100aLb−3. Relative weight Wrm = 100W/(amLbm) can be used for comparing the condition of individuals across populations, where am is the geometric mean of a and bm is the mean of b across all available weight–length relationships for a given species. Twelve recommendations for proper use and presentation of weight–length relationships, condition factors and relative weight are given
W Hya : molecular inventory by ISO-SWS
Infrared spectroscopy is a powerful tool to probe the inventory of solid
state and molecular species in circumstellar ejecta. Here we analyse the
infrared spectrum of the Asymptotic Giant Branch star W Hya, obtained by the
Short and Long Wavelength Spectrometers on board of the Infrared Satellite
Observatory. These spectra show evidence for the presence of amorphous
silicates, aluminum oxide, and magnesium-iron oxide grains. We have modelled
the spectral energy distribution using laboratory measured optical properties
of these compounds and derive a total dust mass loss rate of 3E-10 Msol/yr. We
find no satisfactory fit to the 13 micron dust emission feature and the
identification of its carrier is still an open issue. We have also modelled the
molecular absorption bands due to H2O, OH, CO, CO2, SiO, and SO2 and estimated
the excitation temperatures for different bands which range from 300 to 3000K.
It is clear that different molecules giving rise to these absorption bands
originate from different gas layers. We present and analyse high resolution
Fabry-Perot spectra of the three CO2 bands in the 15 micron region. In these
data, the bands are resolved into individual Q-lines in emission, which allows
the direct determination of the excitation temperature and column density of
the emitting gas. This reveals the presence of a warm (about 450K) extended
layer of CO2, somewhere between the photosphere and the dust formation zone.
The gas in this layer is cooler than the 1000K CO2 gas responsible for the
low-resolution absorption bands at 4.25 and 15 micron. The rotational and
vibrational excitation temperatures derived from the individual Q-branch lines
of CO2 are different (450K and 150K, respectively) so that the CO2 level
population is not in LTE.Comment: To appear in Astronomy and Astrophysics A reference is adde
- …