Martingale theory for housekeeping heat (Vol. 50, No. 2)

Which universal thermodynamic properties emerge in a nonequilibrium process, in isothermal conditions at temperature T, that result from the violation of detailed balance, and how they may be quantified? The housekeeping heat is the fluctuating heat exchanged between a mesoscopic system and its environment due to the violation of detailed balance. Using the framework of martingale theory widely used in probability theory and finance, we derive a number of universal equalities and inequalities for extreme-value and stopping-time statistics of the housekeeping heat. Our theory provides a quantitative link between minimal models of gambling and financial markets (martingales) and heat fluctuations. The housekeeping heat behaves like a gambler’s fortune in a casino: its expected value in the future is always smaller or equal regardless of its past values. The super-martingale structure of the housekeeping heat implies that certain statistical properties of the housekeeping heat are system-independent, i.e. universal. A particular result of our theory is that the average value of the maximum housekeeping heat that a system absorbs from its environment cannot exceed k_B T, with k_B Boltzmann’s constant.

R. Chétrite, S. Gupta, I. Neri and E. Roldán, Martingale theory for housekeeping heat, EPL 124, 60006 (2018),
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New university ranking system includes the cultural perspective (Vol. 50, No. 2)

A new study proposes a new way of ranking universities, using a more balanced cultural view and based on 24 international editions of Wikipedia

Scientists in France have developed a new way of generating a ranking of the world’s universities that places more emphasis on the cultural perspective. In a study published recently, the authors perform an analysis of Wikipedia editions in 24 languages, collected in May 2017—previous studies pursuing a similar approach focused on data from 2013. Employing well-known ranking algorithms, they establish a Wikipedia Ranking of World Universities based on the relative cultural views of each of the 24 language-specific Wikipedia editions. Thus, they provide a more balanced view that reflects the standpoints of different cultures. Specifically, the authors use (for the first time for this purpose) a new tool for the analysis of online networks, which is based on the PageRank algorithm and known as reduced Google Matrix analysis. In this study, they determine the interactions between leading universities on a scale of ten centuries, which provides insights into the relative influence of specific universities in each country.

C.Coquidé, J.Lages, and D. L. Shepelyansky, World influence and interactions of universities from Wikipedia networks, Eur. Phys. J. B 92, 3(2019)
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3D virtual slicing of an antique violin reveals ancient varnishing methods (Vol. 50, No. 2)

Physicists and chemists use 3D scanning to unlock the forgotten secrets of the multi-layered coating methods that give violins their exceptional tone and look.

Italian violin-making masters of the distant past developed varnishing techniques that lent their instruments both an excellent musical tone and impressive appearance. Few records from this era have survived, as techniques were most often passed down orally to apprentices; only scarce information is available on the original methods used for finishing the instruments. In a new study published recently, the authors use the Elettra synchrotron facility in Trieste to develop a non-invasive 3D-scanning approach, using the Synchrotron Radiation micro-Computed Tomography (SR-micro-CT), that yields insights into the main morphological features of the overlapping finishing layers used on violins. In turn, the morphological images can be used to determine the chemical nature of the coating. This newly developed method could help scientists rediscover the procedures and materials used, and reproduce the multi-layered coating methods of the ancient masters.

They first use the X-ray beam to scan two sets of mock-ups, prepared in their lab to mimic the finishing layers on the historical instruments. Using the mock-ups, they then optimise the 3D scanning settings, boost the spatial resolution and define the parameters required for 3D reconstruction. They then focus on a large fragment removed from a damaged cello made by the 17th-century Italian luthier Andrea Guarneri. Lastly, they compare their findings with those produced by micro-invasive analyses of the varnish to evaluate the merits of the reconstructed volumes and virtual slicing in terms of investigating such layered, complex structures.

G. Fiocco, T. Rovetta, M. Malagodi, M. Licchelli, M. Gulmini, G. Lanzafame, F. Zanini, A. Lo Giudice, and A. Re, Synchrotron radiation micro-computed tomography for the investigation of finishing treatments in historical bowed string instruments: issues and perspectives, Eur. Phys. J. Plus 133, 525 (2018)
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How ion adsorption affects biological membranes’ functions (Vol. 50, No. 2)

A new study presents new models describing how the adsorption of calcium, barium and strontium ions onto biological membranes may affect the functions of cells

Ions with two positive electrical charges, such as calcium ions, play a key role in biological cell membranes. The adsorption of ions in solution onto the membrane surface is so significant that it affects the structural and functional properties of the biological cells. Specifically, ions interact with surface molecules such as a double layer of lipids, or liposomes, formed from phosphatidylcholines (PC). In a new study published recently, the author develops a mathematical model describing the electrical properties of biological membranes when ions such as calcium, barium and strontium adsorb onto them at different pH levels. Her work helps shed light on how ion adsorption reduces the effective surface concentration of add-on molecules with a specific function that can take part in biochemical reactions. These factors need to be taken into account when studying the diverse phenomena that occur at the lipid membrane in living cells, such as ion transport mechanisms.

I. Dobrzyńska, Association equilibria of divalent ions on the surface of liposomes formed from phosphatidylcholines, Eur. Phys. J. E 42, 3 (2019)
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Holey graphene as Holy Grail alternative to silicon chips (Vol. 50, No. 2)

Novel spintronics applications could stem from introducing holes into graphene to form triangular antidot lattices, granting the material new magnetic properties

Graphene, in its regular form, does not offer an alternative to silicon chips for applications in nanoelectronics. It is known for its energy band structure, which leaves no energy gap and no magnetic effects. Graphene antidot lattices, however, are a new type of graphene device that contain a periodic array of holes-- missing several atoms in the otherwise regular single layer of carbon atoms. This causes an energy band gap to open up around the baseline energy level of the material, effectively turning graphene into a semiconductor. In a new study published recently, Iranian physicists investigate the effect of antidot size on the electronic structure and magnetic properties of triangular antidots in graphene. The authors have confirmed the existence of a band gap opening in such antidot graphene lattices, which depends on the electron’s spin degree of freedom, and which could be exploited for applications like spin transistors.

Z. Talebi Esfahani, A. Saffarzadeh, and A. Akhound, A DFT study on the electronic and magnetic properties of triangular graphene antidot lattices, Eur. Phys. J. B 91, 308 (2018)
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Lattice Improvement in Lattice Effective Field Theory (Vol. 50, No. 2)

Lattice calculations using the framework of effective field theory have been applied to a wide range of few-body and many-body systems. One of the challenges of these calculations is to remove systematic errors arising from the nonzero lattice spacing. While the lattice improvement program pioneered by Symanzik provides a formalism for doing this and has already been utilized in lattice effective field theory calculations, the effectiveness of the improvement program has not been systematically benchmarked.

In this work lattice improvement is used to remove lattice errors for a one-dimensional system of bosons with zero-range interactions. To this aim the improved lattice action up to next-to-next-to-leading order is constructed and it is verified that the remaining errors scale as the fourth power of the lattice spacing for observables involving as many as five particles. These results provide a guide for increasing the accuracy of future calculations in lattice effective field theory with improved lattice actions.

N. Klein, D. Lee and U.-G. Meißner,, Lattice improvement in lattice effective field theory, Eur. Phys. J. A 54, 233 (2018)
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Quantifying how much quantum information can be eavesdropped (Vol. 50, No. 2)

New study yields more precise characterisation of monogamous and polygamous entanglement of quantum information units

Encrypted communication is achieved by sending quantum information in basic units called quantum bits, or qubits. The most basic type of quantum information processing is quantum entanglement. However, this process remains poorly understood. Better controlling quantum entanglement could help to improve quantum teleportation, the development of quantum computers, and quantum cryptography. Now, the authors have focused on finding ways to enhance the reliability of quantum secret sharing. In a new study published recently, they provide a much finer characterisation of the distributions of entanglement in multi-qubit systems than previously available. In the context of quantum cryptography, these findings can be used to estimate the quantity of information an eavesdropper can capture regarding the secret encryption key.

Z. Zhang, Y. Luo, and Y. Li , Tighter monogamy and polygamy relations in multiqubit systems, Eur. Phys. J. D 73, 13 (2019)
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Multimodal microscope enables structural and functional cellular imaging (Vol. 50, No. 2)

Answering cell physiology and pharmacology research questions often requires structural and functional information to be obtained from a network of cells. The authors have developed a multi-modal imaging system based on surface plasmon resonance (SPR) that combines several additional imaging modalities including bright-field, epifluorescence, total internal reflection microscopy and SPR fluorescence microscopy. The microscope features a wide field of view that can study ~40 cells simultaneously with subcellular resolution.

SPR is the collective oscillation of free electrons in a metal excited by polarized light. The resonance condition is highly dependent upon the refractive index of the media. Exploiting this allows the detection of both spatial and temporal variations in refractive index (RI) label-free.

In this work the authors describe a detailed design of the microscopy platform including standard tests for characterization of spatial resolution and sensitivity. Using SPR for imaging requires that the cell of interest is closely adhered to the surface. The spatial variation of refractive index was shown to be reasonably homogenous from a cultured neuron. Finally, a prototypical functional imaging experiment is reported where spatiotemporal cellular functions of stem cell-derived cardiomyocytes have been realised by detecting localized contractions.

C. L. Howe, K. F. Webb, S.A. Abayzeed, D. J. Anderson, C. Denning and N. A. Russell, Surface plasmon resonance imaging of excitable cells, J. Phys. D: Appl. Phys. 52, 104001 (2019)
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Spin freezing and the Sachdev-Ye model (Vol. 50, No. 2)

The infinite-range, random-exchange Heisenberg spin model introduced in 1993 by Sachdev and Ye describes a non-Fermi liquid metal without quasi-particles, which resembles the bad-metal state of unconventional superconductors. Because of the somewhat artificial nature of the model, it is however not obvious how to connect this result to phenomena observed in strongly correlated materials. The latter are typically described by the Hubbard model and its multi-orbital extensions. Interestingly, the same non-Fermi liquid exponents as in the Sachdev-Ye model are generically observed in the correlated metallic phase of multi-orbital Hubbard models with Hund coupling. Our analysis suggests that the Sachdev-Ye model can be regarded as an effective description of a spin-freezing crossover regime with fluctuating local moments and that the variance of the random coupling in the Sachdev-Ye model is related to the Hund coupling. This analogy provides new insights into the nature of non-Fermi liquid metals, and into the close connection between spin freezing and unconventional superconductivity.

Ph. Werner, A.J. Kim and Sh. Hoshino, Spin freezing and the Sachdev-Ye model, EPL 124, 57002 (2018)
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Better safeguards for sensitive information (Vol. 50, No. 2)

Study improves the lower boundary and secret key capacity of an encryption channel

The secure encryption of information units based on a method called quantum key distribution (QKD) involves distributing secret keys between two parties—namely, Alice, the sender, and Bob, the receiver—by using quantum systems as information carriers. However, the most advanced quantum technology, QKD, is currently limited by the channel's capacity to send or share secret bits. In a recent study the authors show how to better approach the secret key capacity by improving the channel's lower boundary. They focus on a particular type of channel, called the noisy thermal amplifier channel, where the input signals are amplified together with noise induced by the thermal environment. The authors calculate the highest-known amount of secret information units, or bits, that Alice and Bob can share via such a channel. This is done by injecting controlled noise—made up of well-defined thermal agitation—into the detection apparatuses. By optimising over this noise, they improve the lower boundary of the capacity in the amplifier channel. The authors also confirm that the distribution of secret keys over this channel may occur at higher rates than the transmission of quantum information itself.

G. Wang, C. Ottaviani, H. Guo, and S. Pirandola, Improving the lower bound to the secret-key capacity of the thermal amplifier channel, Eur. Phys. J. D 73, 17 (2019)
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