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 24 publications 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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  • 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.

  • The identification of human Colon Stem Cells and Colon Cancer Stem Cells (2011)

    Batlle Gómez, Eduard (IRB Barcelona)

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    The identification of human Colon Stem Cells and Colon Cancer Stem Cells

    The inner layer of the intestinal tube, the intestinal epithelium, is in a constant process of renewal. Hundreds of millions of terminally differentiated intestinal cells are replaced by new cells every day during the life of an adult organism. This tremendous regenerative power is ultimately sustained by a small population of intestinal stem cells. It is believed that alterations in the biology of human colon stem cells (CoSCs) account for the pathophysiology of various large-bowel disorders, including colorectal cancer (CRC). Yet, the identification of human CoSCs had remained elusive. In these manuscripts we describe for the first time the isolation of stem cells of the human colonic epithelium. Differential cell surface abundance of the receptor EPHB2 allows the purification of different cell types from human normal colon mucosa biopsies. Colon epithelial cells with highest EPHB2 levels exhibit the longest telomeres and express markers characteristic of intestinal stem cells. Using culturing conditions that recreate the intestinal stem cell niche, a substantial proportion of EPHB2-high cells can be expanded in vitro as an undifferentiated and multipotent population. This discovery has profound implications for the field of regenerative medicine as it opens the possibility of using CoSCs as a therapeutic tool. In parallel, my group applied the same method of stem cell purification to human CRCs. These experiments led us to demonstrate that most human CRCs are constituted by cell populations with phenotypes similar to either CoSCs or intestinal differentiated cells organized into well-defined compartments. CoSC-like cells purified from primary CRCs generate tumors in immunodeficient mice with high efficiency and display both self-renewal and differentiation capacity. Remarkably, the expression of the CoSC gene program predicts disease relapse after intended curative therapy in CRC patients. Overall, these results imply that CRC shares a common hierarchy with the intestinal mucosa and that the acquisition of an intestinal stem cell gene program is a central process in the development of metastatic and recurrent CRC.

  • At the origins of human rationality: how infants reason about future unknown events. (2011)

    Bonatti, Luca (UPF)

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    At the origins of human rationality: how infants reason about future unknown events.

    Humans excel at reasoning about novel situations, flexibly combining abstract knowledge and perceptual information from disparate sources in "one-shot" intuitions to predict outcomes of events they have never before directly experienced. To investigate the root of this ability, we study how infants reason in very simple, but completely novel situations which contain potentially conflicting information. Consider a simple situation in which three yellow objects and one blue object bounce inside a container with an opening on its lower side, as in a lottery machine (Figure 1). At a certain point, an occluder hides the container. You have to bet which object will exit first: will it be blue or yellow? The right answer is: it depends.
    The scene contains different kinds of information: about the properties of the objects, such as their colors; about the number of objects in the scene; or about the dynamics of their trajectories, collisions and changes in speed. All such potentially conflicting cues can be relevant to form optimal expectations about which object will exit the container. What is crucial is that the most relevant information can change according to subtle changes in how the scene unfolds.

    We showed prelinguistic infants such lottery-like scenes, by systematically varying the length of the occlusion prior to the exit of the object. When the occlusion was long, infants were surprised when an object of the less numerous category exited the container, irrespectively of its distance from the exit before the occlusion. This makes sense, because if the occlusion is long, the last position of an object is not relevant to predict what object will exit, as objects keep moving behind the occluder. Instead, when they saw the same situation but the occlusion was very short, infants were surprised when a far object exited, regardless of its color. Also this makes sense, because if an object is far from the exit, it becomes physically impossible for it to travel the distance to the exit during the short occlusion. Thus, infants have rational "gut feelings" about future events.
    In our study, we also elaborated a Bayesian ideal observer model which precisely predicts infants' looking times in our experiments and extends to other known results about infant cognition, providing for the first time a unifying explanation of several classic findings. This model shows how powerful pure reasoning capacities could derive from the operation of probabilistic inference mechanisms constrained by abstract principles of how

  • Entangled Photon Stored in Solid (2011)

    de Riedmatten, Hugues (ICFO)

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    Entangled Photon Stored in Solid

    Entanglement is one of the essential characteristics of quantum physics. It leads to very strong correlations between quantum particles that cannot be explained using classical physics. Beyond its fundamental significance, entanglement is also a crucial resource for quantum information science. For example, the strong correlations can be used to perform quantum key distribution, or to distribute an unknown quantum state via quantum state teleportation. As a result, a strong experimental effort is currently devoted worldwide to harnessing entanglement between various physical systems. In this context, entanglement between light and material systems is particularly interesting because it enables a connection between light based flying quantum bits that can be used to communicate over long distances, and matter based stationary quantum bits that can be used to store and process quantum information.
    In this paper, we demonstrate for the first time an entanglement experiment between a photon and a macroscopic, cm long crystal. This is realized by mapping one photon of an entangled pair onto a collective atomic excitation stored in the crystal and shared over ~ 1 billion Neodymium ions. The crystal is cooled down to 3 K in a cryostat in order minimize interactions between the Neodymium atoms and the crystal phonons that would destroy quantum coherence. The successful mapping of entanglement is proved by the violation of a Bell inequality.
    These results represent an important step towards quantum information technologies based on solid-state devices, in particular for quantum repeaters and long-distance quantum information networks. The experimental work has been performed at the University of Geneva. The research has been published in Nature, and highlighted in a News&Views article by Dr Jevon Longdell [2].