# Recent Noteworthy Physics Papers

by ZapperZ
Tags: noteworthy, papers, physics
 Mentor P: 28,801 M. Fridman et al., "Demonstration of temporal cloaking", Nature v.481, p.62 (2012). Abstract:Recent research has uncovered a remarkable ability to manipulate and control electromagnetic fields to produce effects such as perfect imaging and spatial cloaking. To achieve spatial cloaking, the index of refraction is manipulated to flow light from a probe around an object in such a way that a ‘hole’ in space is created, and the object remains hidden. Alternatively, it may be desirable to cloak the occurrence of an event over a finite time period, and the idea of temporal cloaking has been proposed in which the dispersion of the material is manipulated in time, producing a ‘time hole’ in the probe beam to hide the occurrence of the event from the observer. This approach is based on accelerating the front part of a probe light beam and slowing down its rear part to create a well controlled temporal gap—inside which an event occurs—such that the probe beam is not modified in any way by the event. The probe beam is then restored to its original form by the reverse manipulation of the dispersion. Here we present an experimental demonstration of temporal cloaking in an optical fibre-based system by applying concepts from the space–time duality between diffraction and dispersive broadening. We characterize the performance of our temporal cloak by detecting the spectral modification of a probe beam due to an optical interaction and show that the amplitude of the event (at the picosecond timescale) is reduced by more than an order of magnitude when the cloak is turned on. These results are a significant step towards the development of full spatio-temporal cloaking. Also read the News and Views article in the same issue of Nature. Zz.
 P: 4 Parthiban Santhanam et al., "Thermoelectrically Pumped Light-Emitting Diodes Operating above Unity Efficiency", Phys. Rev. Lett. 108, 097403 (2012) Abstract: A heated semiconductor light-emitting diode at low forward bias voltage V
 P: 4 Almost forgot this one :) Brendan McMonigal et al., "Alcubierre warp drive: On the matter of matter", Phys. Rev. D 85, 064024 (2012) Abstract: The Alcubierre warp drive allows a spaceship to travel at an arbitrarily large global velocity by deforming the spacetime in a bubble around the spaceship. Little is known about the interactions between massive particles and the Alcubierre warp drive, or the effects of an accelerating or decelerating warp bubble. We examine geodesics representative of the paths of null and massive particles with a range of initial velocities from -c to c interacting with an Alcubierre warp bubble traveling at a range of globally subluminal and superluminal velocities on both constant and variable velocity paths. The key results for null particles match what would be expected of massive test particles as they approach ±c. The increase in energy for massive and null particles is calculated in terms of vs, the global ship velocity, and vp, the initial velocity of the particle with respect to the rest frame of the origin/destination of the ship. Particles with positive vp obtain extremely high energy and velocity and become “time locked” for the duration of their time in the bubble, experiencing very little proper time between entering and eventually leaving the bubble. When interacting with an accelerating bubble, any particles within the bubble at the time receive a velocity boost that increases or decreases the magnitude of their velocity if the particle is moving toward the front or rear of the bubble, respectively. If the bubble is decelerating, the opposite effect is observed. Thus Eulerian matter is unaffected by bubble accelerations/decelerations. The magnitude of the velocity boosts scales with the magnitude of the bubble acceleration/deceleration. arXiv.org
 Mentor P: 28,801 F. Buscemi, "All Entangled Quantum States Are Nonlocal", Phys. Rev. Lett. v.108, p. 200401 (2012). Abstract: Departing from the usual paradigm of local operations and classical communication adopted in entanglement theory, we study here the interconversion of quantum states by means of local operations and shared randomness. A set of necessary and sufficient conditions for the existence of such a transformation between two given quantum states is given in terms of the payoff they yield in a suitable class of nonlocal games. It is shown that, as a consequence of our result, such a class of nonlocal games is able to witness quantum entanglement, however weak, and reveal nonlocality in any entangled quantum state. An example illustrating this fact is provided. Also see this Viewpoint article, where you can have a free access to download the actual paper. Zz.
 Sci Advisor P: 1,563 Bernhard Wittmann, Sven Ramelow, Fabian Steinlechner, Nathan K Langford, Nicolas Brunner, Howard M Wiseman, Rupert Ursin and Anton Zeilinger, "Loophole-free Einstein–Podolsky–Rosen experiment via quantum steering", New J. Phys. 14, 053030 (2012). Abstract: Tests of the predictions of quantum mechanics for entangled systems have provided increasing evidence against local realistic theories. However, there remains the crucial challenge of simultaneously closing all major loopholes—the locality, freedom-of-choice and detection loopholes—in a single experiment. An important sub-class of local realistic theories can be tested with the concept of 'steering'. The term 'steering' was introduced by Schrödinger in 1935 for the fact that entanglement would seem to allow an experimenter to remotely steer the state of a distant system as in the Einstein–Podolsky–Rosen (EPR) argument. Einstein called this 'spooky action at a distance'. EPR-steering has recently been rigorously formulated as a quantum information task opening it up to new experimental tests. Here, we present the first loophole-free demonstration of EPR-steering by violating three-setting quadratic steering inequality, tested with polarization-entangled photons shared between two distant laboratories. Our experiment demonstrates this effect while simultaneously closing all loopholes: both the locality loophole and a specific form of the freedom-of-choice loophole are closed by having a large separation of the parties and using fast quantum random number generators, and the fair-sampling loophole is closed by having high overall detection efficiency. Thereby, we exclude—for the first time loophole-free—an important class of local realistic theories considered by EPR. Besides its foundational importance, loophole-free steering also allows the distribution of quantum entanglement secure event in the presence of an untrusted party. The paper can be downloaded for free here and as different EPR experiments and their loopholes are discussed here quite often, I thought it would be a good idea to link it here.
 Mentor P: 28,801 E. Kot et al., "Breakdown of the Classical Description of a Local System", Phys. Rev. Lett., v.08, p.233601 (2012). Abstract: We provide a straightforward demonstration of a fundamental difference between classical and quantum mechanics for a single local system: namely, the absence of a joint probability distribution of the position x and momentum p. Elaborating on a recently reported criterion by Bednorz and Belzig [ Phys. Rev. A 83 052113 (2011)] we derive a simple criterion that must be fulfilled for any joint probability distribution in classical physics. We demonstrate the violation of this criterion using the homodyne measurement of a single photon state, thus proving a straightforward signature of the breakdown of a classical description of the underlying state. Most importantly, the criterion used does not rely on quantum mechanics and can thus be used to demonstrate nonclassicality of systems not immediately apparent to exhibit quantum behavior. The criterion is directly applicable to any system described by the continuous canonical variables x and p, such as a mechanical or an electrical oscillator and a collective spin of a large ensemble. Zz.
 Mentor P: 28,801 J.R. Williams et al., "Unconventional Josephson Effect in Hybrid Superconductor-Topological Insulator Devices", Phys. Rev. Lett. 109, 056803 (2012) Abstract: We report on transport properties of Josephson junctions in hybrid superconducting-topological insulator devices, which show two striking departures from the common Josephson junction behavior: a characteristic energy that scales inversely with the width of the junction, and a low characteristic magnetic field for suppressing supercurrent. To explain these effects, we propose a phenomenological model which expands on the existing theory for topological insulator Josephson junctions. Also see a review of this work at APS Physics where you will have a free download to the paper. Zz.
 Mentor P: 28,801 J. Yin et al., "Quantum teleportation and entanglement distribution over 100-kilometre free-space channels" Nature v.488, p.185 (2012). Abstract: Transferring an unknown quantum state over arbitrary distances is essential for large-scale quantum communication and distributed quantum networks. It can be achieved with the help of long-distance quantum teleportation1, 2 and entanglement distribution. The latter is also important for fundamental tests of the laws of quantum mechanics3, 4. Although quantum teleportation5, 6 and entanglement distribution7, 8, 9 over moderate distances have been realized using optical fibre links, the huge photon loss and decoherence in fibres necessitate the use of quantum repeaters10 for larger distances. However, the practical realization of quantum repeaters remains experimentally challenging11. Free-space channels, first used for quantum key distribution12, 13, offer a more promising approach because photon loss and decoherence are almost negligible in the atmosphere. Furthermore, by using satellites, ultra-long-distance quantum communication and tests of quantum foundations could be achieved on a global scale. Previous experiments have achieved free-space distribution of entangled photon pairs over distances of 600 metres (ref. 14) and 13 kilometres (ref. 15), and transfer of triggered single photons over a 144-kilometre one-link free-space channel16. Most recently, following a modified scheme17, free-space quantum teleportation over 16 kilometres was demonstrated18 with a single pair of entangled photons. Here we report quantum teleportation of independent qubits over a 97-kilometre one-link free-space channel with multi-photon entanglement. An average fidelity of 80.4 ± 0.9 per cent is achieved for six distinct states. Furthermore, we demonstrate entanglement distribution over a two-link channel, in which the entangled photons are separated by 101.8 kilometres. Violation of the Clauser–Horne–Shimony–Holt inequality4 is observed without the locality loophole. Besides being of fundamental interest, our results represent an important step towards a global quantum network. Moreover, the high-frequency and high-accuracy acquiring, pointing and tracking technique developed in our experiment can be directly used for future satellite-based quantum communication and large-scale tests of quantum foundations. Zz.
 P: 265 J. M. Hill and B. J. Cox, "Einstein's special relativity beyond the speed of light" Proc. R. Soc. A published ahead of print October 3, 2012, (2012) Abstract: We propose here two new transformations between inertial frames that apply for relative velocities greater than the speed of light, and that are complementary to the Lorentz transformation, giving rise to the Einstein special theory of relativity that applies to relative velocities less than the speed of light. The new transformations arise from the same mathematical framework as the Lorentz transformation, displaying singular behaviour when the relative velocity approaches the speed of light and generating the same addition law for velocities, but, most importantly, do not involve the need to introduce imaginary masses or complicated physics to provide well-defined expressions. Making use of the dependence on relative velocity of the Lorentz transformation, the paper provides an elementary derivation of the new transformations between inertial frames for relative velocities v in excess of the speed of light c, and further we suggest two possible criteria from which one might infer one set of transformations as physically more likely than the other. If the energy–momentum equations are to be invariant under the new transformations, then the mass and energy are given, respectively, by the formulae $m=(p_\infty/c)[(v/c)^2-1]^{-1/2}$ and $\mathcal{E}=mc^2$ where $p_\infty$ denotes the limiting momentum for infinite relative velocity. If, however, the requirement of invariance is removed, then we may propose new mass and energy equations, and an example having finite non-zero mass in the limit of infinite relative velocity is given. In this highly controversial topic, our particular purpose is not to enter into the merits of existing theories, but rather to present a succinct and carefully reasoned account of a new aspect of Einstein's theory of special relativity, which properly allows for faster than light motion. I'll post the full citation when it officially gets published.
 Mentor P: 28,801 K. Borozdin et al., "Cosmic Ray Radiography of the Damaged Cores of the Fukushima Reactors" Phys. Rev. Lett. 109, 152501 (2012). Abstract: The passage of muons through matter is dominated by the Coulomb interaction with electrons and nuclei. The interaction with the electrons leads to continuous energy loss and stopping of the muons. The interaction with nuclei leads to angle “diffusion.” Two muon-imaging methods that use flux attenuation and multiple Coulomb scattering of cosmic-ray muons are being studied as tools for diagnosing the damaged cores of the Fukushima reactors. Here, we compare these two methods. We conclude that the scattering method can provide detailed information about the core. Attenuation has low contrast and little sensitivity to the core. Review of this work can be found here. You may also obtain a free copy of the paper at that link since it is published under a Creative Commons license. Zz.
 Mentor P: 28,801 C. Zu et al.,“State-Independent Experimental Test of Quantum Contextuality in an Indivisible System” Phys. Rev. Lett. 109, 150401 (2012). Abstract: We report the first state-independent experimental test of quantum contextuality on a single photonic qutrit (three-dimensional system), based on a recent theoretical proposal [ Phys. Rev. Lett. 108 030402 (2012)]. Our experiment spotlights quantum contextuality in its most basic form, in a way that is independent of either the state or the tensor product structure of the system. Read a review of this work here. The link also provides a free copy of the actual paper. Zz.
 Sci Advisor P: 1,563 Paula Mellado, Andres Concha, and L. Mahadevan,“Macroscopic Magnetic Frustration” Phys. Rev. Lett. 109, 257203 (2012). Abstract: Although geometrical frustration transcends scale, it has primarily been evoked in the micro- and mesoscopic realm to characterize such phases as spin ice, liquids, and glasses and to explain the behavior of such materials as multiferroics, high-temperature superconductors, colloids, and copolymers. Here we introduce a system of macroscopic ferromagnetic rotors arranged in a planar lattice capable of out-of-plane movement that exhibit the characteristic honeycomb spin ice rules studied and seen so far only in its mesoscopic manifestation. We find that a polarized initial state of this system settles into the honeycomb spin ice phase with relaxation on multiple time scales. We explain this relaxation process using a minimal classical mechanical model that includes Coulombic interactions between magnetic charges located at the ends of the magnets and viscous dissipation at the hinges. Our study shows how macroscopic frustration arises in a purely classical setting that is amenable to experiment, easy manipulation, theory, and computation, and shows phenomena that are not visible in their microscopic counterparts. A review of this work can be found here. This is a very cute experiment. These guys just created a triangular lattice composed of 352 macroscopic magnets and simulated a simplified version of spin ice that way. Also check the supplementary material. It contains a nice video of the magnet system developing towards a steady state. Setting up that experiment must have been lots of fun.
 Sci Advisor P: 1,563 M. Giustina et al., "Bell violation using entangled photons without the fair-sampling assumption" Nature (2013) doi:10.1038/nature12012. (advance online publication) http://www.nature.com/nature/journal...ATURE-20130418 Abstract: The violation of a Bell inequality is an experimental observation that forces the abandonment of a local realistic viewpoint—namely, one in which physical properties are (probabilistically) defined before and independently of measurement, and in which no physical influence can propagate faster than the speed of light1, 2. All such experimental violations require additional assumptions depending on their specific construction, making them vulnerable to so-called loopholes. Here we use entangled photons to violate a Bell inequality while closing the fair-sampling loophole, that is, without assuming that the sample of measured photons accurately represents the entire ensemble3. To do this, we use the Eberhard form of Bell’s inequality, which is not vulnerable to the fair-sampling assumption and which allows a lower collection efficiency than other forms4. Technical improvements of the photon source5, 6 and high-efficiency transition-edge sensors7 were crucial for achieving a sufficiently high collection efficiency. Our experiment makes the photon the first physical system for which each of the main loopholes has been closed, albeit in different experiments.
 Mentor P: 28,801 A. S. Stodolna et al.,"Hydrogen Atoms under Magnification: Direct Observation of the Nodal Structure of Stark States" Abstract: To describe the microscopic properties of matter, quantum mechanics uses wave functions, whose structure and time dependence is governed by the Schrödinger equation. In atoms the charge distributions described by the wave function are rarely observed. The hydrogen atom is unique, since it only has one electron and, in a dc electric field, the Stark Hamiltonian is exactly separable in terms of parabolic coordinates (η, ξ, φ). As a result, the microscopic wave function along the ξ coordinate that exists in the vicinity of the atom, and the projection of the continuum wave function measured at a macroscopic distance, share the same nodal structure. In this Letter, we report photoionization microscopy experiments where this nodal structure is directly observed. The experiments provide a validation of theoretical predictions that have been made over the last three decades. A synopsis of the work can be found here, which also has a link for free access to the actual paper. Zz.
 Mentor P: 28,801 J Heeck, "How Stable is the Photon?", Phys. Rev. Lett. 111, 021801 (2013). Abstract: Yes, the photon. While a nonzero photon mass has been under experimental and theoretical study for years, the possible implication of a finite photon lifetime lacks discussion. The tight experimental upper bound of the photon mass restricts the kinematically allowed final states of photon decay to the lightest neutrino and/or particles beyond the standard model. We discuss the modifications of the well-measured cosmic microwave background spectrum of free streaming photons due to photon mass and lifetime and obtain model-independent constraints on both parameters—most importantly a lower direct bound of 3 yr on the photon lifetime, should the photon mass be at its conservative upper limit. In that case, the lifetime of microwave photons will be time-dilated by a factor order 10^15. You may read a synopsis of this work at the APS Physics and report on it at Physics World. Zz.
 P: 7 O. Firstenberg et al., "Attractive photons in a quantum nonlinear medium" Nature (2013) doi:10.1038/nature12512. http://www.nature.com/nature/journal...ture12512.html Abstract: The fundamental properties of light derive from its constituent particles—massless quanta (photons) that do not interact with one another. However, it has long been known that the realization of coherent interactions between individual photons, akin to those associated with conventional massive particles, could enable a wide variety of novel scientific and engineering applications. Here we demonstrate a quantum nonlinear medium inside which individual photons travel as massive particles with strong mutual attraction, such that the propagation of photon pairs is dominated by a two-photon bound state. We achieve this through dispersive coupling of light to strongly interacting atoms in highly excited Rydberg states. We measure the dynamical evolution of the two-photon wavefunction using time-resolved quantum state tomography, and demonstrate a conditional phase shift exceeding one radian, resulting in polarization-entangled photon pairs. Particular applications of this technique include all-optical switching, deterministic photonic quantum logic and the generation of strongly correlated states of light.
 Mentor P: 28,801 A. D. Wissner-Gross and C. E. Freer, "Causal Entropic Forces", Phys. Rev. Lett. v.110, p.168702 (2013). Abstract: Recent advances in fields ranging from cosmology to computer science have hinted at a possible deep connection between intelligence and entropy maximization, but no formal physical relationship between them has yet been established. Here, we explicitly propose a first step toward such a relationship in the form of a causal generalization of entropic forces that we find can cause two defining behaviors of the human “cognitive niche”—tool use and social cooperation—to spontaneously emerge in simple physical systems. Our results suggest a potentially general thermodynamic model of adaptive behavior as a nonequilibrium process in open systems. Read the synopsis in APS Physics. Zz.
 P: 441 Andreas Reiserer, Stephan Ritter, Gerhard Rempe Nondestructive Detection of an Optical Photon Science DOI: 10.1126/science.1246164 (2013) http://www.sciencemag.org/content/ea...cience.1246164 Paper on arxiv: http://arxiv-web3.library.cornell.edu/abs/1311.3625 Abstract: All optical detectors to date annihilate photons upon detection, thus excluding repeated measurements. Here, we demonstrate a robust photon detection scheme which does not rely on absorption. Instead, an incoming photon is reflected off an optical resonator containing a single atom prepared in a superposition of two states. The reflection toggles the superposition phase which is then measured to trace the photon. Characterizing the device with faint laser pulses, a single-photon detection efficiency of 74% and a survival probability of 66% is achieved. The efficiency can be further increased by observing the photon repeatedly. The large single-photon nonlinearity of the experiment should enable the development of photonic quantum gates and the preparation of novel quantum states of light.

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