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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  • Why is optical refractive index so small? (2021)

    Chang, Darrick (ICFO)

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    Why is optical refractive index so small?

    The refractive index of a material directly determines the minimum size that we can construct optical devices and the smallest length scales that light can resolve. Despite the game-changing implications that an ultrahigh index material would have, it is interesting to observe that all optical materials that we know of have an index that is of order unity. Surprisingly, a deep understanding of the mechanisms that lead to this universal behavior seems to be lacking. Moreover, this observation is difficult to reconcile with the fact that a single isolated atom is known to have a giant optical response, as characterized by a resonant scattering cross section that far exceeds its physical size. We have developed a theory of the maximum index of a disordered atomic ensemble. Interestingly, despite the giant response of an isolated atom, we find that the maximum index does not indefinitely grow with increasing density but rather reaches a limiting value of n ≈ 1.7. This limit arises from the highly non-perturbative nature of multiple light scattering and near-field optical interactions in such a system. Our work is a promising first step to understand the limits of refractive index from a bottom-up, atomic physics perspective. Furthermore, identifying the limiting mechanisms should also pave the way to developing circumventing strategies, in order to realize an ultrahigh index material and open up vast new technological possibilities with light.

  • Disciplinary, gender and geographical biases dominate the production of science about ecosystem conservation and poverty alleviation  (2021)

    Corbera Elizalde, Esteve (UAB)

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    Disciplinary, gender and geographical biases dominate the production of science about ecosystem conservation and poverty alleviation 

    Research into the relationship between ecosystem services, e.g. environmental goods and functions such as carbon sequestration, and human well-being, including poverty alleviation, has blossomed since the early 2000s. We have explored who has produced this knowledge, what collaborative patterns and institutional and funding conditions have underpinned it, and what implications these matters may have for policy and the future of science. Developing a social network analysis of the most prolific writers in this field of knowledge, we have demonstrated that 70% of these authors are men, most are trained in either the biological sciences or economics and almost none in the humanities. Eighty per cent of authors obtained their PhD from universities in the EU or the USA, and they are currently employed in these regions. The co-authorship network is strongly collaborative, without dominant authors, and with the top 30 most cited scholars being based in the USA and co-authoring frequently. These findings may not be surprising to many, of course. They reflect the same geographical and gender biases that characterize knowledge production in other fields, as well as an expertise bias towards natural sciences, economics and engineering that also characterizes the study of other environmental matters, such as energy transitions, or pollution management research, which in turn invisibilizes the potential contribution that other social sciences and the humanities can make in understanding environment-human relations. Overall, the research suggests that embracing gender diversity, promoting extensive collaboration across disciplines, and being committed to knowledge co-production with affected populations is urgent to better understand and address the environmental and human challenges of our time, particularly amidst increasing calls for the decolonisation of conservation and development science.  

  • How do malaria parasites defend themselves from febrile temperatures? (2021)

    Cortés Closas, Alfred (ISGlobal)

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    How do malaria parasites defend themselves from febrile temperatures?

    Periodic fever is the most characteristic clinical symptom of human malaria, but how malaria parasites survive febrile temperatures was not known until now. Here we identified and characterized a transcription factor that regulates the protective response to febrile temperatures in Plasmodium falciparum, the parasite that causes the most severe forms of human malaria. The response of an organism to high temperature is called the heat shock response.

    This transcription factor, which we termed PfAP2-HS, activates the expression of a very small number of genes when the parasite is exposed to febrile temperatures. These genes mainly encode chaperones, which help to maintain proteins correctly folded. A gene encoding HSF1, the conserved regulator of the eukaryotic heat shock response from yeast to humans, is absent from the genome of malaria parasites. PfAP2-HS does not have any structural or sequence similarity with HSF1, but it plays an analogous function activating the expression of chaperone-encoding genes at high temperature.

    Several malarial transcription factors had been previously characterized, but all of them were found to be involved in regulating parasite development. PfAP2-HS is the first transcription factor identified in malaria parasites that drives a rapid protective response to a condition of the environment. In addition to regulating the malarial heat shock response, PfAP2-HS also plays an important role under basal (non-stress) conditions: parasites engineered to lack the PfAP2-HS protein were not only hypersensitive to febrile temperatures, but they also grew poorly at 37ºC, the physiological temperature for P. falciparum, and had to be cultured at 35ºC. These parasites could not maintain their proteins correctly folded even under optimal growth conditions. More importantly, they showed much higher sensitivity to the frontline antimalarial drug artemisinin, indicating that PfAP2-HS is needed for artemisinin resistance.

    Our findings settle a long-standing controversy about whether malaria parasites are able to produce immediate, protective transcriptional responses when changes in their environment occur: yes, they are!

  • Super resolution microscopy reveals how DNA supercoiling shapes the genome (2021)

    Cosma, Maria Pia (CRG)

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    Super resolution microscopy reveals how DNA supercoiling shapes the genome

    The biological mechanisms that enable the DNA to be squeezed into a tight space in each human cell is matter of deep investigation. In this condensed state, the DNA, also known as chromatin, contains many loops that bring together different regions of the genome that would normally be far apart. The resulting physical proximity is important for transcribing DNA into RNA which then makes proteins, making chromatin looping a fundamental biological mechanism for human health and disease.

    In this work we pictured individual loops of DNA. The images reveal how the human genome organises itself in three-dimensional space at much higher resolution than previously possible. Moreover, we found that DNA supercoiling, which is produced as a byproduct of transcription activity, generates a force that causes structural proteins known as cohesins to ‘surf’ across DNA strands and generate loops, finally morphing the overall shape of the genome.

  • SARDO: Rescuing Lost Victims through their Mobile Phones with Drones (2021)

    Costa Perez, Xavier (i2CAT)

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    SARDO: Rescuing Lost Victims through their Mobile Phones with Drones

    Natural disasters affect millions of people every year. Finding missing persons in the shortest possible time is of crucial
    importance to reduce the death toll. This task is especially challenging when victims are sparsely distributed in large and/or
    difficult-to-reach areas and cellular networks are down.

    Unmanned Aerial Vehicles (UAVs) or Drones have recently emerged as a cost-efficient alternative to address emergency scenarios for multiple reasons. First, UAVs can be rapidly deployed in disaster areas providing on-demand mobile networks. Second, UAVs may rapidly approach difficult-to-reach locations, such as mountains, deserts, or devastated areas and cover large search areas with sparse victims distribution. Finally, given the high penetration rate of mobile devices in our society, it can be reasonably assumed that victims are equipped with smart devices, e.g., smart phones and wearables, that can be detected by UAV mobile networks.

    In this work we present SARDO, a drone-based search and rescue solution that leverages the high penetration rate of mobile phones
    in the society to localize missing people. SARDO is an autonomous, all-in-one drone-based mobile network solution that does not
    require infrastructure support or mobile phones modifications. It builds on novel concepts such as pseudo-trilateration combined with
    machine-learning techniques to efficiently locate mobile phones in a given area.

    Our results, with a prototype implementation in a field-trial, show that SARDO rapidly determines the location of mobile phones (~3 min/UE) in a given area with an accuracy of few tens of meters and at a low battery consumption cost (~5%).

    State-of-the-art localization solutions for disaster scenarios rely either on mobile infrastructure support or exploit onboard cameras for
    human/computer vision, IR, thermal-based localization. To the best of our knowledge, SARDO is the first drone-based cellular
    search-and-rescue solution able to accurately localize missing victims through their mobile phones.

  • Towards quantum repeaters: Telecom heralded entanglement between solid-state quantum memories (2021)

    de Riedmatten, Hugues (ICFO)

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    Towards quantum repeaters: Telecom heralded entanglement between solid-state quantum memories

    Entanglement is one of the main properties of quantum physics, which leads to very strong correlations between spatially separated systems. The distribution of entanglement between the nodes of a quantum network will allow new advances e.g. in long distance quantum communication, distributed quantum computing and quantum sensing. On the ground, quantum information can be distributed across the nodes using single photons at telecommunication wavelengths traveling in optical fibers. The maximal distance that can be reached in optical fibers with direct transmission is however limited to a few hundred km due to the loss in the fiber. In classical communications, this problem is solved by placing amplifiers every 50 to 100 km in the fiber network (so called repeaters). However, this kind of repeaters cannot be used with quantum bits, due to the unavoidable noise they produce. One solution to reach longer, continental distances is to use quantum repeaters which use entanglement between quantum memories as main building block.

    In a study published in Nature that was featured on the cover of the issue, ICFO researchers led by ICREA Prof. Hugues de Riedmatten , have achieved entanglement between two solid-state quantum memories located in different laboratories 10 m appart. The entanglement was heralded by a photon at teleccomunication wavelength and stored in the quantum memories in a multiplexed fashion.  These two key features have been achieved together for the first time and define the stepping stone in extending this scheme to much longer distances.

    These results constitute an important milestone towards the practical realization of quantum repeaters using the installed fiber optic telecom network.