6,928 research outputs found
Robustness of Highly Entangled Multi-Qubit States Under Decoherence
We investigate the decay of entanglement, due to decoherence, of multi-qubit
systems that are initially prepared in highly (in some cases maximally)
entangled states. We assume that during the decoherence processes each qubit of
the system interacts with its own, independent environment. We determine, for
systems with a small number of qubits and for various decoherence channels, the
initial states exhibiting the most robust entanglement. We also consider a
restricted version of this robustness optimization problem, only involving
states equivalent under local unitary transformations to the |GHZ> state.Comment: 16 pages, 3 figures. Changes in Sec.
Microwave-assisted synthesis and electrochemical evaluation of VO2 (B) nanostructures
Understanding how intercalation materials change during electrochemical operation is paramount to optimizing their behaviour and function and in situ characterization methods allow us to observe these changes without sample destruction. Here we first report the improved intercalation properties of bronze phase vanadium dioxide VO2 (B) prepared by a microwave-assisted route which exhibits a larger electrochemical capacity (232 mAh g-1) compared with VO2 (B) prepared by a solvothermal route (197 mAh g-1). These electrochemical differences have also been followed using in situ X-ray absorption spectroscopy allowing us to follow oxidation state changes as they occur during battery operation
Brachistochrone of Entanglement for Spin Chains
We analytically investigate the role of entanglement in time-optimal state
evolution as an appli- cation of the quantum brachistochrone, a general method
for obtaining the optimal time-dependent Hamiltonian for reaching a target
quantum state. As a model, we treat two qubits indirectly cou- pled through an
intermediate qubit that is directly controllable, which represents a typical
situation in quantum information processing. We find the time-optimal unitary
evolution law and quantify residual entanglement by the two-tangle between the
indirectly coupled qubits, for all possible sets of initial pure quantum states
of a tripartite system. The integrals of the motion of the brachistochrone are
determined by fixing the minimal time at which the residual entanglement is
maximized. Entan- glement plays a role for W and GHZ initial quantum states,
and for the bi-separable initial state in which the indirectly coupled qubits
have a nonzero value of the 2-tangle.Comment: 9 pages, 4 figure
Time-optimal CNOT between indirectly coupled qubits in a linear Ising chain
We give analytical solutions for the time-optimal synthesis of entangling
gates between indirectly coupled qubits 1 and 3 in a linear spin chain of three
qubits subject to an Ising Hamiltonian interaction with equal coupling plus
a local magnetic field acting on the intermediate qubit. The energy available
is fixed, but we relax the standard assumption of instantaneous unitary
operations acting on single qubits. The time required for performing an
entangling gate which is equivalent, modulo local unitary operations, to the
between the indirectly coupled qubits 1 and 3 is
, i.e. faster than a previous estimate based on a similar
Hamiltonian and the assumption of local unitaries with zero time cost.
Furthermore, performing a simple Walsh-Hadamard rotation in the Hlibert space
of qubit 3 shows that the time-optimal synthesis of the (which acts as the identity when the control qubit 1 is in the state
, while if the control qubit is in the state the target
qubit 3 is flipped as ) also requires the same
time .Comment: 9 pages; minor modification
Multi-Qubit Systems: Highly Entangled States and Entanglement Distribution
A comparison is made of various searching procedures, based upon different
entanglement measures or entanglement indicators, for highly entangled
multi-qubits states. In particular, our present results are compared with those
recently reported by Brown et al. [J. Phys. A: Math. Gen. 38 (2005) 1119]. The
statistical distribution of entanglement values for the aforementioned
multi-qubit systems is also explored.Comment: 24 pages, 3 figure
Scholar-activists in an expanding European food sovereignty movement
This article analyzes the roles, relations, and positions of scholar-activists in the European food sovereignty movement. In doing so, we document, make visible and question the political dimensions of researchers' participation in the movement. We argue that scholar-activists are part of the movement, but are distinct from the affected constituencies, put in place to ensure adequate representation of key movement actors. This is because scholar-activists lack a collective identity, have no processes to formulate collective demands, and no mechanisms for inter-researcher and researchers-movement communication. We reflect on whether and how scholar-activists could organize, and discuss possible pathways for a more cohesive and stronger researcher engagement in the movement.</p
Does the use of health technology assessment have an impact on the utilisation of health care resources? Evidence from two European countries
Objectives: A centralised approach to health technology assessment (HTA) may facilitate optimal use of HTA resources. A regional approach may increase the chances of local implementation of recommendations. This study aimed to compare assessment procedures in England (centralised HTA approach) with Spain (regional HTA approach) discussing key challenges and opportunities from both approaches. Methods: We compared technology assessments of anticancer medicines in the two jurisdictions from 2008 to 2015. In order to assess the implementation of HTA recommendations, we assessed trends in medicine usage using regression methods. We used IQVIA data, from 2011 to 2016, for a sample of 11 medicines. We used CatSalut data from Catalonia to assess the implementation of local recommendations. Results: In England 66 assessments were undertaken by the National Institute for Health and Care Excellence (NICE), using a standardised methodology. In Spain there were 79 reports undertaken by a range of bodies using a shared process and coordinated through the GENESIS collaboration; the assessment methods used varied substantially. Overall, the recommendations in the two jurisdictions were similar. Regression analyses indicate that where there is a positive recommendation by HTA bodies, the usage of the medicine responds most strongly (p<0.001) in Catalonia (=4.892), followed by England ( =3.120) and Spain ( =1.693). Conclusions: This study suggests that medicine utilisation does respond to the positive recommendations of HTA bodies. However, if HTA capacity is organised primarily regionally, considerable effort may be required in coordination, in order to ensure consistent and rigorous assessments and adequate implementation of HTA findings
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