Quantum simulation of cosmic inflation in two-component Bose-Einstein condensates

Generalizing the one-component case, we demonstrate that the propagation of sound waves in two-component Bose-Einstein condensates can also be described in terms of effective sonic geometries under appropriate conditions. In comparison with the one-component case, the two-component setup offers more flexibility and several advantages. In view of these advantages, we propose an experiment in which the evolution of the inflaton field, and thereby the generation of density quantum fluctuations in the very early stages of our universe during inflation, can be simulated, realizing a {\em quantum simulation via analogue gravity models}.Comment: 8 pages of RevTex4, 1 figure; added explanatory material, to appear in Physical Review

Scalable and Energy-Efficient Millimeter Massive MIMO Architectures: Reflect-Array and Transmit-Array Antennas

Hybrid analog-digital architectures are considered as promising candidates for implementing millimeter wave (mmWave) massive multiple-input multiple-output (MIMO) systems since they enable a considerable reduction of the required number of costly radio frequency (RF) chains by moving some of the signal processing operations into the analog domain. However, the analog feed network, comprising RF dividers, combiners, phase shifters, and line connections, of hybrid MIMO architectures is not scalable due to its prohibitively high power consumption for large numbers of transmit antennas. Motivated by this limitation, in this paper, we study novel massive MIMO architectures, namely reflect-array (RA) and transmit-array (TA) antennas. We show that the precoders for RA and TA antennas have to meet different constraints compared to those for conventional MIMO architectures. Taking these constraints into account and exploiting the sparsity of mmWave channels, we design an efficient precoder for RA and TA antennas based on the orthogonal matching pursuit algorithm. Furthermore, in order to fairly compare the performance of RA and TA antennas with conventional fully-digital and hybrid MIMO architectures, we develop a unified power consumption model. Our simulation results show that unlike conventional MIMO architectures, RA and TA antennas are highly energy efficient and fully scalable in terms of the number of transmit antennas.Comment: submitted to IEEE ICC 201

O(N) symmetry-breaking quantum quench: Topological defects versus quasiparticles

We present an analytical derivation of the winding number counting topological defects created by an O(N) symmetry-breaking quantum quench in N spatial dimensions. Our approach is universal in the sense that we do not employ any approximations apart from the large-$N$ limit. The final result is nonperturbative in N, i.e., it cannot be obtained by %the usual an expansion in 1/N, and we obtain far less topological defects than quasiparticle excitations, in sharp distinction to previous, low-dimensional investigations.Comment: 6 pages of RevTex4-1, 1 figure; to be published in Physical Review

Tunneling-induced damping of phase coherence revivals in deep optical lattices

We consider phase coherence collapse and revival in deep optical lattices, and calculate within the Bose-Hubbard model the revival amplitude damping incurred by a finite tunneling coupling of the lattice wells (after sweeping from the superfluid to the Mott phase). Deriving scaling laws for the corresponding decay of first-order coherence revival in terms of filling factor, final lattice depth, and number of tunneling coupling partners, we estimate whether revival-damping related to tunneling between sites can be or even has already been observed in experiment.Comment: 4+epsilon pages of RevTex4; Rapid Communication in Physical Review

Quantum simulator for the Ising model with electrons floating on a helium film

We propose a physical setup that can be used to simulate the quantum dynamics of the Ising model with present-day technology. Our scheme consists of electrons floating on superfluid helium which interact via Coulomb forces. In the limit of low temperatures, the system will stay near the ground state where its Hamiltonian is equivalent to the Ising model and thus shows phenomena such as quantum criticality. Furthermore, the proposed design could be generalized in order to study interacting field theories (e.g., $\lambda\phi^4$) and adiabatic quantum computers.Comment: 4 page

Effect of fluctuations on the superfluid-supersolid phase transition on the lattice

We derive a controlled expansion into mean field plus fluctuations for the extended Bose-Hubbard model, involving interactions with many neighbors on an arbitrary periodic lattice, and study the superfluid-supersolid phase transition. Near the critical point, the impact of (thermal and quantum) fluctuations on top of the mean field grows, which entails striking effects, such as negative superfluid densities and thermodynamical instability of the superfluid phase -- earlier as expected from mean-field dynamics. We also predict the existence of long-lived "supercooled" states with anomalously large quantum fluctuations.Comment: 5 pages of RevTex4; as published in Physical Review

Implementing Movie Control, Access and Management - from a Formal Description to a Working Multimedia System

In this paper we describe the tool-supported specification and implementation of a multimedia communication protocol on parallel hardware. MCAM is an application layer protocol for movie control, access and management. We specify the full MCAM protocol together with ISO presentation and session layers in Estelle. Using a code generator, we derive parallel C++ code from the specification. The code is compiled and executed on a multiprocessor system under OSF/1 and on UNIX workstations. Measurements show the performance speedup gained by several different configurations of parallel units. We also report on experiences with our methodology

Bogoliubov theory of quantum correlations in the time-dependent Bose-Hubbard model

By means of an adapted mean-field expansion for large fillings $n\gg1$, we study the evolution of quantum fluctuations in the time-dependent Bose-Hubbard model, starting in the superfluid state and approaching the Mott phase by decreasing the tunneling rate or increasing the interaction strength in time. For experimentally relevant cases, we derive analytical results for the temporal behavior of the number and phase fluctuations, respectively. This allows us to calculate the growth of the quantum depletion and the decay of off-diagonal long-range order. We estimate the conditions for the observability of the time dependence in the correlation functions in the experimental setups with external trapping present. Finally, we discuss the analogy to quantum effects in the early universe during the inflationary epoch.Comment: 11 pages of RevTex4, 2 figures; significantly extended, with several analytically solvable cases added, to appear in Physical Review