Every year, a committee of experts sits down with a tough job to do: from among all ICREA publications, they must find a handful that stand out from all the others. This is indeed a challenge. The debates are sometimes heated and always difficult but, in the end, a shortlist of  the most outstanding publications of the year is produced. No prize is awarded, and the only additional acknowledge is the honour of being chosen and highlighted by ICREA. Each piece has something unique about it, whether it be a particularly elegant solution, the huge impact it has in the media or the sheer fascination it generates as a truly new idea. For whatever the reason, these are the best of the best and, as such, we are proud to share them here.


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  • Seeing light by electrons (2012)

    van Hulst, Niek F. (ICFO)

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    Seeing light by electrons

    "Fiat Lux - he saw the light and it was good.......". Indeed light is at the basis of our sensations and observations. Yet, oddly enough, light has its limits in seeing how light and matter interact. Specifically, the behaviour of light at the nanoscale is hard to discern by light microscopy. Here we show that electrons are an effective alternative to image light at the true nanoscale. Exciting by electrons, while still detecting light, we have probed the inside of photonic crystals and mapped how light behaves with a spatial resolution of 30 nanometer, far below the wavelength of light.

    Photonic crystals are nanostructures in which two materials with different refractive index are arranged in a regular pattern, giving rise to exotic optical properties. Natural photonic crystals can be found in certain species of butterflies, birds and beetles as well as in opal gemstones where they give rise to beautiful iridescent colors. Due to major advances in nanofabrication techniques it has become possible to fabricate artificial photonic crystals with optical properties that can be accurately engineered. These structures can be used to make high-quality nanoscale optical waveguides and cavities, which are important in telecommunication and sensing applications.

    We constructed 2-dimensional photonic crystals by etching a hexagonal pattern of holes in a very thin silicon nitride membrane. The photonic crystal inhibits light propagation for certain colors of light, which leads to strong reflection of those colors. By leaving out one hole a very small cavity can be defined where the surrounding crystal acts as a mirror for the light, making it possible to strongly confine light within such a "crystal defect cavity".

    Using the electron approach we can now see the finest details of photonic crystals that were simply inaccessible before. Seeing where the various colours are trapped in the cavity provides direct insight into the light - matter interaction on the nanoscale. Such understanding is crucial for the development of enhanced optical devices such as bio-sensors for healthcare and more efficient solar cells and displays.

  • Modernity - understanding the present (2012)

    Wagner, Peter (UB)

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    Modernity - understanding the present

    We live in a modern age, but what does `modern' mean and how can a reflection on `modernity' help us to understand the world today? These are the questions that Peter Wagner sets out to answer in this concise and accessible book.

    Wagner begins by returning to the question of modernity's western origins and its claims to open up a new and better era in the history of humanity. Modernity's claims and expectations have become more prevalent and widely shared, but in the course of their realization and diffusion they have also been radically transformed. In an acute and engaging analysis, Wagner examines, among others, the following key issues:

    * Modernity was based on the hope for freedom and reason, but it created the institutions of contemporary capitalism and democracy. How does the freedom of the citizen relate to the freedom of the buyer and seller today? And what does disaffection with capitalism and democracy entail for the sustainability of modernity?

    * Rather than a single model of modernity, there is now a plurality of forms of modern socio-political organization. What does this entail for our idea of progress and our hope that the future world can be better than the present one?

    * All nuance and broadening notwithstanding, our concept of modernity is in some way inextricably tied to the history of Europe and the West. How can we compare different forms of modernity in a `symmetric', non-biased or non-Eurocentric way? How can we develop a world-sociology of modernity?

    This book is the first major publication from the European Research Council-funded research project "Trajectories of modernity: comparing non-European and European varieties" (TRAMOD), which is based in the Department of Sociological Theory, Philosophy of Law and Methodology of the Social Sciences at the University of Barcelona and of which Peter Wagner is the Principal Investigator. The project TRAMOD aims at advancing the analysis of the contemporary plurality of self-understandings and related institutional structures of societies and polities in the current global context. It analyzes these self-understandings against the background of the historical trajectories of those societies. The analysis of the existing multiple forms of modernity is the major challenge to current social and political theory and comparative-historical and political sociology. It requires a conceptual and empirical analysis of that which is common to different forms of modernity and that which varies between them.

  • Europe's Oldest Cave Art (2012)

    Zilhão, João (UB)

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    Europe's Oldest Cave Art

    Dating rock art has traditionally been made by indirect methods, such as stylistical comparison, or, when buried by sediments that have an archeological content or can themselves be dated, stratigraphic context. Over the last two decades, radiocarbon has been applied to obtain direct dates, namely to black paintings containing organic pigments (mostly charcoal), but most rock art comes in the form of engravings or paintings made with mineral pigments (black manganese, yellow or red ochre). Moreover, curatorial concerns limit the size of such radiocarbon samples, which makes it difficult to deal with decontamination, and an offset may exist between the sample's date and the time of painting (e.g., fossil bone collected from the floor of a cave could have been used many thousands of years later as fuel for the production of the charcoal used for pigment).

    Technical developments in the Uranium-series dating of speleothems, allowing age measurements for samples as small as 10 mg, make it possible to circumvent the problems with radiocarbon and obtain secure minimum dates for Paleolithic art covered by thin calcite films. We applied the technique to a number of sites in Northern Spain, including the World Heritage properties of El Castillo, Altamira and Tito Bustillo. The age of the calcite covering red disks at El Castillo places their execution at a time before 41,000 years ago, possibly significantly earlier. Ages in excess of 37,000 years have also been obtained for calcite-covered hand stencils in the same panel. These results show that the walls of European caves were being decorated many millennia before the time indicated by traditional methods. Additionally, the great antiquity of this cave art implies a strong probability that the first cave painters were the Neanderthals, with attendant implications for current debates concerning their behavioral and cognitive capabilities and evolutionary status -- as a different species altogether or, as suggested by the recent genetic and fossil evidence, simply as a geographical variant of early Homo sapiens.

  • Device-independent quantum key distribution (2011)

    Acín Dal Maschio, Antonio (ICFO)

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    Device-independent quantum key distribution

    A central problem in cryptography is the distribution among distant users of secret keys, which can be used for the secure encryption of messages. This task is impossible in classical cryptography unless assumptions are made on the computational power of the eavesdropper. Quantum key distribution (QKD), on the other hand, offers security against adversaries with unbounded computing power. The ultimate level of security provided by QKD was made possible thanks to a change of paradigm. While in classical cryptography security relies on the hardness of certain mathematical problems, in QKD it relies on the fundamental laws of quantum physics. A side-effect of this change of paradigm, however, is that whereas the security of classical cryptography is based on the mathematical properties of the key itself, in QKD, the security crucially depends on the physical properties of the key generation process. But then, how can one assess the level of security provided by a real-life implementation of QKD, which inevitably differs in inconspicuous ways from the idealized, theoretical description? In fact, technological imperfections have recently been exploited to hack QKD commercial products.

    Device-independent QKD (DIQKD) aims at closing the gap between theoretical analyses and practical realizations of QKD by designing protocols whose security does not require a detailed characterization of the devices used to generate the secret key. These devices are just seen as quantum black-boxes, see the Figure, generating outputs given some inputs. This stronger form of cryptography is possible if it is based on the observation of non-local quantum correlations. In some sense, DIQKD combines the advantages of classical and quantum cryptography: security against unbounded adversaries based on the law of quantum physics but which does not rely on the physical details of the generation process.

    In our work, we provide a general formalism for proving the security of DIQKD protocols. The DIQKD model that we consider, however, is partly restricted as it supposes that the measurement processes generating the different bits of the raw key are causally independent of each other. This independence condition may be hard to meet in practice, but it is assumed in any of the existing security proofs, including those for standard QKD. Our analysis therefore shows that secure QKD is in principle possible independently of the internal working of the devices used in the protocol.

  • A close look to the atoms: advanced electron microscopies the eyes of nanoscience (2011)

    Arbiol Cobos, Jordi (CSIC - ICMAB)

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    A close look to the atoms: advanced electron microscopies the eyes of nanoscience

    New materials for future applications are nowadays being synthesized at nanoscale (ultrathin layers, complex nanoparticles, nanowires, soft matter nanostructures or nanotubes, functionalized for novel applications). As developments in Materials Science are pushing to the size limits of physics and chemistry, there is a critical need for the structural, chemical and morphological characterization of the synthesized nanostructures at atomic scale in order to correlate these results with the physical and chemical properties and functionalities they have. In this way, a worldwide increasing interest for electron microscopy is emerging. Imagine being able to hold an electron beam over a single atom for 2 entire seconds in order to actually directly SEE and acquire information. The answer nowadays is "YES, we can make it and SEE single atoms!". The advent of aberration-corrected transmission electron microscopy technology is now giving resolutions below 0.05 nanometers enabling single atoms to be directly viewed and analyzed. The Group of Advanced Electron Nanoscopy at ICMAB-CSIC, leaded by Prof. Jordi Arbiol is pushing the resolution of electron microscopy to the limits. As consequence of this, looking face to face at single atoms is the daily task of this group. Despite that the advanced facilities they require are placed at the moment abroad, the multiple collaborations they have with some of the top most labs and universities are resulting in the consecution of great results as the ones highlighted here: as visualizing for the first time the atomic structure of dreamed complex nanoparticles, untangling new growth mechanisms or novel electronic properties of nanomaterials or obtaining the structure and chemical properties at the atomic scale of novel nanostructures.

  • Circadian rhythms regulate epidermal stem cell activity (2011)

    Aznar Benitah, Salvador (CRG)

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    Circadian rhythms regulate epidermal stem cell activity

    Adult stem cells maintain tissue homeostasis by continuously replenishing non-functional cells with healthy ones. Most stem cells identified to date are compartmentalized in functionally deterministic niches. From there stem cells are instructed by unique combinations of signals and spatial tensile forces which they translate into a specific behavior. However, how stem cells spatiotemporally coordinate their intrinsic stem cell potential with niche- and systemic cues is still poorly understood. These issues are essential for proper tissue function, since perturbations in the signals that govern stem cell function can cause tissue malfunction such as tumorigenesis and ageing.

    We have identified a mechanism of stem cell regulation based on circadian rhythms. The circadian machinery anticipates and synchronizes the daily function of tissues according to the entrainment by natural changes in light and metabolism. We have observed that the molecular clock fine-tunes the behavior of epidermal stem cells by imposing transcriptional oscillations in the expression of stem cell regulatory genes. This ¨circadian transcriptome signature¨ includes genes involved in development, metabolism, drug-metabolism, and cell invasion and motility, among others. Circadian oscillation of these genes provides epidermal stem cells with a spatiotemporal axis for responding to dormancy, activating, and pro-differentiation cues. Interestingly, circadian regulation creates populations of stem cells co-existing within the same niche which are more predisposed than others to respond to activating cues. In addition, the clock imposes a proper timing of epidermal stem cell function by ensuring that they primarily respond to harmful UV radiation during the hours of peak exposure to light, to subsequently and sequentially become responsive to proliferation and pro-differentiation cues in the evening and night.

    We hypothesize that this mechanism ensures that the right number of stem cells respond at a given time, avoiding unnecessary proliferation and its associated risk of DNA damage. Also, by timing the function of stem cells this mechanism prevents their proliferation in instances of high risk of DNA damage at the hours of peak UV exposure, but stimulates their activation and differentiation when enough energy supply is available. Notably, forced circadian arrhythmia in epidermal stem cells causes severe tissue ageing and modifies the predisposition to tumorigenesis.