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This is a detailed introduction to matter, the elements of the periodic table, atoms, electrons, reactions and bonding, and radioactivity. This volume provides young adults with chemistry examples that reflect their real-world. Key terms, easy experiments, and clear illustrations guide students through subatomic explorations. A chapter about Niels Bohr and his model for the atom honors his contribution to the understanding of atomic structure. Tools and techniques, such as a scanning tunneling microscope, Rutherford's gold foil experiment, and a mass spectrometer, help readers to gain a comprehensive understanding of atoms and molecules.

Bound states of elementary spin waves (magnons) have been predicted to occur in one-dimensional quantum magnets; the observation of two-magnon bound states in a system of ultracold bosonic atoms in an optical lattice is now reported. Quanta at the double Two papers published in this issue of Nature show that the propagation of energy quanta in very different physical systems can exhibit the same, unusual dynamics, where bound pairs of quanta become dominant. Ofer Firstenberg et al . realize coherent interactions between individual photons — quanta of light — which are massless and do not usually interact. They achieve this using a quantum nonlinear medium inside which individual photons pair up and travel as massive particles with strong mutual attraction. Potential applications of this technique include all-optical switching, deterministic photonic quantum logic and the generation of strongly correlated states of light. The second paper deals with magnons, the quanta that carry energy in magnets. More than eighty years ago, Hans Bethe predicted the existence of bound states of elementary spin waves (magnons) in one-dimensional quantum magnets. Experimental observation of the phenomenon remained elusive, but now Takeshi Fukuhara et al . have observed two-magnon bound states in a system of ultracold bosonic atoms in an optical lattice. The results provide a new way of studying fundamental properties of quantum magnets. In an accompanying News and Views, Sougato Bose puts these two independent findings into a general context of quantum many-body dynamics. The existence of bound states of elementary spin waves (magnons) in one-dimensional quantum magnets was predicted almost 80 years ago 1 . Identifying signatures of magnon bound states has so far remained the subject of intense theoretical research 2 , 3 , 4 , 5 , and their detection has proved challenging for experiments. Ultracold atoms offer an ideal setting in which to find such bound states by tracking the spin dynamics with single-spin and single-site resolution 6 , 7 following a local excitation 8 . Here we use in situ correlation measurements to observe two-magnon bound states directly in a one-dimensional Heisenberg spin chain comprising ultracold bosonic atoms in an optical lattice. We observe the quantum dynamics of free and bound magnon states through time-resolved measurements of two spin impurities. The increased effective mass of the compound magnon state results in slower spin dynamics as compared to single-magnon excitations. We also determine the decay time of bound magnons, which is probably limited by scattering on thermal fluctuations in the system. Our results provide a new way of studying fundamental properties of quantum magnets and, more generally, properties of interacting impurities in quantum many-body systems.

Highly excited Rydberg atoms have many exaggerated properties. In particular, the interaction strength between such atoms can be varied over an enormous range. In a mesoscopic ensemble, such strong, long-range interactions can be used for fast preparation of desired many-particle states. We generated Rydberg excitations in an ultra-cold atomic gas and subsequently converted them into light. As the principal quantum number n was increased beyond ~ 70, no more than a single excitation was retrieved from the entire mesoscopic ensemble of atoms. These results hold promise for studies of dynamics and disorder in many-body systems with tunable interactions and for scalable quantum information networks.

Single magnetic atoms on non-magnetic surfaces have magnetic moments that are usually destabilized within a microsecond, too speedily to be useful, but here the magnetic moments of single holmium atoms on a highly conductive metallic substrate can reach lifetimes of the order of minutes. Memory in a moment The magnetic moments of individual magnetic atoms are attractive components for both memory and quantum computing applications. But interactions between such atoms and the substrates on which they are mounted tend to destabilize the magnetic moments, giving them lifetimes of typically less than a few milliseconds. Toshio Miyamachi and colleagues have now identified a system consisting of single atoms of the lanthanide series rare earth element holmium on a highly conductive surface, in which intrinsic symmetries related to the properties of both the atom and the substrate combine to minimize these destabilizing interactions. As a result, the magnetic moments of the atoms can achieve lifetimes of several minutes. Single magnetic atoms, and assemblies of such atoms, on non-magnetic surfaces have recently attracted attention owing to their potential use in high-density magnetic data storage and as a platform for quantum computing 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 . A fundamental problem resulting from their quantum mechanical nature is that the localized magnetic moments of these atoms are easily destabilized by interactions with electrons, nuclear spins and lattice vibrations of the substrate 3 , 4 , 5 . Even when large magnetic fields are applied to stabilize the magnetic moment, the observed lifetimes remain rather short 5 , 6 (less than a microsecond). Several routes for stabilizing the magnetic moment against fluctuations have been suggested, such as using thin insulating layers between the magnetic atom and the substrate to suppress the interactions with the substrate’s conduction electrons 2 , 3 , 5 , or coupling several magnetic moments together to reduce their quantum mechanical fluctuations 7 , 8 . Here we show that the magnetic moments of single holmium atoms on a highly conductive metallic substrate can reach lifetimes of the order of minutes. The necessary decoupling from the thermal bath of electrons, nuclear spins and lattice vibrations is achieved by a remarkable combination of several symmetries intrinsic to the system: time reversal symmetry, the internal symmetries of the total angular momentum and the point symmetry of the local environment of the magnetic atom.

Background and aims: Increased N deposition can break the coupled associations among chemical elements in soil, many of which are essential plant nutrients. We evaluated the effects of four years of N deposition (0, 10, 20, 50 kg N ha−1 yr−1) on the temporal dynamics of the spatial co-variation (i.e., coupling) among ten chemical elements in soils from a semiarid shrubland in central Spain. Methods: Soil element coupling was calculated as the mean of Spearman rank correlation coefficients of all possible pairwise interactions among elemental cycles, in absolute value. We also investigated the role of atomic properties of elements as regulators of coupling. Results: While N deposition impacts on nutrient bioavailability were variable, soil elemental coupling consistently increased in response to N. Coupling responses also varied among elements and N treatments, and four out of ten elemental cycles also responded to N in a season-dependent manner. Atomic properties of elements such as mass, valence orbitals, and electronegativity contributed to explain the spatial coupling of soil elements, most likely due their role on the capacity of elements to interact with one another. Conclusions: The cumulative effects of N deposition can alter the spatial associations among chemical elements in soils, while not having evident consequences on the bioavailability of single elments. These results indicate that considering how multiple elements co-vary in topsoils may provide a useful framework to better understand the simultaneous response of multiple elemental cycles to global change.

Entangling atom pairs Controlled two-particle interaction is a fundamental requirement for quantum computing, and achieving it has long been a goal for research on neutral atom systems. Anderlini et al . have used a system, consisting of arrays of paired ultracold rubidium-87 atoms in an optical lattice of double-well potentials, to induce controlled entangling interactions within each atom pair. Repeated interchange of spin between atoms occupying different vibrational levels occurs with a coherence time of more than ten milliseconds. This demonstrates an essential component of a quantum gate. An optical lattice of double-well potentials is used to isolate and manipulate arrays of paired 87 Rb atoms, inducing controlled entangling interactions within each pair. Repeated interchange of spin between atoms occupying different vibrational levels occurs with a coherence time of more than ten milliseconds. This observation demonstrates the essential component of a quantum gate important for quantum computation. Ultracold atoms trapped by light offer robust quantum coherence and controllability, providing an attractive system for quantum information processing and for the simulation of complex problems in condensed matter physics. Many quantum information processing schemes require the manipulation and deterministic entanglement of individual qubits; this would typically be accomplished using controlled, state-dependent, coherent interactions among qubits. Recent experiments have made progress towards this goal by demonstrating entanglement among an ensemble of atoms 1 confined in an optical lattice. Until now, however, there has been no demonstration of a key operation: controlled entanglement between atoms in isolated pairs. Here we use an optical lattice of double-well potentials 2 , 3 to isolate and manipulate arrays of paired 87 Rb atoms, inducing controlled entangling interactions within each pair. Our experiment realizes proposals to use controlled exchange coupling 4 in a system of neutral atoms 5 . Although 87 Rb atoms have nearly state-independent interactions, when we force two atoms into the same physical location, the wavefunction exchange symmetry of these identical bosons leads to state-dependent dynamics. We observe repeated interchange of spin between atoms occupying different vibrational levels, with a coherence time of more than ten milliseconds. This observation demonstrates the essential component of a neutral atom quantum SWAP gate (which interchanges the state of two qubits). Its ‘half-implementation’, the gate, is entangling, and together with single-qubit rotations it forms a set of universal gates for quantum computation 4 .

Highest-resolution laser spectroscopy has generally been limited to single trapped ion systems because of the rapid decoherence that plagues neutral atom ensembles. Precision spectroscopy of ultracold neutral atoms confined in a trapping potential now shows superior optical coherence without any deleterious effects from motional degrees of freedom, revealing optical resonance linewidths at the hertz level with a good signal-to-noise ratio. The resonance quality factor of 2.4 x 10¹⁴ is the highest ever recovered in any form of coherent spectroscopy. The spectral resolution permits direct observation of the breaking of nuclear spin degeneracy for the ¹S₀ and ³P₀ optical clock states of ⁸⁷Sr under a small magnetic bias field. This optical approach for excitation of nuclear spin states allows an accurate measurement of the differential Landé g factor between ¹S₀ and ³P₀. The optical atomic coherence demonstrated for collective excitation of a large number of atoms will have a strong impact on quantum measurement and precision frequency metrology.

In recent years, quantum fluids have been studied largely in gaseous form, such as the Bose-Einstein condensates (BECs) of alkali atoms and related species. Quantum liquids, other than liquid helium, have been comparatively more difficult to come by. Cabrera et al. combined two BECs and manipulated the atomic interactions to create droplets of a quantum liquid (see the Perspective by Ferrier-Barbut and Pfau). Because the interactions were not directional, the droplets had a roughly round shape. The simplicity of this dilute system makes it amenable to theoretical modeling, enabling a better understanding of quantum fluids. Science , this issue p. 301 ; see also p. 274 Tuning interatomic interactions in two ultracold gases of potassium atoms creates quantum liquid droplets. Quantum droplets are small clusters of atoms self-bound by the balance of attractive and repulsive forces. Here, we report on the observation of droplets solely stabilized by contact interactions in a mixture of two Bose-Einstein condensates. We demonstrate that they are several orders of magnitude more dilute than liquid helium by directly measuring their size and density via in situ imaging. We show that the droplets are stablized against collapse by quantum fluctuations and that they require a minimum atom number to be stable. Below that number, quantum pressure drives a liquid-to-gas transition that we map out as a function of interaction strength. These ultradilute isotropic liquids remain weakly interacting and constitute an ideal platform to benchmark quantum many-body theories.

Networking is integral to quantum communications 1 and has significant potential for upscaling quantum computer technologies 2 . Recently, it was realized that the sensing performances of multiple spatially distributed parameters may also be enhanced through the use of an entangled quantum network 3 – 10 . Here, we experimentally demonstrate how sensing of an averaged phase shift among four distributed nodes benefits from an entangled quantum network. Using a four-mode entangled continuous-variable state, we demonstrate deterministic quantum phase sensing with a precision beyond what is attainable with separable probes. The techniques behind this result can have direct applications in a number of areas ranging from molecular tracking to quantum networks of atomic clocks. Experiments demonstrate quantum phase sensing with a four-mode entangled state, reaching a measurement precision that is beyond what can be achieved by separate individual probes.

Large arrays of individually controlled atoms trapped in optical tweezers are a very promising platform for quantum engineering applications. However, deterministic loading of the traps is experimentally challenging. We demonstrate the preparation of fully loaded two-dimensional arrays of up to ~50 microtraps, each containing a single atom and arranged in arbitrary geometries. Starting from initially larger, half-filled matrices of randomly loaded traps, we obtain user-defined target arrays at unit filling. This is achieved with a real-time control system and a moving optical tweezers, which together enable a sequence of rapid atom moves depending on the initial distribution of the atoms in the arrays. These results open exciting prospects for quantum engineering with neutral atoms in tunable two-dimensional geometries.