Current behavior of a quantum Hamiltonian ratchet in resonance

We investigate the ratchet current that appears in a kicked Hamiltonian system when the period of the kicks corresponds to the regime of quantum resonance. In the classical analogue, a spatial-temporal symmetry should be broken to obtain a net directed current. It was recently discovered that in quantum resonance the temporal symmetry can be kept, and we prove that breaking the spatial symmetry is a necessary condition to find this effect. Moreover, we show numerically and analytically how the direction of the motion is dramatically influenced by the strength of the kicking potential and the value of the period. By increasing the strength of the interaction this direction changes periodically, providing us with a non-expected source of current reversals in this quantum model. These reversals depend on the kicking period also, though this behavior is theoretically more difficult to analyze. Finally, we generalize the discussion to the case of a non-uniform initial condition.Comment: 6 pages, 4 figure

Ultra-Long-Haul WDM Transmission Using NANF Hollow-Core Fiber

Hollow-core fiber NANF prototypes have recently achieved lower loss and wider bandwidth than SMF. Theory predicts further progress may be possible. We investigate the potential impact of future high-performance NANFs on long-haul optical communication systems

Novel applications of fibre tapers

In this talk we will discuss new applications for fibre tapers ranging from femtosecond pulse manipulation to optical clock generation in micro-coil resonators. Fundamental to these interactions is the enhanced nonlinearity arising from the smaller cores making compact optical devices possible

Opportunities and Challenges for Long-Distance Transmission in Hollow-Core Fibres

Anti-resonant hollow-core fiber of the Nested Antiresonant Nodeless type (NANF) has been showing a steady decrease in loss over the last few years, gradually approaching that of standard Single-Mode Fiber (SMF). It already by far outperforms SMF as to non-linear effects, which are three to four orders of magnitude lower in NANF than in SMF. Theoretical predictions and experimental evidence also hint at a much wider usable bandwidth than SMF, potentially amounting to several tens of THz. Propagation speed is 50% faster, a key feature in certain contexts. In this paper we investigate the potential impact of possible future high-performance NANF on long-haul optical communication systems, assuming NANF continues on its current steady path towards better performance. We look at the system throughput in different long-haul scenarios, addressing links of various length, from 100~km to 4,000~km, and different NANF optical bandwidths, loss and total launch power. We compare such throughput with a benchmark state-of-the-art SMF Raman-amplified C+L system. We found that NANF might enable relative throughput gains vs.~the benchmark on the order of 1.5x to 5x, at reasonable NANF and system parameter values. We also study the problem of the impact of NANF Inter-Modal-Interference (IMI) on system performance and show that a value of -60~dB/km, close to the currently best reported values, is low enough to have no substantial harmful effect. We finally look at a more long-term scenario in which NANF loss gets below that of SMF and we show that in this context repeterless or even completely amplifierless systems might be possible, delivering 300-400 Tb/s per NANF, over 200 to 300~km distances. The system simplification and ease of wideband exploitation implied by these systems might prove quite attractive especially in densely populated regions where inter-node distances are modest. While several technological hurdles remain towards NANF systems becoming practical contenders, in our opinion NANF appears to have the potential to become an attractive and possibly disruptive alternative to conventional solid-core silica fibers

Steering Bose-Einstein condensates despite time symmetry

A Bose-Einstein condensate in an oscillating spatially asymmetric potential is shown to exhibit a directed current for unbiased initial conditions despite time symmetry. This phenomenon occurs only if the interaction between atoms, treated in mean-field approximation, exceeds a critical value. Our findings can be described with a three-mode model (TMM). These TMM results corroborate well with a many-body study over a time scale which increases with increasing atom number. The duration of this time scale probes the validity of the used mean-field approximation.Comment: 4 pages, 5 figure

RGB generation by four-wave mixing in small-core holey fibers

We report the generation of white light comprising red, green, and blue spectral bands from a frequency-doubled fiber laser by an efficient four-wave mixing process in submicron-sized cores of microstructured holey fibers. A master-oscillator power amplifier (MOPA) source based on Yb-doped fiber is employed to generate 80 ps pulses at 1060 nm wavelength with 32 MHz repetition rate, which are then frequency-doubled in an LBO crystal to generate up to 2 W average power of green light. The green pump is then carefully launched into secondary cores of the cladding of photonic bandgap fibers. These secondary cores with diameters of about 400 to 800 nm act as highly nonlinear waveguides. At the output, we observe strong red and blue sidebands which, together with the remaining green pump light, form a visible white light source of about 360 mW. The generating process is identified as four-wave mixing where phase matching is achieved by birefringence in the secondary cores which arises from non-symmetric deformation during the fiber fabrication. Numerical models of the fiber structure and of the nonlinear processes confirm our interpretation. Finally, we discuss power scaling and limitations of the white light source due to the damage threshold of silica fibers

Social contact structures and time use patterns in the Manicaland Province of Zimbabwe.

BACKGROUND: Patterns of person-to-person contacts relevant for infectious diseases transmission are still poorly quantified in Sub-Saharan Africa (SSA), where socio-demographic structures and behavioral attitudes are expected to be different from those of more developed countries. METHODS AND FINDINGS: We conducted a diary-based survey on daily contacts and time-use of individuals of different ages in one rural and one peri-urban site of Manicaland, Zimbabwe. A total of 2,490 diaries were collected and used to derive age-structured contact matrices, to analyze time spent by individuals in different settings, and to identify the key determinants of individuals' mixing patterns. Overall 10.8 contacts per person/day were reported, with a significant difference between the peri-urban and the rural site (11.6 versus 10.2). A strong age-assortativeness characterized contacts of school-aged children, whereas the high proportion of extended families and the young population age-structure led to a significant intergenerational mixing at older ages. Individuals spent on average 67% of daytime at home, 2% at work, and 9% at school. Active participation in school and work resulted the key drivers of the number of contacts and, similarly, household size, class size, and time spent at work influenced the number of home, school, and work contacts, respectively. We found that the heterogeneous nature of home contacts is critical for an epidemic transmission chain. In particular, our results suggest that, during the initial phase of an epidemic, about 50% of infections are expected to occur among individuals younger than 12 years and less than 20% among individuals older than 35 years. CONCLUSIONS: With the current work, we have gathered data and information on the ways through which individuals in SSA interact, and on the factors that mostly facilitate this interaction. Monitoring these processes is critical to realistically predict the effects of interventions on infectious diseases dynamics

Comment on "Coherent Ratchets in Driven Bose-Einstein Condensates"

C. E. Creffield and F. Sols (Phys. Rev. Lett. 103, 200601 (2009)) recently reported finite, directed time-averaged ratchet current, for a noninteracting quantum particle in a periodic potential even when time-reversal symmetry holds. As we explain in this Comment, this result is incorrect, that is, time-reversal symmetry implies a vanishing current.Comment: revised versio

Bleeding Meckel's diverticulum diagnosis: an unusual indication for computed tomography

Despite the wide use of modern investigation techniques, the diagnosis of complications related to Meckel's diverticulum (MD) remains difficult. Arteriography is commonly indicated for acute bleeding, and radionuclide scans may help in identifying the site of intestinal hemorrhage. In contrast, computed tomography (CT) is usually considered little use in the diagnosis of bleeding MD. We present the case of a young patient with massive gastrointestinal hemorrhage, in whom the diagnosis of MD bleeding was preoperatively made with contrast-enhanced CT after two negatives arteriographie