Witnessing entanglement in hybrid systems

We extend the definition of entanglement witnesses based on structure factors to the case in which the position of the scatterers is quantized. This allows us to study entanglement detection in hybrid systems. We provide several examples that show how these extra degrees of freedom affect the detection of entanglement by directly contributing to the measurement statistics. We specialize the proposed witness operators for a chain of trapped ions. Within this framework, we show how the collective vibronic state of the chain can act as an undesired quantum environment and how ions quantum motion can affect the entanglement detection. Finally, we investigate some specific cases where the method proposed leads to detection of hybrid entanglement.Comment: 6 pages, 4 figure

Entanglement Trapping in Structured Environments

The entanglement dynamics of two independent qubits each embedded in a structured environment under conditions of inhibition of spontaneous emission is analyzed, showing entanglement trapping. We demonstrate that entanglement trapping can be used efficiently to prevent entanglement sudden death. For the case of realistic photonic band-gap materials, we show that high values of entanglement trapping can be achieved. This result is of both fundamental and applicative interest since it provides a physical situation where the entanglement can be preserved and manipulated, e.g. by Stark-shifting the qubit transition frequency outside and inside the gap.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Lett. on Friday 16 May 200

A simple trapped-ion architecture for high-fidelity Toffoli gates

We discuss a simple architecture for a quantum Toffoli gate implemented using three trapped ions. The gate, which in principle can be implemented with a single laser-induced operation, is effective under rather general conditions and is strikingly robust (within any experimentally realistic range of values) against dephasing, heating and random fluctuations of the Hamiltonian parameters. We provide a full characterization of the unitary and noise-affected gate using three-qubit quantum process tomography

Limits in the characteristic function description of non-Lindblad-type open quantum systems

In this paper I investigate the usability of the characteristic functions for the description of the dynamics of open quantum systems focussing on non-Lindblad-type master equations. I consider, as an example, a non-Markovian generalized master equation containing a memory kernel which may lead to nonphysical time evolutions characterized by negative values of the density matrix diagonal elements [S.M. Barnett and S. Stenholm, Phys. Rev. A {\bf 64}, 033808 (2001)]. The main result of the paper is to demonstrate that there exist situations in which the symmetrically ordered characteristic function is perfectly well defined while the corresponding density matrix loses positivity. Therefore nonphysical situations may not show up in the characteristic function. As a consequence, the characteristic function cannot be considered an {\it alternative complete} description of the non-Lindblad dynamics.Comment: Revised version. 4 pages, 1 figur

Driven harmonic oscillator as a quantum simulator for open systems

We show theoretically how a driven harmonic oscillator can be used as a quantum simulator for non-Markovian damped harmonic oscillator. In the general framework, the results demonstrate the possibility to use a closed system as a simulator for open quantum systems. The quantum simulator is based on sets of controlled drives of the closed harmonic oscillator with appropriately tailored electric field pulses. The non-Markovian dynamics of the damped harmonic oscillator is obtained by using the information about the spectral density of the open system when averaging over the drives of the closed oscillator. We consider single trapped ions as a specific physical implementation of the simulator, and we show how the simulator approach reveals new physical insight into the open system dynamics, e.g. the characteristic quantum mechanical non-Markovian oscillatory behavior of the energy of the damped oscillator, usually obtained by the non-Lindblad-type master equation, can have a simple semiclassical interpretation.Comment: 10 pages, 4 figures. V2: Minor modifications and added 2 appendixes for more details about calculation

Dynamics of quantum correlations in two-qubit systems within non-Markovian environments

Knowledge of the dynamical behavior of correlations with no classical counterpart, like entanglement, nonlocal correlations and quantum discord, in open quantum systems is of primary interest because of the possibility to exploit these correlations for quantum information tasks. Here we review some of the most recent results on the dynamics of correlations in bipartite systems embedded in non-Markovian environments that, with their memory effects, influence in a relevant way the system dynamics and appear to be more fundamental than the Markovian ones for practical purposes. Firstly, we review the phenomenon of entanglement revivals in a two-qubit system for both independent environments and a common environment. We then consider the dynamics of quantum discord in non-Markovian dephasing channel and briefly discuss the occurrence of revivals of quantum correlations in classical environments.Comment: 20 pages, 4 figures. Review article, in press in Int. J. Mod. Phys. B, special issue "Classical Vs Quantum correlations in composite systems", edited by L. Amico, S. Bose, V. Korepin and V. Vedra

Association Between Participant Retention and the Proportion of Included Elderly People in Rheumatology Trials: Results From a Series of Exploratory Meta‐Regression Analyses

Objective. The elderly, a population defined by an age of ≥65 years, are underrepresented in rheumatology trials, possibly due to investigators' concerns of increased premature discontinuations in higher age groups. The present study was undertaken to evaluate whether the proportion of included elderly individuals (PE) is independently associated with participant retention in rheumatology trials. Methods. Medline was searched for randomized controlled trials (RCTs) in rheumatoid arthritis (RA) and osteoarthritis (OA) of any intervention (years 2016 and 2017). PE was either extracted from the research manuscript or estimated from an assumed (truncated) normal distribution. We used mixed-effects meta-regression models including several covariates to assess whether there is an independent association between PE and participant retention. Using sensitivity analyses, we evaluated whether associations were connected to attrition due to lack of efficacy (LoE) or adverse events (AE). Results. In total, 243 RCTs comprising >48,000 participants were included. Pooled participant retention was 88%. PE was not associated with retention in the unadjusted (P = 0.97) or adjusted (all: P ≥0.14) models. Of all covariates, only study duration and type of intervention were associated with retention (both: P < 0.001). Post hoc analyses allowing for interaction revealed a small but statistically significant positive association between PE and retention in pharmacologic interventions and a negative association in physical/physiotherapeutic interventions (overall P for interaction = 0.05). No associations were found for PE and attrition due to LoE or AE. Conclusion. Participant retention in RA and OA trials is high and not associated with PE. These findings should motivate investigators to include more elderly participants in rheumatology trials

Role of non-Markovianity and backflow of information in the speed of quantum evolution

We consider a two-level open quantum system undergoing pure dephasing, dissipative, or multiply decohering dynamics and show that whenever the dynamics is non-Markovian, the initial speed of evolution is a monotonic function of the relevant physical parameter driving the transition between the Markovian and non-Markovian behavior of the dynamics. In particular, within the considered models, a speed increase can only be observed in the presence of backflow of information from the environment to the system