The mesoscopic magnetron as an open quantum system

Motivated by the emergence of materials with mean free paths on the order of microns, we propose a novel class of solid state radiation sources based on reimplementing classical vacuum tube designs in semiconductors. Using materials with small effective masses, these devices should be able to access the terahertz range. We analyze the DC and AC operation of the simplest such device, the cylindrical diode magnetron, using effective quantum models. By treating the magnetron as an open quantum system, we show that it continues to operate as a radiation source even if its diameter is only a few tens of magnetic lengths.Comment: 11 pages, 7 figures; submitted to Physical Review Applie

Tunneling in the self-trapped regime of a two-well Bose-Einstein condensate

Starting from a mean-field model of the Bose-Einstein condensate dimer, we reintroduce classically forbidden tunneling through a Bohr-Sommerfeld quantization approach. We find closed-form approximations to the tunneling frequency more accurate than those previously obtained using different techniques. We discuss the central role that tunneling in the self-trapped regime plays in a quantitatively accurate model of a dissipative dimer leaking atoms to the environment. Finally, we describe the prospects of experimental observation of tunneling in the self-trapped regime, both with and without dissipation.We wish to thank Wolfgang Muessel, Markus Oberthaler, Kaspar Sakmann, Andrea Trombettoni, Stephanos Venakides, and Tilman Zibold for helpful discussions. We are also grateful for the hospitality of Joshua E. S. Socolar and the Duke University Physics Department. This work was supported in part by Boston University. D.W. acknowledges support from the Helmholtz Association (Grant No. VH-NG-1025). (Boston University; VH-NG-1025 - Helmholtz Association)First author draf

Dynamics of entanglement in a dissipative Bose-Hubbard dimer

We study the connection between the semiclassical phase space of the Bose-Hubbard dimer and inherently quantum phenomena in this model, such as entanglement and dissipation-induced coherence. Near the semiclassical self-trapping fixed points, the dynamics of Einstein-Podolski-Rosen (EPR) entanglement and condensate fraction consists of beats among just three eigenstates. Since persistent EPR entangled states arise only in the neighborhood of these fixed points, our analysis explains essentially all of the entanglement dynamics in the system. We derive accurate analytical approximations by expanding about the strong-coupling limit; surprisingly, their realm of validity is nearly the entire parameter space for which the self-trapping fixed points exist. Finally, we show significant enhancement of entanglement can be produced by applying localized dissipation.We thank Luca d'Alessio, Pjotrs Gri. sons, and especially Anatoli Polkovnikov for helpful discussions. This work was supported in part by Boston University, by the US National Science Foundation under Grant No. PHYS-1066293, and by a grant of the Max Planck Society to the MPRG Network Dynamics. H. H. acknowledges support by the German Research Foundation under Grant No. HE 6312/1-1. We are also grateful for the hospitality of the Aspen Center for Physics. (Boston University; PHYS-1066293 - US National Science Foundation; Max Planck Society; HE 6312/1-1 - German Research Foundation)First author draf

Holistic Approach: paradigm shift in the research agenda for digitalisation of healthcare in Sub-Saharan Africa

Despite significant resources employed in the digitalisation agenda in the healthcare sector in Sub-Saharan Africa, the transformative impact of information and communication technologies has not been realised. This article makes two contributions towards developing an understanding of this failure. First, it provides a review of a rich body of academic literature and practitioner accounts regarding barriers to digitalisation and organises them using an established framework. Second, recognising the continuing struggle that digitalisation presents, it proposes a paradigmatic shift in thinking about barriers to digitalisation and suggests the existence of a more fundamental barrier related to inappropriate incentives within the international community. Ultimately, it argues that unpacking the complex contextual reality of healthcare delivery systems is a fundamental but still unaddressed antecedent to any successful digitalisation endeavour. Thus, both the academia and the practitioners should direct their efforts to developing new approaches, which could remove this underlying obstacle

Reentrant phase transitions involving glassy and superfluid orders in the random hopping Bose-Hubbard model

We study a system of strongly correlated bosons with off-diagonal disorder, i.e., randomness in the kinetic energy, and find a family of reentrant phase transitions that occur as a function of the on-site interaction. We model the system using the paradigmatic Bose-Hubbard Hamiltonian with a random hopping term and solve it employing the replica trick and Trotter-Suzuki expansion known from quantum spin-glasses. From subsequent numerical calculations, we find three distinct phase boundaries at which the reentrant transitions occur: between glass and disordered phase, between superglass and superfluid ones, and between superfluid and disordered phases. All three happen at temperatures slightly above critical temperatures of corresponding non-interacting systems. When the emerging and disappearing order is glassy, this corresponds to the interplay of the thermal energy and the spread of hoppings. When superfluidity is involved, thermal fluctuations must slightly overcome the mean hopping in turn for the reentrance to occur.Comment: 9 pages, 4 figure

Closed-Form Derivations of ISI and MUI for Time-Reversed Ultra Wideband

Through transmitter pre-filtering, a time reversed UWB system is capable if harnessing a multipath channel to achieve temporal and spatial focusing. Unfortunately, large RMS channel delay spread leads to significant intersymbol and multiuser interference. This paper presents closed-form expressions for self and multi-user interference for a UWB system utilizing a time-reversed approach. The influence of user multiplexing codes is taken to account through incorporation of a ‘separation probability’, which characterizes a family of hopping sequences. The standardized IEEE 802.15.3a channel model is applied, and the derived performances are compared with that of a simulated time hopped time-reversed UWB system

Time evolution of the reaction front in a subdiffusive system

Using the quasistatic approximation, we show that in a subdiffusion--reaction system the reaction front $x_{f}$ evolves in time according to the formula $x_{f} \sim t^{\alpha/2}$, with $\alpha$ being the subdiffusion parameter. The result is derived for the system where the subdiffusion coefficients of reactants differ from each other. It includes the case of one static reactant. As an application of our results, we compare the time evolution of reaction front extracted from experimental data with the theoretical formula and we find that the transport process of organic acid particles in the tooth enamel is subdiffusive.Comment: 18 pages, 3 figure

Equivariant wave maps exterior to a ball

We consider the exterior Cauchy-Dirichlet problem for equivariant wave maps from 3+1 dimensional Minkowski spacetime into the three-sphere. Using mixed analytical and numerical methods we show that, for a given topological degree of the map, all solutions starting from smooth finite energy initial data converge to the unique static solution (harmonic map). The asymptotics of this relaxation process is described in detail. We hope that our model will provide an attractive mathematical setting for gaining insight into dissipation-by-dispersion phenomena, in particular the soliton resolution conjecture.Comment: 16 pages, 9 figure