Infinite number of quantum particles gives clues to big-picture behaviour at large scale (Vol. 50, No. 3)

Scientists gain a deeper understanding of phenomena at macroscopic scale by simulating the consequences of having an infinite number of physical phenomena at quantum scale.

In quantum mechanics, the Heisenberg uncertainty principle prevents an external observer from measuring both the position and speed (referred to as momentum) of a particle at the same time. They can only know with a high degree of certainty either one or the other—unlike what happens at large scales where both are known. To identify a given particle’s characteristics, physicists introduced the notion of quasi-distribution of position and momentum. This approach was an attempt to reconcile quantum-scale interpretation of what is happening in particles with the standard approach used to understand motion at normal scale, a field dubbed classical mechanics. In a new study published recently, the authors reverse this approach; starting with quantum mechanical rules, they explore how to derive an infinite number of quasi-distributions, to emulate the classical mechanics approach. This approach is also applicable to a number of other variables found in quantum-scale particles, including particle spin.

J. S. Ben-Benjamin, L. Cohen and M. O. Scully, From von Neumann to Wigner and beyond, Eur. Phys. J. Spec. Top. 227, 2171 (2019)
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Information stored in quantum states of water fragments (Vol. 48, No. 5-6)

Does water have memory? Well, not in the usual sense. But it is known, that if you tear water molecules apart, the remaining fragments can tell you a story about how it happened. To investigate this phenomenon, a plasma reactor producing miniature lightnings in direct contact with water level was constructed. The electrical discharges are powerful enough to cause dissociation of water molecules in various ways. To facilitate the electrical breakdown, the atmosphere in the reactor was replaced by argon.

The water molecule can be broken by impact of sufficiently fast electron, absorption of deep UV photon or previously excited argon atom. Each of these processes has a different energy balance and the remaining energy is partially conserved in quantum states of the water fragments. By careful analysis of the light emitted by the relaxing OH radicals, we can disentangle the respective contributions to the total spectrum and calculate the portion of water molecules undergoing various dissociation mechanisms.

The water fragments really remember what preceded their creation and they let us know by emitting photons. The time scale for "forgetting" depends on the collisional rate, i.e. the pressure. At atmospheric pressure, the information can be kept for several nanoseconds.

J. Voráč, P. Synek, V. Procházka and T. Hoder, State-by-state emission spectra fitting for non-equilibrium plasmas: OH spectra of surface barrier discharge at argon/water interface, J. Phys. D: Appl. Phys. 50, 294002 (2017)
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Inhibitory neurons have two types of impact on brain oscillations (Vol. 50, No. 4)

A certain type of neuron, called inhibitory neurons, can have two types of overall effect on oscillations in the brain

Studying the brain involves measuring the activity of billions of individual brain cells called neurons. Consequently, many brain measurement techniques produce data that is averaged to reflect the activity of large populations of these neurons. If all of the neurons are behaving differently, this will average out. But, when the behaviour of individual neurons is synchronized, it produces clearly visible oscillations. Synchronisation is important to understanding how neurons behave, which is particularly relevant with regard to brain diseases like Alzheimer’s, epilepsy and Parkinson’s. Now, a group of researchers has used a combination of two computer models to study the ways different kinds of neurons can impact synchronisation. The study has been published recently. To study the effects on synchronisation, the authors examined neurons called inhibitory neurons – which work to slow down or stop the activity of other neurons. Moreover, they explored the likelihood of these inhibitory neurons firing either spontaneously or not at all within the network.

P.-X. Lin, Ch.-Y. Wang and Z.-Xi Wu (2019), Two-fold effects of inhibitory neurons on the onset of synchronization in Izhikevich neuronal networks, Eur. Phys. J. B 92, 113 (2019)
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Inhibitory neurons retrieving physical properties from two-colour laser experiments (Vol. 51, No. 1)

Useful information about ultrafast light-matter interactions is buried deep in the signals produced by two-colour pump-probe experiments, and requires sophisticated techniques to disentangle it.

When photons of light interact with particles of matter, a diverse variety of physical processes can unfold in ultrafast timescales. To explore them, physicists currently use ‘two-colour pump-probe’ experiments, in which an ultrashort, infrared laser pulse is first fired at a material, causing its constituent electrons to move. After a controllable delay, this pulse is followed by a train of similarly short, extreme-ultraviolet pulses, ionising the material. By measuring the total ionisation following the pulses along with the resulting electron energy spectra, physicists can theoretically learn more about ultrafast, light-matter interactions. In new research published in EPJ D, an international team of physicists, led by Eric Suraud at the University of Toulouse, discovered that these signals are in fact dominated by the less interesting interplay between electrons and the initial infrared laser. They show that more useful information is buried deeper within these signals.

T. Brabec, P. M. Dinh, C. Z. Gao, C. R. McDonald, P-G. Reinhard, E. Suraud, Physical mechanisms encoded in photoionization yield from IR+XUV setups, European Physical Journal D 73, 212 (2019)
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Initial state entanglement in inflationary cosmology (Vol. 47 No. 1)

Recent observational data indicate that inflationary cosmology gives an excellent description of the very early universe. Inflationary cosmology assumes that quantum fluctuations seed the observed large scale structure in the universe. So we may be able to test the initial quantum state of the universe observationally in the future. Especially, if primordial vacuum state is entangled, the effect of entanglement could then be observed.

We give a new interpretation of the effect of initial state entanglement on the spectrum of vacuum fluctuations. We consider an initially entangled state between two free massive scalar fields in de Sitter space. We construct the initial state by making use of a Bogoliubov transformation between the Bunch-Davies vacuum and a four-mode squeezed state, and then derive the exact power spectrum for one of the scalar fields. We demonstrate that an oscillatory spectrum hardly appears for the initially entangled state unless an ad hoc absolute value of the Bogoliubov coefficients is chosen. We stress that, on the contrary, an initially non-entangled state may naturally produce an oscillatory spectrum due to quantum interference if the initial state deviates from the Bunch-Davies vacuum.

S. Kanno, A note on initial state entanglement in inflationary cosmology, EPL 111, 60007 (2015)
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Injection coupling in quantum-cascade lasers (Vol. 42, No. 5)

(a) Typical conduction band structure of terahertz QCLs investigated here. The injection coupling (Δ_l'4) varies from 1.6 to 10 meV. The light grey regions represent doping layers. (b) Calculated JNDR, Jpeak gain and ΔJ as a function of injection coupling. (c) Peak gain for different injection coupling structures.

Effect of injection coupling strength on terahertz quantum-cascade lasers
H. Li and J.C. Cao, Semicond. Sci. Technol. 26, 095029 (2011)
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Inner electrons behave differently in aromatic hydrocarbons (Vol. 50, No. 4)

When an electron from one of the lower energy levels in an atom is knocked out of the atom, it creates a space which can be filled by one of the higher-energy electrons, also releasing excess energy. This energy is released in an electron called an Auger electron - and produces an effect known as Auger decay. Now, the authors have studied the Auger effect in four hydrocarbon molecules: benzene, cyclohexane, hexatriene and hexadiene. These molecules were chosen because they exhibit different characteristics of aromaticity. They found that molecules containing pi bonds have a lower threshold for Auger decay. Potential applications of this decay effect include a treatment called Auger therapy, which is used to help cancer patients.

G. Zhao, T. Miteva, and N. Sisourat, Inner-valence Auger decay in hydrocarbon molecules, Eur. Phys. J. D 73, 69 (2019)
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Insect-like vibrating winged NAV (Vol. 49 No.4)

This work presents an original concept using the combination of two resonant vibration modes of the flexible wings of a Nano-Air Vehicle to reproduce insect wings kinematics and generate lift. Since insects use for flying a kinematics which combines flapping and twisting motions with a specific phase shift, the main goal of this study is to design the artificial wings such that they feature two vibrations modes which are producing flapping and twisting deformations and to combine them with the appropriate phase shift. For this purpose, a polymeric prototype was micromachined with a wingspan of 3 cm, flexible wings and a single electromagnetic actuator as illustrated in the figure.

An optimal wings configuration was determined with a modelling and validated through experimental analyses to observe the vibrating behaviour of the prototype. A dedicated lift force measurement bench was then used to demonstrate a lift force equivalent to the prototype weight. Finally, at the maximum lift frequency, high-speed camera measurements confirmed a kinematics of the flexible wings with flapping and twisting motions combined in the expected phase shift.

D. Faux, O. Thomas, E. Cattan and S. Grondel, Two modes resonant combined motion for insect wings kinematics reproduction and lift generation, EPL 121, 66001 (2018)
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Instanton filtering for the stochastic Burgers equation (Vol. 44 No. 3)

Comparison of the filtered velocity field (new direct method, top) and the instanton field (Chernykh-Stepanov method, bottom) as a space-time contour plot

T. Grafke, R. Grauer and T. Schäfer, ‘Instanton filtering for the stochastic Burgers equation’, J.Phys.A: Math. Theor., 46, 062002 (2013)
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Intelligent metamaterials behave like electrostatic chameleons (Vol. 50, No. 3)

A chameleon can flexibly change its colour to match its surroundings. And a similar phenomenon can now be seen in a new class of smart materials called metamaterials. The trouble is that these metamaterials lack the ability to respond to nearby objects due to their physical characteristics. To remedy this shortcoming, the authors have developed so-called 'metashells': hollow shells made of metamaterials and capable of carrying materials in their core. The advantage is that their physical characteristics, such as permittivity--the extent to which a material can store charge within an electrical field--change with the electromagnetic properties of the material they contain. In a theoretical study published recently, they describe how they have developed an entire class of these chameleon-like metashells. These intelligent metashells could become an all-purpose material to satisfy different permittivity requirements under different conditions. The next stages will focus on experimental research, and on industrial applications.

L. Xu and J. Huang, Electrostatic chameleons: Theory of intelligent metashells with adaptive response to inside objects, Eur. Phys. J. B 92, 53 (2019)
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Interaction control in ultracold Fermi-Fermimixtures (Vol. 42, No. 3)

Interaction control in a trapped fermi-fermimixture of ⁶li and ⁴⁰K atoms. a feshbach resonancemanifests itself in a pronounced dependence of the scattering length a on themagnetic field strength b. This facilitateswide interaction tuning between the trapped species.

Feshbach resonances in the ⁶Li-⁴⁰K Fermi-Fermimixture: Elastic versus inelastic Interactions
D. Naik et al., Eur. Phys. J. D 61, Special issue "Cold quantummatter …" (2011)
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Ionisation mechanisms of captive atoms struck by light matter (Vol. 48, No. 3)

Physicists elucidate the effects of light rays falling onto hydrogen atoms trapped in a carbon atom cage.

Light interacting with hydrogen atoms enclosed in hollow cages composed of carbon atoms—referred to as fullerene material—produces ionisation. This phenomenon, which has been the subject of intense theoretical scrutiny, is particularly interesting because the light rays can have dramatic effects in inducing small external energy potentials. Specifically, they alter the structural and dynamic properties of the atoms confined within the fullerene molecule. The authors have just published a study explaining the theory behind the ionisation. Applications of this process include drug delivery, quantum computation, photovoltaics and hydrogen storage.

A. L. Frapiccini, G. Gasaneo and D. M. Mitnik, Generalized Sturmians in the time-dependent frame: effect of a fullerene confining potential, Eur. Phys. J. D 71, 40 (2017)
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Is the relation between mass and energy universal? (Vol. 49 No.5-6)

The energy of ordinary particles is related to their mass through the famous relation E=mC², where m is both the inertial and the gravitational mass of the particle. This energy is minimum when the momentum p of the particle is p=0. Things are completely different if the energy is minimum for a momentum p=po≠0. The inertial mass density of a gas of such particles is then ρ=npo²/(3kBT), where n is the density of particles, and T the temperature of the gas. It is not related to the energy density.

Condensed matter gives an example of such particles. Rotons, which are excitations of superfluid 4He, have their energy minimum at a finite momentum. They largely contribute to the inertial mass density of the “normal fluid” in the two fluid model of superfluid ⁴He. Nothing similar has been evidenced, up to now, within the cosmological particles, but one can raise the question: would the gravitational mass be related to the energy or the inertial mass? Assuming that gravitational and inertial mass densities are the same gives for the gas of such particles properties close to those expected for Dark Energy. This work is a discussion about these questions.

B. Castaing, What is the gravitational mass when energy and inertial mass are not equivalent?, EPL 123, 20003 (2018)
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Is there memory for the memoryless? (Vol. 50, No. 4)

Inertia plays a role on the evolution of Brownian particles. Nevertheless, the interplay of inertial time-scale contributions and an overdamped dynamics with non-Markovian stochastic forces leads to contradictions that make equilibration impossible. This is due to assuming memory correlations for the dissipation, which seems to be inconsistent with the overdamped approximation, where thermal fluctuations adjust instantaneously to the state of the particle. Effectively, by taking the noise correlation time-scale to be zero (no memory) we certainly recover the expected physical behaviour of the problem, e.g., the equilibrium distribution. On the other hand, we can deal with the contradiction by inserting another source of noise, of Markovian type, and with “effective temperature” different from the non-Markovian noise. As a result, the stationary state may be regularized and the equilibrium recovered if both noises have same temperatures, even for finite memory time-scales. The additional white noise brings the system back to equilibrium, no matter how small the new noise intensity is.

E. S. Nascimento and W. A. M. Morgado, Non-Markovian effects on overdamped systems, EPL 126, 10002 (2019)
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Jamming meets Constraint Satisfaction (Vol. 47 No. 3)

Amorphous packing of spheres is also known as jamming. Jamming points are reached through a-thermal compression, when the space where particles can move shrinks to zero. This system is mechanically marginally stable and critical. Differently from usual phase transitions, the critical exponents –which are nontrivial- do not appear to depend on space dimension and have been computed in mean field theory. Our paper connects jamming to the Constraint Satisfaction Problems (CSP) of optimization theory, in the case of continuous variables. We study a neural network, the Perceptron, close to the capacity limit for storing random patterns: the jamming transition of the model. Parameters dependent, one has a convex or a non-convex optimization problem: jamming is non-critical in the former but critical in the latter. Physically in the convex case jamming is approached from a liquid phase while in the non-convex case it occurs from a marginal glass phase. Surprisingly we find the same exponents as in spheres. We conjecture a unique super-universality class for continuous non-convex CSP depending on the glassy nature of the configurations in the vicinity of jamming.

S. Franz and G. Parisi, The simplest model of jamming, J. Phys. A: Math. Theor. 49, 145001 (2016)
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