Quantum manipulation power for quantum information processing gets a boost (Vol. 49, No. 1)

Improving the efficiency of quantum heat engines involves reducing the number of photons in a cavity, ultimately impacting quantum manipulation power.

Traditionally, heat engines produce heat from the exchange between high-temperature and low-temperature baths. Now, imagine a heat engine that operates at quantum scale, and a system made up of an atom interacting with light (photons) confined in a reflective cavity of sub-atomic dimensions. This setup can either be at a high or low temperature, emulating the two baths found in conventional heat engines. Controlling the parameters influencing how such quantum heat engine models work could dramatically increase our power to manipulate the quantum states of the coupled atom-cavity, and accelerate our ability to process quantum information. In order for this to work, we have to find new ways of improving the efficiency of quantum heat engines. In a study published recently, the authors show methods for controlling the output power and efficiency of a quantum thermal engine based on the two-atom cavity.

K.W. Sun, R. Li and G.-F. Zhang, A quantum heat engine based on the Tavis-Cummings model, Eur. Phys. J. D 71, 230 (2017)
[Abstract]

Quasi-effective medium theory for multilayered magneto-dielectric structures (Vol. 45 No.2)

Object-independent cloaking

Maxwell–Garnett theory is the most widely used effective medium theory for the determination of the permittivity/permeability of nano-composite materials. However, it places a serious restriction on the physical dimensions of the constituents, that is, the feature sizes must be smaller than the incident wavelength. Thus, its applicability is limited to the quasi-static regime. An alternative theory has now been proposed by the authors which uses mode-dependent quasi-effective impedances to allow exact calculations of the far field scattering/extinction of complex multi-shell structures regardless of the object physical dimensions.

To demonstrate the physical insights that are to be gained based on this quasi-effective medium theory the authors have studied two practical examples: (i) they consider the problem of surface plasmon hybridization in concentric multi-shell particles, and (ii) they have applied the theory to design an object-independent cloak which consists of an arbitrary shaped object enclosed by a set of concentric shells. The most significant advantages of this new theory are its simplicity, ease of implementation and the important insights the theory provides into the optical properties of complex systems.

D. A. Genov and P. C. Mundru, "Quasi-effective medium theory for multi-layered magneto-dielectric structures", J. Opt. 16, 015101 (2014)
[Abstract]

Rare events in “noisy” networks (Vol. 49, No. 3)

Bringing diseases to extinction and mitigating the effects of human-caused environmental changes which accelerate the rate of species extinction are issues of worldwide importance. Both phenomena are typically rare events, relying on the interplay between network topology, nonlinear dynamics, and random fluctuations from the environment and interactions. However, the prediction of such rare events in general stochastic networks was an unsolved problem, despite extensive work in network dynamics. Here we solve the problem of predicting rare events as large fluctuations from metastable states with a general theory that combines mean-field approximations, large-deviation techniques and network topology. A benefit of our approach is its flexibility in describing the effects of multiple sources of different continuous and discrete noise. Using our theory, we demonstrate that networks with both internal interaction noise and external parameter noise exhibit a cross-over where the familiar exponential scaling of rare-event times with the number of nodes in the network is lost, and parametric noise dominates.

J. Hindes and I. B. Schwartz, Rare events in networks with internal and external noise, EPL 120, 56004 (2017)
[Abstract]

Reading between the lines of highly turbulent plasmas (Vol. 48, No. 3)

Study shows how to identify highly turbulent plasma signatures in the broadening of the shapes of lines emitted by ions and atoms within.

Plasma, the ionised state of matter found in stars, is still not fully understood, largely due to its instability. Astrophysicists have long-since sought to develop models that can account for the turbulent motions inside plasma, based on observing line shapes emitted by atoms and ions in the plasma. Turbulences are typically detected through the observation of broadened lines due to the Doppler effect, similar to the principle behind radar. In a new study published recently, the authors develop an iterative simulation model that accurately predicts, for the first time, the changes to the line shape in the presence of strong plasma turbulence. Ultimately, the authors aim to provide a system for assessing plasma turbulence that is valid for both a stellar atmosphere and the ITER tokamak designed to generate fusion energy.

R. Stamm, I. Hannachi, M. Meireni, H. Capes, L. Godbert-Mouret, M. Koubiti, J. Rosato, Y. Marandet, M. Dimitrijević and Z. Simić, Line shapes in turbulent plasmas, Eur. Phys. J. D 71, 68 (2017)
[Abstract]

Refraction index of shock compressed water at megabar pressure (Vol. 47 No. 1)

Compressing water at megabar pressure through laser-driven shocks induces phase changes and may finally produce a metallic fluid. Such phase is particularly relevant for planetology since water is one of the main constituents of the mantles of giant planets like Uranus and Neptune, and its metallization has been recognized as a possible source of the magnetic field of such planets. In this work we study the transition of water by looking at its optical properties. Increasing pressure water changes from transparent to opaque (absorbing) and finally reflecting. We provide the first quantitative measurement of water refractive index in the megabar range, a measurement, which can give information on how the material is approaching gap closure (metallization). We also performed measurements on water precompressed at 10 kbar, allowing getting off-Hugoniot states at high pressure but low temperature. Refraction index for transparent and opaque water was measured using a VISAR system. At high compression a sharp increase of the real and imaginary part of the refraction index was observed. Experiments were performed at the LULI and RAL laboratories.

D. Batani, K. Jakubowska, A. Benuzzi-Mounaix, C. Cavazzoni, C. Danson, T. Hall, M. Kimpel, D. Neely, J. Pasley, M. Rabec Le Gloahec and B.Telaro, Refraction index of shock compressed water in the megabar pressure range, EPL 112, 36001 (2015)
[Abstract]

Relativity Matters: Two opposing views of the magnetic force reconciled (Vol. 49, No. 2)

How magnetic force acts on charged subatomic particles near the speed of light

Current textbooks often refer to the Lorentz-Maxwell force governed by the electric charge. But they rarely refer to the extension of that theory required to explain the magnetic force on a point particle. For elementary particles, such as muons or neutrinos, the magnetic force applied to such charges is unique and immutable. However, unlike the electric charge, the magnetic force strength is not quantised. For the magnetic force to act on them, the magnetic field has to be inhomogeneous. Hence this force is more difficult to understand in the context of particles whose speed is near the speed of light. Moreover, our understanding of how a point-particle carrying a charge moves in presence of an inhomogeneous magnetic field relied until now on two theories that were believed to differ. The first stems from William Gilbert's study of elementary magnetism in 16th century, while the second relies on André-Marie Ampère electric currents. In a new study just published, the authors succeeded in resolving this ambiguity between Amperian and Gilbertian forms of magnetic force. Their solution makes it possible to characterise the interaction of particles whose speed is close to the speed of light in the presence of inhomogeneous electromagnetic fields.

J. Rafelski, M. Formanek, and A. Steinmetz, Relativistic Dynamics of Point Magnetic Moment, Eur. Phys. J. C 78, 6 (2018)
[Abstract]

Removing complexity layers from the universe’s creation (Vol. 44 No. 5)

A schematic depiction of the combined motion that a Brownian particle executes in a background polycrystalline space.

P. Jizba and F. Scardigli, ‘Special relativity induced by granular space’, Eur. Phys. J. C 73, 2491 (2013)
[Abstract]

Renormalisation group for 3-body interactions in 1D (Vol. 42, No. 6)

The three-body interaction induced by the renormalization group evolution starting from a Hamiltonian with only two-body forces. The three-body potential is plotted as a function of initial and final relative momentum variables, for three values of the flow parameter. It can be seen to approach a diagonal form as the flow progresses.

The Similarity Renormalization Group for Three-Body Interactions in One Dimension
O. Åkerlund , E.J. Lindgren, J. Bergsten, B. Grevholm, P. Lerner, R. Linscott, C. Forssén, L. Platter, EPJ A, 47, 1 (2011)
[Abstract]

Replica techniques can predict learning curves (Vol. 44 No. 1)

Predicted mean square errors vs number of examples per vertex, for different noise levels σ² (triangles), agree very well with numerical simulations (circles, graphs with 500 vertices) and improve significantly on existing approximations (dashed line).

M. J. Urry and P. Sollich, ‘Replica theory for learning curves for Gaussian processes on random graphs’, J. Phys. A: Math. Theor. 45, 425005 (2012)
[Abstract]

Repulsive Casimir forces at quantum criticality (Vol. 47 No. 3)

Casimir forces act between macroscopic objects immersed in a fluctuating entity, which may be the quantum vacuum or material medium in a state hosting sizable fluctuations. These forces were first discussed in 1948 as an observable manifestation of the quantum nature of the vacuum, lying at the heart of quantum electrodynamics. At a somewhat later stage it was realized that a material medium brought to the vicinity of a critical state induces analogous interactions once some macroscopic bodies become immersed therein.

In most of the known cases the Casimir force is attractive in situations where the bodies in question are identical. This however turns out not to be a general rule.

In our theoretical work we addressed a system of bosonic particles in the vicinity of a quantum critical state, where both thermal and quantum fluctuations are strong. As our exact analysis indicates, the sign of the Casimir force between two bodies immersed in such a medium may be changed by varying the ratio between their separation D and the thermal de Broglie length λ. In the thermal regime D>>λ the force in question is attractive, however, by varying the system setup so that D<<λ one crosses over to a regime admitting repulsive Casimir interactions.

P. Jakubczyk, M. Napiórkowski and T. Sęk, Repulsive Casimir forces at quantum criticality, EPL 113, 30006 (2016)
[Abstract]

Resolving tension on the surface of polymer mixes (Vol. 49, No. 1)

A new study finds a simple formula to explain what happens on the surface of melted mixes of short- and long-strand polymers.

Better than playing with Legos, throwing polymer chains of different lengths into a mix can yield surprising results. In a new study published recently, physicists focus on how a mixture of chemically identical chains into a melt produces unique effects on their surface. That’s because of the way short and long polymer chains interact with each other. In these kinds of melts, polymer chain ends have, over time, a preference for the surface. Now, the authors have studied the effects of enriching long-chain polymer melts with short-chain polymers. They performed numerical simulations to explain the decreased tension on the surface of the melt, due to short chains segregating at the surface over time as disorder grows in the melt. They found an elegant formula to calculate the surface tension of such melts, connected to the relative weight of their components.

P. Mahmoudi and M.W. Matsen, Entropic segregation of short polymers to the surface of a polydisperse melt, Eur. Phys. J. E 40, 85 (2017)
[Abstract]

Revealing the microscopic origin of φ0 Josephson junctions (Vol. 46 No. 5-6)

The dependence of φ0 on the length L of the N bridge (see inset) with an intrinsic SOC and spin-splitting field

F. S. Bergeret and I. V. Tokatly, Theory of diffusive φ0 Josephson junctions in the presence of spin-orbit coupling, EPL, 110, 57005 (2015)
[Abstract]

Revisiting quantum effects in MEMS  (Vol. 45 No. 1)

“Example of MEMS”
Credit: United States Government Work

Micro- and nano-electromechanical devices, referred to as MEMS and NEMS, are ubiquitous. They are found in car airbags and smart phones. The trouble is that, as their size decreases, forces typically experienced at the quantum level start to matter in these nanodevices. The authors have studied the mechanical and electrical stability of MEMS and NEMS, depending on the plate thickness and the nature of the material used. They show that previous works overestimated the operating conditions of the devices by not taking into account this Casimir/van der Waals effect. In addition, they demonstrate that the stability of these devices under the Casimir force changes depending on the nature and thickness of the metal coatings used. It also depends on the variation of concentration of the free charges in the silicon used, which changes with doping levels.

R. Esquivel-Sirvent and R. Perez-Pascual, “Geometry and charge carrier induced stability in Casimir actuated nanodevices”, Eur. Phys. J. B, 86, 467 (2013)
[Abstract]

Robustness of states at topological insulator interfaces (Vol. 48, No. 5-6)

Topological phases of matter are characterized by invariant numbers. In two-dimensional time-reversal symmetric electronic systems, a Z₂ valued (0 or 1) invariant distinguishes trivial insulators from non-trivial ones. Interfaces between trivial and non-trivial topological insulators are known to host conductive channels protected against disorder. The protection of these states originates from the necessity of a gap closure in order to change topology. However, if the two regions are of the same topological phase, there is no such requirement.

Using a multi-orbital model, it is shown in this study that conductive states can also emerge at the interface between two non-trivial topological insulators characterized by opposite spin Chern numbers, another invariant. In general, these states are sensitive to disorder. However, it is possible under some conditions to reduce the effect of disorder, or even to cancel it. These conditions are clarified. Since analogues of topological insulators can be presently made with polaritons, ultracold atomic gases, phononic or photonic materials, these conclusions should motivate experimental studies in many directions.

A. Tadjine and Ch. Delerue, Robustness of states at the interface between topological insulators of opposite spin Chern number, EPL 118, 67003 (2017)
[Abstract]

Rodeo in liquid crystal (Vol. 46 No. 3)

Scientists have achieved an unprecedented level of control over defects in liquid crystals that can be engineered for applications in liquid matter photonics.

Sitting with a joystick in the comfort of their chairs, scientists can play “rodeo” on a screen magnifying what is happening under their microscope. They use very strong laser tweezers to locally melt the liquid crystal into a phase where the molecules are oriented in all directions, encircling one part of the fibre. They subsequently switch-off the laser light, resulting in molecules reverting back from being oriented in all directions to being parallel to each other, creating several pairs of defects—akin to localised disruptions of the crystal’s ordering field —forming a ring. The defect ring is used as a non-material "rope" to entangle and strongly bind a microsphere and long fibre of micrometric diameter. The results of this work have been published recently by the authors. They believe that their findings could ultimately open the door to controlling the flow of light using light of a specific frequency in the Gigahertz range in liquid crystal photonic microdevices.

M. Nikkhou, M. Škarabot and I. Muševič,, Topological binding and elastic interactions of microspheres and fibres in a nematic liquid crystal, Eur. Phys. J. E, 38, 23 (2015)
[Abstract]