11,708 research outputs found
Cutting thin sections of bone
Medical equipment for obtaining repetitive planoparallel sections of bone to study healing of bone structure under high gravity stress is described. Device consists of modified saw with diamond cutting edges. Construction of device and manner of use are explained
Testing product states, quantum Merlin-Arthur games and tensor optimisation
We give a test that can distinguish efficiently between product states of n
quantum systems and states which are far from product. If applied to a state
psi whose maximum overlap with a product state is 1-epsilon, the test passes
with probability 1-Theta(epsilon), regardless of n or the local dimensions of
the individual systems. The test uses two copies of psi. We prove correctness
of this test as a special case of a more general result regarding stability of
maximum output purity of the depolarising channel. A key application of the
test is to quantum Merlin-Arthur games with multiple Merlins, where we obtain
several structural results that had been previously conjectured, including the
fact that efficient soundness amplification is possible and that two Merlins
can simulate many Merlins: QMA(k)=QMA(2) for k>=2. Building on a previous
result of Aaronson et al, this implies that there is an efficient quantum
algorithm to verify 3-SAT with constant soundness, given two unentangled proofs
of O(sqrt(n) polylog(n)) qubits. We also show how QMA(2) with log-sized proofs
is equivalent to a large number of problems, some related to quantum
information (such as testing separability of mixed states) as well as problems
without any apparent connection to quantum mechanics (such as computing
injective tensor norms of 3-index tensors). As a consequence, we obtain many
hardness-of-approximation results, as well as potential algorithmic
applications of methods for approximating QMA(2) acceptance probabilities.
Finally, our test can also be used to construct an efficient test for
determining whether a unitary operator is a tensor product, which is a
generalisation of classical linearity testing.Comment: 44 pages, 1 figure, 7 appendices; v6: added references, rearranged
sections, added discussion of connections to classical CS. Final version to
appear in J of the AC
Extremal eigenvalues of local Hamiltonians
We apply classical algorithms for approximately solving constraint
satisfaction problems to find bounds on extremal eigenvalues of local
Hamiltonians. We consider spin Hamiltonians for which we have an upper bound on
the number of terms in which each spin participates, and find extensive bounds
for the operator norm and ground-state energy of such Hamiltonians under this
constraint. In each case the bound is achieved by a product state which can be
found efficiently using a classical algorithm.Comment: 5 pages; v4: uses standard journal styl
Building a Bird: Musculoskeletal Modeling and Simulation of Wing-Assisted Incline Running during Avian Ontogeny
Flapping flight is the most power-demanding mode of locomotion, associated with a suite of anatomical specializations in extant adult birds. In contrast, many developing birds use their forelimbs to negotiate environments long before acquiring “flight adaptations,” recruiting their developing wings to continuously enhance leg performance and, in some cases, fly. How does anatomical development influence these locomotor behaviors? Isolating morphological contributions to wing performance is extremely challenging using purely empirical approaches. However, musculoskeletal modeling and simulation techniques can incorporate empirical data to explicitly examine the functional consequences of changing morphology by manipulating anatomical parameters individually and estimating their effects on locomotion. To assess how ontogenetic changes in anatomy affect locomotor capacity, we combined existing empirical data on muscle morphology, skeletal kinematics, and aerodynamic force production with advanced biomechanical modeling and simulation techniques to analyze the ontogeny of pectoral limb function in a precocial ground bird (Alectoris chukar). Simulations of wing-assisted incline running (WAIR) using these newly developed musculoskeletal models collectively suggest that immature birds have excess muscle capacity and are limited more by feather morphology, possibly because feathers grow more quickly and have a different style of growth than bones and muscles. These results provide critical information about the ontogeny and evolution of avian locomotion by (i) establishing how muscular and aerodynamic forces interface with the skeletal system to generate movement in morphing juvenile birds, and (ii) providing a benchmark to inform biomechanical modeling and simulation of other locomotor behaviors, both across extant species and among extinct theropod dinosaurs
Profile-directed specialisation of custom floating-point hardware
We present a methodology for generating
floating-point arithmetic hardware
designs which are, for suitable applications, much reduced in size, while still
retaining performance and IEEE-754 compliance. Our system uses three
key parts: a profiling tool, a set of customisable
floating-point units and a
selection of system integration methods.
We use a profiling tool for
floating-point behaviour to identify arithmetic
operations where fundamental elements of IEEE-754
floating-point may be
compromised, without generating erroneous results in the common case.
In the uncommon case, we use simple detection logic to determine when
operands lie outside the range of capabilities of the optimised hardware.
Out-of-range operations are handled by a separate, fully capable,
floatingpoint
implementation, either on-chip or by returning calculations to a host
processor. We present methods of system integration to achieve this errorcorrection.
Thus the system suffers no compromise in IEEE-754 compliance,
even when the synthesised hardware would generate erroneous results.
In particular, we identify from input operands the shift amounts required
for input operand alignment and post-operation normalisation. For operations
where these are small, we synthesise hardware with reduced-size
barrel-shifters. We also propose optimisations to take advantage of other
profile-exposed behaviours, including removing the hardware required to
swap operands in a floating-point adder or subtractor, and reducing the
exponent range to fit observed values.
We present profiling results for a range of applications, including a selection
of computational science programs, Spec FP 95 benchmarks and the
FFMPEG media processing tool, indicating which would be amenable to
our method. Selected applications which demonstrate potential for optimisation
are then taken through to a hardware implementation. We show up
to a 45% decrease in hardware size for a
floating-point datapath, with a
correctable error-rate of less then 3%, even with non-profiled datasets
Mechanisms of nonstoichiometry in HfN<sub>1-<i>x</i></sub>
Density functional theory is used to calculate defect structures that can accommodate nonstoichiometry in hafnium nitride: HfN1-x, 0 ≤ X ≤ 0.25. It is predicted that a mechanism assuming simple distributions of nitrogen vacancies can accurately describe the variation in the experimentally observed lattice parameter with respect to the nitrogen nonstoichiometry. Although the lattice parameter changes are remarkably small across the whole nonstoichiometry range, the variations in the bulk modulus are much greater
Embodied carbon and construction cost differences between Hong Kong and Melbourne buildings
Limiting the amount of embodied carbon in buildings can help minimize the damaging impacts of global warming through lower upstream emission of CO2. This study empirically investigates the embodied carbon footprint of new-build and refurbished buildings in both Hong Kong and Melbourne to determine the embodied carbon profile and its relationship to both embodied energy and construction cost. The Hong Kong findings suggest that mean embodied carbon for refurbished buildings is 33-39% lower than new-build projects, and the cost for refurbished buildings is 22-50% lower than new-build projects (per square metre of floor area). The Melbourne findings, however, suggest that mean embodied carbon for refurbished buildings is 4% lower than new-build projects, and the cost for refurbished buildings is 24% higher than new-build projects (per square metre of floor area). Embodied carbon ranges from 645-1,059 kgCO2e/m2 for new-build and 294-655 kgCO2e/m2 for refurbished projects in Hong Kong, and 1,138-1,705 kgCO2e/m2 for new-build and 900-1,681 kgCO2e/m2 for refurbished projects in Melbourne. The reasons behind these locational discrepancies are explored and critiqued. Overall, a very strong linear relationship between embodied energy and construction cost in both cities was found and can be used to predict the former, given the latter
Melanism as a potential thermal benefit in eastern fox squirrels (Sciurus niger)
Melanistic fox squirrels (Sciurus niger) have expanded westward and increased in frequency in the Omaha, Nebraska, and Council Bluffs, Iowa, metropolitan areas. The selective advantage of melanism is currently unknown, but thermal advantages have been hypothesized, especially in winter. No difference in metabolic response curves were measured between melanistic (black) and rufus (orange) fox squirrels. When exposed to sunny skies, both melanistic and rufus squirrels had higher surface (skin and fur) temperature as ambient temperatures increased. Melanistic squirrel surface temperatures did not differ when squirrels were exposed to sunny or cloudy skies. However, rufus individuals showed significantly lower increases in surface temperatures when under cloudy skies. During fall months, rufus individuals were about 1.5 times more active throughout the day than melanistic individuals. However, in winter, melanistic fox squirrels were approximately 30% more active in the mornings (before 13:00) compared to rufus squirrels. Pre-winter body condition was higher in melanistic (25.5 ± 1.8 g/cm) compared to rufus (20.30 ± 3.6 g/cm) fox squirrels; however, there were no significant differences between melanistic (22.8 ± 1.4 g/cm) and rufus (23.9 ± 0.8 g/cm) fox squirrel post-winter body condition. The results of this study indicate that melanistic fox squirrels may have a slight winter thermal advantage over rufus fox squirrels by maintaining higher skin temperatures
Variational Matrix Product Ansatz for Nonuniform Dynamics in the Thermodynamic Limit
We describe how to implement the time-dependent variational principle for
matrix product states in the thermodynamic limit for nonuniform lattice
systems. This is achieved by confining the nonuniformity to a (dynamically
growable) finite region with fixed boundary conditions. The suppression of
unphysical quasiparticle reflections from the boundary of the nonuniform region
is also discussed. Using this algorithm we study the dynamics of localized
excitations in infinite systems, which we illustrate in the case of the spin-1
anti-ferromagnetic Heisenberg model and the model.Comment: 8 pages, 5 figures, tensor network diagrams. Code available at
http://amilsted.github.io/evoMPS
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