Showing papers in "Nature Physics in 2010"
TL;DR: In this paper, the authors report the first observation of the behaviour of a PT optical coupled system that judiciously involves a complex index potential, and observe both spontaneous PT symmetry breaking and power oscillations violating left-right symmetry.
Abstract: One of the fundamental axioms of quantum mechanics is associated with the Hermiticity of physical observables 1 . In the case of the Hamiltonian operator, this requirement not only implies real eigenenergies but also guarantees probability conservation. Interestingly, a wide class of non-Hermitian Hamiltonians can still show entirely real spectra. Among these are Hamiltonians respecting parity‐time (PT) symmetry 2‐7 . Even though the Hermiticity of quantum observables was never in doubt, such concepts have motivated discussions on several fronts in physics, including quantum field theories 8 , nonHermitian Anderson models 9 and open quantum systems 10,11 , to mention a few. Although the impact of PT symmetry in these fields is still debated, it has been recently realized that optics can provide a fertile ground where PT-related notions can be implemented and experimentally investigated 12‐15 . In this letter we report the first observation of the behaviour of a PT optical coupled system that judiciously involves a complex index potential. We observe both spontaneous PT symmetry breaking and power oscillations violating left‐right symmetry. Our results may pave the way towards a new class of PT-synthetic materials with intriguing and unexpected properties that rely on non-reciprocal light propagation and tailored transverse energy flow. Before we introduce the concept of spacetime reflection in optics, we first briefly outline some of the basic aspects of this symmetry within the context of quantum mechanics. In general, a Hamiltonian HD p 2 =2mCV(x
3,097 citations
TL;DR: This paper showed that the most efficient spreaders are not always necessarily the most connected agents in a network, and that the position of an agent relative to the hierarchical topological organization of the network might be as important as its connectivity.
Abstract: Spreading of information, ideas or diseases can be conveniently modelled in the context of complex networks. An analysis now reveals that the most efficient spreaders are not always necessarily the most connected agents in a network. Instead, the position of an agent relative to the hierarchical topological organization of the network might be as important as its connectivity.
2,618 citations
TL;DR: In this paper, it was shown that a designed strain aligned along three main crystallographic directions induces strong gauge fields that effectively act as a uniform magnetic field exceeding 10'T, similar to the case of a topological insulator.
Abstract: Owing to the fact that graphene is just one atom thick, it has been suggested that it might be possible to control its properties by subjecting it to mechanical strain. New analysis indicates not only this, but that pseudomagnetic behaviour and even zero-field quantum Hall effects could be induced in graphene under realistic amounts of strain. Among many remarkable qualities of graphene, its electronic properties attract particular interest owing to the chiral character of the charge carriers, which leads to such unusual phenomena as metallic conductivity in the limit of no carriers and the half-integer quantum Hall effect observable even at room temperature1,2,3. Because graphene is only one atom thick, it is also amenable to external influences, including mechanical deformation. The latter offers a tempting prospect of controlling graphene’s properties by strain and, recently, several reports have examined graphene under uniaxial deformation4,5,6,7,8. Although the strain can induce additional Raman features7,8, no significant changes in graphene’s band structure have been either observed or expected for realistic strains of up to ∼15% (refs 9, 10, 11). Here we show that a designed strain aligned along three main crystallographic directions induces strong gauge fields12,13,14 that effectively act as a uniform magnetic field exceeding 10 T. For a finite doping, the quantizing field results in an insulating bulk and a pair of countercirculating edge states, similar to the case of a topological insulator15,16,17,18,19,20. We suggest realistic ways of creating this quantum state and observing the pseudomagnetic quantum Hall effect. We also show that strained superlattices can be used to open significant energy gaps in graphene’s electronic spectrum.
1,623 citations
TL;DR: The surprising discovery of high-temperature superconductivity in a material containing a strong magnet (iron) has led to thousands of publications as discussed by the authors, and it becomes clear what we know and where we are headed.
Abstract: The surprising discovery of high-temperature superconductivity in a material containing a strong magnet—iron—has led to thousands of publications. By placing all the data in context, it becomes clear what we know and where we are headed.
1,224 citations
TL;DR: In this article, a study of the topological insulating Bi2Se3 thin films finds that a gap in these gapless surface states opens up in films below a certain thickness, which suggests that in thicker films, gapless states exist on both upper and lower surfaces.
Abstract: The gapless surface states of topological insulators could enable quantitatively different types of electronic device. A study of the topological insulating Bi2Se3 thin films finds that a gap in these states opens up in films below a certain thickness. This in turn suggests that in thicker films, gapless states exist on both upper and lower surfaces.
1,201 citations
TL;DR: In this article, it was shown that the Jaynes-cummings model breaks down in the regime of ultrastrong coupling between light and matter, and that higher-order processes are possible.
Abstract: The Jaynes–Cummings model describes the interaction between a two-level system and a small number of photons. It is now shown that the model breaks down in the regime of ultrastrong coupling between light and matter. The spectroscopic response of a superconducting artificial atom in a waveguide resonator indicates higher-order processes.
1,180 citations
TL;DR: Empirical data is used to show that the predictions of the CTRW models are in systematic conflict with the empirical results, and two principles that govern human trajectories are introduced, allowing for a statistically self-consistent microscopic model for individual human mobility.
Abstract: Individual human trajectories are characterized by fat-tailed distributions of jump sizes and waiting times, suggesting the relevance of continuous-time random-walk (CTRW) models for human mobility. However, human traces are barely random. Given the importance of human mobility, from epidemic modelling to traffic prediction and urban planning, we need quantitative models that can account for the statistical characteristics of individual human trajectories. Here we use empirical data on human mobility, captured by mobile-phone traces, to show that the predictions of the CTRW models are in systematic conflict with the empirical results. We introduce two principles that govern human trajectories, allowing us to build a statistically self-consistent microscopic model for individual human mobility. The model accounts for the empirically observed scaling laws, but also allows us to analytically predict most of the pertinent scaling exponents.
1,174 citations
TL;DR: The Peregrine soliton was observed experimentally for the first time by using femtosecond pulses in an optical fiber as mentioned in this paper, which gave some insight into freak waves that can appear out of nowhere before simply disappearing.
Abstract: The Peregrine soliton — a wave localized in both space and time — is now observed experimentally for the first time by using femtosecond pulses in an optical fibre. The results give some insight into freak waves that can appear out of nowhere before simply disappearing.
1,158 citations
TL;DR: In this paper, it was shown that when graphite's graphene sheets are rotated out of their usual alignment, it can generate low-energy Van Hove singularities for which the position is controlled by the angle of rotation.
Abstract: When a Van Hove singularity exists near the Fermi energy of a solid’s density of states, it can cause a variety of exotic phenomena to emerge. Scanning tunnelling microscope measurements indicate that when graphite’s graphene sheets are rotated out of their usual alignment, it can generate low-energy Van Hove singularities for which the position is controlled by the angle of rotation.
1,014 citations
TL;DR: Viewing the brain in terms of collective dynamics is one approach now yielding some insight.
Abstract: Is the brain on the edge of criticality? Understanding the inner workings of the brain is a task made difficult by the number of elements involved: a hundred billion neurons and a hundred trillion synapses. Viewing the brain in terms of collective dynamics is one approach now yielding some insight.
1,011 citations
TL;DR: In this paper, the authors demonstrate the information-to-energy conversion by feedback control has been demonstrated experimentally and demonstrate that feedback can enable the transformation of information into energy without violating the second law of thermodynamics.
Abstract: Feedback mechanisms such as the ‘demon’ in Maxwell’s well-known thought experiment can, in principle, enable the transformation of information into energy, without violating the second law of thermodynamics. Such information-to-energy conversion by feedback control has now been demonstrated experimentally.
TL;DR: In this paper, the Coulomb repulsion was used to enhance the effects of spin-orbit coupling on the surface of a strongly correlated insulator, leading to gapless spin-only excitations.
Abstract: Mott insulators are driven by strong Coulomb repulsion and topological insulators by strong spin–orbit coupling. Although these effects are normally in competition, in some cases the Coulomb interaction can enhance the effects of spin–orbit coupling. Together these interactions could lead to gapless spin-only excitations on the surface of a strongly correlated insulator.
TL;DR: In this article, the authors proposed a universal quantum simulator for spin models with high-order Rydberg atoms, including higher order spin-liquid phases, which can be used to simulate other quantum systems with many-body interactions and strongly correlated ground states.
Abstract: A universal quantum simulator is a controlled quantum device that reproduces the dynamics of any other many-particle quantum system with short-range interactions. This dynamics can refer to both coherent Hamiltonian and dissipative open-system evolution. Here we propose that laser-excited Rydberg atoms in large-spacing optical or magnetic lattices provide an efficient implementation of a universal quantum simulator for spin models involving n-body interactions, including such of higher order. This would allow the simulation of Hamiltonians of exotic spin models involving n-particle constraints, such as the Kitaev toric code, colour code and lattice gauge theories with spin-liquid phases. In addition, our approach provides the ingredients for dissipative preparation of entangled states based on engineering n-particle reservoir couplings. The basic building blocks of our architecture are efficient and high-fidelity n-qubit entangling gates using auxiliary Rydberg atoms, including a possible dissipative time step through optical pumping. This enables mimicking the time evolution of the system by a sequence of fast, parallel and high-fidelity n-particle coherent and dissipative Rydberg gates. Building on recent experimental advances in controlling individual Rydberg atoms, theoretical work proposes a ‘Rydberg quantum simulator’. Such a system would be suitable for efficiently simulating other quantum systems with many-body interactions and strongly correlated ground states.
TL;DR: In this article, the authors extend the Heisenberg uncertainty principle to include the case of quantum memories, and should provide a guide for quantum information applications. But they do not consider the case where a memory device stores quantum information.
Abstract: The Heisenberg uncertainty principle bounds the uncertainties about the outcomes of two incompatible measurements on a quantum particle. This bound, however, changes if a memory device is involved that stores quantum information. New work now extends the uncertainty principle to include the case of quantum memories, and should provide a guide for quantum information applications.
TL;DR: In this article, the authors analyzed entanglement in multi-chromophoric light harvesting complexes, and established methods for quantification of entenglement by presenting necessary and sufficient conditions for entanglements and by deriving a measure of global entengement.
Abstract: Light harvesting components of photosynthetic organisms are complex, coupled,
many-body quantum systems, in which electronic coherence has recently been
shown to survive for relatively long time scales despite the decohering effects
of their environments. Within this context, we analyze entanglement in
multi-chromophoric light harvesting complexes, and establish methods for
quantification of entanglement by presenting necessary and sufficient
conditions for entanglement and by deriving a measure of global entanglement.
These methods are then applied to the Fenna-Matthews-Olson (FMO) protein to
extract the initial state and temperature dependencies of entanglement. We show
that while FMO in natural conditions largely contains bipartite entanglement
between dimerized chromophores, a small amount of long-range and multipartite
entanglement exists even at physiological temperatures. This constitutes the
first rigorous quantification of entanglement in a biological system. Finally,
we discuss the practical utilization of entanglement in densely packed
molecular aggregates such as light harvesting complexes.
TL;DR: For an ideal topological insulator, the metallic surface states should be easy to measure using transport techniques; however, the bulk is not completely insulating as mentioned in this paper, and improving the "leaky" bulk state proves crucial for measuring the surface Dirac fermions, including correlation effects.
Abstract: For an ideal topological insulator, the metallic surface states should be easy to measure using transport techniques; however, the bulk is not completely insulating. Improving the ‘leaky’ bulk state proves crucial for measuring the surface Dirac fermions, including correlation effects.
TL;DR: In this paper, the decoupling of the nuclear spin from the electronic angular momentum is used to implement many-body systems with an unprecedented degree of symmetry, characterized by the SU(N)group withNaslargeas10.
Abstract: Fermionic alkaline-earth atoms have unique properties that make them attractive candidates for the realization of atomic clocks and degenerate quantum gases. At the same time, they are attracting considerable theoretical attention in the context of quantum information processing. Here we demonstrate that when such atoms are loaded in optical lattices, they can be used as quantum simulators of unique many-body phenomena. In particular, we show that the decoupling of the nuclear spin from the electronic angular momentum can be used to implement many-body systems with an unprecedented degree of symmetry, characterizedbytheSU(N)groupwithNaslargeas10.Moreover,theinterplayofthenuclearspinwiththeelectronicdegreeof freedomprovidedbyastableopticallyexcitedstateshouldenablethestudyofphysicsgovernedbythespin‐orbitalinteraction. Such systems may provide valuable insights into the physics of strongly correlated transition-metal oxides, heavy-fermion materials and spin-liquid phases.
TL;DR: In this paper, the metal/organic interface is found to be key for spin injection in organic semiconductors, and the authors investigated how to optimize the injection of spin into these materials.
Abstract: Organic semiconductors are attractive candidates for spintronics applications because of their long spin lifetimes. But few studies have investigated how to optimize the injection of spin into these materials. A new study suggests that the metal/organic interface is key.
TL;DR: In this paper, an in situ technique that corrects for wavefront aberrations and allows X-rays to be focused to a spot just 7 nm wide could provide a solution.
Abstract: X-ray sources such as free-electron lasers offer the potential to study matter at unprecedented spatial and temporal resolution. But that potential is limited by the poor quality of conventional X-ray optical elements. An in situ technique that corrects for wavefront aberrations and allows X-rays to be focused to a spot just 7 nm wide could provide a solution.
TL;DR: In this paper, the attosecond contributions of the two highest occupied molecular orbitals are analyzed and the conditions where they are disentangled in real and imaginary parts of the emission dipole moment are determined.
Abstract: A strong laser field may tunnel ionize a molecule from several orbitals simultaneously, forming an attosecond electron–hole wavepacket. Both temporal and spatial information on this wavepacket can be obtained through the coherent soft X-ray emission resulting from the laser-driven recollision of the liberated electron with the core. By characterizing the emission from aligned N 2 molecules, we demonstrate the attosecond contributions of the two highest occupied molecular orbitals. We determine conditions where they are disentangled in the real and imaginary parts of the emission dipole moment. This allows us to carry out a tomographic reconstruction of both orbitals with angstrom spatial resolution. Their coherent superposition provides experimental images of the attosecond wavepacket created in the ionization process. Our results open the prospect of imaging ultrafast intramolecular dynamics combining attosecond and angstrom resolutions.
TL;DR: In this article, coherent coupling between two electron spins separated by almost 10 nm has been demonstrated, which might enable the construction of a network of connected quantum registers at room temperature, at this distance, the spins can be addressed individually.
Abstract: Nitrogen–vacancy centres in diamond have emerged as a promising platform for quantum information processing at room temperature. Now, coherent coupling between two electron spins separated by almost 10 nm has been demonstrated. At this distance, the spins can be addressed individually, which might enable the construction of a network of connected quantum registers.
TL;DR: The epitaxial films of lead and indium represent the thinnest superconductors possible as discussed by the authors, and have been shown to have superconductivity on silicon substrates.
Abstract: There are many two-dimensional superconductors, but only now have monolayers of metallic atoms shown superconductivity. Grown on silicon substrates, epitaxial films of lead and indium represent the thinnest superconductors possible.
TL;DR: In this paper, a long-lived polariton condensates can propagate well beyond the area of their initial excitation while still maintaining spatial coherence, which enables direct and controllable manipulation of the condensate wavefunction.
Abstract: Long-lived polariton condensates can propagate well beyond the area of their initial excitation while still maintaining spatial coherence. This enables direct and controllable manipulation of the condensate wavefunction.
TL;DR: In this paper, the velocity of nanometric acoustic waves in supercritical fluid argon at high pressures was determined by inelastic X-ray scattering and molecular dynamics simulations, showing that the supercritical region is actually divided into two regions that, although not connected by a first-order singularity, can be identified by different dynamical regimes: gas-like and liquid-like.
Abstract: According to textbook definitions 1 , there exists no physical observable able to distinguish a liquid from a gas beyond the critical point, and hence only a single fluid phase is defined. There are, however, some thermophysical quantities, having maxima that define a line emanating from the critical point, named ‘the Widom line’ 2 in the case of the constant-pressure specific heat. We determined the velocity of nanometric acoustic waves in supercritical fluid argon at high pressures by inelastic X-ray scattering and molecular dynamics simulations. Our study reveals a sharp transition on crossing the Widom line demonstrating how the supercritical region is actually divided into two regions that, although not connected by a first-order singularity, can be identified by different dynamical regimes: gas-like and liquid-like, reminiscent of the subcritical domains.
TL;DR: In this article, the authors demonstrate that the intensity of these X-rays can be as bright as that generated by conventional third-generation synchrotrons, in a device a fraction of the size and cost.
Abstract: Betratron oscillations of electrons driven through a plasma by a high-intensity laser generate coherent X-rays. A new study demonstrates the intensity of these X-rays can be as bright as that generated by conventional third-generation synchrotrons, in a device a fraction of the size and cost.
TL;DR: In this paper, the authors describe a scheme to create optical beams with isolated optical vortex loops in the forms of knots and links using algebraic topology and apply a numerical optimization algorithm to increase the contrast in light intensity.
Abstract: Guided by a general framework for wavefront engineering, experiments demonstrate that in a light field, lines of zero intensity can be shaped into knotted and linked loops of arbitrary topology. Natural and artificially created light fields in three-dimensional space contain lines of zero intensity, known as optical vortices1,2,3. Here, we describe a scheme to create optical beams with isolated optical vortex loops in the forms of knots and links using algebraic topology. The required complex fields with fibred knots and links4 are constructed from abstract functions with braided zeros and the knot function is then embedded in a propagating light beam. We apply a numerical optimization algorithm to increase the contrast in light intensity, enabling us to observe several optical vortex knots. These knotted nodal lines, as singularities of the wave’s phase, determine the topology of the wave field in space, and should have analogues in other three-dimensional wave systems such as superfluids5 and Bose–Einstein condensates6,7.
TL;DR: When doped with copper, the topological insulator Bi2Se3 becomes superconducting, but for new physics and applications the search is not for just any superconductor; the material must retain its topological character.
Abstract: When doped with copper, the topological insulator Bi2Se3 becomes superconducting. But for new physics and applications the search is not for just any superconductor; the material must retain its topological character. And indeed that is the case with doped Bi2Se3.
TL;DR: A theoretical model and experiments demonstrate here that the polarization/alignment of stress-fibers within stem cells is a non-monotonic function of matrix rigidity, and offer a first physical insight for the dependence of stem cell differentiation on tissue elasticity.
Abstract: The shape and differentiation of human mesenchymal stem cells is especially sensitive to the rigidity of their environment; the physical mechanisms involved are unknown. A theoretical model and experiments demonstrate here that the polarization/alignment of stress-fibers within stem cells is a non-monotonic function of matrix rigidity. We treat the cell as an active elastic inclusion in a surrounding matrix whose polarizability, unlike dead matter, depends on the feedback of cellular forces that develop in response to matrix stresses. The theory correctly predicts the monotonic increase of the cellular forces with the matrix rigidity and the alignment of stress-fibers parallel to the long axis of cells. We show that the anisotropy of this alignment depends non-monotonically on matrix rigidity and demonstrate it experimentally by quantifying the orientational distribution of stress-fibers in stem cells. These findings offer a first physical insight for the dependence of stem cell differentiation on tissue elasticity.
TL;DR: In this article, it was shown that entanglement is necessary for steering, but steering can be achieved, as has now been demonstrated experimentally, with states that cannot violate a Bell inequality and therefore non-locality.
Abstract: Erwin Schrodinger introduced in 1935 the concept of ‘steering’, which generalizes the famed Einstein–Podolsky–Rosen paradox. Steering sits in between quantum entanglement and non-locality — that is, entanglement is necessary for steering, but steering can be achieved, as has now been demonstrated experimentally, with states that cannot violate a Bell inequality (and therefore non-locality).
TL;DR: In this paper, a platform based on spin qubits connected through arrays of nanoelectromechanical resonators is proposed to reconcile the conflicting requirements of information leakage in a quantum computer.
Abstract: In a quantum computer, the data carriers (or qubits) must be well isolated from their environment to avoid information leakage. At the same time they have to interact with one another to process information. A proposed platform based on spin qubits connected through arrays of nanoelectromechanical resonators should be able to reconcile these conflicting requirements.