3,488 research outputs found
Entanglement cost of generalised measurements
Bipartite entanglement is one of the fundamental quantifiable resources of
quantum information theory. We propose a new application of this resource to
the theory of quantum measurements. According to Naimark's theorem any rank 1
generalised measurement (POVM) M may be represented as a von Neumann
measurement in an extended (tensor product) space of the system plus ancilla.
By considering a suitable average of the entanglements of these measurement
directions and minimising over all Naimark extensions, we define a notion of
entanglement cost E_min(M) of M.
We give a constructive means of characterising all Naimark extensions of a
given POVM. We identify various classes of POVMs with zero and non-zero cost
and explicitly characterise all POVMs in 2 dimensions having zero cost. We
prove a constant upper bound on the entanglement cost of any POVM in any
dimension. Hence the asymptotic entanglement cost (i.e. the large n limit of
the cost of n applications of M, divided by n) is zero for all POVMs.
The trine measurement is defined by three rank 1 elements, with directions
symmetrically placed around a great circle on the Bloch sphere. We give an
analytic expression for its entanglement cost. Defining a normalised cost of
any d-dimensional POVM by E_min(M)/log(d), we show (using a combination of
analytic and numerical techniques) that the trine measurement is more costly
than any other POVM with d>2, or with d=2 and ancilla dimension 2. This
strongly suggests that the trine measurement is the most costly of all POVMs.Comment: 20 pages, plain late
Evidence of a past disc-disc encounter: HV and DO Tau
© 2018 The Author(s). Published by Oxford University Press on behalf of The Royal Astronomical Society. Theory and observations suggest that star formation occurs hierarchically due to the fragmentation of giant molecular clouds. In this case we would expect substructure and enhanced stellar multiplicity in the primordial cluster. This substructure is expected to decay quickly in most environments, however historic stellar encounters might leave imprints in a protoplanetary disc (PPD) population. In a low-density environment such as Taurus, tidal tails from violent star-disc or disc-disc encounters might be preserved over timescales sufficient to be observed. In this work, we investigate the possibility that just such an event occurred between HV Tau C (itself a component of a triple system) and DO Tau ~0.1 Myr ago, as evidenced by an apparent 'bridge' structure evident in the 160 Ό m emission. By modelling the encounter using smoothed particle hydrodynamics (SPH) we reproduce the main features of the observed extended structure ('V'-shaped emission pointing west of HV Tau and a tail-like structure extending east of DO Tau). We suggest that HV Tau and DO Tau formed together in a quadruple system on a scale of ~5000 au (0.025 pc)
Forming short-period substellar companions in 47 Tucanae - I. Dynamical model and brown dwarf tidal capture rates
ABSTRACT
Stars in globular clusters formed and evolved in the most extreme environment: high density and low metallicity. If the formation of stars and planets is at all sensitive to environmental conditions, this should therefore be evident in globular clusters. Observations have indicated that hot Jupiters are at least an order of magnitude less prevalent in the central region of the globular cluster 47 Tucanae (Tuc) than in the field. In this work, we explore the claims in the literature for additional consequences for the low-mass stellar initial mass function. Tidal capture, the mechanism that produces X-ray binaries in globular clusters, applies also to brown dwarfs (BDs). This process produces tight stellarâBD binaries that would be detectable by transit surveys. Applying a Monte Carlo dynamical evolution model, we compute the overall BD capture rates. We find that the number of captures is lower than previous estimates. Capture efficiency increases steeply with stellar mass, which means that mass segregation reduces capture efficiency as BDs and low-mass stars occupy the same regions. The result of this effect is that the current constraints on the short-period companion fraction remain marginally consistent with initially equal numbers of BDs and stars. However, our findings suggest that expanding the sample in 47 Tuc or surveying other globular clusters for close substellar companions can yield constraints on the substellar initial mass function in these environments. We estimate the capture rates in other globular clusters and suggest that 47 Tuc remains a promising target for future transit surveys.</jats:p
The FRIED grid of mass-loss rates for externally irradiated protoplanetary discs
We present an open access grid of 3930 calculations of externally evaporating
protoplanetary discs. This spans a range of disc sizes (1-400AU), disc masses,
UV field strengths (10-10G) and stellar masses (0.05-1.9M). The
grid is publicly available for download, and offers a means of cheaply
including external photoevaporation in disc evolutionary calculations. It can
also be queried using an online tool for quick estimates of instantaneous mass
loss rates (e.g for convenient evaluation of real observed systems). The
`FRIED' grid itself illustrates that for discs around stars M
external photoevaporation is effective down to small radii (AU) down to UV
fields at least as weak as 10G. At the other end of the scale, in a
G environment photoevaporation is effective down to 1AU even for
stellar masses at least as high as 1.9M. We also illustrate in which
regimes CO survives in the photoevaporative outflow for significant mass loss
rates; marking a system a good candidate to detect external photoevaporation in
weak-intermediate UV environments through sub-Keplerian rotation. Finally we
make illustrative mass loss rate estimates for discs in Taurus based on the
Guilloteau et al. (2011) star-disc parameters, finding that around half are
expected to have both significant mass loss and retain CO in the
photoevaporative outflow.Comment: 16 pages, 9 figures. Accepted for publication in MNRA
Protoplanetary disc truncation mechanisms in stellar clusters: Comparing external photoevaporation and tidal encounters
Most stars form and spend their early life in regions of enhanced stellar
density. Therefore the evolution of protoplanetary discs (PPDs) hosted by such
stars are subject to the influence of other members of the cluster. Physically,
PPDs might be truncated either by photoevaporation due to ultraviolet flux from
massive stars, or tidal truncation due to close stellar encounters. Here we aim
to compare the two effects in real cluster environments. In this vein we first
review the properties of well studied stellar clusters with a focus on stellar
number density, which largely dictates the degree of tidal truncation, and far
ultraviolet (FUV) flux, which is indicative of the rate of external
photoevaporation. We then review the theoretical PPD truncation radius due to
an arbitrary encounter, additionally taking into account the role of eccentric
encounters that play a role in hot clusters with a 1D velocity dispersion
km/s. Our treatment is then applied statistically to varying
local environments to establish a canonical threshold for the local stellar
density ( pc) for which encounters can play a significant
role in shaping the distribution of PPD radii over a timescale Myr. By
combining theoretical mass loss rates due to FUV flux with viscous spreading in
a PPD we establish a similar threshold for which a massive disc is completely
destroyed by external photoevaporation. Comparing these thresholds in local
clusters we find that if either mechanism has a significant impact on the PPD
population then photoevaporation is always the dominating influence.ERC Advanced Grant grant agreement 34113
External photoevaporation of protoplanetary discs in Cygnus OB2: Linking discs to star formation dynamical history
Many stars form in regions of enhanced stellar density, where stellar neighbours can have a strong influence on a protoplanetary disc (PPD) population. In particular, far-ultraviolet (FUV) flux from massive stars drives thermal winds from the outer edge of PPDs, accelerating disc destruction. Here, we present a novel technique for constraining the dynamical history of a star-forming environment using PPD properties in a strongly FUV-irradiated environment. Applying recent models for FUV-induced mass-loss rates to the PPD population of Cygnus OB2, we constrain the time since primordial gas expulsion. This is 0.5 Myr ago if the Shakura & Sunyaev α-viscosity parameter is α = 10 â2 (corresponding to a viscous timescale of Ï visc â 0.5 Myr for a disc of scale radius 40 au around a 1 M star). This value of α is effectively an upper limit, since it assumes efficient extinction of FUV photons throughout the embedded phase. This gas expulsion time-scale is consistent with a full dynamical model that fits kinematic and morphological data as well as disc fractions. We suggest Cygnus OB2 was originally composed of distinct massive clumps or filaments, each with a stellar mass âŒ10 4 M. Finally we predict that in regions of efficient FUV-induced mass-loss, disc mass M disc as a function of stellar host mass m star follows a power law with M disc â m ÎČstar , where ÎČ exceeds âŒ2.7 â steeper than correlations observed in regions of moderate FUV flux (1 < ÎČ < 1.9). This difference offers a promising diagnostic of the influence of external photoevaporation in a given region.This work has been supported by the DISCSIM project, grant agreement 341137 funded by the European Research Council under ERC-2013-ADG. AJW thanks the Science and Technology Facilities Council (STFC) for their studentship
Classes of adolescents with disruptive behaviors in a general population sample
Background To study disruptive behaviors adequately, we need to distinguish between individuals with different types of problems that may have a different etiology. The availability of a taxonomic system that helps in identifying homogeneous groups of individuals, with similar patterns of disruptive behaviors, is crucial to achieve this goal. Therefore, we examine which classes of preadolescents with symptoms of Attention Deficit/Hyperactivity Disorder (ADHD), Oppositional Defiant Disorder ( ODD), and Conduct Disorder (CD) can be identified in the general population. Methods Disruptive behaviors of 2,230 10-12 year olds from the Dutch general population were assessed with the Child Behavior Checklist and Youth. Self-Report. Results Latent class analysis revealed three classes of preadolescents: the first characterized by high scores on ADHD, ODD, and CD items; a second by high probabilities of ADHD and ODD symptoms; a third with low scores on all items. Conclusions Because classes of preadolescents with symptoms of only one type of disruptive behavior problems could not be identified, it can be questioned how useful separate diagnostic distinctions are in general population studies
Two new species of Zospeum Bourguignat, 1856 from the Basque-Cantabrian Mountains, Northern Spain (Eupulmonata, Ellobioidea, Carychiidae)
Two new species of the genus Zospeum Bourguignat, 1856 are described from caves in the Sierra de Aitzgorri (Gipuzkoa) and the Sierra Salvada (Burgos) in Northern Spain. The taxa Z. vasconicum sp. n. and Z. zaldivarae sp. n. have recently, without a formal name, been included in a molecular study of worldwide members of the Carychiidae. In the present paper, the shell morphology and variation of these species is described and illustrated
Structure and Function of the α-Hydroxylation Bimodule of the Mupirocin Polyketide Synthase
\ua9 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.Mupirocin is a clinically important antibiotic produced by a trans-AT Type I polyketide synthase (PKS) in Pseudomonas fluorescens. The major bioactive metabolite, pseudomonic acid A (PAâA), is assembled on a tetrasubstituted tetrahydropyran (THP) core incorporating a 6-hydroxy group proposed to be introduced by α-hydroxylation of the thioester of the acyl carrier protein (ACP) bound polyketide chain. Herein, we describe an in vitro approach combining purified enzyme components, chemical synthesis, isotopic labelling, mass spectrometry and NMR in conjunction with in vivo studies leading to the first characterisation of the α-hydroxylation bimodule of the mupirocin biosynthetic pathway. These studies reveal the precise timing of hydroxylation by MupA, substrate specificity and the ACP dependency of the enzyme components that comprise this α-hydroxylation bimodule. Furthermore, using purified enzyme, it is shown that the MmpA KS0 shows relaxed substrate specificity, suggesting precise spatiotemporal control of in trans MupA recruitment in the context of the PKS. Finally, the detection of multiple intermodular MupA/ACP interactions suggests these bimodules may integrate MupA into their assembly
Unforgeable Quantum Encryption
We study the problem of encrypting and authenticating quantum data in the
presence of adversaries making adaptive chosen plaintext and chosen ciphertext
queries. Classically, security games use string copying and comparison to
detect adversarial cheating in such scenarios. Quantumly, this approach would
violate no-cloning. We develop new techniques to overcome this problem: we use
entanglement to detect cheating, and rely on recent results for characterizing
quantum encryption schemes. We give definitions for (i.) ciphertext
unforgeability , (ii.) indistinguishability under adaptive chosen-ciphertext
attack, and (iii.) authenticated encryption. The restriction of each definition
to the classical setting is at least as strong as the corresponding classical
notion: (i) implies INT-CTXT, (ii) implies IND-CCA2, and (iii) implies AE. All
of our new notions also imply QIND-CPA privacy. Combining one-time
authentication and classical pseudorandomness, we construct schemes for each of
these new quantum security notions, and provide several separation examples.
Along the way, we also give a new definition of one-time quantum authentication
which, unlike all previous approaches, authenticates ciphertexts rather than
plaintexts.Comment: 22+2 pages, 1 figure. v3: error in the definition of QIND-CCA2 fixed,
some proofs related to QIND-CCA2 clarifie
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