Table of contents

In our original paper (Altin et al 2011 New J. Phys.13 065020), we presented the results from a Ramsey atom interferometer operating with an optically trapped sample of up to 10⁶ Bose-condensed ⁸⁷Rb atoms in the m_F = 0 clock states. We were unable to observe projection noise fluctuations on the interferometer output, which we attribute to the stability of our microwave oscillator and background magnetic field. Numerical simulations of the Gross–Pitaevskii equations for our system show that dephasing due to spatial dynamics driven by interparticle interactions accounts for much of the observed decay in fringe visibility at long interrogation times. The simulations show good agreement with the experimental data when additional technical decoherence is accounted for, and suggest that the clock states are indeed immiscible. With smaller samples of 5 × 10⁴ atoms, we observe a coherence time of τ = 1.0^+0.5_−0.3 s.

The accurate control of the relative phase of multiple distinct sources of radiation produced by high harmonic generation is of central importance in the continued development of coherent extreme UV (XUV) and attosecond sources. Here, we present a novel approach which allows extremely accurate phase control between multiple sources of high harmonic radiation generated within the Rayleigh range of a single-femtosecond laser pulse using a dual-gas, multi-jet array. Fully ionized hydrogen acts as a purely passive medium and allows highly accurate control of the relative phase between each harmonic source. Consequently, this method allows quantum path selection and rapid signal growth via the full coherent superposition of multiple HHG sources (the so-called quasi-phase-matching). Numerical simulations elucidate the complex interplay between the distinct quantum paths observed in our proof-of-principle experiments.

We describe a realistic scheme for coupling atoms or other quantum emitters with an array of coupled optical cavities. We consider open Fabry–Perot microcavities coupled to the emitters. Our central innovation is to connect the microcavities to waveguide resonators, which are in turn evanescently coupled to each other on a photonic chip to form a coupled cavity chain. In this paper, we describe the components, their technical limitations and the factors that need to be determined experimentally. This provides the basis for a detailed theoretical analysis of two possible experiments to realize quantum squeezing and controlled quantum dynamics. We close with an outline of more advanced applications.

We show that attosecond metrology has evolved from proof-of-principle experiments to a level where complex processes can be resolved in time that cannot be accessed using any other existing technique. The cascaded Auger decay following ionization and excitation of the 3d-subshell in Kr with subfemtosecond 94 eV soft x-ray pulses has been energy- and time-resolved in an x-ray pump–infrared probe experiment. This Auger cascade reveals rich multi-electron dynamics, which despite the fact that there are many experimental and theoretical data available, is not yet fully understood. We present time-resolved data showing the sequence of the temporal dynamics in the cascaded Auger decay. The decay time of several groups of lines has been measured, including the lines at the low-energy part of the spectrum, which are predominantly produced by the second-step Auger transitions. Our experimental data reveal long lifetimes (up to 70 fs) of the subvalence excited ionic (intermediate) states in the cascaded resonant Auger decay. Extensive theoretical calculations within the multiconfiguration Dirac–Fock (MCDF) approach show that the observed long lifetime may be attributed to the second-step Auger decay of the resonantly excited 3d⁻¹np states with n = 6,7. Furthermore, our experimental data show that the electrons with a kinetic energy around 25 eV (generally assigned as M_4,5N₁N₁ ¹S₀ normal Auger lines) have a component corresponding to the second-step Auger decay of the ion after resonant Auger transition 3d⁻¹np → 4s² 4p³ 4dnp → 4s² 4p⁴ with a lifetime of 26 ± 4 fs.

Electron spin resonance (ESR) investigation of graphene nanoribbons (GNRs) prepared through longitudinal unzipping of multi-walled carbon nanotubes indicates the presence of C-related dangling bond centers, exhibiting paramagnetic features. ESR signal broadening from pristine or oxidized GNRs is explained in terms of unresolved hyperfine structure, and in the case of reduced GNRs, the broadening of the ESR signal can be due to enhancement of conductivity upon reduction. The spin dynamics observed from ESR line width-temperature data reflect a variable range hopping mechanism through localized states, consistent with resistance-temperature data.

Weak localization is studied in two high-quality epitaxial graphene samples grown on silicon-faced 6H-SiC substrates. Following the methodology of Kozikov et al (2010 Phys. Rev. B 82 075424), we measured the temperature dependence of carrier conductivity at zero and low magnetic (B) fields. In both samples, a logarithmic temperature dependence of the carrier conductivity was observed at B = 0 and its amplitude was larger than predicted by a single-particle model, suggesting that electron–electron interaction plays an important role in electron transport in epitaxial graphene films.

Quantum point contacts are fundamental building blocks for mesoscopic transport experiments and play an important role in recent interference and fractional quantum Hall experiments. However, it is unclear how electron–electron interactions and the random disorder potential influence the confinement potential and give rise to phenomena such as the mysterious 0.7 anomaly. Novel growth techniques of Al_XGa_1−XAs heterostructures for high-mobility two-dimensional electron gases enable us to investigate quantum point contacts with a strongly suppressed disorder potential. These clean quantum point contacts indeed show transport features that are obscured by disorder in standard samples. From these transport data, we are able to extract those parameters of the confinement potential that describe its shape in the longitudinal and transverse directions. Knowing the shape (and hence the slope) of the confinement potential might be crucial for predicting which interaction-induced states can best form in quantum point contacts.

We address the security of continuous-variable quantum key distribution with squeezed states upon the realistic conditions of noisy and lossy environment and limited reconciliation efficiency. Considering the generalized preparation scheme and clearly distinguishing between classical and quantum resources, we investigate the effect of finite squeezing on the tolerance of the protocol to untrusted channel noise. For a long-distance strongly attenuating channel and the consequent low reconciliation efficiency, we show that feasible limited squeezing is surprisingly sufficient to provide the security of Gaussian quantum key distribution in the presence of untrusted noise. We explain the effect by the behaviour of the Holevo quantity, which describes the information leakage and is effectively minimized by the squeezed states.

An exciting theme in condensed matter physics is the search for new states of matter. Over the last few years, a new class of topological states has been discovered—the topological insulator—which greatly expands our knowledge about quantum states. One important topic is the transition from the new topological state to other known states, such as the superconductor or normal band insulator. Graphene at filling factor ν = 0 was known to be a topological insulator (called a quantum Hall metal (QH-metal)) protected by the electron–hole symmetry. A recent surprising experiment indicates that graphene can also be a normal band insulator (called a quantum Hall insulator (QH-insulator)) at ν = 0 in a strong magnetic field. Here we show that a transition from a topological insulator to a band insulator can occur in graphene at ν = 0. The topological transition results from the competition between the magnetic field-driven Peierls-type lattice distortion (originating from the Landau level degeneracy) and random bond fluctuations from the intrinsic sheet-buckling. The critical field that separates a QH-metal from a QH-insulator depends on the strength of bond fluctuation. The picture explains well why the field required for observing the QH-insulator is lower for a cleaner sample.

We investigated the doping of zinc oxide (ZnO) microcrystals with iron and nickel via in situ coherent x-ray diffractive imaging (CXDI) in vacuum. Evaporated thin metal films were deposited onto the ZnO microcrystals. A single crystal was selected and tracked through annealing cycles. A solid state reaction was observed in both iron and nickel experiments using CXDI. A combination of the shrink wrap and guided hybrid–input–output phasing methods were applied to retrieve the electron density. The resolution was 33 nm (half order) determined via the phase retrieval transfer function. The resulting images are nevertheless sensitive to sub-angstrom displacements. The exterior of the microcrystal was found to degrade dramatically. The annealing of ZnO microcrystals coated with metal thin films proved an unsuitable doping method. In addition the observed defect structure of one crystal was attributed to the presence of an array of defects and was found to change upon annealing.

Periodic shunted piezoelectric patches are employed for the design of a tunable, one-dimensional metamaterial. The configuration considered encompasses a beam undergoing longitudinal and transverse motion, and a periodic array of piezoelectric patches with electrodes connected to a resonant electric circuit. The resulting acousto-electrical system is characterized by an internal resonant behavior that occurs at the tuning frequency of the shunting circuits, and is analogous in its operation to other internally resonating systems previously proposed, with the addition of its simple tunability. The performance of the beam is characterized through the application of the transfer matrix approach, which evaluates the occurrence of bandgaps at the tuning frequencies and estimates wave attenuation within such bands. Moreover, a homogenization study is conducted to illustrate the internal resonant characteristics of the system within an analytical framework. Experiments performed on the considered beam structure validate the theoretical predictions and illustrate its internal resonant characteristics and the formation of the related bandgaps.

The simulation of quantum effects requires certain classical resources, and quantifying them is an important step to characterize the difference between quantum and classical physics. For a simulation of the phenomenon of state-independent quantum contextuality, we show that the minimum amount of memory used by the simulation is the critical resource. We derive optimal simulation strategies for important cases and prove that reproducing the results of sequential measurements on a two-qubit system requires more memory than the information-carrying capacity of the system.

The readout of a classical memory can be modelled as a problem of quantum channel discrimination, where a decoder retrieves information by distinguishing the different quantum channels encoded in each cell of the memory (Pirandola 2011 Phys. Rev. Lett.106 090504). In the case of optical memories, such as CDs and DVDs, this discrimination involves lossy bosonic channels and can be remarkably boosted by the use of nonclassical light (quantum reading). Here we generalize these concepts by extending the model of memory from single-cell to multi-cell encoding. In general, information is stored in a block of cells by using a channel-codeword, i.e. a sequence of channels chosen according to a classical code. Correspondingly, the readout of data is realized by a process of 'parallel' channel discrimination, where the entire block of cells is probed simultaneously and decoded via an optimal collective measurement. In the limit of a large block we define the quantum reading capacity of the memory, quantifying the maximum number of readable bits per cell. This notion of capacity is nontrivial when we suitably constrain the physical resources of the decoder. For optical memories (encoding bosonic channels), such a constraint is energetic and corresponds to fixing the mean total number of photons per cell. In this case, we are able to prove a separation between the quantum reading capacity and the maximum information rate achievable by classical transmitters, i.e. arbitrary classical mixtures of coherent states. In fact, we can easily construct nonclassical transmitters that are able to outperform any classical transmitter, thus showing that the advantages of quantum reading persist in the optimal multi-cell scenario.

The colonization of unoccupied territory by invading species, known as range expansion, is a spatially heterogeneous non-equilibrium growth process. We introduce a two-species Eden growth model to analyze the interplay between uni-directional (irreversible) mutations and selection at the expanding front. While the evolutionary dynamics leads to coalescence of both wild-type and mutant clusters, the non-homogeneous advance of the colony results in a rough front. We show that roughening and domain dynamics are strongly coupled, resulting in qualitatively altered bulk and front properties. For beneficial mutations the front is quickly taken over by mutants and growth proceeds Eden-like. In contrast, if mutants grow slower than wild-types, there is an antagonism between selection pressure against mutants and growth by the merging of mutant domains with an ensuing absorbing state phase transition to an all-mutant front. We find that surface roughening has a marked effect on the critical properties of the absorbing state phase transition. While reference models, which keep the expanding front flat, exhibit directed percolation critical behavior, the exponents of the two-species Eden model strongly deviate from it. In turn, the mutation-selection process induces an increased surface roughness with exponents distinct from that of the classical Eden model.

We propose and characterize a two-photon (2P) source that emits in a highly polarized, monochromatic and directional beam, realized by means of a quantum dot embedded in a linearly polarized cavity. In our scheme, the cavity frequency is tuned to half the frequency of the biexciton (two excitons with opposite spins) and largely detuned from the excitons thanks to the large biexciton binding energy. We show how the emission can be Purcell enhanced by several orders of magnitude into the 2P channel for available experimental systems.

In the task cryptographers call bit commitment, one party encrypts a prediction in a way that cannot be decrypted until they supply a key, but has only one valid key. Bit commitment has many applications, and has been much studied, but completely and provably secure schemes have remained elusive. Here we report a new development in physics-based cryptography which gives a completely new way of implementing bit commitment that is perfectly secure. The technique involves sending a quantum state (for instance one or more photons) at light speed in one of two or more directions, either along a secure channel or by quantum teleportation. Its security proof relies on the no-cloning theorem of quantum theory and the no-superluminal-signalling principle of special relativity.

The dynamical spin susceptibility in the pseudogap phase of high-T_c cuprates is examined for two different scenarios: the phase fluctuation (PF) of pairing and the d-density wave (DDW) order. In the PF scenario, while the resonant feature of the spin response at Q = (π,π) is preserved at weak PF, the feature is lost as the fluctuation is increased. The dispersion of the spin response in the PF scenario is simply a broadened version of the spectrum in the d-wave superconducting (DSC) state at all the strengths of PFs being discussed. In the DDW scenario, there is no spin resonant mode found at the AFM wave vector in nature, and the spin response exhibits the almost dispersionless feature markedly different from that in the PF scenario. As far as the spin dynamics in the pseudogap state is concerned, our results support the scenario that the pseudogap phenomenon is derived from a distinct normal-state order.

We study experimentally and theoretically the controlled transfer of harmonically trapped ultracold gases between different quantum states. In particular, we experimentally demonstrate a fast decompression and displacement of both a non-interacting gas and an interacting Bose–Einstein condensate, which are initially at equilibrium. The decompression parameters are engineered such that the final state is identical to that obtained after a perfectly adiabatic transformation despite the fact that the fast decompression is performed in the strongly non-adiabatic regime. During the transfer the atomic sample goes through strongly out-of-equilibrium states, while the external confinement is modified until the system reaches the desired stationary state. The scheme is theoretically based on the invariants of motion and scaling equation techniques and can be generalized to decompression trajectories including an arbitrary deformation of the trap. It is also directly applicable to arbitrary initial non-equilibrium states.

We propose an experimental setup to efficiently measure the dynamic structure factor of ultracold quantum gases. Our method uses the interaction of the trapped atomic system with two different cavity modes, which are driven by external laser fields. By measuring the output fields of the cavity, the dynamic structure factor of the atomic system can be determined. In contrast to previous approaches, the atomic system is not destroyed during the measurement process.

Using molecular-dynamics simulations, we study the processes underlying the stopping of energetic clusters upon impact in matter. We investigate self-bombardment of both a metallic (Cu) and a van-der-Waals bonded (frozen Ar) target. Clusters with sizes up to N = 10⁴ atoms and with energies per atom of E/N = 0.1–1600 eV atom⁻¹ were studied. We find that the stopping force exerted on a cluster follows an N^2/3-dependence with cluster size N; thus large clusters experience less stopping than equi-velocity atoms. In the course of being stopped, the cluster is strongly deformed and attains a roughly pancake shape. Due to the cluster inertia, maximum deformation occurs later than the maximum stopping force. The time scale of projectile stopping is set by t₀, the time the cluster needs to cover its own diameter before impacting the target; it thus depends on both cluster size and velocity. The time when the cluster experiences its maximum stopping force is around (0.7–0.8)t₀. We find that the cluster is deformed with huge strain rates of around 1/2t₀; this amounts to 10¹¹–10¹³ s⁻¹ for the cases studied here.

We present a systematic comparison of conditional structure functions in nine turbulent flows. The flows studied include forced isotropic turbulence simulated on a periodic domain, passive grid wind tunnel turbulence in air and in pressurized SF₆, active grid wind tunnel turbulence (in both synchronous and random driving modes), the flow between counter-rotating discs, oscillating grid turbulence and the flow in the Lagrangian exploration module (in both constant and random driving modes). We compare longitudinal Eulerian second-order structure functions conditioned on the instantaneous large-scale velocity in each flow to assess the ways in which the large scales affect the small scales in a variety of turbulent flows. Structure functions are shown to have larger values when the large-scale velocity significantly deviates from the mean in most flows, suggesting that dependence on the large scales is typical in many turbulent flows. The effects of the large-scale velocity on the structure functions can be quite strong, with the structure function varying by up to a factor of 2 when the large-scale velocity deviates from the mean by ±2 standard deviations. In several flows, the effects of the large-scale velocity are similar at all the length scales we measured, indicating that the large-scale effects are scale independent. In a few flows, the effects of the large-scale velocity are larger on the smallest length scales.

Spatial entropy redistribution plays a key role in adiabatic cooling of ultra-cold lattice gases. We show that high-spin fermions with a spatially variable quadratic Zeeman coupling may allow for the creation of an inner spin-1/2 core surrounded by high-spin wings. The latter are always more entropic than the core at high temperatures and, remarkably, at all temperatures in the presence of frustration. Combining thermodynamic Bethe Ansatz with local density approximation, we study the spatial entropy distribution for the particular case of one-dimensional spin-3/2 lattice fermions in the Mott phase. Interestingly, this spatially dependent entropy opens a possible path for an adiabatic cooling technique that, in contrast to previous proposals, would specifically target the spin degree of freedom. We discuss a possible realization of this adiabatic cooling, which may allow for a highly efficient entropy decrease in the spin-1/2 core and help access antiferromagnetic order in experiments on ultracold spinor fermions.

We demonstrate millikelvin thermometry of laser-cooled trapped ions with high-resolution imaging. This equilibrium approach is independent of the cooling dynamics and has lower systematic error than Doppler thermometry, with ±5 mK accuracy and ±1 mK precision. We used it to observe the highly anisotropic dynamics of a single ion, finding temperatures of <60 mK and >15 K simultaneously along different directions. This thermometry technique can offer new insights into quantum systems sympathetically cooled by ions, including atoms, molecules, nanomechanical oscillators and electric circuits.

We propose a new class of gratings having multiple spatial frequencies. Their design relies on the use of small aperiodic grating sequences as unit cells whose repetition forms a superlattice. The superlattice provides well-defined Fourier components, while the choice of the unit cell structure enables the selection, modulation or suppression of certain Fourier components. Using these gratings to provide distributed feedback in mid-infrared quantum cascade lasers, we demonstrate simultaneous lasing on multiple well-defined and isolated longitudinal modes, each one having a sidemode suppression ratio of about 20 dB.

We propose that the acousto-optical (electro-elastic) coupling of the electric field to strain fields localized around defects in disordered ⁴He causes an increase of the dielectric function with decreasing temperature due to the arrested dynamics of defect excitations. A distribution of such low-energy excitations can be described within the framework of a glass susceptibility of a small volume fraction inside solid ⁴He. Upon lowering the temperature the relaxation time τ(T) of defects increases and an anomaly occurs in the dielectric function (ω,T) when ωτ(T) ∼ 1. Since (ω,T) satisfies the Kramers–Kronig relation, we predict an accompanying peak in the imaginary part of (ω,T) at the same temperature that the largest change in amplitude occurs at a fixed frequency. We also discuss recent measurements of the amplitude of the dynamic dielectric function that indicate a low-temperature anomaly similar to that seen in the resonance frequency of the torsional oscillator and shear modulus experiments.

We investigate a computational device that harnesses the effects of Bose–Einstein condensation to accelerate the speed of finding the solution of optimization problems. Many computationally difficult problems, including NP-complete problems, can be formulated as a ground state search problem. In a Bose–Einstein condensate, below the critical temperature, bosonic particles have a natural tendency to accumulate in the ground state. Furthermore, the speed of attaining this configuration is enhanced as a result of final state stimulation. We propose a physical device that incorporates these basic properties of bosons into the optimization problem, such that an optimized solution is found by a simple cooling of the physical temperature of the device. Using a semiclassical model to calculate the equilibration time for reaching the ground state, we found that this can be sped up by a factor of N, where N is the boson number per site. This allows for the annealing times for reaching a particular error to be systematically decreased by increasing the boson number per site.

We report on the first experimental evidence of localized structures (dissipative solitons) in a one-dimensional optical Fabry–Pérot passive Kerr cavity. The Kerr-like medium is a non-instantaneous, diffusive ultra-thin film of liquid crystal inserted in a low-finesse cavity. Solitons with oscillating tails are experimentally observed in this system that can lock together to form complexes of solitons. The numerical simulations carried out on an infinite-dimensional map describing the intra-cavity field dynamics fully agree with the experimental observations.

A diode is an element blocking flow in one direction, but letting it pass in the other. The most prominent realization of a diode is an electrical rectifier. In this paper, we demonstrate a thermal diode based on standard silicon processing technology using rectification of phonon transport. We use a recently developed detection method to directly visualize the heat flow through such a device fabricated in a thin silicon membrane. The diode consists of an array of differently shaped holes milled into the membrane by focused ion beam processing. In our experiment, we achieve a rectification ratio of the heat current of 1.7 at a temperature of 150 K.

Decoupling of the graphene layer from the ferromagnetic substrate via intercalation of sp metal has recently been proposed as an effective way to realize a single-layer graphene-based spin-filter. Here, the structural and electronic properties of the prototype system, graphene/Al/Ni(111), are investigated via a combination of electron diffraction and spectroscopic methods. These studies are accompanied by state-of-the-art electronic structure calculations. The properties of this prospective Al-intercalation-like system and its possible implementations in future graphene-based devices are discussed.

A novel planar Penning trap is presented, which results from the projection of the well-known three-dimensional cylindrical trap onto the surface of a chip. The introduced trap is also a coplanar-waveguide cavity, similar to those used in circuit quantum electrodynamics experiments with superconducting two-level systems. It opens up the possibility of integrating a single trapped electron, or geonium atom, into quantum circuits. The trap is an elliptical Penning trap, with the magnetic field parallel to the chip's surface. A design procedure is described, which permits the compensation of electric anharmonicities up to sixth order. This should render possible the observation of a single trapped electron and the accurate measurement of its eigenfrequencies, a sine qua non requirement for a useful planar geonium technology.

Despite there being a large body of work, the exact molecular details underlying ion selectivity and transport in the potassium channel have not been fully uncovered. One major reason has been the lack of experimental methods that can probe these mechanisms dynamically on their biologically relevant timescales. Recently, it was suggested that quantum coherence and its interplay with thermal vibration might be involved in mediating ion selectivity and transport. In this paper, we present an experimental strategy for using time-resolved infrared spectroscopy to investigate these effects. We show the feasibility by demonstrating the infrared (IR) absorption and Raman spectroscopic signatures of the potassium-binding model molecules that mimic the transient interactions of potassium with binding sites of the selectivity filter during ion conduction. In addition to guiding our experiments on the real system, we have performed molecular dynamic-based simulations of the FTIR and two-dimensional IR (2DIR) spectra of the entire KcsA complex, which is the largest complex for which such modeling has been performed. We found that by combining isotope labeling with 2DIR spectroscopy, the signatures of potassium interaction with individual binding sites would be experimentally observable, and we identified specific labeling combinations that would maximize our expected experimental signatures.

We formulate a multi-band generalization of the time-dependent Gutzwiller theory. This approach allows for the calculation of general two-particle response functions, which are crucial for an understanding of various experiments in solid-state physics. As a first application, we study the momentum- and frequency-resolved magnetic susceptibility in a two-band Hubbard model. As in the case of underlying ground-state approaches, we find significant differences between the results of our method and those from a time-dependent Hartree–Fock approximation.

We calculated and measured the density distribution and cloud size of a trapped two-dimensional (2D) ⁶Li Fermi gas near a Feshbach resonance at low temperatures. Density distributions and cloud sizes were calculated for a wide range of interaction parameters using a local density approximation (LDA) and a zero-temperature equation of state obtained from quantum Monte Carlo simulations reported by Bertaina and Giorgini (2011 Phys. Rev. Lett.106 110403). We found that LDA predictions agree well with experimental measurements across a Feshbach resonance. Theoretical results for Tan's contact parameter in a trapped gas are reported here along with predictions for the static structure factor at large momentum which could be measured in future Bragg spectroscopy experiments on 2D Fermi gases.

We introduce a concept of squeezing in collective qutrit systems through a geometrical picture connected to the deformation of the isotropic fluctuations of su(3) operators when evaluated in a coherent state. This kind of squeezing can be generated by Hamiltonians nonlinear in the generators of su(3) algebra. A simplest model of such a nonlinear evolution is analyzed in terms of semiclassical evolution of the SU(3) Wigner function.

A master equation derived from non-Markovian quantum state diffusion is used to calculate the excitation energy transfer in the photosynthetic Fenna–Matthews–Olson pigment–protein complex at various temperatures. This approach allows us to treat spectral densities that explicitly contain the coupling to internal vibrational modes of the chromophores. Moreover, the method is very efficient and as a result the transfer dynamics can be calculated within about 1 min on a standard PC, making systematic investigations w.r.t. parameter variations tractable. After demonstrating that our approach is able to reproduce the results of the numerically exact hierarchical equations of motion approach, we show how the inclusion of vibrational modes influences the transfer.

The characteristics of a corona discharge in atmospheric pressure air are studied using pulsed power generators that produce voltage pulses of different durations, polarities and shapes. The characteristics are measured in the single pulse, batch, and repetitively pulsed modes. It is shown that no matter what the voltage pulse polarity is, a corona discharge starts developing as a conical diffuse discharge near the electrode tip with a voltage rate of increase of ∼10¹⁵ V s⁻¹ across an electrode of small curvature radius. With lower voltage rate of increase (∼10¹³ V s⁻¹ or lower), one or several diffuse jets develop from this electrode. The diameter of the jets at their front is less than 1 mm and depends on many factors (voltage pulse amplitude and increase, inter-electrode gap width, pulse repetition rate, etc). It is found that at long voltage pulse durations, the radiation spectrum of the corona discharge changes, and the bands and lines of the material of the electrode appear in the UV region at 200–300 nm. It is demonstrated that a runaway electron beam in a corona discharge is generated and detected at a distance several times greater than the brightly glowing plasma region of the corona discharge. It is shown that x-rays are generated from a corona discharge at high pulse repetition rates of up to 1 kHz.

We use the mathematical language of sheaf theory to give a unified treatment of non-locality and contextuality, in a setting that generalizes the familiar probability tables used in non-locality theory to arbitrary measurement covers; this includes Kochen–Specker configurations and more. We show that contextuality, and non-locality as a special case, correspond exactly to obstructions to the existence of global sections. We describe a linear algebraic approach to computing these obstructions, which allows a systematic treatment of arguments for non-locality and contextuality. We distinguish a proper hierarchy of strengths of no-go theorems, and show that three leading examples—due to Bell, Hardy and Greenberger, Horne and Zeilinger, respectively—occupy successively higher levels of this hierarchy. A general correspondence is shown between the existence of local hidden-variable realizations using negative probabilities, and no-signalling; this is based on a result showing that the linear subspaces generated by the non-contextual and no-signalling models, over an arbitrary measurement cover, coincide. Maximal non-locality is generalized to maximal contextuality, and characterized in purely qualitative terms, as the non-existence of global sections in the support. A general setting is developed for the Kochen–Specker-type results, as generic, model-independent proofs of maximal contextuality, and a new combinatorial condition is given, which generalizes the 'parity proofs' commonly found in the literature. We also show how our abstract setting can be represented in quantum mechanics. This leads to a strengthening of the usual no-signalling theorem, which shows that quantum mechanics obeys no-signalling for arbitrary families of commuting observables, not just those represented on different factors of a tensor product.

Even before the experimental discovery of spin- and charge-stripe order in La_2−x−yNd_ySr_xCuO₄ and La_2−xBa_xCuO₄ at x = 1/8, stripe formation was predicted from theoretical considerations. Nevertheless, a consistent description of the complex coexistence of stripe order with superconductivity has remained a challenge. Here we introduce a Hartree–Fock decoupling scheme that unifies previous approaches and allows for a detailed analysis of the competition between antiferromagnetism and superconductivity in real and momentum space. We identify two distinct parameter regimes, where spin-stripe order coexists with either one- or two-dimensional superconductivity; experiments on different striped cuprates are compatible with either the former or the latter regime. We argue that the cuprates at x = 1/8 fall into an intermediate coupling regime with a crossover to long-range phase coherence between individual superconducting stripes.

We study the scenarios where a finite set of non-demolition von Neumann measurements is available. We note that, in some situations, the repeated application of such measurements allows estimation of an infinite number of parameters of the initial quantum state, and we illustrate the point with a physical example. We also study how the system under consideration is perturbed after several rounds of projective measurements. While in the finite dimensional case the effect of this perturbation always saturates, there are instances of infinite dimensional systems where such a perturbation is accumulative, and the act of retrieving information about the system increases its energy indefinitely (i.e. we have 'heat vision'). We analyze this effect and discuss a specific physical system with two dichotomic von Neumann measurements where heat vision is expected to show.

We present a detailed analysis of the upper critical field for CeCoIn₅ under high pressure. We show that, consistent with other measurements, this system shows a decoupling between the maximum of the superconducting transition temperature T_c and the maximum pairing strength. We propose a model in which, in order to account for the discrepancy in pressure between the maximum of the upper critical field and the maximum of T_c, we introduce magnetic pair-breaking effects, already widely suggested by other measurements. We found that within the Eliashberg frame work, the unusual shape of H_c2(T) can be completely reproduced when magnetic pair breaking is taken into account. Surprisingly, we found that the maximum of pair breaking and of pair coupling coincide in pressure, suggesting that both mechanisms originate from quantum criticality. Our model implies that CeCoIn₅ is the first compound of its family that shows clear decoupling between the maximum of T_c and quantum criticality.

Reactive oxygen species (ROS) can be produced by electrical discharges and can be transported in uncharged regions by gas flows, in the so-called afterglows. These species are well known to have bactericidal effects but interaction mechanisms that occur with living micro-organisms remain misunderstood. In order to better understand these interactions, new analysis approaches are necessary. High-lateral-resolution secondary ion mass spectrometry (NanoSIMS) is one of the most promising ways of retrieving additional information on bacteria plasma inactivation mechanisms by combining isotopic imaging of plasma-treated bacteria and the use of ¹⁸O₂ as process gas. Indeed, this technology combines a lateral resolution of a few tens of nanometres that is sufficient to image the interior of bacteria, and a high mass resolution allowing detection of isotopes present in low quantities (a few ppm or lower) within the bacteria. The present paper deals with Ar–¹⁸O₂ (2%) plasma treatment, through low-pressure microwave late afterglows, of Escherichia coli bacteria and their elemental and isotopic imaging by NanoSIMS. E. coli bacteria have been exposed to this reactive medium for varying treatment duration while keeping all other parameters unchanged. Our main goal is to determine whether the quantity of ¹⁸O fixed in treated bacteria and the NanoSIMS50 lateral resolution are sufficient to give additional information on E. coli bacteria–plasma interaction.

EuC₂ is a ferromagnet with a Curie temperature of T_C ≃ 15 K. It is semiconducting with the particularity that the resistivity drops by about five orders of magnitude on cooling through T_C, which is therefore called a metal–insulator transition. In this paper, we study the magnetization, specific heat, thermal expansion and the resistivity around this ferromagnetic transition on high-quality EuC₂ samples. At T_C we observe well-defined anomalies in the specific heat c_p(T) and thermal expansion α(T) data. The magnetic contributions of c_p(T) and α(T) can satisfactorily be described within a mean-field theory, taking into account the magnetization data. In zero magnetic field, the magnetic contributions of the specific heat and thermal expansion fulfil a Grüneisen scaling, which is not preserved in finite fields. From an estimation of the pressure dependence of T_C via Ehrenfest's relation, we expect a considerable increase of T_C under applied pressure due to a strong spin–lattice coupling. Furthermore, the influence of weak off-stoichiometries δ in EuC_2±δ was studied. It is found that δ strongly affects the resistivity, but hardly changes the transition temperature. In all these aspects, the behaviour of EuC₂ strongly resembles that of EuO.

We experimentally demonstrate that a superconducting nanowire single-photon detector is deterministically controllable by bright illumination. We found that bright light can temporarily make a large fraction of the nanowire length normally conductive, can extend deadtime after a normal photon detection, and can cause a hotspot formation during the deadtime with a highly nonlinear sensitivity. As a result, although based on different physics, the superconducting detector turns out to be controllable by virtually the same techniques as avalanche photodiode detectors. As demonstrated earlier, when such detectors are used in a quantum key distribution system, this allows an eavesdropper to launch a detector control attack to capture the full secret key without this being revealed by too many errors in the key.

Quantum Hamiltonian complexity, an emerging area at the intersection of condensed matter physics and quantum complexity theory, studies the properties of local Hamiltonians and their ground states. In this paper we focus on a seemingly specialized technical tool, the detectability lemma (DL), introduced in the context of the quantum PCP challenge (Aharonov et al 2009 arXiv:0811.3412), which is a major open question in quantum Hamiltonian complexity. We show that a reformulated version of the lemma is a versatile tool that can be used in place of the celebrated Lieb–Robinson (LR) bound to prove several important results in quantum Hamiltonian complexity. The resulting proofs are much simpler, more combinatorial and provide a plausible path toward tackling some fundamental open questions in Hamiltonian complexity. We provide an alternative simpler proof of the DL that removes a key restriction in the original statement (Aharonov et al 2009 arXiv:0811.3412), making it more suitable for the broader context of quantum Hamiltonian complexity. Specifically, we first use the DL to provide a one-page proof of Hastings' result that the correlations in the ground states of gapped Hamiltonians decay exponentially with distance (Hastings 2004 Phys. Rev. B 69 104431). We then apply the DL to derive a simpler and more intuitive proof of Hastings' seminal one-dimensional (1D) area law (Hastings 2007 J. Stat. Mech. (2007) P8024) (both these proofs are restricted to frustration-free systems). Proving the area law for two and higher dimensions is one of the most important open questions in the field of Hamiltonian complexity, and the combinatorial nature of the DL-based proof holds out hope for a possible generalization. Indeed, soon after the first publication of the methods presented here, they were applied to derive exponential improvements to Hastings' result (Arad et al 2011, Aharonov et al 2011) in the case of frustration-free 1D systems. Finally, we also provide a more general explanation of how the DL can be used to replace the LR bound.

We report our results on the experimental total cross sections (TCSs) for positron scattering from the isoelectronic molecules N₂, CO and C₂H₂. Where possible, for each species, comparison is made between the present results and those from earlier measurements and calculations. The agreement between the present and earlier experimental results, within the overall uncertainties on the data, is typically satisfactory for energies greater than about 8 eV, but is only marginal at lower positron impact energies. While N₂, CO and C₂H₂ possess 14 electrons each, we find significant differences in the magnitudes of their respective TCSs and subtle differences in the energy dependence of these TCSs. These details are discussed in depth in this paper.

The role of quantum mechanics in biological organisms has been a fundamental question of twentieth-century biology. It is only now, however, with modern experimental techniques, that it is possible to observe quantum mechanical effects in bio-molecular complexes directly. Indeed, recent experiments have provided evidence that quantum effects such as wave-like motion of excitonic energy flow, delocalization and entanglement can be seen even in complex and noisy biological environments (Engel et al 2007 Nature446 782; Collini et al 2010 Nature463 644; Panitchayangkoon et al 2010 Proc. Natl Acad. Sci. USA107 12766). Motivated by these observations, theoretical work has highlighted the importance of an interplay between environmental noise and quantum coherence in such systems (Mohseni et al 2008 J. Chem. Phys.129 174106; Plenio and Huelga 2008 New J. Phys.10 113019; Olaya-Castro et al 2008 Phys. Rev. B 78 085115; Rebentrost et al 2009 New J. Phys.11 033003; Caruso et al 2009 J. Chem. Phys.131 105106; Ishizaki and Fleming 2009 J. Chem. Phys.130 234111). All of this has led to a surge of interest in the exploration of quantum effects in biological systems in order to understand the possible relevance of non-trivial quantum features and to establish a potential link between quantum coherence and biological function. These studies include not only exciton transfer across light harvesting complexes, but also the avian compass (Ritz et al 2000 Biophys. J.78 707), and the olfactory system (Turin 1996 Chem. Sens.21 773; Chin et al 2010 New J. Phys.12 065002).

These examples show that the full understanding of the dynamics at bio-molecular length (10 Å) and timescales (sub picosecond) in noisy biological systems can uncover novel phenomena and concepts and hence present a fertile ground for truly multidisciplinary research.

Quantum entanglement offers considerable advantages in how the sensitivity of interferometric precision scales with the resources available. However, when using entangled states it is important that a balance is found between the phase sensitivity of the state and its robustness to particle loss. Recent work (Dorner et al 2009 Phys. Rev. Lett.102 040403) has optimized this balance and found the ideal initial state for any given loss rate in a two-path interferometer. Here we describe a route towards achieving precisions close to that of the theoretical optimum by using beam splitters with variable reflectivity. We also discuss how adopting a multipath approach may prove to be important in future metrology schemes that aim to surpass the standard quantum limit.

We investigate imaging by spherically symmetric absolute instruments that provide perfect imaging in the sense of geometrical optics. We derive a number of properties of such devices, present a general method for designing them and use this method to propose several new absolute instruments, in particular a lens providing a stigmatic image of an optically homogeneous region and having a moderate refractive index range.

We investigate the optical properties of periodic composites containing metamaterial inclusions in a normal material matrix. We consider the case when these inclusions have sharp corners and, following Hetherington and Thorpe, use analytic results to argue that it is then possible to deduce the shape of the corner (its included angle) by measurements of the absorptance of such composites when the scale size of the inclusions and period cell is much finer than the wavelength. These analytic arguments are supported by highly accurate numerical results for the effective permittivity function of such composites as a function of the permittivity ratio of inclusions to the matrix. The results show that this function has a continuous spectral component with limits independent of the area fraction of inclusions, and with the same limits for both square and staggered square arrays. For staggered arrays where the squares are almost touching, the absorption spectrum is an extremely sensitive probe of the inclusion separation distance and acts like a Vernier scale.

In this paper, we present a method for the evaluation of the signal-to-noise ratio in magnetic resonance imaging (MRI) coils loaded with resonant ring metamaterial lenses, in the presence of a conducting phantom resembling human tissue. The method accounts for the effects of the discrete and finite structure of the metamaterial. Numerical computations are validated with experimental results, including laboratory measurements and MRI experiments.

We describe a new method to measure atom beam velocity in an atom interferometer using phase choppers. Phase choppers are analogous to mechanical chopping discs, but rather than being transmitted or blocked by mechanical choppers, an atom receives different differential phase shifts (e.g. zero or π radians) from phase choppers. Phase choppers yield 0.1% uncertainty measurements of beam velocity in our interferometer with 20 min of data and enable new measurements of polarizability with unprecedented precision.

We explore the phase structure of a holographic toy model of superfluid states in non-relativistic conformal field theories. At low background mass density, we found a familiar second-order transition to a superfluid phase at finite temperature. Increasing the chemical potential for the probe charge density drives this transition strongly first order as the low-temperature superfluid phase merges with a thermodynamically disfavored high-temperature condensed phase. At high background mass density, the system re-enters the normal phase as the temperature is lowered further, hinting at a zero-temperature quantum phase transition as the background density is varied. Given the unusual thermodynamics of the background black hole, however, it seems likely that the true ground state is another configuration altogether.

Integrated quantum photonics is an appealing platform for quantum information processing, quantum communication and quantum metrology. In all these applications it is necessary not only to be able to create and detect Fock states of light but also to programme the photonic circuits that implement some desired logical operation. Here we demonstrate a reconfigurable controlled two-qubit operation on a chip using a multiwaveguide interferometer with a tunable phase shifter. We found excellent agreement between theory and experiment, with a 0.98 ± 0.02 average similarity between measured and ideal operations.

Neutrons are suitable tools for quantum experiments since they are massive, experience nuclear, electromagnetic and gravitational interaction and are easy to manipulate and to detect. Perfect crystal interferometry opened new possibilities to explore quantum phenomena on a new ground. The 4π-symmetry of spinor wave functions, the spin-superposition law, topological quantum phases and various gravitational effects have been examined using this method. Experiments exploiting contextuality and Kochen–Specker phenomena exhibit intrinsic entanglement in single particle systems. This may have consequences for a deeper understanding of quantum physics and for applications in future quantum communication systems.

We present a detailed study of quantum simulations of coupled spin systems in surface-electrode (SE) ion-trap arrays, and illustrate our findings with a proposed implementation of the hexagonal Kitaev model (Kitaev A 2006 Ann. Phys.321 2). The effective (pseudo)spin interactions making up such quantum simulators are found to be proportional to the dipole–dipole interaction between the trapped ions, and are mediated by motion that can be driven by state-dependent forces. The precise forms of the trapping potentials and the interactions are derived in the presence of an SE and a cover electrode. These results are the starting point to derive an optimized SE geometry for trapping ions in the desired honeycomb lattice of Kitaev's model, where we design the dipole–dipole interactions in a way that allows for coupling all three bond types of the model simultaneously, without the need for time discretization. Finally, we propose a simple wire structure that can be incorporated into a microfabricated chip to generate localized state-dependent forces which drive the couplings prescribed by this particular model; such a wire structure should be adaptable to many other situations.

Quantum degenerate Fermi gases and Bose–Einstein condensates give access to a vast new class of quantum states. The resulting multi-particle correlations place extreme demands on the detection schemes. Here we introduce diffractive dark-ground imaging as a novel ultra-sensitive imaging technique. Using only moderate detection optics, we image clouds of less than 30 atoms with near-atom shot-noise-limited signal-to-noise ratio and show Stern–Gerlach separated spinor condensates with a minority component of only seven atoms. This presents an improvement of more than one order of magnitude when compared to our standard absorption imaging. We also examine the optimal conditions for absorption imaging, including saturation and fluorescence contributions. Finally, we discuss potentially serious imaging errors of small atom clouds whose size is near the resolution of the optics.

Clinical diagnosis of skin cancers is based on several morphological criteria, among which is the presence of microstructures (e.g. dots and nests) sparsely distributed within the tumour lesion. In this study, we demonstrate that these patterns might originate from a phase separation process. In the absence of cellular proliferation, in fact, a binary mixture model, which is used to represent the mechanical behaviour of skin cancers, contains a cell–cell adhesion parameter that leads to a governing equation of the Cahn–Hilliard type. Taking into account a reaction–diffusion coupling between nutrient consumption and cellular proliferation, we show, with both analytical and numerical investigations, that two-phase models may undergo a spinodal decomposition even when considering mass exchanges between the phases. The cell–nutrient interaction defines a typical diffusive length in the problem, which is found to control the saturation of a growing separated domain, thus stabilizing the microstructural pattern. The distribution and evolution of such emerging cluster morphologies, as predicted by our model, are successfully compared to the clinical observation of microstructural patterns in tumour lesions.

Cancer results from a sequence of genetic and epigenetic changes that lead to a variety of abnormal phenotypes including increased proliferation and survival of somatic cells and thus to a selective advantage of pre-cancerous cells. The notion of cancer progression as an evolutionary process has been attracting increasing interest in recent years. A great deal of effort has been made to better understand and predict the progression to cancer using mathematical models; these mostly consider the evolution of a well-mixed cell population, even though pre-cancerous cells often evolve in highly structured epithelial tissues. In this study, we propose a novel model of cancer progression that considers a spatially structured cell population where clones expand via adaptive waves. This model is used to assess two different paradigms of asexual evolution that have been suggested to delineate the process of cancer progression. The standard scenario of periodic selection assumes that driver mutations are accumulated strictly sequentially over time. However, when the mutation supply is sufficiently high, clones may arise simultaneously on distinct genetic backgrounds, and clonal adaptation waves interfere with each other. We find that in the presence of clonal interference, spatial structure increases the waiting time for cancer, leads to a patchwork structure of non-uniformly sized clones and decreases the survival probability of virtually neutral (passenger) mutations, and that genetic distance begins to increase over a characteristic length scale L_c. These characteristic features of clonal interference may help us to predict the onset of cancers with pronounced spatial structure and to interpret spatially sampled genetic data obtained from biopsies. Our estimates suggest that clonal interference likely occurs in the progression of colon cancer and possibly other cancers where spatial structure matters.

We present results illustrating the construction of three-dimensional (3D) topological cluster states with coherent state logic. Such a construction would be ideally suited for wave-guide implementations of optical quantum information processing. We investigate the use of a deterministic controlled-Z gate, showing that given large enough initial cat states, it is possible to build large 3D cluster states. We model X and Z basis measurements by displaced photon number detections and x-quadrature homodyne detections, respectively. We investigate whether teleportation can aid in cluster state construction and whether this introduction of located loss errors fits within the topological cluster state framework.