• Letter |

    Quantum metrology aims to use entanglement and other quantum resources to improve precision measurement, resulting in Heisenberg limited sensitivity. However, theory suggests that interactions among particles may allow scaling beyond this limit. This study proves experimentally that this can occur in a nonlinear, non-destructive measurement of the magnetization of an atomic ensemble. The work shows that interparticle interactions could be a useful resource for quantum metrology, although the relative performance of nonlinear versus linear measurements has yet to be explored more generally.

    • M. Napolitano
    • , M. Koschorreck
    •  & M. W. Mitchell
  • Letter |

    Neutron scattering measurements of spin fluctuations in hole-doped high-Tc copper oxides have revealed an unusual 'hour-glass' feature in the momentum-resolved magnetic spectrum. There is no widely accepted explanation for this feature. One possibility is that it derives from a pattern of alternating spin and charge stripes. Many copper oxides without stripe order, however, also exhibit an hour-glass spectrum. This paper reports the observation of an hour-glass magnetic spectrum in a hole-doped antiferromagnet from outside the family of superconducting copper oxides. The system has stripe correlations and is an insulator, which means its magnetic dynamics can conclusively be ascribed to stripes. The results provide compelling evidence that the hour-glass spectrum in the copper-oxide superconductors arises from fluctuating stripes.

    • A. T. Boothroyd
    • , P. Babkevich
    •  & P. G. Freeman
  • Letter |

    Inelastic light scattering spectroscopy is a powerful tool in materials science to probe elementary excitations. In a quantum-mechanical picture, these excitations are generated by the incident photons via intermediate electronic transitions. It is now shown that it is possible to manipulate these intermediate 'quantum pathways' using electrostatically doped graphene. A surprising effect is revealed where blocking one pathway results in an increased intensity, unveiling a mechanism of destructive quantum interference between different Raman pathways. The study refines understanding of Raman scattering in graphene and indicates the possibility of controlling quantum pathways to produce unusual inelastic light scattering phenomena.

    • Chi-Fan Chen
    • , Cheol-Hwan Park
    •  & Feng Wang
  • Letter |

    The ease with which waves can travel through a disordered system is theoretically encapsulated in the separation and width of the energy levels, or modes, describing that system. However, extracting this information is experimentally challenging due to the spectral overlap of these modes. Here it is shown how these modal properties can be reconstructed from measurements of the 'speckle' pattern of radiation transmitted through a disordered medium.

    • Jing Wang
    •  & Azriel Z. Genack
  • Article |

    Ultracold atoms in optical lattices provide a versatile tool to investigate fundamental properties of quantum many-body systems. This paper demonstrates control at the most fundamental level, using a laser beam and microwave field to flip the spin of individual atoms at specific sites of an optical lattice. The technique should enable studies of entropy transport and the quantum dynamics of spin impurities, the implementation of novel cooling schemes, engineering of quantum many-body phases and various quantum information processing applications.

    • Christof Weitenberg
    • , Manuel Endres
    •  & Stefan Kuhr
  • Letter |

    There is a strong drive towards observing quantum effects in macroscopic mechanical systems, as this could lead to new insights in quantum-limited measurements as well as test fundamental questions regarding the impossible consequences of quantum physics at a macroscopic scale. To obtain sufficiently long-lived mechanical states the usual approach is to couple a mechanical oscillator to an electromagnetic resonance in a cavity. This study presents a new design for such a system where a free-standing flexible aluminium membrane (like a drum) is incorporated in a cavity defined by a superconducting circuit, and demonstrates a coupling strength that is two orders of magnitude higher than achieved before. The approach shows the way to observing long-lived quantum states that could survive for hundreds of microseconds.

    • J. D. Teufel
    • , Dale Li
    •  & R. W. Simmonds
  • Letter |

    Simulations of interacting particles in classical systems generally involve the Metropolis algorithm. A quantum version of this approach has been hindered by the lack of a means to simulate the equilibrium and static properties of quantum systems. This study overcomes this problem. Its quantum version of the Metropolis algorithm could find widespread application in many body quantum physics, such as computing the binding energies of complex molecules or determining hadron masses in gauge theories.

    • K. Temme
    • , T. J. Osborne
    •  & F. Verstraete
  • Letter |

    Direct observations over the past four centuries show that the number of sunspots observed on the Sun's surface varies periodically. After sunspot cycle 23, the Sun went into a prolonged minimum characterized by a very weak polar magnetic field and an unusually large number of days without sunspots. This study reports kinematic dynamo simulations which demonstrate that a fast meridional flow in the early half of a cycle, followed by a slower flow in the latter half, reproduces both characteristics of the minimum of sunspot cycle 23.

    • Dibyendu Nandy
    • , Andrés Muñoz-Jaramillo
    •  & Petrus C. H. Martens
  • Letter |

    Spin–orbit coupling describes the interaction between a quantum particle's spin and its momentum, and is important for many areas of physics such as spintronics and topological insulators. However, in systems of ultracold neutral atoms, there is no coupling between the spin and the centre of mass motion of the atom. This study uses lasers to engineer such spin–orbit coupling in a neutral atomic Bose–Einstein condensate, the first time this has been achieved for any bosonic system. This should lead to the realization of topological insulators in fermionic neutral atom systems.

    • Y.-J. Lin
    • , K. Jiménez-García
    •  & I. B. Spielman
  • Letter |

    The harmonic oscillator is a simple and ubiquitous physical system. This paper reports a new realization in the quantum regime, achieving direct controllable coupling between quantized mechanical oscillators. The oscillators are ions held in trapping potentials (separated by 40 micrometres) and coupled through their mutual Coulomb interaction. The system could be used as a building block for quantum computers and simulators.

    • K. R. Brown
    • , C. Ospelkaus
    •  & D. J. Wineland
  • Letter |

    A quantum mechanical link between two mechanical oscillators has never been directly demonstrated. This study achieves direct coupling between single ions oscillating in traps separated over a distance of 54 micrometres. Additional ions are employed as antennae to amplify the coupling. The system may provide a building block for quantum computers and opportunities for coupling different types of quantum systems, such as trapped Rydberg atoms.

    • M. Harlander
    • , R. Lechner
    •  & W. Hänsel
  • Article |

    Impressive progress has been achieved in isolating quantum systems from the environment and coherently controlling their dynamics. However, engineering the dynamics of many particles by a controlled coupling to an environment (in an 'open' quantum system) remains largely unexplored. Here, an approach is demonstrated based on ion-trap technology for simulating an open quantum system with up to five qubits. By adding controlled dissipation to coherent operations, the work offers novel prospects for open-system quantum simulation and computation.

    • Julio T. Barreiro
    • , Markus Müller
    •  & Rainer Blatt
  • Letter |

    In the area of metamaterials it is shown that electromagnetic properties can be achieved that are not attainable with natural materials. The main research efforts have been directed towards experimentally realizing materials with negative refractive index, but to extend the potential and design flexibility for novel 'transformation optics' applications, it is of considerable interest to produce a material with unnaturally high refractive index. A broadband, flexible terahertz metamaterial with unprecedented high refractive index, reaching a value of 38.6, is now demonstrated.

    • Muhan Choi
    • , Seung Hoon Lee
    •  & Bumki Min
  • Letter |

    In a significant step forward in complexity and capability for bottom-up assembly of nanoelectronic circuits, this study demonstrates scalable and programmable logic tiles based on semiconductor nanowire transistor arrays. The same logic tile, consisting 496 configurable transistor nodes in an area of about 960 μm2, could be programmed and operated as, among other functions, a full-adder, full-subtractor and multiplexer. The promise is that these logic tiles can be cascaded to realize fully integrated nanoprocessors with computing, memory and addressing capabilities.

    • Hao Yan
    • , Hwan Sung Choe
    •  & Charles M. Lieber
  • Letter |

    Here, the full two-year Hubble Ultra Deep Field (HUDF09) data are used to conduct an ultra-deep search for z≈10 galaxies in the heart of the reionization epoch, only 500 million years after the Big Bang. One possible z≈10 galaxy candidate is found. It is also shown that regardless of source detections, the star formation rate density is much smaller (∼10%) at this time than it is just ∼200 million years later at z≈8. The 100–200 million years prior to z≈10 is clearly a crucial phase in the assembly of the earliest galaxies.

    • R. J. Bouwens
    • , G. D. Illingworth
    •  & L. Bradley
  • Letter |

    Supermassive black holes have been detected in all galaxies that contain bulge components. Bigger black holes are found in bigger bulges, implying that black-hole growth and bulge formation regulate each other. Reports of a similar correlation between black holes and the dark matter haloes suggest that unknown, exotic physics controls black-hole growth. Here it is shown that there is almost no correlation between dark matter and parameters that measure black holes unless the galaxy also contains a bulge. It is concluded that black holes do not correlate directly with dark matter, and that black holes coevolve only with bulges.

    • John Kormendy
    •  & Ralf Bender
  • Letter |

    The masses of supermassive black holes are known to correlate with the properties of the bulge components of their host galaxies. In contrast, they appear not to correlate with galaxy disks. Disk-grown pseudobulges are intermediate in properties between bulges and disks. This paper reports pseudobulge classifications for a sample of nearby galaxies, and combines them with recent measurements of velocity dispersions in the biggest bulgeless galaxies to confirm that black holes do not correlate with disks, and that they correlate little or not at all with pseudobulges.

    • John Kormendy
    • , R. Bender
    •  & M. E. Cornell
  • Letter |

    On rough metallic surfaces hotspots appear under optical illumination that concentrate light to tens of nanometres. This effect can be used to detect molecules, as weak fluorescence signals are strongly enhanced by the hotspots. Such hotspots are associated with localized electromagnetic modes, caused by the randomness of the surface texture, but the detailed profile of the local electromagnetic field is unknown. Here, an ingenious approach is described, making use of the Brownian motion of single molecules to probe the local field. The study succeeds in imaging the fluorescence enhancement profile of single hotspots on the surface of aluminium thin-film and silver nanoparticle clusters with accuracy down to one nanometre, and finds that the field distribution in a hotspot follows an exponential decay.

    • Hu Cang
    • , Anna Labno
    •  & Xiang Zhang
  • Letter |

    This study measures 'puddles' of charge in a fractional quantum Hall device and finds new evidence for the existence of quarter charge particles, thereby boosting confidence in the prospects for topological quantum computation.

    • Vivek Venkatachalam
    • , Amir Yacoby
    •  & Ken West
  • Letter |

    Nuclei with equal neutron (N) and proton (Z) numbers show enhanced correlations that have been predicted to favour an unusual type of pairing, distinct from normal nuclear superfluidity. Here, technically challenging observations are reported of excited states in the N = Z = 46 nucleus 92Pd, from which evidence is inferred for a type of spin-aligned structure in the ground and low-lying excited states, not established in nuclei before and differing from previous predictions.

    • B. Cederwall
    • , F. Ghazi Moradi
    •  & S. Williams
  • Letter |

    Motion of electrons can influence their spins through a fundamental effect called the spin–orbit interaction. Here, a spin–orbit quantum bit (qubit) is implemented in an indium arsenide nanowire, which should offer significant advantages for quantum computing. The spin–orbit qubit is electrically controllable, and information can be stored in the spin. Moreover, nanowires can serve as one dimensional templates for scalable qubit registers, and are suited for both electronic and optical devices.

    • S. Nadj-Perge
    • , S. M. Frolov
    •  & L. P. Kouwenhoven
  • Letter |

    Electron microscopy has advanced to the stage where individual elements can be identified with atomic resolution. Here it is shown to be possible to get fine-structure spectroscopic information of individual light atoms such as those of carbon, and so also probe their chemical state. This capability is illustrated by investigating the edges of a graphene sample, where it is possible to discriminate between single-, double- and triple-coordinated carbon atoms.

    • Kazu Suenaga
    •  & Masanori Koshino
  • Letter |

    Observations of the 21-centimetre line of atomic hydrogen in the early Universe directly probe the history of the reionization of the gas between galaxies. If reionization happened rapidly, there will be a characteristic signature visible against the smooth foreground in an all-sky spectrum. Here, an all-sky average spectrum between 100 and 200 MHz is reported, corresponding to the redshift range 6 < z < 13 for the 21-centimetree line. The data exclude a rapid reionization timescale of Δz < 0.06 at the 95% confidence level.

    • Judd D. Bowman
    •  & Alan E. E. Rogers
  • Letter |

    A primordial carbon-to-oxygen ratio (C/O) greater than 0.8 in an exoplanet causes a carbide-dominated interior, as opposed to the silicate-dominated composition found on Earth; the atmospheres also can differ from those in the Solar System. The solar C/O is 0.54. This study reports an analysis of spectra from the transiting hot Jupiter WASP-12b that reveals that C/O>1 in its atmosphere, based upon the observed concentrations of the prominent molecules CO, CH4 and H2O.

    • Nikku Madhusudhan
    • , Joseph Harrington
    •  & Richard G. West
  • Letter |

    The stellar initial mass function describes the mass distribution of stars at the time of their formation. This study reports observations of the Na I doublet and the Wing-Ford molecular FeH band in the spectra of elliptical galaxies. These lines are strong in stars with masses <0.3 solar masses and weak or absent in all other types of stars. The direct detection of the light of low-mass stars implies that they are very abundant in elliptical galaxies, making up >80% of the total number of stars and contributing >60% of the total stellar mass.

    • Pieter G. van Dokkum
    •  & Charlie Conroy
  • 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