Highly indistinguishable and strongly entangled photons from symmetric GaAs quantum dots

The development of scalable sources of non-classical light is fundamental to unlocking thetechnological potential of quantum photonics. Semiconductor quantum dots are emerging asnear-optimal sources of indistinguishable single photons. However, their performance assources of entangled-photon pairs are still modest compared to parametric down converters.Photons emitted from conventional Stranski–Krastanov InGaAs quantum dots have shownnon-optimal levels of entanglement and indistinguishability. For quantum networks, bothcriteria must be met simultaneously. Here, we show that this is possible with a system thathas received limited attention so far: GaAs quantum dots. They can emit triggered polar-ization-entangled photons with high purity (g(2)(0) = 0.002±0.002), high indistinguish-ability (0.93±0.07 for 2 ns pulse separation) and high entanglement fidelity(0.94±0.01). Our results show that GaAs might be the material of choice for quantum-dotentanglement sources in future quantum technologie

Measuring the effective complexity of cosmological models

We introduce a statistical measure of the effective model complexity, called the Bayesian complexity. We demonstrate that the Bayesian complexity can be used to assess how many effective parameters a set of data can support and that it is a useful complement to the model likelihood (the evidence) in model selection questions. We apply this approach to recent measurements of cosmic microwave background anisotropies combined with the Hubble Space Telescope measurement of the Hubble parameter. Using mildly non-informative priors, we show how the 3-year WMAP data improves on the first-year data by being able to measure both the spectral index and the reionization epoch at the same time. We also find that a non-zero curvature is strongly disfavored. We conclude that although current data could constrain at least seven effective parameters, only six of them are required in a scheme based on the Lambda-CDM concordance cosmology.Comment: 9 pages, 4 figures, revised version accepted for publication in PRD, updated with WMAP3 result

Bayesian analysis of Friedmannless cosmologies

Assuming only a homogeneous and isotropic universe and using both the 'Gold' Supernova Type Ia sample of Riess et al. and the results from the Supernova Legacy Survey, we calculate the Bayesian evidence of a range of different parameterizations of the deceleration parameter. We consider both spatially flat and curved models. Our results show that although there is strong evidence in the data for an accelerating universe, there is little evidence that the deceleration parameter varies with redshift.Comment: 7 pages, 3 figure

Highly indistinguishable single photons from incoherently and coherently excited GaAs quantum dots

Semiconductor quantum dots are converging towards the demanding requirements of photonic quantum technologies. Among different systems, quantum dots with dimensions exceeding the free-exciton Bohr radius are appealing because of their high oscillator strengths. While this property has received much attention in the context of cavity quantum electrodynamics, little is known about the degree of indistinguishability of single photons consecutively emitted by such dots and on the proper excitation schemes to achieve high indistinguishability. A prominent example is represented by GaAs quantum dots obtained by local droplet etching, which recently outperformed other systems as triggered sources of entangled photon pairs. On these dots, we compare different single-photon excitation mechanisms, and we find (i) a "phonon bottleneck" and poor indistinguishability for conventional excitation via excited states and (ii) photon indistinguishablilities above 90% for both strictly resonant and for incoherent acoustic- and optical-phonon-assisted excitation. Among the excitation schemes, optical phonon-assisted excitation enables straightforward laser rejection without a compromise on the source brightness together with a high photon indistinguishability

The impact of an extra background of relativistic particles on the cosmological parameters derived from the cosmic microwave background

Recent estimates of cosmological parameters derived from cosmic microwave background (CMB) anisotropies are based on the assumption that we know the precise amount of energy density in relativistic particles in the Universe, ωrel, at all times. There are, however, many possible mechanisms that can undermine this assumption. In this paper we investigate the effect that removing this assumption has on the determination of the various cosmological parameters. We obtain fairly general bounds on the redshift of equality, . We show that ωrel is nearly degenerate with the amount of energy in matter, ωm, and that its inclusion in CMB parameter estimation also affects the present constraints on other parameters such as the curvature or the scalar spectral index of primordial fluctuations. This degeneracy has the effect of limiting the precision of parameter estimation from the MAP satellite, but it can be broken by measurements on smaller scales such as those provided by the Planck satellite missio

Engineering of quantum dot photon sources via electro-elastic fields

The possibility to generate and manipulate non-classical light using the tools of mature semiconductor technology carries great promise for the implementation of quantum communication science. This is indeed one of the main driving forces behind ongoing research on the study of semiconductor quantum dots. Often referred to as artificial atoms, quantum dots can generate single and entangled photons on demand and, unlike their natural counterpart, can be easily integrated into well-established optoelectronic devices. However, the inherent random nature of the quantum dot growth processes results in a lack of control of their emission properties. This represents a major roadblock towards the exploitation of these quantum emitters in the foreseen applications. This chapter describes a novel class of quantum dot devices that uses the combined action of strain and electric fields to reshape the emission properties of single quantum dots. The resulting electro-elastic fields allow for control of emission and binding energies, charge states, and energy level splittings and are suitable to correct for the quantum dot structural asymmetries that usually prevent these semiconductor nanostructures from emitting polarization-entangled photons. Key experiments in this field are presented and future directions are discussed.Comment: to appear as a book chapter in a compilation "Engineering the Atom-Photon Interaction" published by Springer in 2015, edited by A. Predojevic and M. W. Mitchel

Standardizing type Ia supernovae optical brightness using near-infrared rebrightening time

Accurate standardization of Type Ia supernovae (SNIa) is instrumental to the usage of SNIa as distance indicators. We analyse a homogeneous sample of 22 low-z SNIa, observed by the Carnegie Supernova Project in the optical and near-infrared (NIR). We study the time of the second peak in the J band, t2, as an alternative standardization parameter of SNIa peak optical brightness, as measured by the standard SALT2 parameter mB. We use BAHAMAS, a Bayesian hierarchical model for SNIa cosmology, to estimate the residual scatter in the Hubble diagram. We find that in the absence of a colour correction, t2 is a better standardization parameter compared to stretch: t2 has a 1\u3c3 posterior interval for the Hubble residual scatter of \u3c3\u394\u3bc = (0.250, 0.257) mag, compared to \u3c3\u394\u3bc = (0.280, 0.287) mag when stretch (x1) alone is used. We demonstrate that when employed together with a colour correction, t2 and stretch lead to similar residual scatter. Using colour, stretch and t2 jointly as standardization parameters does not result in any further reduction in scatter, suggesting that t2 carries redundant information with respect to stretch and colour. With a much larger SNIa NIR sample at higher redshift in the future, t2 could be a useful quantity to perform robustness checks of the standardization procedure

Los significados y las prácticas en tres cooperativas agropecuarias de hoy

The agricultural cooperative movement in Argentina has experienced changes that need to be analyzed from cultural interpretations of relationships and social life of the cooperative members in their cooperatives. This paper explores the meanings and practices that are built on cooperatives. The construction of these objects of study is done on the basis of how articulate the conditions of existence of cooperatives and the discourse of the members producers on them. It appears that in general the members of agricultural cooperatives producing scholars argue that the cooperative promotes cooperative values, however the senses that give are different. Mostly partners with profitable agrarian structure itself and imagine a cooperative with differentiated service and corporate look, an image difficult to imagine from the cooperative principies because the emphasis is on the partner and not in the cooperative. For its part, the partners own no land and cost structure as a result of competition that consider experience, imagine the cooperative as a business enterprise, an image that emphasizes the cooperative, a cooperative must seduce the partner. This leaves the cooperative identity narrative which holds that builds cooperative partner rather than the reverse.El cooperativismo agropecuario en Argentina ha experimentado cambios que requieren ser analizados a partir de interpretaciones culturales de las relaciones y la vida social de los socios cooperativistas con sus cooperativas. El presente trabajo indaga los significados y prácticas que se construyen en las cooperativas sobre las cooperativas. La construcción de estos objetos de estudio se realiza atendiendo de manera articulada las condiciones de existencia de las cooperativas y el discurso de los socios - productores sobre ellas. Se observa que en general los socios productores de las cooperativas agropecuarias estudiadas sostienen que su cooperativa promueve los valores cooperativos, sin embargo los sentidos que le otorgan son diferentes.  Mayormente los socios con estructura agraria propia y rentable imaginan una cooperativa con atenciones diferenciadas y mirada empresarial, una imagen difícil de imaginar desde los principios cooperativos porque el énfasis está en el socio y no en la cooperativa. Por su parte, los socios sin estructura agraria propia y rentable, como consecuencia de la competencia que consideran que experimenta, imaginan la cooperativa como una empresa comercial; una imagen que pone énfasis en la cooperativa, una cooperativa que debe seducir al socio. Con esto abandona el relato identitario cooperativo que sostiene que el socio construye la cooperativa y no al revés