Nature Physics. doi:10.1038/nphys1026

Authors: Arti Garg, Mohit Randeria & Nandini Trivedi

Strong correlations are central to the problem of high-temperature superconductivity in the cuprates. Correlations are responsible for both the Mott insulating, antiferromagnetic state in the parent compounds and for the d-wave superconducting state that arises on doping with mobile charge carriers. An important experimental fact about the superconducting state is its insensitivity to disorder, in marked contrast with conventional theories of d-wave pairing, which predict just the opposite. Here, we generalize the theory of the strongly correlated superconducting ground state based on projected wavefunctions to include impurity effects and find the remarkable result that correlations play a central role in making the superconductor robust against disorder. The nodal quasiparticles, which are the low-energy electronic excitations, are protected against disorder leading to characteristic signatures in scanning tunnelling spectroscopy and angle-resolved photoemission.

Nature Physics. doi:10.1038/nphys1025

Authors: Asaph Widmer-Cooper, Heidi Perry, Peter Harrowell & David R. Reichman

The transition of a fluid to a rigid glass on cooling is a common route of transformation from liquid to solid that embodies the most poorly understood features of both phases. From the liquid perspective, the puzzle is to understand stress relaxation in the disordered state. From the perspective of solids, the challenge is to extend our description of structure and its mechanical consequences to materials without long-range order. Using computer simulations, we show that the localized low-frequency normal modes of a configuration in a supercooled liquid are causally correlated to the irreversible structural reorganization of the particles within this configuration. We also demonstrate that the spatial distribution of these soft local modes can persist in spite of significant particle reorganization. The consequence of these two results is that it is now feasible to construct a theory of relaxation length scales in glass-forming liquids without recourse to dynamics and to explicitly relate molecular properties to their collective relaxation.

Nature Physics. doi:10.1038/nphys1022

Authors: Yuanbo Zhang, Victor W. Brar, Feng Wang, Caglar Girit, Yossi Yayon, Melissa Panlasigui, Alex Zettl & Michael F. Crommie

The honeycomb lattice of graphene is a unique two-dimensional system where the quantum mechanics of electrons is equivalent to that of relativistic Dirac fermions. Novel nanometre-scale behaviour in this material, including electronic scattering, spin-based phenomena and collective excitations, is predicted to be sensitive to charge-carrier density. To probe local, carrier-density-dependent properties in graphene, we have carried out atomically resolved scanning tunnelling spectroscopy measurements on mechanically cleaved graphene flake devices equipped with tunable back-gate electrodes. We observe an unexpected gap-like feature in the graphene tunnelling spectrum that remains pinned to the Fermi level (EF) regardless of graphene electron density. This gap is found to arise from a suppression of electronic tunnelling to graphene states near EF and a simultaneous giant enhancement of electronic tunnelling at higher energies due to a phonon-mediated inelastic channel. Phonons thus act as a ‘floodgate’ that controls the flow of tunnelling electrons in graphene. This work reveals important new tunnelling processes in gate-tunable graphitic layers.

Nature Physics. doi:10.1038/nphys1020

Authors: Cyril Branciard, Nicolas Brunner, Nicolas Gisin, Christian Kurtsiefer, Antia Lamas-Linares, Alexander Ling & Valerio Scarani

Quantum theory predicts and experiments confirm that nature can produce correlations between distant events that are non-local in the sense of violating a Bell inequality. Nevertheless, Bell’s strong sentence ‘Correlations cry out for explanations’ (ref. 2) remains relevant. The maturing of quantum information science and the discovery of the power of non-local correlations, for example for cryptographic key distribution beyond the standard quantum key distribution schemes, strengthen Bell’s wish and make it even more timely. In 2003, Leggett proposed an alternative model for non-local correlations that he proved to be incompatible with quantum predictions. We present here a new approach to this model, along with new inequalities for testing it. These inequalities can be derived in a very simple way, assuming only the non-negativity of probability distributions; they are also stronger than previously published and experimentally tested Leggett-type inequalities. The simplest of the new inequalities is experimentally violated. Then we go beyond Leggett’s model, and show that we cannot ascribe even partially defined individual properties to the components of a maximally entangled pair.

Nature Physics. doi:10.1038/nphys1019

Authors: M. Grajcar, S. H. W. van der Ploeg, A. Izmalkov, E. Il’ichev, H.-G. Meyer, A. Fedorov, A. Shnirman & Gerd Schön

Laser cooling of atoms paved the way for remarkable achievements in quantum optics, including Bose–Einstein condensation and trapping in optical lattices. Recently, superconducting qubits—micrometre-size superconducting circuits—were shown to act as artificial atoms, exhibiting quantum effects such as Rabi oscillations and Ramsey fringes. Coupling superconducting circuits to resonators brought them into the realm of quantum electrodynamics and opened up perspectives for using them as micro-coolers or to create a population inversion inducing lasing behaviour. Here, we demonstrate so-called Sisyphus cooling and amplification of an LC resonator, which consists of an inductor L and a capacitor C, by a superconducting qubit, furthering the analogies between optical and circuit quantum electrodynamics. In quantum optics, the motion of the atom is cooled or amplified by a laser driving its electronic degrees of freedom. In our system, the roles of the two degrees of freedom are played by the levels of the resonator and the qubit. Red-detuned high-frequency driving of the qubit produces cooling, because the low-frequency LC circuit carries out work in the forward and backward oscillation cycle, always increasing the energy of the qubit. For blue-detuning, the same mechanism leads to Sisyphus amplification and a precursor of lasing. Parallel to the experimental demonstration, we analyse these processes theoretically, quantitatively confirming our interpretation.

Nature Physics. doi:10.1038/nphys1021

Authors: W. D. Wise, M. C. Boyer, Kamalesh Chatterjee, Takeshi Kondo, T. Takeuchi, H. Ikuta, Yayu Wang & E. W. Hudson

One of the main challenges in understanding high-Tc superconductivity is to disentangle the rich variety of states of matter that may coexist, cooperate or compete with d-wave superconductivity. At centre stage is the pseudogap phase, which occupies a large portion of the cuprate phase diagram surrounding the superconducting dome. Using scanning tunnelling microscopy, we find that a static, non-dispersive, ‘checkerboard’-like electronic modulation exists in a broad regime of the cuprate phase diagram and exhibits strong doping dependence. The continuous increase of checkerboard periodicity with hole density strongly suggests that the checkerboard originates from charge-density-wave formation in the antinodal region of the cuprate Fermi surface. These results reveal a coherent picture for static electronic orderings in the cuprates and shed important new light on the nature of the pseudogap phase.

Nature Physics. doi:10.1038/nphys1017

Authors: N. Bergeal, J. Lesueur, M. Aprili, G. Faini, J. P. Contour & B. Leridon

The phase diagram of high-temperature superconductors is still to be understood. In the low-carrier-doping regime, a loss of spectral weight in the electronic excitation spectrum—the so-called pseudogap—is observed above the superconducting temperature Tc, and below a characteristic temperature T* (ref. 2). First observed in the spin channel by NMR measurements, the pseudogap has also been observed in the charge channel by scanning probe microscopy and photoemission experiments, for instance. An important issue to address is whether this phenomenon is related to superconductivity or to a competing ‘hidden’ order. In the superconductivity case, it has been suggested that superconducting pairing fluctuations may be responsible, but this view remains to be tested experimentally. Here, we have designed a Josephson-like experiment to probe directly the fluctuating pairs in the normal state. We show that fluctuations survive only in a restricted range of temperature above Tc, well below T*, and therefore cannot explain the opening of the pseudogap at higher temperature.

Nature Physics. doi:10.1038/nphys1016

Authors: Frank Deppe, Matteo Mariantoni, E. P. Menzel, A. Marx, S. Saito, K. Kakuyanagi, H. Tanaka, T. Meno, K. Semba, H. Takayanagi, E. Solano & R. Gross

Superconducting qubits behave as artificial two-level atoms and are used to investigate fundamental quantum phenomena. In this context, the study of multiphoton excitations occupies an important role. Moreover, coupling superconducting qubits to onchip microwave resonators has given rise to the field of circuit quantum electrodynamics (QED). In contrast to quantum-optical cavity QED (refs 16, 17, 18, 19), circuit QED offers the tunability inherent to solid-state circuits. Here, we report on the observation of key signatures of a two-photon-driven Jaynes–Cummings model, which unveils the upconversion dynamics of a superconducting flux qubit coupled to an on-chip resonator. Our experiment and theoretical analysis show clear evidence for the coexistence of one- and two-photon-driven level anticrossings of the qubit–resonator system. This results from the controlled symmetry breaking of the system hamiltonian, causing parity to become a not-well-defined property. Our study provides fundamental insight into the interplay of multiphoton processes and symmetries in a qubit–resonator system.

Nature Physics. doi:10.1038/nphys1002

Authors: S. Nakatsuji, K. Kuga, Y. Machida, T. Tayama, T. Sakakibara, Y. Karaki, H. Ishimoto, S. Yonezawa, Y. Maeno, E. Pearson, G. G. Lonzarich, L. Balicas, H. Lee & Z. Fisk

A long-standing question in the field of superconductivity is whether pairing of electrons can arise in some cases as a result of magnetic interactions instead of electron–phonon-induced interactions as in the conventional Bardeen–Cooper–Schrieffer theory. A major challenge to the idea of magnetically mediated superconductivity has been the dramatically different behaviour of the cerium and ytterbium heavy-fermion compounds. The cerium-based systems are often found to be superconducting, in keeping with a magnetic pairing scenario, but corresponding ytterbium systems, or hole analogues of the cerium systems, are not. Despite searches over two decades there has been no evidence of heavy-fermion superconductivity in an ytterbium system, casting doubt on our understanding of the electron–hole parallelism between the cerium and the ytterbium compounds. Here we present the first empirical evidence that superconductivity is indeed possible in an ytterbium-based heavy-fermion system. In particular, we observe a superconducting transition at Tc=80 mK in high-purity single crystals of YbAlB4 in the new structural β phase. We also observe a novel type of non-Fermi-liquid state above Tc that arises without chemical doping, in zero applied magnetic field and at ambient pressure, establishing β-YbAlB4 as a unique system showing quantum criticality without external tuning.

Nature Physics. doi:10.1038/nphys997

Authors: S. Ospelkaus, A. Pe’er, K.-K. Ni, J. J. Zirbel, B. Neyenhuis, S. Kotochigova, P. S. Julienne, J. Ye & D. S. Jin

Polar molecules have bright prospects for novel quantum gases with long-range and anisotropic interactions, and could find uses in quantum information science and in precision measurements. However, high-density clouds of ultracold polar molecules have so far not been produced. Here, we report a key step towards this goal. We start from an ultracold dense gas of loosely bound 40K87Rb Feshbach molecules with typical binding energies of a few hundred kilohertz, and coherently transfer these molecules in a single transfer step into a vibrational level of the ground-state molecular potential bound by more than 10 GHz. Starting with a single initial state prepared with Feshbach association, we achieve a transfer efficiency of 84%. Given favourable Franck–Condon factors, the presented technique can be extended to access much more deeply bound vibrational levels and those exhibiting a significant dipole moment.

Nature Physics. doi:10.1038/nphys992

Authors: M. Poggio, M. P. Jura, C. L. Degen, M. A. Topinka, H. J. Mamin, D. Goldhaber-Gordon & D. Rugar

Recent advances in the fabrication of microelectromechanical systems and their evolution into nanoelectromechanical systems have enabled researchers to measure extremely small forces, masses and displacements. In particular, researchers have developed position transducers with resolution approaching the uncertainty limit set by quantum mechanics. The achievement of such resolution has implications not only for the detection of quantum behaviour in mechanical systems, but also for a variety of other precision experiments including the bounding of deviations from newtonian gravity at short distances and the measurement of single spins. Here, we demonstrate the use of a quantum point contact as a sensitive displacement detector capable of sensing the low-temperature thermal motion of a nearby micromechanical cantilever. Advantages of this approach include versatility due to its off-board design, compatibility with nanoscale oscillators and, with further development, the potential to achieve quantum-limited displacement detection.

Nature Physics. doi:10.1038/nphys986

Authors: C. Thaury, H. George, F. Quéré, R. Loch, J.-P. Geindre, P. Monot & Ph. Martin

Coherent ultrashort X-ray pulses provide new ways to probe matter and its ultrafast dynamics. One of the promising paths to generate these pulses consists of using a nonlinear interaction with a system to strongly and periodically distort the waveform of intense laser fields, and thus produce high-order harmonics. Such distortions have so far been induced by using the nonlinear polarizability of atoms, leading to the production of attosecond light bursts, short enough to study the dynamics of electrons in matter. Shorter and more intense attosecond pulses, together with higher harmonic orders, are expected by reflecting ultraintense laser pulses on a plasma mirror—a dense (≈1023 electrons cm−3) plasma with a steep interface. However, short-wavelength-light sources produced by such plasmas are known to generally be incoherent. In contrast, we demonstrate that like in usual low-intensity reflection, the coherence of the light wave is preserved during harmonic generation on plasma mirrors. We then exploit this coherence for interferometric measurements and thus carry out a first study of the laser-driven coherent dynamics of the plasma electrons.

Nature Physics. doi:10.1038/nphys983

Authors: Yasuyuki Kato, Qi Zhou, Naoki Kawashima & Nandini Trivedi

Cold atoms in optical lattices provide a unique laboratory for investigating quantum phase transitions between strongly correlated superfluid and Mott insulator phases. One of the major bottlenecks in the analysis of experiments is a clear set of criteria for identifying the superfluid phase. A ‘sharp’ interference pattern in time-of-flight experiments has been widely adopted as a signature of superfluidity. Here, we show that sharp peaks are not a reliable diagnostic of superfluidity. Using large-scale quantum Monte Carlo simulations of the Bose–Hubbard model in three dimensions with up to N=1.4×104 particles, we calculate the momentum distribution n(k) as a function of temperature T and t/U, the ratio of hopping to the onsite repulsion. We find that even above the transition temperature Tc where both superfluid density and condensate fraction vanish, the interference pattern can nevertheless have sharp peaks riding over a broad background. We identify the true signature of the superfluid and give a deeper understanding of why such sharp peaks appear in the normal state.

Nature Physics. doi:10.1038/nphys1024

Authors: Rebecca Flint, M. Dzero & P. Coleman

Nature Physics. doi:10.1038/nphys1018

Authors: Stefano Pirandola, Stefano Mancini, Seth Lloyd & Samuel L. Braunstein

Nature Physics. doi:10.1038/nphys1027

Authors: Nicholas P. Robins, Cristina Figl, Matthew Jeppesen, Graham R. Dennis & John D. Close

Nature Physics. doi:10.1038/nphys999

Authors: D. F. Agterberg & H. Tsunetsugu

Nature Physics. doi:10.1038/nphys994

Authors: G. P. Lansbergen, R. Rahman, C. J. Wellard, I. Woo, J. Caro, N. Collaert, S. Biesemans, G. Klimeck, L. C. L. Hollenberg & S. Rogge

Nature Physics. doi:10.1038/nphys993

Authors: Björn Zimmermann, Daniel Rolles, Burkhard Langer, Rainer Hentges, Markus Braune, Slobodan Cvejanovic, Oliver Geßner, Franz Heiser, Sanja Korica, Toralf Lischke, Axel Reinköster, Jens Viefhaus, Reinhard Dörner, Vincent McKoy & Uwe Becker