# Vol. 42 No. 5 - Highlights

## The twin paradox in a cosmological context (Vol. 42, No. 5)

The twin paradox has been a source of debate since it was discovered by Einstein. It can be analytically verified assuming the existence of global nonrotating inertial frames.

The natural nonrotating frame and its identification with "fixed stars" is an aspect of Mach's Principle, which holds that the totality of matter in the universe determines the inertial frames.

Grøn and Braeck first note that the experiment by Hafele and Keating (1972), who flew atomic clocks eastward and westward around the Earth in commercial aircraft, also shows agreement with the expected result, assuming an inertial frame which is nonrotating with respect to "fixed stars." The authors then show that in the case of two observers in an otherwise empty universe (i.e., without "fixed stars") moving at different speeds on a circular path yield different twin paradox results, depending on whether one or the other – or neither – observer is assumed to be at rest.

The authors ultimately take a fresh look at the work of Brill and Cohen, who studied the geometry inside a massive rotating shell, and conclude that in the black hole limit, such a mass distribution will drag the frames around at its own rotation rate.

Taken together, and given the entire universe closely satisfying the black hole condition, this paper lends further support to the Mach Principle.

**The twin paradox in a cosmological context**

∅. Grøn and S. Braeck, *Eur. Phys. J. Plus*, **126**, 79 (2011)

**[Abstract]**

## Discovering Technicolor (Vol. 42, No. 5)

At present there are no known elementary scalar fields. A possible candidate is the as yet undiscovered Higgs particle; however it could well be that this elusive particle is instead composite. This possibility is exhaustively examined in this article, which is both tutorial and extensive review, classifying the diverse technicolor models as extensions of the Standard Model of particle physics. These model extensions are then compared with electroweak precision data, the spectrum of states common to most such models are identified, and their decays and associated experimental signals for the LHC illustrated, including the implementation in event generators important for searches at the LHC. This timely review provides the most complete and up-to-date benchmarks for the potential discovery of technicolor models.

**Discovering Technicolor**

J.R. Andersen, O. Antipin, G. Azuelos, L. Del Debbio, E. Del Nobile, S. Di Chiara, T. Hapola, M. Järvinen, P.J. Lowdon, Y. Maravin, I. Masina, M. Nardecchia, C. Pica, and F. Sannino, *Eur. Phys. J. Plus*, **126**: 81 (2011)

**[Abstract]**

## Fast domain-wall propagation in magnetic nanotracks (Vol. 42, No. 5)

Magnetic domain walls are the interfaces separating magnetic domains of opposite magnetizations, which are used to store binary information in magnetic media. The current developments of magnetic data storage and processing technologies make highly desirable to control a fast and reproducible motion of these domain walls in nanoscale magnetic tracks. This can be performed using either magnetic field, or electrical current.

For this purpose, nanotracks defined in ultrathin magnetic films with out-of-plane magnetic anisotropy seem to be particularly good candidates to obtain an efficient propagation under a low excitation. However, in most of the out-of-plane metallic nanosystems that were studied up to now, domain-wall pinning on the track’s defects was shown to significantly reduce the domain-wall velocity, as compared to the one measured in the corresponding plain magnetic film.

In this work, nanotracks are etched in a Pt/Co/Pt thin film with out-of-plane magnetic anisotropy, where pinning has been artificially reduced by weak He^{+}-ion irradiation. It is shown that in these tracks, domain walls propagate as fast and under magnetic field as low as in the corresponding plain irradiated film.

Moreover, when magnetic-field and electrical-current pulses are simultaneously applied to the track, a considerably faster magnetization reversal is observed, which is due to a Joule-heating-induced thermomagnetic effect when current flows into the track.

**Fast propagation of weakly pinned domain walls and current-assisted magnetization reversal in He ^{+}-irradiated Pt/Co/Pt nanotracks**

M. Cormier, A. Mougin, J. Ferré, J.-P. Jamet, R. Weil, J. Fassbender, V. Baltz and B. Rodmacq,

*J. Phys. D: Appl. Phys.*

**44**, 215002 (2011)

**[Abstract]**

## Controlling qubit arrays with anisotropic Heisenberg interaction (Vol. 42, No. 5)

Quantum-control methods are employed to manipulate physical and chemical processes using time-dependent fields. In particular they can be used to develop quantum logic gates thus helping us achieve a major goal of modern physics, the realization of scalable quantum computation.

A large body of work in quantum control has been devoted to the study of interacting spin-1/2 chains since these are effective models of qubit arrays. While interactions between qubits are necessary for realizing entangling two-qubit gates, standard approaches for controlling such arrays by acting on each qubit do not make an explicit use of these interactions. However for some particular types of interaction it suffices to control only a small subsystem of a given system, the idea underpinning the local-control approach.

In this paper we have explored anisotropic Heisenberg interactions, relevant for the use of Josephson junction based superconducting charge-qubit arrays. This example is particularly interesting as the concept of local control can be taken to the extreme -- controlling just one end qubit in an array. We investigated how time-dependent control fields acting on the first qubit in an array can be selected in order to realize relevant quantum logic operations (e.g. controlled-NOT, square-root-of-SWAP) in the shortest possible times.

Further extending the idea of local control, we showed that in building some quantum gates the degree of control over the chosen end qubit can be further reduced by acting with a control field in only one direction (say, x direction). Most remarkably, we demonstrated that in the parameter regime of interest for superconducting charge qubits this reduced control can lead to a more time-efficient realization of relevant gates than the approach involving alternate x and y control fields. We anticipate that our findings will facilitate implementations of quantum computation in superconducting qubit arrays.

**Controlling qubit arrays with anisotropic XXZ Heisenberg interaction by acting on a single qubit**

R. Heule, C. Bruder, D. Burgarth, and V.M. Stojanovic, *Eur. Phys. J. D* **63**, 41 (2011)

**[Abstract]**

## Delayed dynamic triggering of earthquakes (Vol. 42, No. 5)

In recent years, evidence has been accumulating that seismic waves generated by a large earthquake can produce additional quakes far away of the main shock. This may not be surprising when the secondary quakes occur right at the occurrence of the seismic waves. However, in general these dynamically triggered earthquakes occur hours or days after the main shock, namely when the seismic waves have elapsed.

In order to understand the origin of this phenomenon, we have adapted a recently proposed statistical model of seismicity that takes into account the existence of plastic relaxation processes within the faults. This kind of model has been used before to obtain realistic sequences of earthquakes, including in particular aftershocks. By appropriately defining a perturbation (assumed to be the occurrence of seismic waves), it is observed that the seismic activity in the system has a sharp increase following the perturbation, well after it has vanished.

The origin of a temporarily delayed effect in the model is tightly related to the existence of relaxation processes, as the effect does not exist in the case in which relaxation is absent. In this respect delayed dynamically triggered earthquakes are in some sense similar to aftershocks: the latter are delayed events triggered by a permanent perturbation (the change in the stress field caused by the main shock) while the former are delayed events triggered by a transient perturbation (the passage of seismic waves), once the perturbation has vanished.

The present investigation suggests that both aftershocks and delayed dynamically triggered quakes originate in the same kind of physical mechanism, namely internal relaxation mechanisms within the faults, that the present model appropriately captures.

**Delayed dynamic triggering of earthquakes: Evidence from a statistical model of seismicity**

E. A. Jagla, *EPL* **93** 19001 (2011)

**[Abstract]**

## Observing different quantum trajectories in cavity QED (Vol. 42, No. 5)

Quantum systems, as isolated as they can be, always interact with their surrounding environment. This interaction can lead to correlations between system and environment and, when the states of the reservoir are inaccessible to observation, to an irreversible loss of information on the system. This deleterious decoherence effect is at the heart of the quantum theory of measurement and plays an essential role in explaining the emergence of classical behaviour in quantum systems.

However, this system-reservoir interaction can be exploited in a completely different way when the environment can be monitored. In this case, extracting information from the environment causes the state of the system to change stochastically, conditioned on the measurement record. This can then be used to manipulate the system dynamics, being an important strategy in quantum dynamical control.

While it is well known that there are infinitely many possible ways of unravelling the decoherence process in terms of stochastic trajectories, it is not always clear how to interpret these trajectories in terms of concrete physical measurements on the environment. In this paper we show how to produce, in a controllable manner, a variety of quantum trajectories in realistic cavity quantum electrodynamics setups. In the microwave regime, we show how the detection of atoms that have crossed a cavity can induce a jump in the field proportional to its quadrature. In this case, the field dynamics is quite different from the usual photodetection monitoring and can be used to produce conditional four-component cat-like states. Alternatively, in the optical domain, the detection of photons can be used to protect entangled states of atoms that have interacted with the cavity field.

This proposal for the implementation of new stochastic trajectories in terms of continuous measurements in realistic systems certainly expands the possibility of engineering quantum states of lights and atoms.

**Observing different quantum trajectories in cavity QED**

M.F. Santos and A.R.R. Carvalho, *EPL*, **94**, 64003 (2011)

**[Abstract]**

## Injection coupling in quantum-cascade lasers (Vol. 42, No. 5)

_{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.

High-performance quantum-cascade lasers (QCLs) emitting at terahertz frequencies are highly expected for some practical applications, such as imaging and sensing. Though mid-infrared QCLs operating in room temperature and continuous wave mode had been achieved, the wavelength extension to terahertz is pretty difficult due to the large free carrier absorption loss and small subband energy spacing. The performance improvements of terahertz QCLs need advances in active region and waveguide designs.

Recently the injection coupling parameter has attracted much interest because it not only affects the injection efficiency but also determines the laser gain shape and peak gain values. A density-matrix based calculation showed that a much stronger injection coupling (18 meV) should be employed for high performance mid-infrared QCL designs and the simulation has been verified by experiments.

Since the injection coupling plays an important role in the design and realization of high performance mid-infrared QCLs, it could also take effect in terahertz range. The research team investigated the effect of injection coupling strength on terahertz QCLs using an ensemble Monte Carlo method. An optimal injection coupling strength of 7.5 meV for dynamic lasing range and peak gain has been obtained for a 3.7-THz QCL. It is worth noting that the optimal injection coupling value is strongly dependent on the wavelength of the specific terahertz QCL design.

**Effect of injection coupling strength on terahertz quantum-cascade lasers**

H. Li and J.C. Cao, *Semicond. Sci. Technol.* **26**, 095029 (2011)

**[Abstract]**

## Fast growth of high mobility ZnO:Al by cathodic arc deposition (Vol. 42, No. 5)

^{2}/Vs, a very high value for doped ZnO on glass. The film is transparent not only in the visible but for most of the solar spectrum, which is also shown.

Transparent conducting oxides (TCOs) are increasingly important materials as the demand for high efficiency solar cells, displays, and smart windows increases. Currently, indium tin oxide (ITO) is the preferred material for its properties of low resistance and high visible light transmittance. Excessive indium demand justifies the search for abundant and low cost alternative materials to satisfy the growing need of coating (>108 m2/year). The best ZnO films doped with Ga or Al typically deposited at low rate by magnetron sputtering, are attractive but their performance is usually inferior to ITO.

Using a lesser known technique, called dc filtered cathodic arc deposition, we have shown that very high quality Al-doped ZnO (AZO) can be grown at rates 10 times higher than the rates for sputtering. Filtered cathodic arc produces a flux of fully ionized material, in stark contrast to sputtering which occurs at much lower power and predominantly produces a flux of neutral atoms. The arc-produced ions bring significant potential and kinetic energies to the surface, which leads to heating of the growing film just where the growth occurs while the substrate as a whole can remain at a much lower temperature. If the ion flux is high, the surface heat accumulates and anneals the film as it grows. The result is high quality AZO with electron mobility approaching the theoretical limit for polycrystalline AZO films. High electron mobility is what allows TCOs to transmit light throughout the solar spectrum while maintaining high conductivity since the electron concentration does not have to be very high.

**Achieving high mobility ZnO:Al at very high growth rates by dc filtered cathodic arc deposition**

R.J. Mendelsberg, S.H.N. Lim, Y.K. Zhu, J. Wallig, D.J. Milliron and A. Anders, *J. Phys. D: Appl. Phys.* **44**, 232003 (2011)

**[Abstract]**

## Eigenvalue problem in 2D for an irregular boundary (Vol. 42, No. 5)

_{o}

^{2}) with Neumann condition for the first 7 states.

The Helmholtz equation arises in a number of physical contexts as one reduces the wave equation by considering single frequency propagation. One such application appears in studying wave behaviour in waveguides. While waveguides in technology are carefully engineered to have, for instance, constant and simple cross-sectional geometries, natural waveguides relax such engineering constraints. (Natural waveguides range across systems from atmospheric and oceanic ducts, to biological systems such as seal and polar bear hair fibres.) In particular the cross section of natural waveguides tends to have a complex boundary shape that is far from the ideal circular or rectangular form of engineered waveguides.

The present team proposes a new approach to the irregular boundary 2-d Helmholtz equation with Neumann boundary conditions (specified normal derivatives of the field at the boundary). This boundary condition has significant physical importance: it is the natural one for electromagnetic propagation in the TM mode in waveguides.

To date the most successful efforts to solve the irregular boundary Helmholtz equation have been computational, but even this general method has its drawbacks. The present analytic perturbative approach solves the irregular boundary problem via a perturbative series. The authors explicitly work out several nontrivial examples. The benefits of this approach include, most importantly, an analytical understanding of the behaviour of the solution as the amplitude of the boundary distortion is increased.

Another important feature of Panda et al.’s expression is its analytic precision in the terms computed and its analytic error estimates for the terms truncated from the series. Together these give the analytic methods a much larger dynamic range than available computationally.

**Eigenvalue problem in two dimensions for an irregular boundary: Neumann condition**

S. Panda, S. Chakraborty, and S.P. Khastgir, *Eur. Phys. J. Plus*, **126**, 62 (2011)

**[Abstract]**

## XUV-FEL spectroscopy: He two-photon ionization cross-sections (Vol. 42, No. 5)

^{+}and H

_{2}

^{+}recorded at λ = 61.4, 58.4, 56.0 and 53.4 nm.

Non-linear optical processes of atoms and molecules such as multiphoton absorption and tunnelling ionization are very attractive issues in current atomic, molecular and optical sciences. The recent development of free electron laser (FEL) sources enabled us to investigate such non-linear optical processes in the extreme ultraviolet (XUV) wavelength regions Our group demonstrated that we can determine absolute values of a two-photon ionization cross section of atomic species and its wavelength dependence by using an XUV FEL light source. This was achieved by introducing an internal reference for the cross section measurements and by the frequency tunability of the FEL light source.

The FEL light source we used is the SPring-8 Compact SASE Source test accelerator in RIKEN, Harima Institute, equipped with a couple of compact vacuum undulators, having a unique advantage of its high peak intensity and frequency tunability in the 50 ~ 62 nm region.

We measured the wavelength dependence and the light field intensity dependence of the absolute values of two-photon ionization cross section of He at 53.4, 58.4, 56.0 and 61.4 nm, covering the 1s2p and 1s3p resonances in the light field intensities range of 5×1012 ~ 5×1013 W/cm2 by measuring simultaneously one-photon ionization signal of H2 mixed in the sample as reference.

We showed through the critical comparison with the theoretically obtained cross sections that, in the resonance wavelength regions, dressed state formation through the strong coupling between the intermediate 1snp resonance state and the 1s2 ground state needs to be taken into account if the XUV light field intensity becomes larger than ~1012 W/cm2. We are now entering into the stage of quantitative non-linear spectroscopy in the XUV wavelength region.

**Determination of absolute two-photon ionization cross section of He by XUV Free Electron Laser**

T. Sato, A. Iwasaki, I. Kazuki, T. Okino, K. Yamanouchi, J. Adachi, A. Yagishita, H. Yazawa, F. Kannari, M. Aoyma, K. Yamakawa, K. Midorikawa, H. Nakano, M. Yabashi, M. Nagasono, A. Higashiya and T. Ishikawa, *J. Phys. B: At. Mol. Opt. Phys.*, **44**, 161001 (2011)

**[Abstract]**

## Electromagnetic force density and energy-momentum tensor in any continuous medium (Vol. 42, No. 5)

For more than a century, physicists have searched for a unique and general form for the force density that an electromagnetic field imposes on a medium. The existing expressions for this quantity, obtained, e.g., by Minkowski, Einstein and Laub, Abraham, and Helmholtz, are different, and, as such, give different predictions in particular physical situations. The theories of Abraham and Minkowski, for example, ignore the existence of electro- and magnetostriction. Moreover, real media with dispersion, dissipation, and nonlinearities have not been addressed much.

We present an unambiguous general equation for the electromagnetic force density **f** = -∇**T**- (d**G**/dt) expressed in terms of a new three-dimensional energy-momentum tensor **T** and momentum density **G** of the field. The tensor **T** can be written as T = T_{M} + I_{V}, where **T _{M}** is the Minkowski tensor, I the unit tensor, and

*V*the density of the field-matter interaction potential that is responsible for electro- and magnetostriction. Remarkably, if the medium is not magnetic, the momentum density

**G**is given by Abraham’s expression

**G**=

**ExH**/c

^{2}. If the material obeys the Clausius-Mossotti law, the tensor T becomes the Helmholtz tensor that to our knowledge has not been contradicted in any experiment so far.

The general equation obtained for the force density can be applied to essentially any natural or designed material whether inhomogeneous, anisotropic, nonlinear, dispersive, or dissipative, and even to materials providing optical gain. We also calculate the rate of work done on a medium by an electromagnetic field, and using the result, obtain the four-dimensional energy-momentum tensor **T _{4}** in spacetime. Interestingly, this tensor is physically very close to the almost forgotten tensor of Einstein and Laub.

**Electromagnetic force density and energy–momentum tensor in an arbitrary continuous medium**

A. Shevchenko and M. Kaivola, *J. Phys. B: At. Mol. Opt. Phys.* **44**, 175401 (2011)

**[Abstract]**

## Pionic Deuterium (Vol. 42, No. 5)

A new precise measurement of the pD(3p-1s) X-ray transition in the pionic deuterium atom has been performed at the PSI accelerator in Switzerland. The pionic deuterium is a short lifetime atom, where the negative pion (p^{-}) replaces the electron, resulting in an atomic size scaled down by the ratio of the pion mass over electron mass, a factor of about 270.

The experiment makes use of a high intensity decelerated beam of ^{-} stopping in a cooled deuterium gas target where the p- is captured. Following the capture an atomic de-excitation quantum cascade of 0.1ns duration takes place and the atom ends up in the 1s ground state as shown in the Figure. A Bragg spectrometer equipped with a bent Silicon crystal and pixel semiconductor detectors provides the precise X-ray detection in the appropriate keV region.

The measurement of the energy of the X-ray emitted in the pD(3p-1s) transition leads to a new value of 3075.583 ± 0.030 eV. A new and updated calculation of this transition energy assuming a pure electromagnetic system (pure QED - no strong interaction) leads to a value of 3077.939 ± 0.008 eV. The difference between these two quantities gives exactly the hadronic shift e_{1s} = -2.336 ± 0.031 eV. The line-shape has been analysed, providing a new and precise value of the hadronic broadening G_{1s} = 1.171+0.023 -0.049 eV.

The accuracy of 1.3% achieved for the shift e1s leads to a more precise determination of the isoscalar scattering length a+ (pD being an isoscaler object). The new precise value obtained for the hadronic broadening G1s leads to a new determination of the threshold parameter a, the transition strenght for a S-wave pion, with unprecedented accuracy.

**Pionic Deuterium**

Th. Strauch, F.D. Amaro, D.F. Anagnostopoulos, P. BÅNuhler, D.S. Covita, H. Gorke, D. Gotta, A. Gruber, A. Hirtl, P. Indelicato, E.-O. Le Bigot, M. Nekipelov, J.M.F. dos Santos, Ph. Schmid, S. Schlesser, L.M. Simons, M. Trassinelli, J.F.C.A. Veloso and J. Zmeskal, *Eur. Phys. J. A*, **47**, 88 (2011)

**[Abstract]**