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<jats:p>Trapped atomic ions are standards for quantum information processing, serving as quantum memories, hosts of quantum gates in quantum computers and simulators, and nodes of quantum communication networks. Quantum bits based on trapped ions enjoy a rare combination of attributes: They have exquisite coherence properties, they can be prepared and measured with nearly 100% efficiency, and they are readily entangled with each other through the Coulomb interaction or remote photonic interconnects. The outstanding challenge is the scaling of trapped ions to hundreds or thousands of qubits and beyond, at which scale quantum processors can outperform their classical counterparts in certain applications. We review the latest progress and prospects in that effort, with the promise of advanced architectures and new technologies, such as microfabricated ion traps and integrated photonics.</jats:p>

<jats:p>The performance of superconducting qubits has improved by several orders of magnitude in the past decade. These circuits benefit from the robustness of superconductivity and the Josephson effect, and at present they have not encountered any hard physical limits. However, building an error-corrected information processor with many such qubits will require solving specific architecture problems that constitute a new field of research. For the first time, physicists will have to master quantum error correction to design and operate complex active systems that are dissipative in nature, yet remain coherent indefinitely. We offer a view on some directions for the field and speculate on its future.</jats:p>

<jats:p>The past decade has seen remarkable progress in isolating and controlling quantum coherence using charges and spins in semiconductors. Quantum control has been established at room temperature, and electron spin coherence times now exceed several seconds, a nine–order-of-magnitude increase in coherence compared with the first semiconductor qubits. These coherence times rival those traditionally found only in atomic systems, ushering in a new era of ultracoherent spintronics. We review recent advances in quantum measurements, coherent control, and the generation of entangled states and describe some of the challenges that remain for processing quantum information with spins in semiconductors.</jats:p>

<jats:p>Quantum computation requires controlled engineering of quantum states to perform tasks that go beyond those possible with classical computers. Topological quantum computation aims to achieve this goal by using non-Abelian quantum phases of matter. Such phases allow for quantum information to be stored and manipulated in a nonlocal manner, which protects it from imperfections in the implemented protocols and from interactions with the environment. Recently, substantial progress in this field has been made on both theoretical and experimental fronts. We review the basic concepts of non-Abelian phases and their topologically protected use in quantum information processing tasks. We discuss different possible realizations of these concepts in experimentally available solid-state systems, including systems hosting Majorana fermions, their recently proposed fractional counterparts, and non-Abelian quantum Hall states.</jats:p>

<jats:title>Limits in Percolation Models</jats:title> <jats:p> Slight changes in the number of connections within a network that form at random (for example, connections in social networks) can lead to a huge increase in connectivity, a phenomenon termed "explosive percolation." These percolation transitions are often studied with Erdös and Rényi models, in which edges connecting pairs of vertices in a network are added randomly or according to a rule. Whether these transitions are continuous in nature has been the subject of several recent studies. <jats:bold> Cho <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6124" page="1185" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1230813">1185</jats:related-article> ; see the Perspective by <jats:bold>Ziff</jats:bold> ) examined the effect of avoiding bridge bonds that create a spanning cluster (one that completes the percolation path) on the continuity of transitions for a <jats:italic>d</jats:italic> -dimensional lattice (up to six dimensions). Analytical arguments and numerical studies reveal a critical value for the number of bonds <jats:italic>m</jats:italic> below which the percolation transition is continuous and above which it is discontinuous. The critical value depends on <jats:italic>d</jats:italic> and on the fractal dimension of the bridge bonds of the clusters. </jats:p>

<jats:title>Characterizing Quantum Measurement</jats:title> <jats:p> In the classical world, the result of measurement is often viewed as independent of the experimental apparatus. In the quantum world, however, the act of measurement has an effect through processes such as back-action, entanglement, and decoherence. Looking at the scattering of single electrons from a single ion and the evolution of the spin state of that ion, <jats:bold> Glickman <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6124" page="1187" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1229650">1187</jats:related-article> ) probed how these processes are intertwined and characterize how the result of quantum measurement emerges from the system's interaction with its environment. </jats:p>

<jats:title>S-T-R-E-T-C-H Me</jats:title> <jats:p> Most metals show elastic strain limits well below 1%, beyond which permanent plastic deformation occurs. Metal nanowires can be elastically stretched to much higher strains, on the order of 4 to 7%. However, when placed inside a metal matrix to form a composite, these nanowires can no longer be stretched to the same extent, even when the nanowires are well distributed and show good bonding with the matrix. <jats:bold> Hao <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6124" page="1191" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1228602">1191</jats:related-article> ; see the Perspective by <jats:bold>Zhou</jats:bold> ) used a shape memory alloy as the matrix material to produce a much better (more elastic) composite. </jats:p>

<jats:title>Earth's Ingredients</jats:title> <jats:p> What was the composition of the earliest terrestrial starting blocks? The answer lies in understanding how Earth's interior separated into mantle and core components. <jats:bold> Siebert <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6124" page="1194" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1227923">1194</jats:related-article> , published online 10 January) performed a series of high pressure and temperature experiments to track how chromium and vanadium, which have a slight affinity for iron, partition into metal and silicate fractions. Combined with accretionary models, the data suggest that Earth accreted under the same relatively oxidizing conditions under which the most common types of meteorites formed. Transferring oxygen in the form of FeO from the mantle to the core could have gradually reduced the mantle to its present-day oxidation state. </jats:p>

<jats:title>Exceptional Now</jats:title> <jats:p> The climate has been warming since the industrial revolution, but how warm is climate now compared with the rest of the Holocene? <jats:bold> Marcott <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6124" page="1198" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1228026">1198</jats:related-article> ) constructed a record of global mean surface temperature for more than the last 11,000 years, using a variety of land- and marine-based proxy data from all around the world. The pattern of temperatures shows a rise as the world emerged from the last deglaciation, warm conditions until the middle of the Holocene, and a cooling trend over the next 5000 years that culminated around 200 years ago in the Little Ice Age. Temperatures have risen steadily since then, leaving us now with a global temperature higher than those during 90% of the entire Holocene. </jats:p>

<jats:title>Bees Get That Caffeine "Buzz"</jats:title> <jats:p> Caffeine improves memory in humans, millions of whom find that their daily dose enhances clarity, focus, and alertness. The human relationship with caffeine is relatively recent, however, and thus its impact on our brains is likely a by-product of its true ecological role. Caffeine occurs naturally in the floral nectar of <jats:italic>Coffea</jats:italic> and <jats:italic>Citrus</jats:italic> plants. <jats:bold> Wright <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6124" page="1202" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1228806">1202</jats:related-article> ; see the Perspective by <jats:bold>Chittka and Peng</jats:bold> ) found that caffeine presented at naturalistic levels significantly improved the ability of bees to remember and locate a learned floral scent and potentiated the responses of neurons involved in olfactory learning and memory. </jats:p>