2,237 research outputs found
New Protocols and Lower Bound for Quantum Secret Sharing with Graph States
We introduce a new family of quantum secret sharing protocols with limited
quantum resources which extends the protocols proposed by Markham and Sanders
and by Broadbent, Chouha, and Tapp. Parametrized by a graph G and a subset of
its vertices A, the protocol consists in: (i) encoding the quantum secret into
the corresponding graph state by acting on the qubits in A; (ii) use a
classical encoding to ensure the existence of a threshold. These new protocols
realize ((k,n)) quantum secret sharing i.e., any set of at least k players
among n can reconstruct the quantum secret, whereas any set of less than k
players has no information about the secret. In the particular case where the
secret is encoded on all the qubits, we explore the values of k for which there
exists a graph such that the corresponding protocol realizes a ((k,n)) secret
sharing. We show that for any threshold k> n-n^{0.68} there exists a graph
allowing a ((k,n)) protocol. On the other hand, we prove that for any k<
79n/156 there is no graph G allowing a ((k,n)) protocol. As a consequence there
exists n_0 such that the protocols introduced by Markham and Sanders admit no
threshold k when the secret is encoded on all the qubits and n>n_0
Incorporating spatial correlations into multispecies mean-field models
In biology, we frequently observe different species existing within the same environment. For example, there are many cell types in a tumour, or different animal species may occupy a given habitat. In modeling interactions between such species, we often make use of the mean-field approximation, whereby spatial correlations between the locations of individuals are neglected. Whilst this approximation holds in certain situations, this is not always the case, and care must be taken to ensure the mean-field approximation is only used in appropriate settings. In circumstances where the mean-field approximation is unsuitable, we need to include information on the spatial distributions of individuals, which is not a simple task. In this paper, we provide a method that overcomes many of the failures of the mean-field approximation for an on-lattice volume-excluding birth-death-movement process with multiple species. We explicitly take into account spatial information on the distribution of individuals by including partial differential equation descriptions of lattice site occupancy correlations. We demonstrate how to derive these equations for the multispecies case and show results specific to a two-species problem. We compare averaged discrete results to both the mean-field approximation and our improved method, which incorporates spatial correlations. We note that the mean-field approximation fails dramatically in some cases, predicting very different behavior from that seen upon averaging multiple realizations of the discrete system. In contrast, our improved method provides excellent agreement with the averaged discrete behavior in all cases, thus providing a more reliable modeling framework. Furthermore, our method is tractable as the resulting partial differential equations can be solved efficiently using standard numerical techniques
The self-assembly of DNA Holliday junctions studied with a minimal model
In this paper, we explore the feasibility of using coarse-grained models to
simulate the self-assembly of DNA nanostructures. We introduce a simple model
of DNA where each nucleotide is represented by two interaction sites
corresponding to the phosphate-sugar backbone and the base. Using this model,
we are able to simulate the self-assembly of both DNA duplexes and Holliday
junctions from single-stranded DNA. We find that assembly is most successful in
the temperature window below the melting temperatures of the target structure
and above the melting temperature of misbonded aggregates. Furthermore, in the
case of the Holliday junction, we show how a hierarchical assembly mechanism
reduces the possibility of becoming trapped in misbonded configurations. The
model is also able to reproduce the relative melting temperatures of different
structures accurately, and allows strand displacement to occur.Comment: 13 pages, 14 figure
Survival of entanglement in thermal states
We present a general sufficiency condition for the presence of multipartite
entanglement in thermal states stemming from the ground state entanglement. The
condition is written in terms of the ground state entanglement and the
partition function and it gives transition temperatures below which
entanglement is guaranteed to survive. It is flexible and can be easily adapted
to consider entanglement for different splittings, as well as be weakened to
allow easier calculations by approximations. Examples where the condition is
calculated are given. These examples allow us to characterize a minimum gapping
behavior for the survival of entanglement in the thermodynamic limit. Further,
the same technique can be used to find noise thresholds in the generation of
useful resource states for one-way quantum computing.Comment: 6 pages, 2 figures. Changes made in line with publication
recommendations. Motivation and concequences of result clarified, with the
addition of one more example, which applies the result to give noise
thresholds for measurement based quantum computing. New author added with new
result
The uses and functions of ageing celebrity war reporters
This article starts from the premise that recognition of professional authority and celebrity status depends on the embodiment and performance of field-specific dispositional practices: there’s no such thing as a natural, though we often talk about journalistic instinct as something someone simply has or doesn’t have. Next, we have little control over how we are perceived by peers and publics, and what we think are active positioning or subjectifying practices are in fact, after Bourdieu, revelations of already-determined delegation. The upshot is that two journalists can arrive at diametrically opposed judgements on the basis of observation of the same actions of a colleague, and as individuals we are blithely hypocritical in forming (or reciting) evaluations of the professional identity of celebrities. Nowhere is this starker than in the discourse of age-appropriate behaviour, which this paper addresses using the examples of ‘star’ war reporters John Simpson, Kate Adie and Martin Bell. A certain rough-around-the-edges irreverence is central to dispositional authenticity amongst war correspondents, and for ageing hacks this incorporates gendered attitudes to sex and alcohol as well as indifference to protocol. And yet perceived age-inappropriate sexual behaviour is also used to undermine professional integrity, and the paper ends by outlining the phenomenological context that makes possible this effortless switching between amoral and moralising recognition by peers and audiences alike
Properties of implanted and CVD incorporated nitrogen-vacancy centers: preferential charge state and preferential orientation
The combination of the long electron state spin coherence time and the optical coupling of the ground electronic states to an excited state manifold makes the nitrogen-vacancy (NV) center in diamond an attractive candidate for quantum information processing. To date the best spin and optical properties have been found in centers deep within the diamond crystal. For useful devices it will be necessary to engineer NVs with similar properties close to the diamond surface. We report on properties including charge state control and preferential orientation for near surface NVs formed either in CVD growth or through implantation and annealing
The maximally entangled symmetric state in terms of the geometric measure
The geometric measure of entanglement is investigated for permutation
symmetric pure states of multipartite qubit systems, in particular the question
of maximum entanglement. This is done with the help of the Majorana
representation, which maps an n qubit symmetric state to n points on the unit
sphere. It is shown how symmetries of the point distribution can be exploited
to simplify the calculation of entanglement and also help find the maximally
entangled symmetric state. Using a combination of analytical and numerical
results, the most entangled symmetric states for up to 12 qubits are explored
and discussed. The optimization problem on the sphere presented here is then
compared with two classical optimization problems on the S^2 sphere, namely
Toth's problem and Thomson's problem, and it is observed that, in general, they
are different problems.Comment: 18 pages, 15 figures, small corrections and additions to contents and
reference
Integrated Diamond Optics for Single Photon Detection
Optical detection of single defect centers in the solid state is a key
element of novel quantum technologies. This includes the generation of single
photons and quantum information processing. Unfortunately the brightness of
such atomic emitters is limited. Therefore we experimentally demonstrate a
novel and simple approach that uses off-the-shelf optical elements. The key
component is a solid immersion lens made of diamond, the host material for
single color centers. We improve the excitation and detection of single
emitters by one order of magnitude, as predicted by theory.Comment: 10 pages, 3 figure
Nondispersive hole transport in a spin-coated dendrimer film measured by the charge-generation-layer time-of-flight method
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