Berezinskii-Kosterlitz-Thouless transition in two-dimensional dipolar stripes

A two-dimensional quantum system of dipoles, with a polarization angle not perpendicular to the plane, shows a transition from a gas to a stripe phase. We have studied the thermal properties of these two phases using the path-integral Monte Carlo (PIMC) method. By simulating the thermal density matrix, PIMC provides exact results for magnitudes of interest such as the superfluid fraction and the one-body density matrix. As it is well known, in two dimensions the superfluid-to-normal phase transition follows the Berezinskii-Kosterlitz-Thouless (BKT) scenario. Our results show that both the anisotropic gas and the stripe phases follow the BKT scaling laws. At fixed density and increasing the tilting angle, the transition temperature decreases in going from the gas to the stripe phase. Superfluidity in the perpendicular direction to the stripes is rather small close to the critical temperature but it becomes larger at lower temperatures, mainly close to the transition to the gas. Our results are in qualitative agreement with the supersolidity observed recently in a quasi-one-dimensional array of dipolar droplets.Postprint (published version

Gapped spectrum in pair-superfluid bosons

We study the ground state of a bilayer system of dipolar bosons with dipoles oriented by an external field perpendicularly to the two parallel planes. By decreasing the interlayer distance, for a fixed value of the strength of the dipolar interaction, the system undergoes a quantum phase transition from an atomic to a pair superfluid. We investigate the excitation spectrum on both sides of this transition by using two microscopic approaches. Quantum Monte Carlo methods are employed to obtain the static structure factors and intermediate scattering functions in imaginary time. The dynamic response is calculated using both the correlated basis functions (CBF) method and the approximate inversion of the Laplace transform of the quantum Monte Carlo imaginary time data. In the atomic phase, both the density and spin excitations are gapless. However, in the pair-superfluid phase a gap opens in the excitation energy of the spin mode. For small separation between layers, the minimal spin excitation energy equals the binding energy of a dimer and is twice the gap value.Postprint (author's final draft

A customizable open-source framework for measuring and equalizing e2e delays in shared video watching

[EN] Low-latency and media sync are essential requirements to enable interactive multi-party services, such as Social TV. In this work, we present an open-source and customizable framework that allows measuring end-to-end (e2e) video delays and provides support for different types of media sync, including Inter-Destination Media Sync (IDMS). This framework can be used by researchers to investigate the suitability of different techniques for optimizing the system performance in terms of e2e delays and media sync.This work has been funded, partially, by UPV under its R&D Support Program in PAID-01-10 Project and by CWI under EU/FP7 REVERIE Project (ICT-2011-7-287723).Montagud, M.; Boronat, F.; Cesar, P. (2014). A customizable open-source framework for measuring and equalizing e2e delays in shared video watching. ACM. 95-96. https://doi.org/10.6084/m9.figshare.1032656S959

Droplets of trapped quantum dipolar bosons

Strongly interacting systems of dipolar bosons in three dimensions confined by harmonic traps are analyzed using the exact path integral ground-state Monte Carlo method. By adding a repulsive two-body potential, we find a narrow window of interaction parameters leading to stable ground-state configurations of droplets in a crystalline arrangement. We find that this effect is entirely due to the interaction present in the Hamiltonian without resorting to additional stabilizing mechanisms or specific three-body forces. We analyze the number of droplets formed in terms of the Hamiltonian parameters, relate them to the corresponding s-wave scattering length, and discuss a simple scaling model for the density profiles. Our results are in qualitative agreement with recent experiments showing a quantum Rosensweig instability in trapped Dy atoms.Peer ReviewedPostprint (published version

Arqueología informática: diseño e implementación de la calculadora mecánica Millionaire con Scratch

[ES] El objetivo principal de este proyecto es realizar una virtualización de la máquina de cálculo Millionaire. En el inicio de esta memoria, se estudia el lenguaje de programación Scratch, explicando sus orígenes y su funcionamiento, así como sus características, sus ventajas y sus limitaciones. Para este capítulo se ha hecho uso de las guías de referencia de Scratch. A continuación, se realiza un repaso histórico de la evolución de las distintas herramientas de cálculo, analizando también el contexto socioeconómico que propició la invención de la calculadora Millionaire. La memoria continúa haciendo un exhaustivo análisis del funcionamiento de la máquina y sus orígenes, explicando cada una de las operaciones que puede realizar el aparato y el mecanismo interno que lo permite. Por último, con los conocimientos adquiridos en Scratch y en la máquina, se ha programado y documentado el código de la calculadora.[CA] El objectiu principal d’aquest projecte es realitzar una virtualització de la màquina de càlcul Millionaire. En l’inici d’aquesta memòria, s’estudia el llenguatge de programació Scratch, explicant el seus orígens i el seu funcionament, així com les seues característiques, els seus avantatges y les seues limitacions. Per a este capítol s’ha fet ús de les guíes de referència de Scratch. A continuació, es realitza un repàs històric de l’evolució de les distintes eines de càlcul, tot analitzant també el context socioeconòmic que va propiciar la invenció de la calculadora Millionaire. La memòria continua fent un exhaustiu anàlisi del funcionament de la màquina i els seus orígens, explicant cadascuna de les operacions que pot realitzar l’aparell i el mecanisme intern que ho permet. Finalment, amb els coneixements adquirits en Scratch i en la màquina, s’ha programat i documentat el codi de la calculadora.[EN] The main objective of this project is to realize virtualization of the Millionaire calculation machine. IV At the beginning of this report, the Scratch programming language is studied, explaining its origins and operation, as well as its features, advantages and limitations. This chapter was written with the help of the Scratch reference guides. The next part is a historical review of the evolution of the different calculation tools and the economic context that led to the invention of the Millionaire calculator. The report continues with an exhaustive analysis of the operation of the machine and its origins, explaining each of the operations that the device can make and the internal mechanism that allows it. Finally, with the knowledge acquired in Scratch and the machine, the calculator code has been programmed and documented.Baeza Boronat, P. (2021). Arqueología informática: diseño e implementación de la calculadora mecánica Millionaire con Scratch. Universitat Politècnica de València. http://hdl.handle.net/10251/174676TFG

Design, Development and Assessment of Control Schemes for IDMS in a Standardized RTCP-based Solution

[EN] Currently, several media sharing applications that allow social interactions between distributed users are gaining momentum. In these networked scenarios, synchronized playout between the involved participants must be provided to enable truly interactive and coherent shared media experiences. This research topic is known as Inter-Destination Media Synchronization (IDMS). This paper presents the design and development of an advanced IDMS solution, which is based on extending the capabilities of RTP/RTCP standard protocols. Particularly, novel RTCP extensions, in combination with several control algorithms and adjustment techniques, have been specified to enable an adaptive, highly accurate and standard compliant IDMS solution. Moreover, as different control or architectural schemes for IDMS exist, and each one is best suited for specific use cases, the IDMS solution has been extended to be able to adopt each one of them. Simulation results prove the satisfactory responsiveness of our IDMS solution in a small scale scenario, as well as its consistent behavior, when using each one of the deployed architectural schemes.This work has been financed, partially, by Universitat Politecnica de Valencia (UPV), under its R&D Support Program in PAID-01-10. TNO's work has been partially funded by European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. ICT-2011-8-318343 (STEER Project). CWI's work has been partially funded by the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. ICT-2011-7-287723 (REVERIE Project).Montagud Aguar, M.; Boronat Segui, F.; Stokking, H.; Cesar, P. (2014). Design, Development and Assessment of Control Schemes for IDMS in a Standardized RTCP-based Solution. Computer Networks. 70:240-259. https://doi.org/10.1016/j.comnet.2014.06.004S2402597

Diffusion Monte Carlo methods for spin-orbit-coupled ultracold Bose gases

We present two diffusion Monte Carlo (DMC) algorithms for systems of ultracold quantum gases featuring synthetic spin-orbit interactions. The first one is a spin-integrated DMC method which provides fixed-phase energy estimates. The second one is a discrete spin generalisation of the T-moves spin-orbit DMC [Melton et al., J. Chem. Phys. 144, 244113 (2016)], which provides an upper bound to the fixed-phase energy. The former is a more accurate method but it is restricted to spin-independent two-body interactions. We report a comparison between both algorithms for different systems. As a check of the efficiency of both methods, we compare the DMC energies with results obtained with other numerical methods, finding agreement between both estimations.Peer ReviewedPostprint (published version

Supersolid stripes enhanced by correlations in a Raman spin-orbit-coupled system

A Bose gas under the effect of Raman spin-orbit coupling (SOC) is analyzed using the discrete spin T-moves diffusion Monte Carlo method. Upon computing the energy as well as the static structure factor and the superfluid fraction of the system, the emergence of an energetically favorable supersolid stripe state is observed, which is in agreement with recent observations. A significant enhancement of the stability of the stripe phase with respect to the mean-field prediction is observed when the strength of the interatomic correlations is increased. We also quantify and characterize the degree of superfluidity of the stripes and show that this quantity is mostly determined by the ratio between the Raman coupling and the square of the momentum difference between the pair of SOC-inducing laser beams.Postprint (published version

One-dimensional harmonically confined SU(N) fermions

We study the momentum distributions and spatial correlations of few harmonically confined SU(N) fermions using quantum Monte Carlo methods. In our study, we vary the spin degeneracy N from 2 to 6 and the total number of particles from 6 to 18. Only balanced mixtures, with the same number of atoms per spin type, and repulsive unlike-spin contact interactions are considered. Going from N=2 to N=6, with the same occupancy of each spin state, we observe an increase of atom-atom correlations. This effect is particularly significant in the momentum distributions, which show fatter tails at large k (ks>5, s being the oscillator length) when N grows, in agreement with experimental findings. Those tails also show the expected k-4 decay related to the Tan contact for different values of the spin degeneracy. According to our results, the local spin ordering and the spin-spin correlations are mainly determined by N via the Pauli exclusion principle, with minor influences from the particle-particle interactions, irrespective of the total number of confined atoms.Postprint (published version

A customizable open-source framework for measuring and equalizing e2e delays in shared video watching

Low-latency and media sync are essential requirements to enable interactive multi-party services, such as Social TV. In this work, we present an open-source and customizable framework that allows measuring end-to-end (e2e) video delays and provides support for different type