Physics

  • Letter |

    A long-standing question has been the interplay between pseudogap, which is generic to all hole doped copper oxide superconductors, and stripes, whose static form occurs in only one family of copper oxides over a narrow range of the phase diagram. This study reports observations of the spatial reorganization of electronic states with the onset of the pseudogap state at T* in the high temperature superconductor Bi2Sr2CaCu2O8+x using scanning tunnelling microscopy. The onset of the pseudogap phase coincides with the appearance of electronic patterns that have the predicted characteristics of fluctuating stripes. The experiments indicate that stripes are a consequence of pseudogap behaviour rather than its cause.

    • Colin V. Parker
    • , Pegor Aynajian
    •  & Ali Yazdani
  • Letter |

    The acceleration of the expansion of the Universe is attributed to a 'dark energy' component that opposes gravity. These authors report an analysis of the symmetry properties of distant pairs of galaxies from archival data. This allows them to determine that the Universe is flat, and by alternately fixing its spatial geometry and the dark energy equation-of-state parameter, wX, they establish at the 68.3 per cent confidence level that −0.85 > wX > −1.12 and 0.60 < ΩX < 0.80, where ΩX is the abundance of dark energy.

    • Christian Marinoni
    •  & Adeline Buzzi
  • Letter |

    The development of table-top femtosecond electron diffraction sources in recent years has opened up a new way to observe atomic motions in crystalline materials undergoing structural changes. Here, the technique is used to study the charge density wave material 1T-TaS2, where a modulation of the electron density is accompanied by a periodic lattice distortion. In this femtosecond electron diffraction experiment, where atomic motions are observed in response to a 140 femtosecond optical pulse, the periodic lattice distortion is found to collapse on an exceptionally fast timescale (about 250 femtoseconds), indicative of an electronically driven process involving a highly cooperative process.

    • Maximilian Eichberger
    • , Hanjo Schäfer
    •  & R. J. Dwayne Miller
  • Letter |

    Masses of pulsating classical Cepheid supergiants derived from stellar pulsation theory are smaller than the masses derived from stellar evolution theory. An independent determination for a classical Cepheid in a binary system is needed to determine which is correct. These authors report the discovery of a classical Cepheid in the Large Magellanic Cloud. They determine the mass to a precision of one per cent and show that it agrees with its pulsation mass.

    • G. Pietrzyński
    • , I. B. Thompson
    •  & B. Pilecki
  • Letter |

    Bose–Einstein condensation has been observed in several physical systems, but is not predicted to occur for blackbody radiation such as photons. However, it becomes theoretically possible in the presence of thermalization processes that conserve photon number. These authors experimentally realise such conditions, observing Bose–Einstein condensation of photons in a dye-filled optical microcavity. The effect is of interest for fundamental studies and may lead to new coherent ultraviolet sources.

    • Jan Klaers
    • , Julian Schmitt
    •  & Martin Weitz
  • Letter |

    Quantum networks are being developed for computation, communication and simulation. These authors demonstrate a quantum network capable of storing and reading out entanglement among multiple parties. It comprises four atomic memories connected by photonic channels, representing a significant increase in complexity in comparison with previous two-party networks.

    • K. S. Choi
    • , A. Goban
    •  & H. J. Kimble
  • Letter |

    Antihydrogen, the bound state of an antiproton and a positron, has been produced at low energies at CERN since 2002. It is of fundamental interest for testing the standard model of elementary particles and interactions. However, experiments so far have produced antihydrogen that is not confined, precluding detailed study of its structure. Here, trapping of antihydrogen atoms is demonstrated, opening the door to precision measurements on anti atoms.

    • G. B. Andresen
    • , M. D. Ashkezari
    •  & Y. Yamazaki
  • Letter |

    Recent findings indicate that the pseudogap regime in the high-transition-temperature copper oxides constitutes a new phase of matter rather than a mere crossover phenomenon. These authors report inelastic neutron scattering results for HgBa2CuO4+δ that reveal a fundamental collective magnetic mode associated with the unusual order, further supporting this picture. The mode's intensity rises below the pseudogap characteristic temperature and its dispersion is weak.

    • Yuan Li
    • , V. Balédent
    •  & M. Greven
  • Article |

    Within quantum electrodynamics electric charge is energy dependent, and there is a previous claim that charge is affected by gravity (general relativity) with the implication that the charge is reduced at high energies. But that claim has been very controversial. This author reports an analysis demonstrating that quantum gravity corrections to quantum electrodynamics have a quadratic energy dependence that results in the electric charge vanishing at high energies.

    • David J. Toms
  • Letter |

    Single-molecule magnets are molecular complexes with magnetic bistability, and recently it was shown that such a magnetic memory effect is retained for Fe4 clusters when they are wired to a gold surface. These authors have tailored the clusters to have a preferential orientation and form a self-assembled monolayer on the surface. It then becomes possible to observe quantum tunnelling of the magnetization, which shows up as steps in the magnetic hysteresis loop.

    • M. Mannini
    • , F. Pineider
    •  & R. Sessoli
  • Letter |

    The X-ray source M33 X-7 hosts a rapidly spinning, 15.65M¤ black hole orbiting an underluminous, 70M¤ main-sequence companion in a slightly eccentric 3.45-day orbit. Hitherto, there has been no satisfactory explanation for the observed properties. These authors report simulations of evolutionary tracks which reveal that if M33 X-7 started as a primary body of 85M¤–99M¤ and a secondary body of 28M¤–32M¤, in a 2.8–3.1-day orbit, its properties can be consistently explained.

    • Francesca Valsecchi
    • , Evert Glebbeek
    •  & Vassiliki Kalogera
  • Letter |

    Until now, the most distant spectroscopically confirmed galaxies known in the Universe were at redshifts of z = 8.2 and z = 6.96. It is now reported that the galaxy UDFy-38135539 is at a redshift of z = 8.5549 ± 0.0002. The finding has implications for our understanding of the timing, location and nature of the sources responsible for reionization of the Universe after the Big Bang.

    • M. D. Lehnert
    • , N. P. H. Nesvadba
    •  & S. Basa
  • Letter |

    Electrophoresis is a motion of charged dispersed particles relative to a fluid in a uniform electric field. Here it is described how an anisotropic fluid — a nematic liquid crystal — can lead to motion of both charged and neutral particles, even when they are perfectly symmetrical, in any type of electric field. The phenomenon is caused by a distortion in the orientation of the liquid crystals around the particles. The approach could see applications in, for example, display technologies and colloidal assembly and disassembly.

    • Oleg D. Lavrentovich
    • , Israel Lazo
    •  & Oleg P. Pishnyak
  • Letter |

    The peculiar object P/2010 A2, discovered in January 2010, is in an asteroidal orbit in the inner main asteroid belt and was given a cometary designation because of the presence of a trail of material. These authors report observations of P/2010 A2 by the Rosetta spacecraft. They conclude that the trail arose from a single event, an asteroid collision that occurred around 10 February 2009.

    • Colin Snodgrass
    • , Cecilia Tubiana
    •  & K.P. Wenzel
  • Letter |

    High-resolution observations of early galaxies have shown that two-thirds are massive rotating disk galaxies with velocity dispersions typically five times higher than in today's galaxies. These authors report observations of a sample of rare, high-velocity-dispersion disk galaxies. They find that their velocity dispersions are correlated with their star formation rates, but not their masses or gas fractions, suggesting that star formation is the energetic driver of galaxy disk turbulence at all cosmic epochs.

    • Andrew W. Green
    • , Karl Glazebrook
    •  & Robert G. Sharp
  • Letter |

    Many fields would benefit from a simple and efficient method of trapping single particles, but this is extremely difficult when dealing with nanometre-sized objects in solution. These authors show that grooves and pockets etched into fluidic channels that acquire a charge on exposure act as highly effective electrostatic traps. With further optimization, this trapping concept could allow contact-free confinement of single proteins and nanoparticles, their sorting and fractionation, or assembly into high-density arrays.

    • Madhavi Krishnan
    • , Nassiredin Mojarad
    •  & Vahid Sandoghdar
  • Letter |

    Quantum entanglement is a key resource for technologies such as quantum communication and computation. A major question for solid-state quantum information processing is whether an engineered system can display the three-qubit entanglement necessary for quantum error correction. A positive answer to this question is now provided. A circuit quantum electrodynamics device has been used to demonstrate deterministic production of three-qubit entangled states and the first step of basic quantum error correction.

    • L. DiCarlo
    • , M. D. Reed
    •  & R. J. Schoelkopf
  • Letter |

    Nearly forty years ago, Fulde, Ferrell, Larkin and Ovchinnikov (FFLO) proposed an exotic theory of polarized superconductivity. FFLO correlations have never been observed, but it is thought that in one dimension (1D) a state with FFLO correlations occupies a major part of the phase diagram. Now, intriguing measurements are reported of the phase profile of a two-spin mixture of ultracold 6Li atoms trapped in an array of 1D tubes. The findings may provide a hint of FFLO physics.

    • Yean-an Liao
    • , Ann Sophie C. Rittner
    •  & Erich J. Mueller
  • Letter |

    Quantum entanglement is one of the key resources required for quantum computation. In superconducting devices, two-qubit entangled states have been used to implement simple quantum algorithms, but three-qubit states, which can be entangled in two fundamentally different ways, have not been demonstrated. Here, however, three superconducting phase qubits have been used to create and measure these two entangled three-qubit states.

    • Matthew Neeley
    • , Radoslaw C. Bialczak
    •  & John M. Martinis
  • Letter |

    Electron spins generated by phosphorus dopant atoms buried in silicon represent well-isolated quantum bits with long coherence times, but so far the control of such single electrons has been insufficient to use them in this way. These authors report single-shot, time-resolved readout of electron spins in silicon, achieved by coupling the donor atoms to a charge-sensing device called a single-electron transistor. This opens a path to the development of a new generation of quantum computing and spintronic devices in silicon.

    • Andrea Morello
    • , Jarryd J. Pla
    •  & Andrew S. Dzurak
  • Letter |

    Laser cooling has not yet been extended to molecules because of their complex internal structure. At present, the only technique for producing ultracold molecules is to bind ultracold alkali atoms to produce bialkali molecules. These authors experimentally demonstrate laser cooling of the polar molecule strontium monofluoride, reaching temperatures of a few millikelvin or less. The technique should allow the production of molecules at microkelvin temperatures for species that are chemically distinct from bialkalis.

    • E. S. Shuman
    • , J. F. Barry
    •  & D. DeMille
  • Letter |

    A promising approach to realizing a practical quantum bit scheme is the optical control of single electron spins in quantum dots. The reliable preparation and manipulation of the quantum states of such spins have been demonstrated recently. The final challenge is to carry out single-shot measurements of the electron spin without interfering with it. A technique has now been developed that enables such measurement, by coupling one quantum dot to another to produce a quantum dot molecule.

    • A. N. Vamivakas
    • , C.-Y. Lu
    •  & M. Atatüre
  • Letter |

    It was demonstrated recently that passing electrons through a spiral stack of graphite thin films generates an electron beam with orbital angular momentum — analogous to the spiralling wavefronts that can be introduced in photon beams and which have found widespread application. Here, a versatile holographic technique for generating these twisted electron beams is described. Moreover, a demonstration is provided of their potential use in probing a material's magnetic properties.

    • J. Verbeeck
    • , H. Tian
    •  & P. Schattschneider
  • Article |

    Biological systems avoid molecular noise using feedback loops controlling RNA or protein synthesis, but these reactions rely on the stochastic birth and death of molecules. These authors use control and information theory to show that making a genetic network twice as accurate takes 16 times more signalling steps. Nature must therefore call on brute-force solutions to maintain accuracy, and hence does so only when noise suppression is absolutely vital.

    • Ioannis Lestas
    • , Glenn Vinnicombe
    •  & Johan Paulsson
  • Letter |

    Collective motion is a ubiquitous self-organization phenomenon that can be observed in systems ranging from flocks of animals to the cytoskeleton. Similarities between these systems suggest that there are universal underlying principles. This idea can be tested with 'active' or 'driven' fluids, but so far such systems have offered limited parameter control. Here, an active fluid is studied that contains only a few components — actin filaments and molecular motors — allowing the control of all relevant system parameters.

    • Volker Schaller
    • , Christoph Weber
    •  & Andreas R. Bausch
  • Letter |

    There is much interest in graphene for applications in ultrahigh-speed radio-frequency electronics, but conventional device fabrication processes lead to significant defects in graphene. Here a new way of fabricating high-speed graphene transistors is described. A nanowire with a metallic core and insulating shell is placed as the gate electrode on top of graphene, and source and drain electrodes are deposited through a self-alignment process, causing no appreciable damage to the graphene lattice.

    • Lei Liao
    • , Yung-Chen Lin
    •  & Xiangfeng Duan
  • Letter |

    The exciton state in semiconductors, where an electron and hole are paired, has been studied extensively, but the properties of exciton states involving three or more charged particles are largely unknown. These authors use a challenging spectroscopy technique to generate and characterize biexcitons, triexcitons and other, unbound, correlations in a gallium arsenide nanostructure. It was previously unknown whether triexcitons, which involve correlations between six particles, can exist at all.

    • Daniel B. Turner
    •  & Keith A. Nelson
  • Letter |

    Ferroelectric ferromagnets — materials that are both ferroelectric and ferromagnetic — are of significant technological interest. But they are rare, and those that do exist have weak ferroelectric and ferromagnetic properties. Recently a new way of fabricating such materials was proposed, involving strain from the underlying substrate. This route has now been realized experimentally for EuTiO3. The work shows that a single experimental parameter, strain, can simultaneously control multiple order parameters.

    • June Hyuk Lee
    • , Lei Fang
    •  & Darrell G. Schlom
  • Letter |

    Quantum entanglement is widely used in fundamental tests of quantum mechanics and applications such as quantum cryptography. Previous experiments have demonstrated entanglement of optical photons with trapped atoms, ions and atomic ensembles. These authors realize quantum entanglement between the polarization of a single optical photon and a solid-state qubit associated with a single electronic spin. This may provide a key building block for the solid-state realization of quantum optical networks.

    • E. Togan
    • , Y. Chu
    •  & M. D. Lukin
  • 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