APS Physics Tip Sheet – July 21, 2015

In this issue: How Ice Reduces Drag; Pinpointing Qubits; Testing General Relativity with Black Holes; Tuning the Buckling of Structural Beams 

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How Ice Reduces Drag

​(movies of the experiments are available)

An ice coating can halve the drag of an object moving in water by reducing the turbulent wake behind the object. 

Many common activities, from ice-skating to driving on a frozen road, reveal the lubricant effect of ice. This is known to be due to a thin layer of water on the icy surface, but no experiments to date have addressed how the same effect may impact the movement of objects in water—a problem relevant for the modeling of iceberg drifts in warming oceans. A team of researchers in Saudi Arabia and Australia has now found that an ice coating substantially reduces the drag of a sphere falling in water. Inspired by cocktail-drink iceball makers, the researchers coated metallic balls with a 10-mm shell of ice. They then dropped them in water and recorded their fall with a high-speed video camera. The authors observed a drag reduction of up to 50%, which, according to their analysis, was due to the effect of the surface water film on the turbulent wake left by the moving body.

 * Ivan U. Vakarelski (contact author), D.Y.C. Chan and S.T. Thoroddsen, “Drag moderation by the melting of ice surface in contact with water,” Physical Review Letters (expected publication date: July 24)

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Pinpointing Qubits

​Researchers demonstrate the ability to control individual qubits in a three-dimensional 5x5x5 qubit matrix.

To be able to perform useful tasks, a quantum computer would need to work with many qubits. A key challenge in proposed architectures is controlling an individual qubit without disturbing nearby ones. Researchers in the U.S. have demonstrated this capability in a qubit structure made of a three-dimensional (5x5x5) array of cesium atoms trapped in the optical lattice created by laser beams. Using a combination of lasers and microwave pulses, the researchers were able to perform logic operations on single atoms without affecting non-targeted atoms. 

* Y. Wang, David S. Weiss (contact author) et al., “Coherent addressing of individual neutral atoms in a 3D optical lattice,” Physical Review Letters (expected publication date: Jul 21)

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Testing General Relativity with Black Holes 

X ray observations might probe the gravitational field around a black hole and test for a possible breakdown of general relativity. 

General relativity is the current paradigm for describing gravitation and no deviations from its predictions have ever been revealed. However, the theory has been tested extensively only under weak gravitational fields, and it’s unclear whether it holds when gravity becomes strong, like around a black hole. Certain alternative models predict that, in the vicinity of a black hole, spacetime differs considerably from that of general relativity. Researchers at the Institute of Astronomy in Cambridge, UK, have now proposed a way of searching for a possible breakdown of general relativity. According to their calculations, collisions between gas molecules in a black hole’s accretion disk would lead to x ray emission lines that are shaped by the strong-gravity environment. X ray observations could thus be used to test general relativity and alternative gravity theories.

* Christofer J. Moore (contact author) and J.R. Gair, “Testing the “no-hair” property of black holes with X-ray observations of accretion disks,” Physical Review D (expected publication date: Jul 22)

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Tuning the Buckling of Structural Beams

​The mechanical properties of beams buckling under compression can be controlled by adding holes into the beam.

When compressed, beams used as structural elements can undergo buckling, which could lead to instability and mechanical failure. A research group in the Netherlands has now shown that they can adjust a beam’s stability, after it has buckled, by adding a series of elliptical holes into the beam. Guided by numerical simulations, the authors used 3D printing to realize holey beams with different geometries, showing that their response to compressive stress could be tuned by varying design parameters. The approach might lead to strategies for engineering mechanical elements with well-defined buckling properties.

* Corentin Coulais (contact author) et al., “Discontinuous Buckling of Wide Beams and Metabeams,” Physical Review Letters (published July 21)

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Journal articles and preprints are available to journalists on request. 

Contact: Matteo Rini Tel: +1 631 591 4224 (office), +1 646 288 5441 (cell), email: mrini@aps.org

APS Physics Tip Sheet – July 14, 2015

In this issue: Sending Quantum Messages Through Space; The Physics of Puddle Spreading; Terahertz-Controlled Chemistry; Perfect Absorbers of Light

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Sending Quantum Messages Through Space

​Experiments demonstrate that qubits can be reliably transmitted from satellites to Earth, showing the feasibility of global quantum communications in space. 

Quantum communication protocols such as quantum key distribution (QKD) transmit information with absolute security thanks to the laws of quantum mechanics. Demonstrated schemes are typically based on optical fibers but, according to a new study, quantum techniques may soon take advantage of the infrastructure of satellites orbiting the Earth. Researchers at the University of Padua and the Italian Space Agency have demonstrated that qubits encoded in photons can preserve their fragile quantum properties even after having bounced off satellites that are over a thousand kilometers away from Earth. The authors sent light pulses from Earth to five satellites, which reflected them back to Earth. The results showed that different qubit states could be faithfully distinguished after the long journey, achieving an error rate low enough for applications. 

 * G. Vallone, Paolo Villoresi (contact author) et al., “Experimental Satellite Quantum Communications,” Physical Review Letters (expected publication date: July 20)

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The Physics of Puddle Spreading 

A new model explains how a puddle spreads by taking into account the forces between the liquid and the surface on which it lies.

Place a liquid drop on a surface, and it will spread out and then stop, leaving a puddle with sharp boundaries. But existing theories, based on the interplay between gravity, capillary and viscous forces, have not been able to fully explain the process. In particular, in contrast with everyday observations, they predict that the liquid never stops spreading. MIT researchers have now developed a new model that reproduces several experimental observations, including the spreading rate and the resulting shape of the puddle’s profile. The model’s success comes from its ability to account for intermolecular forces between the surface and the liquid, which become significant where the liquid is very thin, such as at the puddle’s edges. The authors plan to use the model to tackle a wide range of problems like flows over rough surfaces or though porous materials.

* A.A. Pahlavan, L. Cueto-Felgueroso, G.H. McKinley and Ruben Juanes (contact author), “Thin films in partial wetting: internal selection of contact-line dynamics,” Physical Review Letters (expected publication date: Jul 17)

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Terahertz-Controlled Chemistry

​(Figure credit: H. Ogasawara/SLAC)

Pulses of Terahertz radiation have been used to selectively drive certain chemical reactions occurring on a metallic surface.

Many chemical reactions occur at a higher rate when the reactants are stuck to a surface, which acts as a catalyst. But sometimes other processes compete with the desired reaction, lowering its efficiency. Researchers from the U.S. and Sweden have demonstrated a new technique for controlling surface chemistry. The researchers used pulses of terahertz-frequency light—generated at Stanford’s Linac Coherent Light Source—to selectively drive the surface reaction by which carbon monoxide (CO) is oxidized, producing carbon dioxide (CO₂). The pulses “kick” oxygen atoms adsorbed on the surface, so that they move along the surface and interact with CO molecules more often. 

* J.L. LaRue, Hirohito Ogasawara (contact author) et al., “THz induced selective catalytic CO oxidation on Ru,” Physical Review Letters (expected publication date: Jul 15)

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Perfect Absorbers of Light

​(Figure credit: Viktar Asadchy, Aalto University) 

A metamaterial made of an array of helical elements absorbs radiation at certain frequencies while transmitting perfectly other frequencies. 

Conventional devices used for absorbing light in a given frequency range often create an unwanted effect: they partly reflect frequencies outside the absorption frequency band. A team of researchers from Finland, Belarus and Japan has now designed and fabricated a thin metamaterial that absorbs electromagnetic waves over a narrow frequency band while producing no reflections at other frequencies. The device, made of an array of alternating right- and left-handed chromium–nickel helices embedded in a plastic-foam slab, could find applications ranging from optical filters to stealth technology.

* Viktar S. Asadchy (contact author) et al., “Broadband reflectionless metasheets:

Frequency-selective transmission and perfect absorption,” Physical Review X (published Jul 14)

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Journal articles and preprints are available to journalists on request. 

Contact: Matteo Rini Tel: +1 631 591 4224 (office), +1 646 288 5441 (cell), email: mrini@aps.org

Single-Photon Transistors, Direct Test of Cosmic Acceleration, A Cooper-Pair Laser, Antineutrinos As Nuclear Watchdogs

APS Physics Tip Sheet – Jul 22, 2014

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Single-Photon Transistors

Two research groups have built optical transistors that turn on in the presence of just one photon.

Optical transistors, which handle photons instead of electrical currents, could be much faster than their electrical counterparts. However, a fundamental obstacle hinders their development: photons cannot easily be used to control other photons because, unlike electrons, they do not interact strongly. Two independent research groups in Germany have now demonstrated all-optical transistor devices in which a “gate” light pulse (consisting of a single photon) acts as a switch that either blocks or transmits another light pulse. Both schemes are based on the strong interaction between photons that can occur in atomic gases prepared by lasers in highly excited states. The transistors might be used to realize more complex logic gates for ultrafast all-optical signal processing.

* Hannes Gorniaczyk (contact author) et al., “Single-photon transistor mediated by interstate Rydberg interactions,”Physical Review Letters (expected publication date: Jul 28)

** D. Tiarks, Stephan Duerr (contact author) et al., “Single-photon transistor using a Förster resonance,” Physical Review Letters (expected publication date: Jul 28)

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Direct Test of Cosmic Acceleration

Future radio surveys of intergalactic hydrogen clouds could offer the first direct measurement of the Universe's acceleration

Measurements of distant supernovae have revealed that the universe is accelerating—a finding most often explained by a yet-to-be-deciphered form of dark energy. But this conclusion is based on assumptions on the Universe’s uniformity and expansion history. A team of researchers in China has now suggested that upcoming radio-telescope surveys could offer a more direct proof of the universe’s acceleration by tracking intergalactic hydrogen clouds. The velocity of these clouds could be determined by observing redshifts of the hydrogen absorption line, which occurs at a wavelength of 21 centimeters. The authors’ calculations suggest a few additional modifications that planned radio surveys should implement to measure—over the course of a decade—cosmically relevant accelerations of around 1 millimeter/second/year

* Hao-Ran Yu, Tong-Jie Zhang (contact author), Ue-Li Pen: Oleg Brandt), “Method for direct measurement of cosmic acceleration by 21-cm absorption systems,” Physical Review Letters (expected publication date: Jul 24)

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A Cooper-Pair Laser

​A superconducting transistor embedded in a microcavity acts like a laser that could be used to generate nonclassical light.

By placing a single emitter (like an atom or a quantum dot) inside a microcavity, researchers can realize lasers whose properties are remarkably different from conventional ones: they can be extremely compact, require little power to operate, and generate quantum states of light, such as single photons. A team of physicists from the US and the UK has built and tested a new single-emitter laser consisting of a superconducting circuit embedded in a microwave cavity. The authors argue the device is particularly suitable to generate “amplitude-squeezed” light, a quantum state of light that has reduced intensity fluctuations compared to conventional lasers—a property useful for high-precision measurements and quantum communication protocols. 

* F. Chen, Alex Rimberg (contact author) et al., “Realization of a Single-Cooper-Pair Josephson Laser,” Physical Review B(expected publication date: Jul 22)

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Antineutrinos As Nuclear Watchdogs

Researchers in the US and Austria have proposed a system for monitoring a nuclear reactor using an antineutrino detector placed right outside the reactor walls. The authors’ calculations suggest that the method could determine the level of fuel enrichment inside the reactor, which could be used to infer whether spent nuclear fuel has been removed for possible weapons development.

* E. Christensen, Patrick Huber (contact author), P. Jaffke, T.E. Shea, “Antineutrino monitoring for heavy water reactors,”Physical Review Letters (expected publication date: Jul 25)

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Journal articles and preprints are available to journalists on request. 

Contact: Matteo Rini Tel: +1 631 591 4224 (office), +1 646 288 5441 (cell), email: mrini@aps.org

Matteo Rini, PhD 

Deputy Editor, Physics 

mrini@aps.org  

http://physics.aps.org

Shear Waves for Medical Imaging, The Mass of a Top Quark, A Wormhole for Electrons

APS Physics Tip Sheet – Jul 15, 2014

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Shear Waves for Medical Imaging

​Magnetic fields and electric currents can generate shear waves in biological tissue—an effect that could lead to novel medical imaging schemes.

The combined application of magnetic fields and electric currents could lead to a new method for imaging biological tissues, as suggested by a study carried out by a team of researchers in France. The group investigated a scheme in which a current is injected in a tissue placed in a magnetic field. Thanks to the Lorentz force (the force arising when charged particles move in a magnetic field), the electrons moving in the tissue generated shear waves that can be detected with ultrasound imaging techniques. The intensity of the observed waves can be related to the stiffness of the material, revealing, for instance, structures that differ from the surrounding tissue, such as tumors.

* Pol Grasland-Mongrain (contact author) et al., “Imaging of shear waves induced by Lorentz force in soft tissues,” Physical Review Letters (expected publication date: Jul 18)

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The Mass of a Top Quark

Researchers at Fermilab have reported the most precise measurement to date of the top quark’s mass.

The top quark, with a mass roughly two hundred times larger than a proton’s, is the heaviest elementary particle. Determining the top quark mass is considered one of the best tests of the standard model of particle physics, so even small refinements of its measured value can be used to constrain or rule out alternative theories. Researchers have now analyzed a large dataset previously collected by the now-shut-down Tevatron particle accelerator at Fermilab. The new value has an uncertainty of about 0.43% -- the most precise value from a single measurement -- and exceeds the precision of the current world average, which is based on several independent experiments.

* The D0 Collaboration (contact author: Oleg Brandt), “Precision measurement of the top quark mass in lepton+jets final states,” Physical Review Letters (expected publication date: Jul 17)

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A Wormhole for Electrons

A team of researchers from Portugal and the U.S. has shown how sheets of graphene can be used to build a new material that acts as a “wormhole” for electrons: a perfect tunnel that connects two regions of space, as if the in-between region did not exist. The scheme may have far reaching implications for graphene-based electronics.

* D.E. Fernandes, N. Engheta, and Mario G. Silveirinha (contact author), “Wormhole for electron waves in graphene,” Physical Review B (published Jul 11)

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Journal articles and preprints are available to journalists on request. 

Contact: Matteo Rini Tel: +1 631 591 4224 (office), +1 646 288 5441 (cell), email: mrini@aps.org

Matteo Rini, PhD 

Deputy Editor, Physics 

mrini@aps.org  

http://physics.aps.org