Synchronized and desynchronized phases of coupled non-equilibrium exciton-polariton condensates

We theoretically analyze the synchronized and desynchronized phases of coupled non-equilibrium polariton condensates within mean field theory. An analytical condition for the existence of a synchronized phase is derived for two coupled wells. The case of many wells in a 2D disordered geometry is studied numerically. The relation to recent experiments on polariton condensation in CdTe microcavities is discussed.Comment: 5 pages, 3 figure

Efficiency of Wastewater Treatment System and Its Use for Irrigation - A Case Study of a private University in Ghana

The reuse or utilization of treated wastewater for irrigation is conventionally considered as a means of mitigating water shortage or abating water pollution. Wastewater treatment plants designed for reuse in irrigation are more appropriate for developing countries striving to enhance access to improved sanitation. It is well known that successive stages of treatment of sewage effluent reduces the quantity of suspended solids, organic matter and nutrient load, bacteria population as well as biological oxygen demand to the extent that the final treated effluent contains virtually a small fraction compared to the influent sewage. A short-term assessment of the decentralised reuse-oriented effluent system of a private University (in Ghana) was carried out to determine its effluent quality for the purpose of irrigating its landscape. The investigation showed that Total Coliform, E. coli and Vibrio spp. were significantly reduced, through the treatment stages, but not to within Internationally accepted guideline values. Salmonella spp. was not significantly reduced. Physical parameters, nutrients as well as biological oxygen demand did not show any variation along the treatment stages. Suspended solids, optimum temperature and pH were identified as contributing to treatment inefficiency of the plant. It is therefore recommended the treated wastewater, prior to disinfection, should be filtered to reduce suspended solids. This will enhance effective chlorination and by extension, significantly reduce bacteria population. Furthermore Regular monitoring and laboratory analysis of the recycled effluent from the plant should be carried out by the EPA or other professional organisation to ensure compliance. Keywords: Reuse, Wastewater, Irrigation, Chlorinatio

Quantum dynamics of a binary mixture of BECs in a double well potential: an Holstein-Primakoff approach

We study the quantum dynamics of a binary mixture of Bose-Einstein condensates (BEC) in a double-well potential starting from a two-mode Bose-Hubbard Hamiltonian. Focussing on the regime where the number of atoms is very large, a mapping onto a SU(2) spin problem together with a Holstein-Primakoff transformation is performed. The quantum evolution of the number difference of bosons between the two wells is investigated for different initial conditions, which range from the case of a small imbalance between the two wells to a coherent spin state. The results show an instability towards a phase-separation above a critical positive value of the interspecies interaction while the system evolves towards a coherent tunneling regime for negative interspecies interactions. A comparison with a semiclassical approach is discussed together with some implications on the experimental realization of phase separation with cold atoms.Comment: 12 pages, 7 figures, accepted for publication in J. Phys.

Arc Simulation in Low Voltage Switching Devices, a Case Study

Arc simulations are becoming a valuable tool in the development of low voltage switching devices. Sim-ulations reveal physical quantities that are experimentally not accessible and help in the investigation of the underlying phenomena. However, the strong interaction between different processes and the intrinsic multi-scale nature of the problem, both in time and space, pose great challenges to accurate and efficient simulations. At ABB Corporate Research, we developed a simulation tool capable of simulating the be-havior of low voltage switchgear. To verify the accuracy and predictive capability of our platform, we validate the simulations by comparing their results with available experimental findings. After describing the tool, we provide here evidence of the good agreement between measured and simulated data on several commercial ABB devices

Second Josephson excitations beyond mean field as a toy model for thermal pressure: exact quantum dynamics and the quantum phase model

A simple four-mode Bose-Hubbard model with intrinsic time scale separation can be considered as a paradigm for mesoscopic quantum systems in thermal contact. In our previous work we showed that in addition to coherent particle exchange, a novel slow collective excitation can be identified by a series of Holstein-Primakoff transformations. This resonant energy exchange mode is not predicted by linear Bogoliubov theory, and its frequency is sensitive to interactions among Bogoliubov quasi-particles; it may be referred to as a second Josephson oscillation, in analogy to the second sound mode of liquid Helium II. In this paper we will explore this system beyond the Gross-Pitaevskii mean field regime. We directly compare the classical mean field dynamics to the exact full quantum many-particle dynamics and show good agreement over a large range of the system parameters. The second Josephson frequency becomes imaginary for stronger interactions, however, indicating dynamical instability of the symmetric state. By means of a generalized quantum phase model for the full four-mode system, we then show that, in this regime, high-energy Bogoliubov quasiparticles tend to accumulate in one pair of sites, while the actual particles preferentially occupy the opposite pair. We interpret this as a simple model for thermal pressure

Dynamical Realization of Macroscopic Superposition States of Cold Bosons in a Tilted Double Well

We present exact expressions for the quantum sloshing of Bose-Einstein condensates in a tilted two-well potential. Tunneling is suppressed by a small potential difference between wells, or tilt. However, tunneling resonances occur for critical values of the tilt when the barrier is high. At resonance, tunneling times on the order of 10-100 ms are possible. Furthermore, such tilted resonances lead to a dynamical scheme for creating few-body NOON-like macroscopic superposition states which are protected by the many body wavefunction against potential fluctuations.Comment: 6 pages, 5 figures, final version, only minor changes from previous arXiv versio

Nonlinear quantum model for atomic Josephson junctions with one and two bosonic species

We study atomic Josephson junctions (AJJs) with one and two bosonic species confined by a double-well potential. Proceeding from the second quantized Hamiltonian, we show that it is possible to describe the zero-temperature AJJs microscopic dynamics by means of extended Bose-Hubbard (EBH) models, which include usually-neglected nonlinear terms. Within the mean-field approximation, the Heisenberg equations derived from such two-mode models provide a description of AJJs macroscopic dynamics in terms of ordinary differential equations (ODEs). We discuss the possibility to distinguish the Rabi, Josephson, and Fock regimes, in terms of the macroscopic parameters which appear in the EBH Hamiltonians and, then, in the ODEs. We compare the predictions for the relative populations of the Bose gases atoms in the two wells obtained from the numerical solutions of the two-mode ODEs, with those deriving from the direct numerical integration of the Gross-Pitaevskii equations (GPEs). Our investigations shows that the nonlinear terms of the ODEs are crucial to achieve a good agreement between ODEs and GPEs approaches, and in particular to give quantitative predictions of the self-trapping regime.Comment: Accepted for the publication in J. Phys. B: At. Mol. Opt. Phy