Nature Physics. doi:10.1038/nphys1127

Authors: Ophir M. Auslaender, Lan Luan, Eric W. J. Straver, Jennifer E. Hoffman, Nicholas C. Koshnick, Eli Zeldov, Douglas A. Bonn, Ruixing Liang, Walter N. Hardy & Kathryn A. Moler

Superconductors often contain quantized microscopic whirlpools of electrons, called vortices, that can be modelled as one-dimensional elastic objects. Vortices are a diverse area of study for condensed matter because of the interplay between thermal fluctuations, vortex–vortex interactions and the interaction of the vortex core with the three-dimensional disorder landscape. Although vortex matter has been studied extensively, the static and dynamic properties of an individual vortex have not. Here, we use magnetic force microscopy (MFM) to image and manipulate individual vortices in a detwinned YBa2Cu3O6.991 single crystal, directly measuring the interaction of a moving vortex with the local disorder potential. We find an unexpected and marked enhancement of the response of a vortex to pulling when we wiggle it transversely. In addition, we find enhanced vortex pinning anisotropy that suggests clustering of oxygen vacancies in our sample and demonstrates the power of MFM to probe vortex structure and microscopic defects that cause pinning.

Nature Physics. doi:10.1038/nphys1133

Authors: J. S. Lundeen, A. Feito, H. Coldenstrodt-Ronge, K. L. Pregnell, Ch. Silberhorn, T. C. Ralph, J. Eisert, M. B. Plenio & I. A. Walmsley

Measurement connects the world of quantum phenomena to the world of classical events. It has both a passive role—in observing quantum systems—and an active one, in preparing quantum states and controlling them. In view of the central status of measurement in quantum mechanics, it is surprising that there is no general recipe for designing a detector that measures a given observable. Compounding this, the characterization of existing detectors is typically based on partial calibrations or elaborate models. Thus, experimental specification (that is, tomography) of a detector is of fundamental and practical importance. Here, we present the realization of quantum detector tomography. We identify the positive-operator-valued measure describing the detector, with no ancillary assumptions. This result completes the triad, state, process and detector tomography, required to fully specify an experiment. We characterize an avalanche photodiode and a photon-number-resolving detector capable of detecting up to eight photons. This creates a new set of tools for accurately detecting and preparing non-classical light.

Nature Physics. doi:10.1038/nphys1128

Authors: K. Sugawara, T. Sato & T. Takahashi

The discovery of superconductivity in C6Yb and C6Ca (ref. 1) has activated fierce debates on whether it is described within the conventional Bardeen–Cooper–Schrieffer scheme or some other exotic mechanisms are involved, because the superconducting transition temperature (Tc) is significantly higher than that of the alkali-metal graphite intercalation compounds intensively studied in the 1980s (refs 2, 3, 4). The key to understand the mechanism of superconductivity lies in the superconducting energy gap associated with the formation of superconducting pairs. Here, we report the first direct observation of a superconducting gap in C6Ca by high-resolution angle-resolved photoemission spectroscopy. We found that the superconducting gap of 1.8–2.0 meV opens on the intercalant Fermi surface, whereas the gap is very small or absent on the Fermi surface derived from the π* band of graphene layers. These experimental results unambiguously establish that the interlayer band has an essential role for the high-Tc superconductivity in C6Ca.

Nature Physics. doi:10.1038/nphys1109

Authors: R. Daou, Nicolas Doiron-Leyraud, David LeBoeuf, S. Y. Li, Francis Laliberté, Olivier Cyr-Choinière, Y. J. Jo, L. Balicas, J.-Q. Yan, J.-S. Zhou, J. B. Goodenough & Louis Taillefer

A fundamental question for high-temperature superconductors is the nature of the pseudogap phase, which lies between the Mott insulator at zero doping and the Fermi liquid at high doping p (refs 1, 2). Here we report on the behaviour of charge carriers near the zero-temperature onset of this phase, namely at the critical doping p*, where the pseudogap temperature T* goes to zero, accessed by investigating a material in which superconductivity can be fully suppressed by a steady magnetic field. Just below p*, the normal-state resistivity and Hall coefficient of La1.6−xNd0.4SrxCuO4 are found to rise simultaneously as the temperature drops below T*, suggesting a change in the Fermi surface with a large associated drop in conductivity. At p*, the resistivity shows a linear temperature dependence as the temperature approaches zero, a typical signature of a quantum critical point. These findings impose new constraints on the mechanisms responsible for inelastic scattering and Fermi-surface transformation in theories of the pseudogap phase.

Nature Physics. doi:10.1038/nphys1112

Authors: Ruifang Dong, Mikael Lassen, Joel Heersink, Christoph Marquardt, Radim Filip, Gerd Leuchs & Ulrik L. Andersen

The distribution of entangled states between distant parties in an optical network is crucial for the successful implementation of various quantum communication protocols such as quantum cryptography, teleportation and dense coding. However, owing to the unavoidable loss in any real optical channel, the distribution of loss-intolerant entangled states is inevitably afflicted by decoherence, which causes a degradation of the transmitted entanglement. To combat the decoherence, entanglement distillation, a process of extracting a small set of highly entangled states from a large set of less entangled states, can be used. Here we report on the distillation of deterministically prepared light pulses entangled in continuous variables that have undergone non-Gaussian noise. The entangled light pulses are sent through a lossy channel, where the transmission is varying in time similarly to light propagation in the atmosphere. By using linear optical components and global classical communication, the entanglement is probabilistically increased.

Nature Physics. doi:10.1038/nphys1110

Authors: Boris Hage, Aiko Samblowski, James DiGuglielmo, Alexander Franzen, Jaromír Fiurášek & Roman Schnabel

The distribution of entangled states of light over long distances is a major challenge in the field of quantum information. Optical losses, phase diffusion and mixing with thermal states lead to decoherence and destroy the non-classical states after some finite transmission-line length. Quantum repeater protocols, which combine quantum memory, entanglement distillation and entanglement swapping, were proposed to overcome this problem. Here we report on the experimental demonstration of entanglement distillation in the continuous-variable regime. Entangled states were first disturbed by random phase fluctuations and then distilled and purified using interference on beam splitters and homodyne detection. Measurements of covariance matrices clearly indicate a regained strength of entanglement and purity of the distilled states. In contrast to previous demonstrations of entanglement distillation in the complementary discrete-variable regime, our scheme achieved the actual preparation of the distilled states, which might therefore be used to improve the quality of downstream applications such as quantum teleportation.

Nature Physics. doi:10.1038/nphys1106

Authors: A. Camjayi, K. Haule, V. Dobrosavljević & G. Kotliar

Strong correlation effects, such as a marked increase in the effective mass of the carriers of electricity, recently observed in the low-density electron gas have provided spectacular support for the existence of a sharp metal–insulator transition in dilute two-dimensional electron gases. Here, we show that strong correlations, normally expected only for narrow integer-filled bands, can be effectively enhanced even far away from integer-filling, owing to incipient charge ordering driven by non-local Coulomb interactions. This general mechanism is illustrated by solving an extended Hubbard model using dynamical mean-field theory. Our findings account for the key aspects of the experimental phase diagram, and reconcile the early viewpoints of Wigner and Mott. The interplay of short-range charge order and local correlations should result in a three-peak structure in the electron spectral function, which can be observed in tunnelling and optical spectroscopy. These experiments will discriminate between the Wigner–Mott scenario and the alternative perspective that views disorder as the main driving force for the two-dimensional metal–insulator transition.

Nature Physics. doi:10.1038/nphys1103

Authors: E. García Saiz, G. Gregori, D. O. Gericke, J. Vorberger, B. Barbrel, R. J. Clarke, R. R. Freeman, S. H. Glenzer, F. Y. Khattak, M. Koenig, O. L. Landen, D. Neely, P. Neumayer, M. M. Notley, A. Pelka, D. Price, M. Roth, M. Schollmeier, C. Spindloe, R. L. Weber, L.  van Woerkom, K. Wünsch & D. Riley

One of the grand challenges of contemporary physics is understanding strongly interacting quantum systems comprising such diverse examples as ultracold atoms in traps, electrons in high-temperature superconductors and nuclear matter. Warm dense matter, defined by temperatures of a few electron volts and densities comparable with solids, is a complex state of such interacting matter. Moreover, the study of warm dense matter states has practical applications for controlled thermonuclear fusion, where it is encountered during the implosion phase, and it also represents laboratory analogues of astrophysical environments found in the core of planets and the crusts of old stars. Here we demonstrate how warm dense matter states can be diagnosed and structural properties can be obtained by inelastic X-ray scattering measurements on a compressed lithium sample. Combining experiments and ab initio simulations enables us to determine its microscopic state and to evaluate more approximate theoretical models for the ionic structure.

Nature Physics. doi:10.1038/nphys1101

Authors: Hefei Hu, A. Strybulevych, J. H. Page, S. E. Skipetrov & B. A. van Tiggelen

After exactly half a century of Anderson localization, the subject is more alive than ever. Direct observation of Anderson localization of electrons was always hampered by interactions and finite temperatures. Yet, many theoretical breakthroughs were made, highlighted by finite-size scaling, the self-consistent theory and the numerical solution of the Anderson tight-binding model. Theoretical understanding is based on simplified models or approximations and comparison with experiment is crucial. Despite a wealth of new experimental data, with microwaves and light, ultrasound and cold atoms, many questions remain, especially for three dimensions. Here, we report the first observation of sound localization in a random three-dimensional elastic network. We study the time-dependent transmission below the mobility edge, and report ‘transverse localization’ in three dimensions, which has never been observed previously with any wave. The data are well described by the self-consistent theory of localization. The transmission reveals non-Gaussian statistics, consistent with theoretical predictions.

Nature Physics. doi:10.1038/nphys1096

Authors: Meng Khoon Tey, Zilong Chen, Syed Abdullah Aljunid, Brenda Chng, Florian Huber, Gleb Maslennikov & Christian Kurtsiefer

Many quantum information processing protocols require efficient transfer of quantum information from a flying photon to a stationary quantum system. To transfer information, a photon must first be absorbed by the quantum system. This can be achieved, with a probability close to unity, by an atom residing in a high-finesse cavity. However, it is unclear whether a photon can be absorbed effectively by an atom in a free space. Here, we report on an observation of substantial extinction of a light beam by a single 87Rb atom through focusing light to a small spot with a single lens. The measured extinction values can be directly compared to the predictions of existing free-space photon–atom coupling models. Our result should open a new perspective on processing quantum information carried by light using atoms, in particular for experiments that require strong absorption of single photons by an atom in free space.

Nature Physics. doi:10.1038/nphys1090

Authors: M. A. Castellanos-Beltran, K. D. Irwin, G. C. Hilton, L. R. Vale & K. W. Lehnert

It has recently become possible to encode the quantum state of superconducting qubits and the position of nanomechanical oscillators into the states of microwave fields. However, to make an ideal measurement of the state of a qubit, or to detect the position of a mechanical oscillator with quantum-limited sensitivity, requires an amplifier that adds no noise. If an amplifier adds less than half a quantum of noise, it can also squeeze the quantum noise of the electromagnetic vacuum. Highly squeezed states of the vacuum can be used to generate entanglement or to realize back-action-evading measurements of position. Here we introduce a general-purpose parametric device, which operates in a frequency band between 4 and 8 GHz. It adds less than half a noise quantum, it amplifies quantum noise above the added noise of commercial amplifiers and it squeezes quantum fluctuations by 10 dB.

Nature Physics. doi:10.1038/nphys1094

Authors: B. A. Piot, Z. Jiang, C. R. Dean, L. W. Engel, G. Gervais, L. N. Pfeiffer & K. W. West

When a strong magnetic field is applied perpendicularly (along z) to a sheet confining electrons to two dimensions (x–y), highly correlated states emerge as a result of the interplay between electron–electron interactions, confinement and disorder. These so-called fractional quantum Hall liquids form a series of states that ultimately give way to a periodic electron solid that crystallizes at high magnetic fields. This quantum phase of electrons has been identified previously as a disorder-pinned two-dimensional Wigner crystal with broken translational symmetry in the x–y plane. Here, we report our discovery of a new insulating quantum phase of electrons when, in addition to a perpendicular field, a very high magnetic field is applied in a geometry parallel (y direction) to the two-dimensional electron sheet. Our data point towards this new quantum phase being an electron solid in a ‘quasi-three-dimensional’ configuration induced by orbital coupling with the parallel field.