Highlights

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.

LIST OF SCIENTIFIC HIGHLIGHTS

Format: yyyy

  • Generation of a macroscopic singlet state (2014)

    Mitchell, Morgan W. (ICFO)

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    Generation of a macroscopic singlet state

    The ancient Greek philosopher Epicurus posed the deeply scientific question "what microscopic entities make up the world we see ?"  His speculations gave us the idea of atoms, and indeed the word "atom."  Only in the 20th century did measurements of Brownian motion, and arguments by Albert Einstein, prove beyond any doubt the existence of atoms.  Today, a twin question continues to resist rational explanation and to challenge our best experimentalists.  The question is "how do these microscopic entities  produce the behaviour we see ?"Striking macroscopic phenomena, including high-temperature superconductivity and exotic magnetic phases, are thought to be produced by large-scale entanglement of spins in matter. These massively entangled states cannot, however, be simulated even on our most powerful computers, so the question remains open whether these hypothesized states of matter really exist. A promising experimental approach, known as quantum simulation, aims to produce and study highly-entangled states in artificial material systems.  Here we have taken a step in this direction, by producing a "macroscopic spin singlet," (MSS) in a cloud of about one million cold atoms.  The MSS is an archetypal entangled state, consisting of a macroscopic number of atoms organized into spin singlets, the same kind of entanglement believed to underlie superconductivity and some exotic magnetic phases.  Our experiment found the majority of the atoms, more than 500.000, participating in the entangled state.  The article, published in Physical Review Letters, highlighted by the editors, and popularized in Scientific American, describes how we used quantum non-demolition measurements to both produce and detect large-scale entanglement.  This brings a very powerful quantum optical technique to the study of quantum many-body physics.

  • Unveiling the mechanisms controlling the regenerative capacity loss during aging (2014)

    Muñoz-Cánoves, Pura (UPF)

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    Unveiling the mechanisms controlling the regenerative capacity loss during aging

    Dr. Pura Muñoz-Cánoves’ Group recently published in Nature the description of an irreversible ageing process responsible for the loss of regeneration capacity in very old muscle stem cells. In geriatric mice, corresponding to humans of 75 years of age and beyond, muscle stem cells were found to lose their regenerative capacity due to activation of a signaling pathway associated with cellular senescence, a state characterized by the incapacity of the cell to divide. Similar processes may also be involved in muscle degeneration associated with advanced ageing in humans (sarcopenia).Regeneration of skeletal muscle depends on a population of adult stem cells (satellite cells) that are normally in a dormant ‘quiescent’ state, which allows the cells to be activated and divide in response to damage or stress, to eventually form new muscle fibers and repair the damaged muscle. The regenerative functions of these cells are known to decline with ageing. The causes, however, are poorly understood. Pura Muñoz-Cánoves and colleagues have reported that geriatric satellite cells suffer intrinsic irreversible changes that cause the cells to switch from a quiescent state (responsive to external damage and allowing cell expansion) into a permanent senescent state (where satellite cell expansion no longer occurs). This work shows that satellite in old mice (up to 24 months of age) maintain this quiescent state through repressing the expression of p16INK4a (an inhibitor of cell division and proliferation which also induces cell senescence).  In contrast, satellite cells in geriatric mice (28 months and beyond) no longer repress p16INK4a , and this drives cells to enter senescence, constituting a point of “no-return”. Geriatric satellite cells are therefore refractory to activation by tissue damage and incapable of forming new muscle.As cellular senescence mediated by 16INK4a  is also dysregulated in human geriatric muscle stem cells, the findings may provide a basis for attenuating loss of muscle regenerative capacity in geriatric humans.  

  • Infall-driven Protostellar Accretion and the Solution to the Luminosity Problem. (2014)

    Padoan, Paolo (UB)

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    Infall-driven Protostellar Accretion and the Solution to the Luminosity Problem.

    In this work we provide a definitive solution to the protostellar luminosity problem. For many years, astronomers have been puzzled by the low luminosity of protostars, the progenitors of stars. They have also wondered about the origin of the large scatters in the observed luminosities. We have shown that the problem arised from an oversimplification of current star formation models. We overcame the problem, and were able to accuratly reproduce the observed protostellar luminosities, by developing the largest star formation simulation to date, thanks to a very large supercomputer award from NASA High-End Computing.

  • Entanglement between spin and pseudospin in graphene: An unprecedented effect yielding spin relaxation and spin manipulation possibility (2014)

    Roche, Stephan (ICN2)
    Valenzuela, Sergio O. (ICN2)

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    Entanglement between spin and pseudospin in graphene: An unprecedented effect yielding spin relaxation and spin manipulation possibility

    The prospect of transporting spin information over long distances in graphene, possible because of its small intrinsic spin–orbit coupling and vanishing hyperfine interaction, has stimulated intense research exploring spintronics applications. However, measured spin relaxation times are orders of magnitude smaller than initially predicted, while the main physical process for spin dephasing and its charge-density and disorder dependences remain unconvincingly described by conventional mechanisms.

    Our team has unraveled an unprecedented spin relaxation mechanism for non-magnetic samples that follows from an entanglement between spin and pseudospin driven by random spin-orbit coupling, unique to graphene.

    The mixing between spin and pseudospin-related Berry's phases results in fast spin dephasing even when approaching the ballistic limit, with increasing relaxation times away from the Dirac point, as observed experimentally. The origin of spin-orbit coupling can stem from adatoms, ripples or even the substrate, suggesting novel spin manipulation strategies based on the pseudospin degree of freedom.

    Such possibilities suggest unprecedented approaches for the emergence of non-charge-based information processing and computing, resulting in a new generation of active (CMOS compatible) spintronic devices together with non-volatile low-energy MRAM memories. This phenomenon not only revisits years of experimental and theoretical controversies, but also opens a new window into the formidable challenge of manipulating spin degree of freedom in future information-processing technologies.

  • Catalase-peroxidase reaction mechanism deciphered (2014)

    Rovira Virgili, Carme (UB)

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    Catalase-peroxidase reaction mechanism deciphered

    When it comes to enzymes and hydrogen peroxide, there are catalases and peroxidases. The former degrade hydrogen peroxide; the latter use it to oxidize other molecules. The heme-containing catalase-peroxidases (KatGs) perform both activities, but as the enzymes’ active sites tend to resemble peroxidases (which have poor catalase activity), it has never been clear how they work. Now Ignacio Fita, Carme Rovira  and colleagues describe a reaction mechanism that resolves this mystery.Traditional catalases degrade hydrogen peroxide via a mechanism involving a heme group and proton transfer from a key histidine residue. KatGs lack that amino acid, but they do have a methionine-tryptophan-tyrosine (MWY) “covalent adduct” near the heme ring, as well as a critical arginine residue that alternates between “in” and “out” orientations. Using quantum mechanical/molecular mechanics calculations and x-ray crystallography of the active site, the authors propose an eight-step mechanism in which the MWY complex and mobile arginine act as an “electronic switch” that drives conversion of hydrogen peroxide to water and oxygen.

  • 60-year old theory from Alan Turing explains finger formation (2014)

    Sharpe, James (CRG)

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    60-year old theory from Alan Turing explains finger formation

    Alan Turing, the British mathematician (1912-1954), is famous for a number of breakthroughs: the foundations of computer science, cracking the Enigma code, and helping to found the field of artificial intelligence. His contribution to mathematical biology is much less famous, but was no less profound. He published just one paper (1952), but it triggered a whole new field of mathematical enquiry into pattern formation. He discovered that a system with just 2 reacting and diffusing molecules could spontaneously self-organise into repetitive spatial patterns of concentrations, such as spots or stripes. His theory has come to be accepted as an explanation of simple patterns such as zebra stripes and leopard spots, but in embryology it has been resisted for decades as an explanation of how structures such as fingers are formed. The group of ICREA Research Prof. James Sharpe at the CRG in Barcelona, has now provided data supporting the idea that fingers and toes are indeed patterned by a Turing mechanism. They had previously provided evidence that Hox genes and FGF signaling modulated a hypothetical Turing system (Science 338:1476, 2012), but the identity of the Turing molecules themselves was still a mystery. The new study completes the picture, by revealing which signaling molecules act as the Turing system. It was achieved by a systems biology approach – combining experimental work with computational modelling. In this way, the two equal-first authors of the paper were able to iterate between the empirical and the theoretical: the lab-work of Jelena Raspopovic providing experimental data for the model, and the computer simulations of Luciano Marcon making predictions to be tested back in the lab. By screening for the expression of many different genes, they found that two signalling pathways stood out as having the required activity patterns: BMPs and WNTs. They gradually constructed the minimal possible mathematical model compatible with all the data, and were able to make computational predictions about how inhibiting these 2 pathways should alter the pattern of fingers. Strikingly, when the same experiments were done on small pieces of limb bud tissue cultured in a petri dish the same alterations in embryonic finger pattern were observed, strongly supporting the hypothesized model. Their result goes beyond the question of finger development. It challenges the dominance of an important theory for embryology called positional information, which states that cel

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