Quantum hierarchic models for information processing

Both classical and quantum computations operate with the registers of bits. At nanometer scale the quantum fluctuations at the position of a given bit, say, a quantum dot, not only lead to the decoherence of quantum state of this bit, but also affect the quantum states of the neighboring bits, and therefore affect the state of the whole register. That is why the requirement of reliable separate access to each bit poses the limit on miniaturization, i.e, constrains the memory capacity and the speed of computation. In the present paper we suggest an algorithmic way to tackle the problem of constructing reliable and compact registers of quantum bits. We suggest to access the states of quantum register hierarchically, descending from the state of the whole register to the states of its parts. Our method is similar to quantum wavelet transform, and can be applied to information compression, quantum memory, quantum computations.Comment: 14 pages, LaTeX, 1 eps figur

Spin- and entanglement-dynamics in the central spin model with homogeneous couplings

We calculate exactly the time-dependent reduced density matrix for the central spin in the central-spin model with homogeneous Heisenberg couplings. Therefrom, the dynamics and the entanglement entropy of the central spin are obtained. A rich variety of behaviors is found, depending on the initial state of the bath spins. For an initially unpolarized unentangled bath, the polarization of the central spin decays to zero in the thermodynamic limit, while its entanglement entropy becomes maximal. On the other hand, if the unpolarized environment is initially in an eigenstate of the total bath spin, the central spin and the entanglement entropy exhibit persistent monochromatic large-amplitude oscillations. This raises the question to what extent entanglement of the bath spins prevents decoherence of the central spin.Comment: 8 pages, 2 figures, typos corrected, published versio

Consumer attitudes towards organic versus conventional food with specific quality attributes

This paper describes the findings from a consumer survey conducted as part of the EU-funded research project QualityLowInputFood (QLIF). The objective was to segment occasional organic consumers with regard to their preferences for organic, conventional and conventional-plus products, i.e., conventional products with a specific attribute that also applies to organic products. In other words, these conventional-plus products are placed between organic and conventional food products. In addition, we aimed at analysing differences between consumer segments regarding their price sensitivity and attitudes towards food. The survey used choice experiments to investigate occasional organic consumer preferences for the different types of products. In subsequent standardized face-to-face interviews we collected data on consumer attitudes towards food that could explain the observed preferences. The attitudes were summarized in attitude factors, using factor analysis. The responses from the interviews and choice experiments were analysed by latent class models. These econometric models were used to identify segments within a group of individuals for their preference structure and to relate membership in each segment to consumer characteristics. Two segments of occasional organic consumers were identified. Consumers in segment 1 strongly preferred organic products and were less price sensitive. Furthermore, consumers in this segment showed a significantly higher level of agreement with most of the investigated attitude factors than consumers in segment 2. The latter consisted of consumers who were significantly more price sensitive and preferred conventional-plus and conventional products rather than organic products. Communicating quality attributes represents a promising marketing tool of product differentiation and information for both organic and conventional food marketers. The price sensitivity of parts of occasional organic consumers suggests that the perceived price-performance ratio of organic products needs to be increased by targeted pricing and communication strategies integrating product-relevant information. If not, conventional-plus products, representing a cheaper alternative, might be preferred by parts of the occasional organic consumers

COMPLETE SOLUTION OF THE XXZ-MODEL ON FINITE RINGS. DYNAMICAL STRUCTURE FACTORS AT ZERO TEMPERATURE.

The finite size effects of the dynamical structure factors in the XXZ-model are studied in the euclidean time $(\tau)$-representation. Away from the critical momentum $p=\pi$ finite size effects turn out to be small except for the large $\tau$ limit. The large finite size effects at the critical momentum $p=\pi$ signal the emergence of infrared singularities in the spectral $(\omega)$-representation of the dynamical structure factors.Comment: PostScript file with 12 pages + 11 figures uuencoded compresse

Direct Observation of Quantum Coherence in Single-Molecule Magnets

Direct evidence of quantum coherence in a single-molecule magnet in frozen solution is reported with coherence times as long as T2 = 630 ns. We can strongly increase the coherence time by modifying the matrix in which the single-molecule magnets are embedded. The electron spins are coupled to the proton nuclear spins of both the molecule itself and interestingly, also to those of the solvent. The clear observation of Rabi oscillations indicates that we can manipulate the spin coherently, an essential prerequisite for performing quantum computations.Comment: 5 Pages, 4 Figures, final version published in PR

Decoherence in a scalable adiabatic quantum computer

We consider the effects of decoherence on Landau-Zener crossings encountered in a large-scale adiabatic-quantum-computing setup. We analyze the dependence of the success probability, i.e. the probability for the system to end up in its new ground state, on the noise amplitude and correlation time. We determine the optimal sweep rate that is required to maximize the success probability. We then discuss the scaling of decoherence effects with increasing system size. We find that those effects can be important for large systems, even if they are small for each of the small building blocks.Comment: 6 pages (two-column), 1 figur

Quantum integrability and nonintegrability in the spin-boson model

We study the spectral properties of a spin-boson Hamiltonian that depends on two continuous parameters $0\leq\Lambda<\infty$ (interaction strength) and $0\leq\alpha\leq\pi/2$ (integrability switch). In the classical limit this system has two distinct integrable regimes, $\alpha=0$ and $\alpha=\pi/2$. For each integrable regime we can express the quantum Hamiltonian as a function of two action operators. Their eigenvalues (multiples of $\hbar$) are the natural quantum numbers for the complete level spectrum. This functional dependence cannot be extended into the nonintegrable regime $(0<\alpha<\pi/2)$. Here level crossings are prohibited and the level spectrum is naturally described by a single (energy sorting) quantum number. In consequence, the tracking of individual eigenstates along closed paths through both regimes leads to conflicting assignments of quantum numbers. This effect is a useful and reliable indicator of quantum chaos -- a diagnostic tool that is independent of any level-statistical analysis

Transcriptional networks specifying homeostatic and inflammatory programs of gene expression in human aortic endothelial cells.

Endothelial cells (ECs) are critical determinants of vascular homeostasis and inflammation, but transcriptional mechanisms specifying their identities and functional states remain poorly understood. Here, we report a genome-wide assessment of regulatory landscapes of primary human aortic endothelial cells (HAECs) under basal and activated conditions, enabling inference of transcription factor networks that direct homeostatic and pro-inflammatory programs. We demonstrate that 43% of detected enhancers are EC-specific and contain SNPs associated to cardiovascular disease and hypertension. We provide evidence that AP1, ETS, and GATA transcription factors play key roles in HAEC transcription by co-binding enhancers associated with EC-specific genes. We further demonstrate that exposure of HAECs to oxidized phospholipids or pro-inflammatory cytokines results in signal-specific alterations in enhancer landscapes and associate with coordinated binding of CEBPD, IRF1, and NFκB. Collectively, these findings identify cis-regulatory elements and corresponding trans-acting factors that contribute to EC identity and their specific responses to pro-inflammatory stimuli

Resistive transition of hydrogen-rich superconductors

Critical temperature, Tc, and transition width, ΔTc, are two primary parameters of the superconducting transition. The latter parameter reflects the superconducting state disturbance originating from the thermodynamic fluctuations, atomic disorder, applied magnetic field, the presence of secondary crystalline phases, applied pressure, etc. Recently, Hirsch and Marsiglio (2021 Phys. Rev. B 103 134505, doi: 10.1103/PhysRevB.103.134505) performed an analysis of the transition width in several near-room-temperature superconductors and reported that the reduced transition width, ΔTc/Tc, in these materials does not follow the conventional trend of transition width broadening in applied magnetic field observed in low- and high-Tc superconductors. Here, we present a thorough mathematical analysis of the magnetoresistive data, R(T, B), for the high-entropy alloy (ScZrNb)0.65[RhPd]0.35 and hydrogen-rich superconductors of Im-3m-H3S, C2/m-LaH10 and P63/mmc-CeH9. We found that the reduced transition width, ΔTc/Tc, in these materials follows a conventional broadening trend in applied magnetic field. © 2021 IOP Publishing Ltd

Impurity spin relaxation in S=1/2 XX chains

Dynamic autocorrelations $$ (\alpha=x,z) of an isolated impurity spin in a S=1/2 XX chain are calculated. The impurity spin, defined by a local change in the nearest-neighbor coupling, is either in the bulk or at the boundary of the open-ended chain. The exact numerical calculation of the correlations employs the Jordan-Wigner mapping from spin operators to Fermi operators; effects of finite system size can be eliminated. Two distinct temperature regimes are observed in the long-time asymptotic behavior. At T=0 only power laws are present. At high T the x correlation decays exponentially (except at short times) while the z correlation still shows an asymptotic power law (different from the one at T=0) after an intermediate exponential phase. The boundary impurity correlations follow power laws at all T. The power laws for the z correlation and the boundary correlations can be deduced from the impurity-induced changes in the properties of the Jordan-Wigner fermion states.Comment: Final version to be published in Phys. Rev. B. Three references added, extended discussion of relation to previous wor