876 research outputs found
Experimentally generating and tuning robust entanglement between photonic qubits
We generate and study the entanglement properties of novel states composed of
three polarisation-encoded photonic qubits. By varying a single experimental
parameter we can coherently move from a fully separable state to a maximally
robust W state, while at all times preserving an optimally robust, symmetric
entanglement configuration. We achieve a high fidelity with these
configurations experimentally, including the highest reported W state fidelity.Comment: lower print quality for arxiv figure
Coating thickness and elastic modulus measurement using ultrasonic bulk wave resonance
Measurement of the resonant through thickness ultrasonic modes of a homogeneous plate using a fast Fourier transform of the temporal data can be used to calculate plate thickness very accurately. We describe an extension of this principle to two-layer systems, examining a thin coating on a substrate of known properties. The resonant behavior of these systems is predicted and we explain how this approach is used to measure coating thickness and elastic modulus. Noncontact electromagnetic acoustic transducers are used for ultrasonic measurement, as they do not significantly affect the resonant response of the system, unlike alternative contact transducers
Entanglement growth in quench dynamics with variable range interactions
Studying entanglement growth in quantum dynamics provides both insight into
the underlying microscopic processes and information about the complexity of
the quantum states, which is related to the efficiency of simulations on
classical computers. Recently, experiments with trapped ions, polar molecules,
and Rydberg excitations have provided new opportunities to observe dynamics
with long-range interactions. We explore nonequilibrium coherent dynamics after
a quantum quench in such systems, identifying qualitatively different behavior
as the exponent of algebraically decaying spin-spin interactions in a
transverse Ising chain is varied. Computing the build-up of bipartite
entanglement as well as mutual information between distant spins, we identify
linear growth of entanglement entropy corresponding to propagation of
quasiparticles for shorter range interactions, with the maximum rate of growth
occurring when the Hamiltonian parameters match those for the quantum phase
transition. Counter-intuitively, the growth of bipartite entanglement for
long-range interactions is only logarithmic for most regimes, i.e.,
substantially slower than for shorter range interactions. Experiments with
trapped ions allow for the realization of this system with a tunable
interaction range, and we show that the different phenomena are robust for
finite system sizes and in the presence of noise. These results can act as a
direct guide for the generation of large-scale entanglement in such
experiments, towards a regime where the entanglement growth can render existing
classical simulations inefficient.Comment: 17 pages, 7 figure
Experimental quantum computing without entanglement
Entanglement is widely believed to lie at the heart of the advantages offered
by a quantum computer. This belief is supported by the discovery that a
noiseless (pure) state quantum computer must generate a large amount of
entanglement in order to offer any speed up over a classical computer. However,
deterministic quantum computation with one pure qubit (DQC1), which employs
noisy (mixed) states, is an efficient model that generates at most a marginal
amount of entanglement. Although this model cannot implement any arbitrary
algorithm it can efficiently solve a range of problems of significant
importance to the scientific community. Here we experimentally implement a
first-order case of a key DQC1 algorithm and explicitly characterise the
non-classical correlations generated. Our results show that while there is no
entanglement the algorithm does give rise to other non-classical correlations,
which we quantify using the quantum discord - a stronger measure of
non-classical correlations that includes entanglement as a subset. Our results
suggest that discord could replace entanglement as a necessary resource for a
quantum computational speed-up. Furthermore, DQC1 is far less resource
intensive than universal quantum computing and our implementation in a scalable
architecture highlights the model as a practical short-term goal.Comment: 5 pages, 4 figure
The Contribution of Experimental <i>in vivo </i>Models to Understanding the Mechanisms of Adaptation to Mechanical Loading in Bone
Changing loading regimens by natural means such as exercise, with or without interference such as osteotomy, has provided useful information on the structure:function relationship in bone tissue. However, the greatest precision in defining those aspects of the overall strain environment that influence modeling and remodeling behavior has been achieved by relating quantified changes in bone architecture to quantified changes in bones’ strain environment produced by direct, controlled artificial bone loading.Jiri Heřt introduced the technique of artificial loading of bones in vivo with external devices in the 1960s using an electromechanical device to load rabbit tibiae through transfixing stainless steel pins. Quantifying natural bone strains during locomotion by attaching electrical resistance strain gauges to bone surfaces was introduced by Lanyon, also in the 1960s. These studies in a variety of bones in a number of species demonstrated remarkable uniformity in the peak strains and maximum strain rates experienced.Experiments combining strain gauge instrumentation with artificial loading in sheep, pigs, roosters, turkeys, rats and mice has yielded significant insight into the control of strain-related adaptive (re)modeling. This diversity of approach has been largely superseded by non-invasive transcutaneous loading in rats and mice which is now the model of choice for many studies. Together such studies have demonstrated that; over the physiological strain range, bone’s mechanically-adaptive processes are responsive to dynamic but not static strains; the size and nature of the adaptive response controlling bone mass is linearly related to the peak loads encountered; the strain-related response is preferentially sensitive to high strain rates and unresponsive to static ones; is most responsive to unusual strain distributions; is maximized by remarkably few strain cycles and that these are most effective when interrupted by short periods of rest between them
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