Showing papers in "Physical Review Letters in 2010"
TL;DR: The electronic properties of ultrathin crystals of molybdenum disulfide consisting of N=1,2,…,6 S-Mo-S monolayers have been investigated by optical spectroscopy and the effect of quantum confinement on the material's electronic structure is traced.
Abstract: The electronic properties of ultrathin crystals of molybdenum disulfide consisting of N=1,2,…,6 S-Mo-S monolayers have been investigated by optical spectroscopy Through characterization by absorption, photoluminescence, and photoconductivity spectroscopy, we trace the effect of quantum confinement on the material's electronic structure With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by more than 06 eV This leads to a crossover to a direct-gap material in the limit of the single monolayer Unlike the bulk material, the MoS₂ monolayer emits light strongly The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 10⁴ compared with the bulk material
12,822 citations
TL;DR: The measurement of the supercurrent through the junction allows one to discern topologically distinct phases and observe a topological phase transition by simply changing the in-plane magnetic field or the gate voltage, which will be a direct demonstration of the existence of Majorana particles.
Abstract: We propose and analyze theoretically an experimental setup for detecting the elusive Majorana particle in semiconductor-superconductor heterostructures. The experimental system consists of one-dimensional semiconductor wire with strong spin-orbit Rashba interaction embedded into a superconducting quantum interference device. We show that the energy spectra of the Andreev bound states at the junction are qualitatively different in topologically trivial (i.e., not containing any Majorana) and nontrivial phases having an even and odd number of crossings at zero energy, respectively. The measurement of the supercurrent through the junction allows one to discern topologically distinct phases and observe a topological phase transition by simply changing the in-plane magnetic field or the gate voltage. The observation of this phase transition will be a direct demonstration of the existence of Majorana particles.
2,702 citations
TL;DR: It is argued that zero-energy Majorana bound states are formed in various situations when such wires are situated in proximity to a conventional s-wave superconductor when the external magnetic field, the superconducting gap, or the chemical potential vary along the wire.
Abstract: We show that the combination of spin-orbit coupling with a Zeeman field or strong interactions may lead to the formation of a helical electron liquid in single-channel quantum wires, with spin and velocity perfectly correlated. We argue that zero-energy Majorana bound states are formed in various situations when such wires are situated in proximity to a conventional s-wave superconductor. This occurs when the external magnetic field, the superconducting gap, or, most simply, the chemical potential vary along the wire. These Majorana states do not require the presence of a vortex in the system. Experimental consequences of the helical liquid and the Majorana states are also discussed.
2,470 citations
TL;DR: A class of interatomic potential models that can be automatically generated from data consisting of the energies and forces experienced by atoms, as derived from quantum mechanical calculations, are introduced.
Abstract: We introduce a class of interatomic potential models that can be automatically generated from data consisting of the energies and forces experienced by atoms, as derived from quantum mechanical calculations. The models do not have a fixed functional form and hence are capable of modeling complex potential energy landscapes. They are systematically improvable with more data. We apply the method to bulk crystals, and test it by calculating properties at high temperatures. Using the interatomic potential to generate the long molecular dynamics trajectories required for such calculations saves orders of magnitude in computational cost.
1,923 citations
TL;DR: The heterostructure proposed is a semiconducting thin film sandwiched between an s-wave superconductor and a magnetic insulator which can be used as the platform for topological quantum computation by virtue of the existence of non-Abelian Majorana fermions.
Abstract: We show that a film of a semiconductor in which $s$-wave superconductivity and Zeeman splitting are induced by the proximity effect, supports zero-energy Majorana fermion modes in the ordinary vortex excitations Since time-reversal symmetry is explicitly broken, the edge of the film constitutes a chiral Majorana wire The heterostructure we propose---a semiconducting thin film sandwiched between an $s$-wave superconductor and a magnetic insulator---is a generic system which can be used as the platform for topological quantum computation by virtue of the existence of non-Abelian Majorana fermions
1,494 citations
TL;DR: The transmission matrix of a thick random scattering sample is determined and it is shown that this matrix exhibits statistical properties in good agreement with random matrix theory and allows light focusing and imaging through the random medium.
Abstract: We introduce a method to experimentally measure the monochromatic transmission matrix of a complex medium in optics. This method is based on a spatial phase modulator together with a full-field interferometric measurement on a camera. We determine the transmission matrix of a thick random scattering sample. We show that this matrix exhibits statistical properties in good agreement with random matrix theory and allows light focusing and imaging through the random medium. This method might give important insight into the mesoscopic properties of a complex medium.
1,455 citations
TL;DR: A necessary and sufficient condition for the existence of nonzero quantum discord for any dimensional bipartite states is obtained, and a geometrical way of quantifying quantum discord is proposed.
Abstract: Quantum discord characterizes "nonclassicality" of correlations in quantum mechanics. It has been proposed as the key resource present in certain quantum communication tasks and quantum computational models without containing much entanglement. We obtain a necessary and sufficient condition for the existence of nonzero quantum discord for any dimensional bipartite states. This condition is easily experimentally implementable. Based on this, we propose a geometrical way of quantifying quantum discord. For two qubits this results in a closed form of expression for discord. We apply our results to the model of deterministic quantum computation with one qubit, showing that quantum discord is unlikely to be the reason behind its speedup.
1,101 citations
TL;DR: These methods are specialized for quantum states that are fairly pure, and they offer a significant performance improvement on large quantum systems, and are able to reconstruct an unknown density matrix of dimension d and rank r using O(rdlog²d) measurement settings, compared to standard methods that require d² settings.
Abstract: We establish methods for quantum state tomography based on compressed sensing. These methods are specialized for quantum states that are fairly pure, and they offer a significant performance improvement on large quantum systems. In particular, they are able to reconstruct an unknown density matrix of dimension d and rank r using O(rdlog^2d) measurement settings, compared to standard methods that require d^2 settings. Our methods have several features that make them amenable to experimental implementation: they require only simple Pauli measurements, use fast convex optimization, are stable against noise, and can be applied to states that are only approximately low rank. The acquired data can be used to certify that the state is indeed close to pure, so no a priori assumptions are needed.
1,084 citations
TL;DR: It is conjecture that this duality persists to all quantum loop orders and can thus be used to obtain multiloop gravity amplitudes easily from gauge-theory ones.
Abstract: In a previous paper we observed that (classical) tree-level gauge-theory amplitudes can be rearranged to display a duality between color and kinematics. Once this is imposed, gravity amplitudes are obtained using two copies of gauge-theory diagram numerators. Here we conjecture that this duality persists to all quantum loop orders and can thus be used to obtain multiloop gravity amplitudes easily from gauge-theory ones. As a nontrivial test, we show that the three-loop four-point amplitude of N=4 super-Yang-Mills theory can be arranged into a form satisfying the duality, and by taking double copies of the diagram numerators we obtain the corresponding amplitude of N=8 supergravity. We also remark on a nonsupersymmetric two-loop test based on pure Yang-Mills theory resulting in gravity coupled to an antisymmetric tensor and dilaton.
1,046 citations
TL;DR: This work demonstrates, for the first time, a spatially dependent metamaterial perfect absorber operating in the infrared regime, and achieves an experimental absorption of 97% at a wavelength of 6.0 μm.
Abstract: We demonstrate, for the first time, a spatially dependent metamaterial perfect absorber operating in the infrared regime We achieve an experimental absorption of 97% at a wavelength of 60 microns, and our results agree well with numerical full-wave simulations By using two different metamaterial sublattices we experimentally demonstrate a spatial and frequency varying absorption which may have many relevant applications including hyperspectral sub-sampling imaging
1,045 citations
TL;DR: This work investigates the nonlinear optical properties of graphene flakes using four-wave mixing and finds the corresponding third-order optical susceptibility to be remarkably large and only weakly dependent on the wavelength in the near-infrared frequency range.
Abstract: We investigate the nonlinear optical properties of graphene flakes using four-wave mixing. The corresponding third-order optical susceptibility is found to be remarkably large and only weakly dependent on the wavelength in the near-infrared frequency range. The magnitude of the response is in good agreement with our calculations based on the nonlinear quantum response theory.
TL;DR: A method to efficiently sort orbital angular momentum states of light using two static optical elements that perform a Cartesian to log-polar coordinate transformation, converting the helically phased light beam corresponding to OAM states into a beam with a transverse phase gradient.
Abstract: We present a method to efficiently sort orbital angular momentum (OAM) states of light using two static optical elements. The optical elements perform a Cartesian to log-polar coordinate transformation, converting the helically phased light beam corresponding to OAM states into a beam with a transverse phase gradient. A subsequent lens then focuses each input OAM state to a different lateral position. We demonstrate the concept experimentally by using two spatial light modulators to create the desired optical elements, applying it to the separation of eleven OAM states.
TL;DR: An optical clock with a fractional frequency inaccuracy of 8.6x10{-18}, based on quantum logic spectroscopy of an Al+ ion, is constructed, consistent with the accuracy limit of the older clock.
Abstract: We have constructed an optical clock with a fractional frequency inaccuracy of 8.6x10{sup -18}, based on quantum logic spectroscopy of an Al{sup +} ion. A simultaneously trapped Mg{sup +} ion serves to sympathetically laser cool the Al{sup +} ion and detect its quantum state. The frequency of the {sup 1}S{sub 0}reversible{sup 3}P{sub 0} clock transition is compared to that of a previously constructed Al{sup +} optical clock with a statistical measurement uncertainty of 7.0x10{sup -18}. The two clocks exhibit a relative stability of 2.8x10{sup -15}tau{sup -1/2}, and a fractional frequency difference of -1.8x10{sup -17}, consistent with the accuracy limit of the older clock.
TL;DR: It is shown that copper intercalation in the van der Waals gaps between the Bi2Se3 layers, yielding an electron concentration of approximately 2x10{20} cm{-3}, results in superconductivity at 3.8 K in CuxBi2 Se3 for 0.12
Abstract: Bi2Se3 is one of a handful of known topological insulators. Here we show that copper intercalation in the van der Waals gaps between the Bi2Se3 layers, yielding an electron concentration of ?2×1020??cm-3, results in superconductivity at 3.8 K in CuxBi2Se3 for 0.12?x?0.15. This demonstrates that Cooper pairing is possible in Bi2Se3 at accessible temperatures, with implications for studying the physics of topological insulators and potential devices.
TL;DR: A measure of the local density of chirality of the electromagnetic field determines the asymmetry in the rates of excitation between a small chiral molecule and its mirror image, and applies to molecules in electromagnetic fields with arbitrary spatial dependence.
Abstract: We introduce a measure of the local density of chirality of the electromagnetic field. This optical chirality determines the asymmetry in the rates of excitation between a small chiral molecule and its mirror image, and applies to molecules in electromagnetic fields with arbitrary spatial dependence. A continuity equation for optical chirality in the presence of material currents describes the flow of chirality, in a manner analogous to the Poynting theorem for electromagnetic energy. ``Superchiral'' solutions to Maxwell's equations show larger chiral asymmetry, in some regions of space, than is found in circularly polarized plane waves.
TL;DR: The density dependence of the characteristic temperature Θ( BG) defining the crossover between the two distinct regimes is mapped out, and it is shown that, for all n, ρ(T) scales as a universal function of the normalized temperature T/Θ(BG).
Abstract: We report on the temperature dependent electron transport in graphene at different carrier densities $n$. Employing an electrolytic gate, we demonstrate that $n$ can be adjusted up to $4\ifmmode\times\else\texttimes\fi{}{10}^{14}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$ for both electrons and holes. The measured sample resistivity $\ensuremath{\rho}$ increases linearly with temperature $T$ in the high temperature limit, indicating that a quasiclassical phonon distribution is responsible for the electron scattering. As $T$ decreases, the resistivity decreases more rapidly following $\ensuremath{\rho}(T)\ensuremath{\sim}{T}^{4}$. This low temperature behavior can be described by a Bloch-Gr\"uneisen model taking into account the quantum distribution of the two-dimensional acoustic phonons in graphene. We map out the density dependence of the characteristic temperature ${\ensuremath{\Theta}}_{\mathrm{BG}}$ defining the crossover between the two distinct regimes, and show that, for all $n$, $\ensuremath{\rho}(T)$ scales as a universal function of the normalized temperature $T/{\ensuremath{\Theta}}_{\mathrm{BG}}$.
TL;DR: Two different measures of non-markovianity are introduced that exploit the specific traits of quantum correlations and are suitable for opposite experimental contexts when complete tomographic knowledge about the evolution is available and when no information whatsoever is available.
Abstract: We address the problem of quantifying the non-Markovian character of quantum time evolutions of general systems in contact with an environment. We introduce two different measures of non-Markovianity that exploit the specific traits of quantum correlations and are suitable for opposite experimental contexts. When complete tomographic knowledge about the evolution is available, our measure provides a necessary and sufficient condition to quantify strictly the non-Markovianity. In the opposite case, when no information whatsoever is available, we propose a sufficient condition for non-Markovianity. Remarkably, no optimization procedure underlies our derivation, which greatly enhances the practical relevance of the proposed criteria.
TL;DR: With measured temperature drop across the particle, the speed of self-propulsion is corroborated with the prediction based on accessible parameters and as an application for driving a micromachine, a microrotor is demonstrated.
Abstract: We study self-propulsion of a half-metal coated colloidal particle under laser irradiation. The motion is caused by self-thermophoresis: i.e., absorption of a laser at the metal-coated side of the particle creates local temperature gradient which in turn drives the particle by thermophoresis. To clarify the mechanism, temperature distribution and a thermal slip flow field around a microscale Janus particle are measured for the first time. With measured temperature drop across the particle, the speed of self-propulsion is corroborated with the prediction based on accessible parameters. As an application for driving a micromachine, a microrotor is demonstrated.
TL;DR: A new example of an electronic property arising from the interfacial breaking of inversion symmetry, namely, a large Rashba spin-orbit interaction, whose magnitude can be modulated by the application of an external electric field is laid out.
Abstract: The quasi-two-dimensional electron gas found at the ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ interface offers exciting new functionalities, such as tunable superconductivity, and has been proposed as a new nanoelectronics fabrication platform. Here we lay out a new example of an electronic property arising from the interfacial breaking of inversion symmetry, namely, a large Rashba spin-orbit interaction, whose magnitude can be modulated by the application of an external electric field. By means of magnetotransport experiments we explore the evolution of the spin-orbit coupling across the phase diagram of the system. We uncover a steep rise in Rashba interaction occurring around the doping level where a quantum critical point separates the insulating and superconducting ground states of the system.
TL;DR: A spin one-half Hamiltonian is derived on a honeycomb lattice describing the exchange interactions between Ir4+ ions in a family of layered iridates A2IrO3 (A=Li,Na).
Abstract: We derive and study a spin one-half Hamiltonian on a honeycomb lattice describing the exchange interactions between Ir4+ ions in a family of layered iridates A2IrO3 (A=Li,Na). Depending on the microscopic parameters, the Hamiltonian interpolates between the Heisenberg and exactly solvable Kitaev models. Exact diagonalization and a complementary spin-wave analysis reveal the presence of an extended spin-liquid phase near the Kitaev limit and a conventional Neel state close to the Heisenberg limit. The two phases are separated by an unusual stripy antiferromagnetic state, which is the exact ground state of the model at the midpoint between two limits.
TL;DR: This technique opens the route towards the direct integration of laser-cooled atomic ensembles within fiber networks, an important prerequisite for large scale quantum communication schemes, and is ideally suited to the realization of hybrid quantum systems that combine atoms with, e.g., solid state quantum devices.
Abstract: Trapping and optically interfacing laser-cooled neutral atoms are essential requirements for their use in advanced quantum technologies. Here we simultaneously realize both of these tasks with cesium atoms interacting with a multicolor evanescent field surrounding an optical nanofiber. The atoms are localized in a one-dimensional optical lattice about 200 nm above the nanofiber surface and can be efficiently interrogated with a resonant light field sent through the nanofiber. Our technique opens the route towards the direct integration of laser-cooled atomic ensembles within fiber networks, an important prerequisite for large scale quantum communication schemes. Moreover, it is ideally suited to the realization of hybrid quantum systems that combine atoms with, e.g., solid state quantum devices.
TL;DR: The results show the possibility of inducing a macroscopic ferrimagnetic state in multilayered graphene just by randomly removing single C atoms.
Abstract: Atomic vacancies have a strong impact in the mechanical, electronic, and magnetic properties of graphenelike materials. By artificially generating isolated vacancies on a graphite surface and measuring their local density of states on the atomic scale, we have shown how single vacancies modify the electronic properties of this graphenelike system. Our scanning tunneling microscopy experiments, complemented by tight-binding calculations, reveal the presence of a sharp electronic resonance at the Fermi energy around each single graphite vacancy, which can be associated with the formation of local magnetic moments and implies a dramatic reduction of the charge carriers' mobility. While vacancies in single layer graphene lead to magnetic couplings of arbitrary sign, our results show the possibility of inducing a macroscopic ferrimagnetic state in multilayered graphene just by randomly removing single C atoms.
TL;DR: The first demonstration of a CNOT gate between two individually addressed neutral atoms is presented, using Rydberg blockade interactions between neutral atoms held in optical traps separated by >8 microm.
Abstract: We present the first demonstration of a CNOT gate between two individually addressed neutral atoms. Our implementation of the CNOT uses Rydberg blockade interactions between neutral atoms held in optical traps separated by $g8\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. Using two different gate protocols we measure CNOT fidelities of $F=0.73$ and 0.72 based on truth table probabilities. The gate was used to generate Bell states with fidelity $F=0.48\ifmmode\pm\else\textpm\fi{}0.06$. After correcting for atom loss we obtain an a posteriori entanglement fidelity of $F=0.58$.
TL;DR: The first result from such an axion search using a superconducting first-stage amplifier (SQUID) replacing a conventional GaAs field-effect transistor amplifier is reported, setting the stage for a definitive axions search utilizing near quantum-limited SQUID amplifiers.
Abstract: Axions in the $\ensuremath{\mu}\mathrm{eV}$ mass range are a plausible cold dark-matter candidate and may be detected by their conversion into microwave photons in a resonant cavity immersed in a static magnetic field. We report the first result from such an axion search using a superconducting first-stage amplifier (SQUID) replacing a conventional GaAs field-effect transistor amplifier. This experiment excludes KSVZ dark-matter axions with masses between $3.3\text{ }\text{ }\ensuremath{\mu}\mathrm{eV}$ and $3.53\text{ }\text{ }\ensuremath{\mu}\mathrm{eV}$ and sets the stage for a definitive axion search utilizing near quantum-limited SQUID amplifiers.
TL;DR: This work discusses the problem of the separation of total correlations in a given quantum state into entanglement, dissonance, and classical correlations using the concept of relative entropy as a distance measure of correlations.
Abstract: We discuss the problem of the separation of total correlations in a given quantum state into entanglement, dissonance, and classical correlations using the concept of relative entropy as a distance measure of correlations. This allows us to put all correlations on an equal footing. Entanglement and dissonance, whose definition is introduced here, jointly belong to what is known as quantum discord. Our methods are completely applicable for multipartite systems of arbitrary dimensions. We investigate additivity relations between different correlations and show that dissonance may be present in pure multipartite states.
TL;DR: A compact analytic formula is presented for the two-loop six-particle maximally helicity violating remainder function in N=4 supersymmetric Yang-Mills theory in terms of the classical polylogarithm functions Lik with cross ratios of momentum twistor invariants as their arguments.
Abstract: We present a compact analytic formula for the two-loop six-particle maximally helicity violating remainder function (equivalently, the two-loop lightlike hexagon Wilson loop) in N=4 supersymmetric Yang-Mills theory in terms of the classical polylogarithm functions Lik with cross ratios of momentum twistor invariants as their arguments. In deriving our formula we rely on results from the theory of motives.
TL;DR: This Letter provides a sufficient criterion for realizing time-reversal-invariant topological superconductor in centrosymmetric superconductors with odd-parity pairing, and proposes that CuxBi2Se3 is a good candidate for a topologicalsuperconductor.
Abstract: Topological superconductors have a full pairing gap in the bulk and gapless surface Andreev bound states. In this Letter, we provide a sufficient criterion for realizing time-reversal-invariant topological superconductors in centrosymmetric superconductors with odd-parity pairing. We next study the pairing symmetry of the newly discovered superconductor ${\mathrm{Cu}}_{x}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ within a two-orbital model, and find that a novel spin-triplet pairing with odd parity is favored by strong spin-orbit coupling. Based on our criterion, we propose that ${\mathrm{Cu}}_{x}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ is a good candidate for a topological superconductor. We close by discussing experimental signatures of this new topological phase.
TL;DR: It is shown both analytically and numerically that reducing the coupling between the networks leads to a change from a first order percolation phase transition to a second orderpercolation transition at a critical point.
Abstract: We study a system composed from two interdependent networks A and B, where a fraction of the nodes in network A depends on nodes of network B and a fraction of the nodes in network B depends on nodes of network A. Because of the coupling between the networks, when nodes in one network fail they cause dependent nodes in the other network to also fail. This invokes an iterative cascade of failures in both networks. When a critical fraction of nodes fail, the iterative process results in a percolation phase transition that completely fragments both networks. We show both analytically and numerically that reducing the coupling between the networks leads to a change from a first order percolation phase transition to a second order percolation transition at a critical point. The scaling of the percolation order parameter near the critical point is characterized by the critical exponent � ¼ 1.
TL;DR: In this paper, the first measurement of charged particle elliptic flow in Pb-Pb collisions at root s(NN) p = 2.76 TeV with the ALICE detector at the CERN Large Hadron Collider was performed in the central pseudorapidity region.
Abstract: We report the first measurement of charged particle elliptic flow in Pb-Pb collisions at root s(NN) p = 2.76 TeV with the ALICE detector at the CERN Large Hadron Collider. The measurement is performed in the central pseudorapidity region (vertical bar eta vertical bar < 0.8) and transverse momentum range 0.2 < p(t) < 5.0 GeV/c. The elliptic flow signal v(2), measured using the 4-particle correlation method, averaged over transverse momentum and pseudorapidity is 0.087 +/- 0.002(stat) +/- 0.003(syst) in the 40%-50% centrality class. The differential elliptic flow v(2)(p(t)) reaches a maximum of 0.2 near p(t) = 3 GeV/c. Compared to RHIC Au-Au collisions at root s(NN) = 200 GeV, the elliptic flow increases by about 30%. Some hydrodynamic model predictions which include viscous corrections are in agreement with the observed increase.
TL;DR: A screening mechanism that allows a scalar field to mediate a long-range force of gravitational strength in the cosmos while satisfying local tests of gravity and predicting deviations from general relativity in the solar system that are within reach of next-generation experiments, as well as astrophysically observable violations of the equivalence principle.
Abstract: We present a screening mechanism that allows a scalar field to mediate a long-range ($\ensuremath{\sim}\mathrm{Mpc}$) force of gravitational strength in the cosmos while satisfying local tests of gravity. The mechanism hinges on local symmetry restoration in the presence of matter. In regions of sufficiently high matter density, the field is drawn towards $\ensuremath{\phi}=0$ where its coupling to matter vanishes and the $\ensuremath{\phi}\ensuremath{\rightarrow}\ensuremath{-}\ensuremath{\phi}$ symmetry is restored. In regions of low density, however, the symmetry is spontaneously broken, and the field couples to matter with gravitational strength. We predict deviations from general relativity in the solar system that are within reach of next-generation experiments, as well as astrophysically observable violations of the equivalence principle. The model can be distinguished experimentally from Brans-Dicke gravity, chameleon theories and brane-world modifications of gravity.