Tuning hole mobility in InP nanowires

Transport properties of holes in InP nanowires were calculated considering electron-phonon interaction via deformation potentials, the effect of temperature and strain fields. Using molecular dynamics, we simulate nanowire structures, LO-phonon energy renormalization and lifetime. The valence band ground state changes between light- and heavy-hole character, as the strain fields and the nanowire size are changed. Drastic changes in the mobility arise with the onset of resonance between the LO-phonons and the separation between valence subbands.Comment: 4 pages, 4 figure

Entanglement versus Quantum Discord in Two Coupled Double Quantum Dots

We study the dynamics of quantum correlations of two coupled double quantum dots containing two excess electrons. The dissipation is included through the contact with an oscillator bath. We solve the Redfield master equation in order to determine the dynamics of the quantum discord and the entanglement of formation. Based on our results, we find that the quantum discord is more resistant to dissipation than the entanglement of formation for such a system. We observe that this characteristic is related to whether the oscillator bath is common to both qubits or not and to the form of the interaction Hamiltonian. Moreover, our results show that the quantum discord might be finite even for higher temperatures in the asymptotic limit.Comment: 14 pages, 8 figures (new version is the final version to appear in NJP

Non-unitary versus unitary optimization in the control of open quantum systems

In this work, we compare the performance of the Krotov method for open quantum systems (non-unitary optimization) with the Krotov method for closed quantum systems (unitary optimization) in finding optimal controls aimed at manipulating qubits and qutrits in the presence of the environment. In the case of unitary optimization, the Krotov method is applied to quantum system neglecting its interaction with the environment, afterwards the resulting controls are used to manipulate the system along with the eviromental noise. We consider two distinct control problems: target-state preparation from a given initial state and quantum gate implementation. For the state preparation, we {have found that the performance of the controls obtained from the non-unitary optimization outperform that of the controls obtained from the unitary optimization}. {However, in the case of the implementation of quantum gates, we have found that the optimal controls obtained from the unitary evolution exhibit a mean fidelity similar to that obtained from the non-unitary evolution. Since unitary optimization does not depend on decay rates nor on specific kinds of noise, besides being less computationally demanding, our results suggest that the best current practice to implement quantum gates in open quantum systems is to employ unitary optimization.Comment: 7 pages, 9 figure

Calculation of quantum discord for qubit-qudit or N qubits

Quantum discord, a kind of quantum correlation, is defined as the difference between quantum mutual information and classical correlation in a bipartite system. It has been discussed so far for small systems with only a few independent parameters. We extend here to a much broader class of states when the second party is of arbitrary dimension d, so long as the first, measured, party is a qubit. We present two formulae to calculate quantum discord, the first relating to the original entropic definition and the second to a recently proposed geometric distance measure which leads to an analytical formulation. The tracing over the qubit in the entropic calculation is reduced to a very simple prescription. And, when the d-dimensional system is a so-called X state, the density matrix having non-zero elements only along the diagonal and anti-diagonal so as to appear visually like the letter X, the entropic calculation can be carried out analytically. Such states of the full bipartite qubit-qudit system may be named "extended X states", whose density matrix is built of four block matrices, each visually appearing as an X. The optimization involved in the entropic calculation is generally over two parameters, reducing to one for many cases, and avoided altogether for an overwhelmingly large set of density matrices as our numerical investigations demonstrate. Our results also apply to states of a N-qubit system, where "extended X states" consist of (2^(N+2) - 1) states, larger in number than the (2^(N+1) - 1) of X states of N qubits. While these are still smaller than the total number (2^(2N) - 1) of states of N qubits, the number of parameters involved is nevertheless large. In the case of N = 2, they encompass the entire 15-dimensional parameter space, that is, the extended X states for N = 2 represent the full qubit-qubit system.Comment: 6 pages, 1 figur

Light sensitive memristor with bi-directional and wavelength-dependent conductance control

The authors gratefully acknowledge financial support from the European Union (FPVII (2007-2013) under Grant Agreement No. 318287 Landauer) as well as the State of Bavaria. The Brazilian authors acknowledge the support from CNPq. V.L.-R. acknowledges the support from FAPESP (Grant Nos. 2014/02112-3 and 2015/10765-0).We report the optical control of localized charge on positioned quantum dots in an electro-photo-sensitive memristor. Interband absorption processes in the quantum dot barrier matrix lead to photo-generated electron-hole-pairs that, depending on the applied bias voltage, charge or discharge the quantum dots and hence decrease or increase the conductance. Wavelength-dependent conductance control is observed by illumination with red and infrared light, which leads to charging via interband and discharging via intraband absorption. The presented memristor enables optical conductance control and may thus be considered for sensory applications in artificial neural networks as light-sensitive synapses or optically tunable memories.PostprintPeer reviewe

Temperature tuning from direct to inverted bistable electroluminescence in resonant tunneling diodes

The authors are grateful for financial support by the BMBF via national project EIPHRIK (FKZ: 13N10710), the European Union (FPVII (2007-2013) under grant agreement no. 318287 LANDAUER), and the Brazilian Agencies CNPq and CAPES. S. H. gratefully acknowledges support by the Royal Society and the Wolfson Foundation.We study the electroluminescence (EL) emission of purely n-doped resonant tunneling diodes in a wide temperature range. The paper demonstrates that the EL originates from impact ionization and radiative recombination in the extended collector region of the tunneling device. Bistable current-voltage response and EL are detected and their respective high and low states are tuned under varying temperature. The inversion bistability of the EL intensity can be switched from direct to inverted with respect to the tunneling current and the optical on/off ratio can be enhanced with increasing temperature. One order of magnitude amplification of the optical on/off ratio can be attained compared to the electrical one. Our observation can be explained by an interplay of moderate peak-to-valley current ratios, large resonance voltages, and electron energy loss mechanisms and thus could be applied as an alternative route towards optoelectronic applications of tunneling devices.PostprintPeer reviewe

Mimicking of pulse shape-dependent learning rules with a quantum dot memristor

The authors gratefully acknowledge financial support from the European Union (FPVII (2007-2013) under grant agreement n° 318287 Landauer) as well as the state of Bavaria. The Brazilian authors acknowledge the support of CNPq. V. L.-R. acknowledges the support of FAPESP (grants 2014/02112-3 and 2015/10765-0).We present the realization of four different learning rules with a quantum dot memristor by tuning the shape, the magnitude, the polarity and the timing of voltage pulses. The memristor displays a large maximum to minimum conductance ratio of about 57000 at zero bias voltage. The high and low conductances correspond to different amounts of electrons localized in quantum dots, which can be successively raised or lowered by the timing and shapes of incoming voltage pulses. Modifications of the pulse shapes allow altering the conductance change in dependence on the time difference. Hence, we are able to mimic different learning processes in neural networks with a single device. In addition, the device performance under pulsed excitation is emulated combining the Landauer-Büttiker formalism with a dynamic model for the quantum dot charging, which allows explaining the whole spectrum of learning responses in terms of structural parameters that can be adjusted during fabrication such as gating efficiencies and tunneling rates. The presented memristor may pave the way for future artificial synapses with a stimulus-dependent capability of learning.PostprintPeer reviewe

Locally Inaccessible Information as a Fundamental Ingredient to Quantum Information

Quantum discord (QD) measures the fraction of the pairwise mutual information that is locally inaccessible, in a multipartite system. Fundamental aspects related to two important measures in quantum information theory the Entanglement of Formation (EOF) and the conditional entropy, can be understood in terms of the distribution of this form of Local Inaccessible Information (LII). As such, the EOF for an arbitrarily mixed bipartite system AB can be related to the gain or loss of LII due to the extra knowledge that a purifying ancillary system E has on the pair AB. Similarly, a clear meaning of the negativity of the conditional entropy for AB is given. We exemplify by showing that these relations elucidate important and yet not well understood quantum features, such as the bipartite entanglement sudden death and the distinction between EOF and QD for quantifying quantum correlation. For that we introduce the concept of LII flow which quantifies the LII shared in multipartite system when a sequential local measurements are performed.Comment: We relate the entanglement and the conditional entropy exclusively as a function of quantum discord. Final Versio

Viribus Unitis - Croatian Political Emigration under the Critical View of Zarko Vlaho

Žarko Vlaho, jedan od vodećih hrvatskih katoličkih intelektualaca u Hercegovini, našavši se nakon završetka Drugoga svjetskog rata kao emigrant u Argentini, nije pasivno promatrao politička gibanja među Hrvatima u izbjeglištvu. U knjižici Viribus unitis (1950.) oštro se obračunao s pokušajima bivših pripadnika ustaškoga pokreta da ustroje svoje nove stranačke organizacije u inozemstvu. Stavivši pod svoj katolički kirurški nož političko djelovanje Vladka Mačeka, Vlaho se zauzeo za okupljanje svih hrvatskih emigrantskih snaga oko njega, držeći da je riječ o jedinoj osobi koja svojim autoritetom i međunarodnim ugledom može dovesti do stvaranja samostalne hrvatske države.At the beginning of the 20th century, the Croatian Catholic Movement was founded in the Triune Kingdom of Croatia, Slavonia and Dalmatia, as well as in Bosnia and Herzegovina, the countries within the Austro-Hungarian Monarchy. In 1912/1913 the Croatian Catholic Seniority was established, the leading organization of the entire Catholic movement in Croatian countries. Part of the Seniors (so called "nationals") opted for Yugoslav ideology and actively engaged in the demolition of the Monarchy and the creation of the Kingdom of Serbs, Croats and Slovenes / Yugoslavia during the First World War. After the establishment of a new state, Seniority for Herzegovina (1919) was organized in Mostar, under the leadership of Fr. Dominik Mandic (senior national). One of the most important members of the Seniority was Zarko Vlaho (1895-1960), Mandic\u27s friend and associate. Seniors sought to realize their political ideas through the Croatian People\u27s Party. In the period of multi-party parliamentary democracy, their members were the most controversial opponents of Stjepan Radic and his Croatian Peasant Party. After the end of World War II, Vlaho, after shorter detention in Italy, joined his family in Argentina (1947). Reflecting on the Croatian political emigration, in his book Viribus Unitis (1950), he fiercely criticized the attempts of former Ustashi movement members to organize their parties. Putting under his Catholic scalpel Vladko Macek, the president of HSS, Vlaho took up the task of gathering all the Croatian emigrant forces around him, holding that Macek was the only person who could, through his political authority and Catholic worldview and international reputation, lead to the creation of an independent Croatian state. In his endeavor, Vlaho deliberately neglected the facts that did not support his claims (Macek\u27s non-transparent political statements, his civil marriage etc.). Vlaho\u27s thoughts did not echo among a wider circle of political emigrants scattered from Europe and Africa to North and South America