• Letter |

    Attosecond technology (1 as = 10−18 S) promises the tools needed to directly probe electron motion in real time. These authors report attosecond pump–probe measurements that track the movement of valence electrons in krypton ions. This first proof-of-principle demonstration uses a simple system, but the expectation is that attosecond transient absorption spectroscopy will ultimately also reveal the elementary electron motions that underlie the properties of molecules and solid-state materials.

    • Eleftherios Goulielmakis
    • , Zhi-Heng Loh
    •  & Ferenc Krausz
  • Letter |

    Metamaterials have the counterintuitive optical property of negative refraction index. They have a wide range of possible applications, including 'invisibility cloaks' and perfect lenses, but their performance is severely limited by absorption losses. These authors have incorporated an optical gain medium within a metamaterial as a way to compensate the intrinsic loss, and show that optical pumping leads to a significantly improved negative refraction index and figure of merit within the 722–738-nm visible wavelength range.

    • Shumin Xiao
    • , Vladimir P. Drachev
    •  & Vladimir M. Shalaev
  • Article |

    The quantum Hall effect takes place in a two-dimensional electron gas under a strong magnetic field and involves current flow along the edges of the sample. In the fractional regime, counter-propagating modes that carry energy but not charge — the so-called neutral modes — have been predicted but never observed. These authors report the first direct observation of these elusive modes.

    • Aveek Bid
    • , N. Ofek
    •  & D. Mahalu
  • Letter |

    New methods are emerging that aim to image chemical reactions as they occur using X-ray diffraction, electron diffraction or laser-induced recollision, but spectral selection cannot be used to monitor the reacting molecules for these methods. These authors show that this apparent limitation offers opportunities for recollision-based high-harmonic spectroscopy, in which unexcited molecules can act as local oscillators against which structural and electronic dynamics is observed on an attosecond timescale.

    • H. J. Wörner
    • , J. B. Bertrand
    •  & D. M. Villeneuve
  • Letter |

    Fluctuations arising from Heisenberg's uncertainty principle enable quantum systems to exhibit phase transitions even at zero temperature. For example, a superfluid-to-insulator transition has been observed for weakly interacting bosonic atomic gases. Here the authors report a novel type of quantum phase transition in a strongly interacting, one-dimensional quantum gas of bosonic caesium atoms. The results open up the experimental study of ultracold gases in a new regime.

    • Elmar Haller
    • , Russell Hart
    •  & Hanns-Christoph Nägerl
  • Letter |

    Non-classical states of light, such as entangled photon states, form an essential quantum resource. Entangled photon pairs can be created by spontaneous parametric down-conversion of laser light, but so far it has not been possible to produce photon triplets in this way. These authors report the generation of quantum-correlated photon triplets by cascaded down-conversion of a single pump photon. This should find widespread use in optical quantum technologies.

    • Hannes Hübel
    • , Deny R. Hamel
    •  & Thomas Jennewein
  • Review Article |

    For 350 years after Galileo's discoveries, ground-based telescopes and theoretical modelling furnished everything known about the Sun's planetary retinue. Over the past five decades, data from spacecraft sent to all the planets and some of their satellites have shown the diversity of Solar System bodies. Many planets and satellites have changed substantially since their birth, and violent events punctuate their histories.

    • Joseph A. Burns
  • Letter |

    Hitherto, 21-cm emission has been detected in galaxies only to redshift 0.24, although it is possible to measure the aggregate emission from many more distant, unresolved sources in the 'cosmic web'. Here the authors report a three-dimensional 21-cm intensity field at redshift 0.53–1.12. They co-add neutral-hydrogen emission from the volumes surrounding about 10,000 galaxies to detect the aggregate 21-cm glow at a significance of approximately four standard deviations.

    • Tzu-Ching Chang
    • , Ue-Li Pen
    •  & Jeffrey B. Peterson
  • Letter |

    Topological surface states are a class of electronic states that might be of interest in quantum computing or spintronic applications. They are predicted to be robust against imperfections, but so far there has been no evidence that these states do transmit through naturally occurring surface defects. Here, scanning tunnelling microscopy has been used to show that topological surface states of antimony can be transmitted through naturally occurring barriers that block non-topological surface states of common metals.

    • Jungpil Seo
    • , Pedram Roushan
    •  & Ali Yazdani
  • Letter |

    Circumstellar disks are an essential ingredient of the formation of low-mass stars, but it is unclear whether they are also required for the formation of stars more massive than about 10 solar masses. Clear observational evidence is needed, for example the detection of dusty disks around massive young stellar objects. Here, near-infrared interferometric observations are reported that spatially resolve the distribution of hot material around a high-mass young stellar object.

    • Stefan Kraus
    • , Karl-Heinz Hofmann
    •  & Leonardo Testi
  • Letter |

    Quantum information science requires a source of entangled photon pairs, but existing sources suffer from a low intrinsic efficiency or poor extraction efficiency. Collecting emitted photons from quantum dots can be improved by coupling the dots to an optical cavity, but this is not easy for entangled photon pairs. Now, a suitable optical cavity has been made in the form of a 'photonic' molecule — two identical, connecting microcavities that are deterministically coupled to the optically active modes of a pre-selected quantum dot.

    • Adrien Dousse
    • , Jan Suffczyński
    •  & Pascale Senellart
  • Letter |

    Here, a technically challenging spectroscopic experiment is described: the measurement of the muonic Lamb shift. The results lead to a new determination of the charge radius of the proton. The new value is 5.0 standard deviations smaller than the previous world average, a large discrepancy that remains unexplained. Possible implications of the new finding are that the value of the Rydberg constant will need to be revised, or that the validity of quantum electrodynamics theory is called into question.

    • Randolf Pohl
    • , Aldo Antognini
    •  & Franz Kottmann
  • Article |

    With the start-up of the first X-ray free-electron laser, a new era has begun in dynamical studies of atoms. Here the facility is used to study the fundamental nature of the electronic response in free neon atoms. During a single X-ray pulse, they sequentially eject all their ten electrons to produce fully stripped neon. The authors explain this electron-stripping in a straightforward model, auguring favourably for further studies of interactions of X-rays with more complex systems.

    • L. Young
    • , E. P. Kanter
    •  & M. Messerschmidt
  • Letter |

    Light–matter interactions in semiconductors hold great promise for numerous applications, but as device size is reduced such interactions typically weaken, potentially posing problems for applications at the nanoscale. Here the authors circumvent these limitations by producing colloidal particles with metallic cores and semiconducting shells, in which coupling of the plasmons in the metal to the excitons in the semiconductor is engineered to enhance light–matter interactions in the particle.

    • Jiatao Zhang
    • , Yun Tang
    •  & Min Ouyang
  • Letter |

    When electrons or photons are used to detect the motion of a mechanical resonator, they exert tiny forces on the resonator, subtly changing its motion. Here, through analysis of electrical noise measurements, the authors report a striking example of such back-action where electrons tunnelling through a semiconductor quantum device cause vibrations of the host crystal, which is massive compared with the electrons — an effect comparable to a flea causing metre-scale vibrations in Mount Everest.

    • Joel Stettenheim
    • , Madhu Thalakulam
    •  & A. J. Rimberg
  • Letter |

    A quantum memory would enable storage and retrieval of a quantum state of light without corrupting the information it carries. Previous devices have had low efficiencies of less than 17 per cent, and used weak quantum states with an average photon number of around one. Now a solid-state quantum memory is described with an efficiency of up to 69 per cent, which performs better than a classical device for bright states of up to 500 photons.

    • Morgan P. Hedges
    • , Jevon J. Longdell
    •  & Matthew J. Sellars
  • Letter |

    If the orbital velocity of an extrasolar planet could be determined, the masses of both the planet and its host star could be calculated using Newton's law of gravity. Here, high-dispersion ground-based spectroscopy of a transit of the extrasolar planet HD 209458b is reported. This allowed the radial component of the planet's orbital velocity to be calculated, and thus the masses of star and planet. Moreover, a strong wind flowing from the irradiated dayside to the non-irradiated nightside of the planet is suggested.

    • Ignas A. G. Snellen
    • , Remco J. de Kok
    •  & Simon Albrecht
  • Letter |

    Network theory has become pervasive in all sectors of biology, from biochemical signalling to human societies, but identification of relevant functional communities has been impaired by many nodes belonging to several overlapping groups at once, and by hierarchical structures. These authors offer a radically different viewpoint, focusing on links rather than nodes, which allows them to demonstrate that overlapping communities and network hierarchies are two faces of the same issue.

    • Yong-Yeol Ahn
    • , James P. Bagrow
    •  & Sune Lehmann
  • Letter |

    Over the past few decades, two techniques in particular have opened up new avenues for probing molecular processes: ultrafast spectroscopy and single-molecule detection. The two approaches have now been combined, enabling not only the observation but also the manipulation of vibrational wave-packet interference at ambient conditions. The technique could help to unravel details of molecular function and dynamics in systems as diverse as light-harvesting complexes, photoactive proteins and conjugated polymers.

    • Daan Brinks
    • , Fernando D. Stefani
    •  & Niek F. van Hulst
  • Letter |

    Skyrmions are stable topological textures with particle-like properties — a mathematical concept that was originally used to describe nuclear particles but has since turned up at all scales. Last year, the presence of skyrmions in the magnetic compounds MnSi and Fe1−xCoxSi was confirmed with neutron-scattering experiments. Here, real-space images are presented of a two-dimensional skyrmion lattice in a thin film of the latter compound. The observed nanometre-scale spin topology might reveal new magneto-transport effects.

    • X. Z. Yu
    • , Y. Onose
    •  & Y. Tokura
  • Letter |

    When a bubble on a liquid–gas or solid–gas interface ruptures, the general expectation is that the bubble vanishes. Here, it is shown that in many cases interfacial bubbles do not simply vanish when they rupture, but rather create numerous small bubbles via unexpected folding of the ruptured bubble as it retracts. The process may increase the efficiency of rupture-induced aerosol dispersal.

    • James C. Bird
    • , Riëlle de Ruiter
    •  & Howard A. Stone
  • Letter |

    Attosecond (10−18 s) laser pulses make it possible to peer into the inner workings of atoms and molecules on the electronic timescale — phenomena in solids have already been investigated in this way. Here, an attosecond pump–probe experiment is reported that investigates the ionization and dissociation of hydrogen molecules, illustrating that attosecond techniques can also help explore the prompt charge redistribution and charge localization that accompany photoexcitation processes in molecular systems.

    • G. Sansone
    • , F. Kelkensberg
    •  & M. J. J. Vrakking
  • Letter |

    A quantum computer based on optical processes requires a source of entangled photons that can be delivered efficiently on demand. Such a source has now been developed: it involves a compact light-emitting diode with an embedded quantum dot that can be driven electrically to generate entangled photon pairs.

    • C. L. Salter
    • , R. M. Stevenson
    •  & A. J. Shields
  • Letter |

    A network is frustrated when competing interactions between nodes prevent each bond from being satisfied. Frustration in quantum networks can lead to massively entangled ground states, as occurs in exotic materials such as quantum spin liquids and spin glasses. Here, a quantum simulation of a frustrated spin system is described, in which there are three trapped atomic ions whose interactions are controlled using optical forces.

    • K. Kim
    • , M.-S. Chang
    •  & C. Monroe
  • Letter |

    Atomic nuclei have a shell structure that allows for 'magic numbers' of neutrons and protons, analogous to the noble gases in atomic physics. Knowledge of the properties of single-particle states outside nuclear shell closures in exotic nuclei is important for the fundamental understanding of nuclear structure and nucleosynthesis. Here, a nucleon-transfer technique has been used to measure the single-particle states of 133Sn, revealing the highly magic nature of 132Sn.

    • K. L. Jones
    • , A. S. Adekola
    •  & J. S. Thomas
  • Letter |

    Although compound semiconductors like gallium arsenide (GaAs) offer advantages over silicon for photovoltaic and optoelectronic applications, these do not outweigh the costly process of growing large layers of these materials and transferring them to appropriate substrates. However, a new fabrication approach is now demonstrated: films of GaAs and AlGaAs are grown in thick, multilayered assemblies in a single sequence; the individual layers are then released and distributed over foreign substrates by printing.

    • Jongseung Yoon
    • , Sungjin Jo
    •  & John A. Rogers
  • Letter |

    Interactions between microscopic particles are usually described as two-body interactions, although it has been shown that higher-order multi-body interactions could give rise to new quantum phases with intriguing properties. Here, effective six-body interactions are demonstrated in a system of ultracold bosonic atoms in a three-dimensional optical lattice.

    • Sebastian Will
    • , Thorsten Best
    •  & Immanuel Bloch
  • Letter |

    Electromagnetically induced transparency enables the transmission of a laser pulse through an optically dense medium to be manipulated using a control beam. Here this technique is scaled down to a single atom, which acts as a quantum-optical transistor with the ability to coherently control the transmission of light through a cavity. This may lead to novel quantum applications, such as dynamic control of the photon statistics of propagating light fields.

    • Martin Mücke
    • , Eden Figueroa
    •  & Gerhard Rempe
  • Letter |

    Recent progress in solid-state quantum information processing has stimulated the search for amplifiers and frequency converters with quantum-limited performance in the microwave range. Here, a phase-preserving, superconducting parametric amplifier with ultra-low-noise properties has been experimentally realized.

    • N. Bergeal
    • , F. Schackert
    •  & M. H. Devoret
  • Letter |

    Here it is shown, both theoretically and experimentally, that non-local correlations between entangled quantum particles can be used for a new cryptographic application — the generation of certified private random numbers — that is impossible to achieve classically. The results have implications for future device-independent quantum information experiments and for addressing fundamental issues regarding the randomness of quantum theory.

    • S. Pironio
    • , A. Acín
    •  & C. Monroe
  • Article |

    A quantum spin liquid is a hypothetical system of spins (such as those carried by electrons), the orientations of which continue to fluctuate even at absolute zero. Theoretical and experimental evidence for the existence of such states at the microscopic level is elusive, but these authors have modelled correlated electrons arranged on a honeycomb lattice (such as in graphene), and identified the conditions under which a microscopic quantum spin liquid would be realized in two dimensions.

    • Z. Y. Meng
    • , T. C. Lang
    •  & A. Muramatsu
  • Letter |

    X-ray crystallography has become the most common way for structural biologists to obtain the three-dimensional structures of proteins and protein complexes. However, crystals of large macromolecular complexes often diffract only weakly (yielding a resolution worse than 4 Å), so new methods that work at such low resolution are needed. Here a new method is described by which to obtain higher-quality electron density maps and more accurate molecular models of weakly diffracting crystals.

    • Gunnar F. Schröder
    • , Michael Levitt
    •  & Axel T. Brunger
  • Letter |

    Ultracold polar molecules offer the possibility of exploring quantum gases with interparticle interactions that are strong, long-range and spatially anisotropic. Here, dipolar collisions in an ultracold gas of fermionic potassium–rubidium molecules have been experimentally observed. The results show how the long-range dipolar interaction can be used for electric-field control of chemical reaction rates in an ultracold gas of polar molecules.

    • K.-K. Ni
    • , S. Ospelkaus
    •  & D. S. Jin
  • Article |

    A phase transition occurs when a physical system suddenly changes state, for instance when it melts or freezes. The Dicke model describes a collective matter–light interaction and has been predicted to show a quantum phase transition. Here, this quantum phase transition has been realized in an open system formed by a Bose–Einstein condensate coupled to an optical cavity. Surprisingly, the atoms are observed to self-organize into a supersolid phase.

    • Kristian Baumann
    • , Christine Guerlin
    •  & Tilman Esslinger
  • Letter |

    The precision of interferometers — used in metrology and in the state-of-the-art time standard — is generally limited by classical statistics. Here it is shown that the classical precision limit can be beaten by using nonlinear atom interferometry with Bose–Einstein condensates.

    • C. Gross
    • , T. Zibold
    •  & M. K. Oberthaler
  • Letter |

    Atom chips provide a versatile quantum laboratory for experiments with ultracold atomic gases, but techniques to control atomic interactions and to generate entanglement have been unavailable so far. Here, the experimental generation of multi-particle entanglement on an atom chip is described. The technique is used to produce spin-squeezed states of a two-component Bose–Einstein condensate, which should be useful for quantum metrology.

    • Max F. Riedel
    • , Pascal Böhi
    •  & Philipp Treutlein
  • Letter |

    A two-dimensional gas of electrons is a powerful test-bed for the fundamental physics of interacting particles, and has been much studied in the context of integer and fractional quantum Hall effects. The latest observations of this system reveal prominent structure in the high energy single particle spectrum that cannot be readily explained with existing models of this system.

    • O. E. Dial
    • , R. C. Ashoori
    •  & K. W. West
  • Letter |

    Until now, quantum atomic gases and single trapped ions have been treated separately in experiments. Now a hybrid system has been investigated, involving the immersion of a single trapped ion into a Bose–Einstein condensate of neutral atoms. The two systems could be controlled independently and the fundamental interaction processes were studied. Sympathetic cooling of the single ion by the condensate was observed, hinting at the possibility of using these condensates as refrigerators for ion-trap quantum computers.

    • Christoph Zipkes
    • , Stefan Palzer
    •  & Michael Köhl
  • Article |

    Quantum mechanics provides an accurate description of a wide variety of physical systems but it is very challenging to prove that it also applies to macroscopic (classical) mechanical systems. This is because it has been impossible to cool a mechanical mode to its quantum ground state, in which all classical noise is eliminated. Recently, various mechanical devices have been cooled to a near-ground state, but this paper demonstrates the milestone result of a piezoelectric resonator with a mechanical mode cooled to its quantum ground state.

    • A. D. O’Connell
    • , M. Hofheinz
    •  & A. N. Cleland
  • Letter |

    An insulator does not conduct electricity, and so cannot in general be used to transmit an electrical signal. But an insulator's electrons possess spin in addition to charge, and so can transmit a signal in the form of a spin wave. Here a hybrid metal–insulator–metal structure is reported, in which an electrical signal in one metal layer is directly converted to a spin wave in the insulating layer; this wave is then transmitted to the second metal layer, where the signal can be directly recovered as an electrical voltage.

    • Y. Kajiwara
    • , K. Harii
    •  & E. Saitoh
  • Review Article |

    • T. D. Ladd
    • , F. Jelezko
    •  & J. L. O’Brien
  • Letter |

    The phenomenon of superconductivity continues to intrigue, and several new superconducting materials have been discovered in recent years — but in the case of organic superconductors, no new material system with a high superconducting transition temperature has been identified in the past decade. Now it has been shown that the introduction of potassium into crystals of organic molecule picene can yield superconductivity at temperatures as high as 18 K.

    • Ryoji Mitsuhashi
    • , Yuta Suzuki
    •  & Yoshihiro Kubozono
  • Letter |

    In principle, it is possible to simulate some astrophysical phenomena inside the highly controlled environment of an atomic physics laboratory: previous work on the thermodynamics of a two-component Fermi gas (a system suited for such studies) led to thermodynamic quantities averaged over the trap. Now a general experimental method is reported that yields the equation of state of a uniform gas, providing new physical insights and enabling a detailed comparison with existing theories.

    • S. Nascimbène
    • , N. Navon
    •  & C. Salomon
  • Letter |

    One of the central predictions of general relativity is that a clock in a gravitational potential well runs more slowly than a similar clock outside the well. This effect, known as gravitational redshift, has been measured using clocks on a tower, an aircraft and a rocket, but here, laboratory experiments based on quantum interference of atoms are shown to produce a much more precise measurement.

    • Holger Müller
    • , Achim Peters
    •  & Steven Chu
  • Letter |

    The difference between the mass of an atom and the sum of its building blocks (the binding energy) is a manifestation of Einstein's famous relation E = mc2. Superheavy elements have been observed, but our present knowledge of the binding energy of these nuclides is based only on the detection of their decay products, although they represent the gateway to the predicted 'island of stability'. Here, direct mass measurements of trans-uranium nuclides are reported, providing reliable anchor points en route to the island of stability.

    • M. Block
    • , D. Ackermann
    •  & C. Weber
  • Letter |

    In the study of high-transition-temperature (high-Tc) copper oxide superconductors, a fundamental question is what symmetries are broken when the pseudogap phase sets in below a temperature T*. A large in-plane anisotropy of the Nernst effect is now observed in a high-Tc copper oxide superconductor that sets in precisely at T* throughout the doping phase diagram. It is concluded that the pseudogap phase is an electronic state that strongly breaks four-fold rotational symmetry.

    • R. Daou
    • , J. Chang
    •  & Louis Taillefer
  • Letter |

    The close binary Algol system contains a radio-bright KIV sub-giant star in a very close and rapid orbit with a main sequence B8 star. Evidence points to the existence of an extended, complex coronal magnetosphere originating at the cooler K subgiant, but the detailed morphology of the subgiant's corona and its possible interaction with its companion are unknown. Multi-epoch radio imaging of the Algol system now reveals a large coronal loop suggestive of a persistent asymmetric magnetic field structure aligned between the two stars.

    • W. M. Peterson
    • , R. L. Mutel
    •  & W. M. Goss