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A walk across a quantum lattice A simple two-atom system is used to probe complex quantum phenomena “Since the early days of quantum theory, the wave nature of particles has been essential for the description of single-particle properties in the quantum regime.”

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sible for the speed-up of quantum search algorithms relative to classical search algorithms. Preiss et al. use the quantum walk, which has been observed in various systems, including single neutral atoms (3), to study the influence of quantum statistics and interactions in a two-body quantum system (see the figure). With additional identical particles added to the system, two effects must be taken into account: (i) in the absence of interactions, quantum statistics determining the distribution of particles across the quantum states available; and (ii) attractive or repulsive interactions between particles. Quantum statistics divides the many-body properties of indistinguishable particles into two classes, depending on the value of their total spin. Particles with halfinteger spin value, fermions, obey Pauli’s exclusion principle, imposing a total antisymmetric wave function of a many-body system under exchange of any two particles. In contrast, particles with integer spin value, bosons, have symmetric wave functions under particle exchange. The consequences of the wave function’s symmetry are dramatic: Electrons in the conduction band of a metal have much higher energies than expected from the metal’s

Physics Department and State Research Center, Kaiserslautern University, 67663 Kaiserslautern, Germany. E-mail: [email protected]

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lattice position is dominated by matter wave interference. (B) The effect of quantum statistics on the dynamics of two particles. Bosons tend to bunch and can be found close to each other, while fermions antibunch and avoid each other. (C) Strong interactions can mimic an exclusion principle for bosons. For strong repulsion, the atoms avoid each other and can be described as weakly interacting fermions. sciencemag.org SCIENCE

13 MARCH 2015 • VOL 347 ISSUE 6227

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uantum effects emerge from an intricate interplay of the wave nature of massive particles, quantum statistics, and interactions in many-body systems. Each individual aspect can lead to nonclassical behavior and is often well understood; in many-body systems, however, it can be hard to identify the mutual influence of these three aspects on a single-particle level. On page 1229 of this issue, Preiss et al. (1) present an exoscillations. In pure systems, however, perimental study illustrating the effect of such as cold atoms in optical traps (2) they quantum statistics as well as of various have been observed. Preiss et al. demoninteraction strengths on the matter wave strate such Bloch oscillations, showing a quantum dynamics of two ultracold atoms. dispersion of the atomic probability distriStarting from the matter wave dynamics of bution in position space that subsequently a single atom, adding a second atom allows refocuses to its initial position after one them to observe the two-body dynamics oscillation period, yielding an illustrative of interacting bosons. By controlling the example of particle-wave duality. Furtherinterparticle interaction, they are able to more, matter wave interference in a peribring the atoms into the strongly interactodic potential also modifies the diffusion ing regime, where the system resembles properties of a quantum particle. For a diftwo weakly interacting fermions. The work fusion time t, the probability of finding a by Preiss et al. allows the role of different classical particle at a specific distance from quantum mechanisms to be identified. It the starting position on a line grows as also constitutes a step toward the study ˇñt ; for a quantum system, the probability of more complex quantum systems in a grows proportional to t. It is this difference controlled fashion, where the level of comin the time dependence that is responplexity is not accessible to classical simulations. Since the early days of quantum Atoms undergoing a quantum walk in an optical lattice theory, the wave nature of particles has been essential for the descripMatter wave interference Quantum statistics Interactions tion of single-particle properties in the quantum regime. It lies at the A B C heart of modern electron microscopes, where the resolution can be adjusted via the electrons’ small de Broglie wavelength. In a periBosonic Fermionic odic potential, such as for electrons bunching antibunching moving in the periodic potential of ion cores, matter wave interference has surprising consequences. The first is Bloch oscillations, which are oscillations of a quantum particle within a small length scale in position space when it is exposed to an external force. In typical solid-state Position Position Position systems, scattering of electrons off impurities destroys such quantum Walking and interacting. (A) A single atom undergoing a quantum walk. The probability of finding the atom at a specific

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By Artur Widera

temperature, because all lower-lying energy levels are already occupied and thus further occupation is forbidden. For a measurement of position correlations, the antisymmetry of the wave function results in an apparent antibunching, where fermions seem to avoid each other. Bosons, in contrast, experience an increase in the probability of reaching a state already occupied by other bosons. That is, bosons tend to bunch together. For small systems of particles, the consequences of quantum statistics were exploited in fermionic (4) as well as bosonic (5) systems. Preiss et al. observe this effect of bunching in the position correlations of two identical atoms undergoing a quantum walk. Surprisingly, although this behavior due to quantum statistics seems to be fundamentally fixed, interactions between particles can in fact turn bosonic bunching into fermionic antibunching and vice versa. This has been seen, for example, by association of two fermionic atoms to a bosonic molecule that can undergo Bose-Einstein condensation (6), or pairing of fermions in a many-body system showing superfluid behavior similar to Cooper pairs in a superconductor (7). Preiss et al. go the other way: By increasing repulsive interaction more and more, the bosonic atoms under investigation start to mimic the behavior of weakly interacting fermions, as was observed in one-dimensional Bose gases in the so-called Tonks-Girardeau regime (8, 9). With their superb position resolution, Preiss et al. can track the crossover from the quantum statistics–dominated bosonic bunching to the interaction-dominated antibunching, again extracting position correlations of two atoms doing a quantum walk. The system presented by Preiss et al. allows the study of the interplay of all these aforementioned aspects in a rather simple, paradigmatic system comprising all these effects: two identical, interacting atoms. Beyond the illustration of quantum physics, their system can serve as a basic building block for a bottom-up approach to engineering of complex quantum states atom by atom. ■ REFERENCES

1. P. M. Preiss et al., Science 347, 1229 (2015). 2. M. Ben Dahan, E. Peik, J. Reichel, Y. Castin, C. Salomon, Phys. Rev. Lett. 76, 4508 (1996). 3. M. Karski et al., Science 325, 174 (2009). 4. A. N. Wenz et al., Science 342, 457 (2013). 5. A. M. Kaufman et al., Science 345, 306 (2014). 6. M. Greiner, C. A. Regal, D. S. Jin, Nature 426, 537 (2003). 7. M. W. Zwierlein, J. R. Abo-Shaeer, A. Schirotzek, C. H. Schunck, W. Ketterle, Nature 435, 1047 (2005). 8. T. Kinoshita, T. Wenger, D. S. Weiss, Science 305, 1125 (2004). 9. B. Paredes et al., Nature 429, 277 (2004). 10.1126/science.aaa6885

IMMUNOLOGY

Getting sepsis therapy right Is decreasing inflammation or increasing the host immune response the better approach? By Richard S. Hotchkiss1 and Edward R. Sherwood2

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epsis—a complication of infection—is a factor in at least a third of all hospital deaths—a sobering statistic (1). Patients with sepsis frequently present with fever, shock, and multiorgan failure. Because of this dramatic clinical scenario, investigators have generally assumed that sepsis mortality is due to unbridled inflammation (2). Research in animal models, in which administration of the cytokines tumor necrosis factor–α (TNF-α) and interleukin-1 (IL-1) reproduced many features of sepsis, supported that assertion. Yet, over 40 clinical trials of agents that block cytokines, pathogen recognition, or inflammation-signaling pathways have universally failed (3, 4). However, on page 1260 of this issue, Weber et al. (5) show that blocking a cytokine—specifically, IL-3—can indeed be protective against sepsis. IL-3 is a pleiotropic cytokine that induces proliferation, differentiation, and enhanced function of a broad range of hemopoietic cells (blood cells derived from the bone marrow). Using a mouse abdominal sepsis model, Weber et al. identified IL-3 as a critical driving force of sepsis. The authors observed that the cytokine caused proliferation and mobilization of myeloid cells that generated excessive proinflammatory cytokines, thereby fueling systemic inflammation, organ injury, and death. Blocking IL-3 (by treating wild-type mice with an antibody that blocks the receptor for IL-3 or using IL-3–deficient mice) prevented sepsisinduced increases in the number of circulating neutrophils and inflammatory monocytes and decreased the amount of circulating inflammatory cytokines, thus ameliorating organ injury and improving survival. Additionally, the authors showed a correlation between mortality in septic patients and elevated blood IL-3 concentrations. The findings of Weber et al. are mechanistically analogous to that of another study in which intravenous injection of mesenchymal stem cells (also known as bone marrow stromal cells) into a mouse model of sepsis led to reprogramming of immune cells toward a less inflammatory phenotype, thereby decreasing organ injury and mortality (6). In this scenario, mesenchymal stem

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cells released factors that reprogrammed monocytes and macrophages; the downstream effect was to prevent a damaging, unrestrained immune response. Thus, IL-3 blockade and mesenchymal stem cell–based therapy represent potential approaches for sepsis treatment because of their ability to broadly reshape early immune responses from a proinflammatory, damaging reaction to a more balanced and effective one. However, a few cautionary caveats should be considered before adopting this approach. A phase II clinical trial of granulocyte-macrophage colony-stimulating factor (GM-CSF), a cytokine that increases production, maturation, and function of monocytes, macrophages, and neutrophils, thereby mimicking selected properties of IL-3, was efficacious in treating sepsis and, indeed, a large multicenter trial of GM-CSF in sepsis is under way (7). This is contrary to the findings of Weber et al. that blocking IL-3 can

“Which approach to sepsis… is correct? …there are several clues…” ameliorate sepsis. Two other highly promising agents that are likely to enter clinical trials in sepsis are IL-7 (which promotes CD4+ and CD8+ T lymphocyte proliferation and maturation) and an antibody to programmed death–ligand 1 [(PD-L1), an immunosuppressive protein] (8, 9). Both IL-7 and anti-PD-L1 antibody are immunostimulatory agents that reverse key immunologic defects in lymphocytes and monocytes from septic patients ex vivo and are highly effective in multiple animal models of sepsis (9). Emerging evidence shows correlations between lymphopenia (decrease in lymphocytes) and impaired leukocyte functions with late mortality in patients with sepsis (8, 9). Thus, there is rationale for using approaches that selectively enhance antimicrobial immunity during sepsis. 1

Department of Anesthesiology, Medicine, and Surgery, Washington University School of Medicine, St. Louis, MO, USA. 2Department of Anesthesiology and Pathology, Microbiology and Immunology. Vanderbilt University School of Medicine, Nashville, TN, USA. E-mail: [email protected]; [email protected] 13 MARCH 2015 • VOL 347 ISSUE 6227

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A walk across a quantum lattice Artur Widera Science 347, 1200 (2015); DOI: 10.1126/science.aaa6885

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