Universal pulse sequence to minimize spin dephasing in the central spin decoherence problem

We present a remarkable finding that a recently discovered [G. S. Uhrig, Phys. Rev. Lett. 98, 100504 (2007)] series of pulse sequences, designed to optimally restore coherence to a qubit in the spin-boson model of decoherence, is in fact completely model-independent and generically valid for arbitrary dephasing Hamiltonians given sufficiently short delay times between pulses. The series maximizes qubit fidelity versus number of applied pulses for sufficiently short delay times because the series, with each additional pulse, cancels successive orders of a time expansion for the fidelity decay. The "magical" universality of this property, which was not appreciated earlier, requires that a linearly growing set of "unknowns" (the delay times) must simultaneously satisfy an exponentially growing set of nonlinear equations that involve arbitrary dephasing Hamiltonian operators.Comment: Published in PRL, revise

Concatenated dynamical decoupling in a solid-state spin bath

Concatenated dynamical decoupling (CDD) pulse sequences hold much promise as a strategy to mitigate decoherence in quantum information processing. It is important to investigate the actual performance of these dynamical decoupling strategies in real systems that are promising qubit candidates. In this Rapid Communication, we compute the echo decay of concatenations of the Hahn echo sequence for a solid-state electronic spin qubit in a nuclear spin bath using a cluster expansion technique. We find that each level of concatenation reverses the effect of successive levels of intrabath fluctuations. On the one hand, this advances CDD as a versatile and realistic decoupling strategy. On the other hand, this invalidates, as overly optimistic, results of the simple pair approximation used previously to study restoration, through CDD, of coherence lost to a mesoscopic spin bath

The biomechanical function of periodontal ligament fibres in orthodontic tooth movement

Orthodontic tooth movement occurs as a result of resorption and formation of the alveolar bone due to an applied load, but the stimulus responsible for triggering orthodontic tooth movement remains the subject of debate. It has been suggested that the periodontal ligament (PDL) plays a key role. However, the mechanical function of the PDL in orthodontic tooth movement is not well understood as most mechanical models of the PDL to date have ignored the fibrous structure of the PDL. In this study we use finite element (FE) analysis to investigate the strains in the alveolar bone due to occlusal and orthodontic loads when PDL is modelled as a fibrous structure as compared to modelling PDL as a layer of solid material. The results show that the tension-only nature of the fibres essentially suspends the tooth in the tooth socket and their inclusion in FE models makes a significant difference to both the magnitude and distribution of strains produced in the surrounding bone. The results indicate that the PDL fibres have a very important role in load transfer between the teeth and alveolar bone and should be considered in FE studies investigating the biomechanics of orthodontic tooth movement. © 2014 McCormack et al

Quantum theory of spectral diffusion induced electron spin decoherence

A quantum cluster expansion method is developed for the problem of localized electron spin decoherence due to dipolar fluctuations of lattice nuclear spins. At the lowest order it provides a microscopic explanation for the Lorentzian diffusion of Hahn echoes without resorting to any phenomenological Markovian assumption. Our numerical results show remarkable agreement with recent electron spin echo experiments in phosphorus doped silicon.Comment: 5 pages, 1 figur

Determinants of information behaviour and information literacy related to healthy eating among Internet users in five European countries

Introduction. This study investigates how Europeans seek information related to healthy eating, what determines their information seeking and whether any problems are encountered in doing so. Method. A survey was administered through computer-assisted on-line web-interviewing. Respondents were grouped by age and sex (n=3003, age + 16) in Belgium, Denmark, Italy, Poland, and the UK. Analysis. Descriptive statistics and regression analysis were used to analyse the influence of social, demographic, psychological and economic characteristics on the information seeking of the respondents. Results. Nearly half of those surveyed do not know where to look for information on healthy diets. Men, less well educated, poorer and sicker person know less about where to look for such information and are less likely to attempt finding it. Most of the respondents searching for information on the Internet use Google as a search tool. Conclusions. Individual and environmental factors affect information behaviour and should be taken into account in public campaigns aimed at changing eating habits of the population to increase their effectiveness. More emphasis should be placed on raising health information literacy of the information-poor, men, the uneducated, and the economically disadvantaged

Mental Well-Being and Sexual Intimacy among Men and Gender Diverse People Who Have Sex with Men during the First UK COVID-19 Lockdown: A Mixed-Methods Study

Abstract: This mixed-methods study aimed to explore mental well-being, circumstances and strategies around managing sexual intimacy and risk during the first UK COVID-19 lockdown (Spring 2020) among men and gender diverse people who have sex with men (MGDPSM), commencing while lockdown was in progress. n = 1429 MGDPSM completed the survey and 14 undertook an in-depth interview. Low mental well-being was reported by 49.6% of the survey participants. Low mental well-being was not predicted by relationship and living circumstance, sexual networking app use, or by casual sexual partners. Low mental well-being was associated with more frequent COVID-19 anxiety (OR = 5.08 CI: 3.74, 6.88 p < 0.001) and with younger age (18–24 years OR = 2.23 CI:1.41–3.53 p = 0.001, 25–34 years OR = 1.45 CI:1.04–2.02 p = 0.029, 35–44 years OR = 1.41 CI:1.00–1.99 p = 0.052). The interview participants understood their lockdown experiences as being relative to normalcy, and those experiencing more dramatic changes faced greater challenges. Living with partners was felt to protect well-being. Many participants reported intimacy interruption challenges. The findings indicate that mental well-being is predicted by age and COVID-19 impact, highlighting opportunities for targeting MGDPSM who are most vulnerable to poor mental health. Services that support MGDPSM during COVID-19 recovery efforts must provide non-judgemental and affirming support

Quantum theory for electron spin decoherence induced by nuclear spin dynamics in semiconductor quantum computer architectures: Spectral diffusion of localized electron spins in the nuclear solid-state environment

We consider the decoherence of a single localized electron spin due to its coupling to the lattice nuclear spin bath in a semiconductor quantum computer architecture. In the presence of an external magnetic field and at low temperatures, the dominant decoherence mechanism is the spectral diffusion of the electron spin resonance frequency due to the temporally fluctuating random magnetic field associated with the dipolar interaction induced flip-flops of nuclear spin pairs. The electron spin dephasing due to this random magnetic field depends intricately on the quantum dynamics of the nuclear spin bath, making the coupled decoherence problem difficult to solve. We provide a formally exact solution of this non-Markovian quantum decoherence problem which numerically calculates accurate spin decoherence at short times, which is of particular relevance in solid-state spin quantum computer architectures. A quantum cluster expansion method is developed, motivated, and tested for the problem of localized electron spin decoherence due to dipolar fluctuations of lattice nuclear spins. The method is presented with enough generality for possible application to other types of spin decoherence problems. We present numerical results which are in quantitative agreement with electron spin echo measurements in phosphorus doped silicon. We also present spin echo decay results for quantum dots in GaAs which differ qualitatively from that of the phosphorus doped silicon system. Our theoretical results provide the ultimate limit on the spin coherence (at least, as characterized by Hahn spin echo measurements) of localized electrons in semiconductors in the low temperature and the moderate to high magnetic field regime of interest in scalable semiconductor quantum computer architectures.Comment: 23 pages, 15 figure