arXiv subjects

Michele Modugno

Publications and source records attributed to Michele Modugno.

At least 37 records · Page 2Linked to original sources

arXiv2020

Effective self-similar expansion of a Bose-Einstein condensate: Free space vs confined geometries

We compare the exact evolution of an expanding three-dimensional Bose-Einstein condensate with that obtained from the effective scaling approach introduced in D. Guéry-Odelin [Phys. Rev. A 66, 033613 (2002)]. This approach, which consists in looking for self-similar solutions to be satisfied on average, is tested here in different geometries and configurations. We find that, in case of almost isotropic traps, the effective scaling reproduces with high accuracy the exact evolution dictated by the Gross-Pitaevskii equation for arbitrary values of the interactions, in agreement with the proof-of-concept of M. Modugno, G. Pagnini, and M. A. Valle-Basagoiti [Phys. Rev. A 97, 043604 (2018)]. Conversely, it is shown that the hypothesis of universal self-similarity breaks down in case of strong anisotropies and trapped geometries.

cond-mat.quant-gas↗

arXiv2020

Dual-species Bose-Einstein condensate of 41K and 87Rb in a hybrid trap

We report on the production of a 41K-87Rb dual-species Bose-Einstein condensate in a hybrid trap, consisting of a magnetic quadrupole and an optical dipole potential. After loading both atomic species in the trap, we cool down 87Rb first by magnetic and then by optical evaporation, while 41K is sympathetically cooled by elastic collisions with 87Rb. We eventually produce two-component condensates with more than 10^5 atoms and tunable species population imbalance. We observe the immiscibility of the quantum mixture by measuring the density profile of each species after releasing them from the trap.

cond-mat.quant-gas↗

arXiv2020

A dual-species Bose-Einstein condensate with attractive interspecies interactions

We report on the production of a $^{41}$K-$^{87}$Rb dual-species Bose-Einstein condensate with tunable interspecies interaction and we study the mixture in the attractive regime, i.e. for negative values of the interspecies scattering length $a_{12}$. The binary condensate is prepared in the ground state and confined in a pure optical trap. We exploit Feshbach resonances for tuning the value of $a_{12}$. After compensating the gravitational sag between the two species with a magnetic field gradient, we drive the mixture into the attractive regime. We let the system to evolve both in free space and in an optical waveguide. In both geometries, for strong attractive interactions, we observe the formation of self-bound states, recognizable as quantum droplets. Our findings prove that robust, long-lived droplet states can be realized in attractive two-species mixtures, despite the two atomic components may experience different potentials.

cond-mat.quant-gas↗

arXiv2018

Non-linear mixing of Bogoliubov modes in a bosonic Josephson junction

We revisit the dynamics of a Bose-Einstein condensate in a double-well potential, from the regime of Josephson plasma oscillations to the self-trapping regime, by means of the Bogoliubov quasiparticle projection method. For very small imbalance between left and right wells only the lowest Bogoliubov mode is significantly occupied. In this regime the system performs plasma oscillations at the corresponding frequency, and the evolution of the condensate is characterized by a periodic transfer of population between the ground and the first excited state. As the initial imbalance is increased, more excited modes -- though initially not macroscopically occupied -- get coupled during the evolution of the system. Since their population also varies with time, the frequency spectrum of the imbalance turns out to be still peaked around a single frequency, which is continuously shifted towards lower values. The nonlinear mixing between Bogoliubov modes eventually drives the system into the the self-trapping regime, when the population of the ground state can be transferred completely to the excited states at some time during the evolution. For simplicity, here we consider a one-dimensional setup, but the results are expected to hold also in higher dimensions.

cond-mat.quant-gas↗

arXiv2018

Effective self-similar expansion for the Gross-Pitaevskii equation

We consider an effective scaling approach for the free expansion of a one-dimensional quantum wave packet, consisting in a self-similar evolution to be satisfied on average, i.e. by integrating over the coordinates. A direct comparison with the solution of the Gross-Pitaevskii equation shows that the effective scaling reproduces with great accuracy the exact evolution - the actual wave function is reproduced with a fidelity close to unity - for arbitrary values of the interactions. This result represents a proof-of-concept of the effectiveness of the scaling ansatz, which has been used in different forms in the literature but never compared with the exact evolution.

cond-mat.quant-gas↗

arXiv2017

Correspondence between a shaken honeycomb lattice and the Haldane model

We investigate the correspondence between the tight-binding Floquet Hamiltonian of a periodically modulated honeycomb lattice and the Haldane model. We show that - though the two systems share the same topological phase diagram, as reported in a breakthrough experiment with ultracold atoms in a stretched honeycomb lattice [Jotzu et al., Nature 515, 237 (2014)] - the corresponding Hamiltonians are not equivalent, the one of the shaken lattice presenting a much richer structure.

cond-mat.quant-gas↗

arXiv2016

Josephson plasma oscillations and the Gross-Pitaevskii equation: Bogoliubov approach vs two-mode model

We show that the Josephson plasma frequency for a condensate in a double-well potential, whose dynamics is described by the Gross-Pitaevskii (GP) equation, can be obtained with great precision by means of the usual Bogoliubov approach, whereas the two-mode model - commonly constructed by means of a linear combinations of the low-lying states of the GP equation - generally provides accurate results only for weak interactions. A proper two-mode model in terms of the Bogoliubov functions is also discussed, revealing that in general a two-mode approach is formally justified only for not too large interactions, even in the limit of very small amplitude oscillations. Here we consider specifically the case of a one-dimensional system, but the results are expected to be valid in arbitrary dimensions.

cond-mat.quant-gas↗

arXiv2016

Tight-binding models for ultracold atoms in optical lattices: general formulation and applications

Tight-binding models for ultracold atoms in optical lattices can be properly defined by using the concept of maximally localized Wannier functions for composite bands. The basic principles of this approach are reviewed here, along with different applications to lattice potentials with two minima per unit cell, in one and two spatial dimensions. Two independent methods for computing the tight-binding coefficients - one ab initio, based on the maximally localized Wannier functions, the other through analytic expressions in terms of the energy spectrum - are considered. In the one dimensional case, where the tight-binding coefficients can be obtained by designing a specific gauge transformation, we consider both the case of quasi resonance between the two lowest bands, and that between s and p orbitals. In the latter case, the role of the Wannier functions in the derivation of an effective Dirac equation is also reviewed. Then, we consider the case of a two dimensional honeycomb potential, with particular emphasis on the Haldane model, its phase diagram, and the breakdown of the Peierls substitution. Tunable honeycomb lattices, characterized by movable Dirac points, are also considered. Finally, general considerations for dealing with the interaction terms are presented.

cond-mat.quant-gas↗

arXiv2015

Ab initio analysis of the topological phase diagram of the Haldane model

We present an ab initio analysis of a continuous Hamiltonian that maps into the celebrated Haldane model. The tunnelling coefficients of the tight-binding model are computed by means of two independent methods - one based on the maximally localized Wannier functions, the other through analytic expressions in terms of gauge-invariant properties of the spectrum - that provide a remarkable agreement and allow to accurately reproduce the exact spectrum of the continuous Hamiltonian. By combining these results with the numerical calculation of the Chern number, we are able to draw the phase diagram in terms of the physical parameters of the microscopic model. Remarkably, we find that only a small fraction of the original phase diagram of the Haldane model can be accessed, and that the topological insulator phase is suppressed in the deep tight-binding regime.

cond-mat.mes-hall↗

arXiv2014

Fault-tolerant breathing pattern in optical lattices as a dynamical quantum memory

Proposals for quantum information processing often require the development of new quantum tech- nologies. However, here we build quantum memory by ultracold atoms in one-dimensional optical lattices with existing state-of-the-art technology. Under a parabolic external field, we demonstrate that an arbitrary initial state at an end of the optical lattices can time-evolve and revive, with very high fidelity, at predictable discrete time intervals. Physically, the parabolic field, can catalyze a breathing pattern. The initial state is memorized by the pattern and can be retrieved at any of the revival time moments. In comparison with usual time-independent memory, we call this a dynamical memory. Furthermore, we show that the high fidelity of the quantum state at revival time moments is fault-tolerant against the fabrication defects and even time-dependent noise.

quant-ph↗

arXiv2014

One-dimensional s-p superlattice

The physics of one dimensional optical superlattices with resonant $s$-$p$ orbitals is reexamined in the language of appropriate Wannier functions. It is shown that details of the tight binding model realized in different optical potentials crucially depend on the proper determination of Wannier functions. We discuss the properties of a superlattice model which quasi resonantly couples $s$ and $p$ orbitals and show its relation with different tight binding models used in other works.

cond-mat.quant-gas↗

arXiv2014

Breakdown of the Peierls substitution for the Haldane model with ultracold atoms

We present two independent calculations of the tight-binding parameters for a specific realization of the Haldane model with ultracold atoms. The tunneling coefficients up to next-to-nearest neighbors are computed ab-initio by using the maximally localized Wannier functions, and compared to analytical expressions written in terms of gauge invariant, measurable properties of the spectrum. The two approaches present a remarkable agreement and evidence the breakdown of the Peierls substitution: (i) the phase acquired by the next-to-nearest tunneling amplitude $t_{1}$ presents quantitative and qualitative differences with respect to that obtained by the integral of the vector field A, as considered in the Peierls substitution, even in the regime of low amplitudes of A; (ii) for larger values, also $|t_{1}|$ and the nearest-neighbor tunneling $t_{0}$ have a marked dependence on A. The origin of this behavior and its implications are discussed.

cond-mat.quant-gas↗

arXiv2014

On the effective Dirac equation for ultracold atoms in optical lattices: role of the localization properties of the Wannier functions

We review the derivation of the effective Dirac equation for ultracold atoms in one-dimensional bichromatic optical lattices, following the proposal by Witthaut et al. Phys. Rev. A 84, 033601 (2011). We discuss how such a derivation - based on a suitable rotation of the Bloch basis and on a coarse graining approximation - is affected by the choice of the Wannier functions entering the coarsening procedure. We show that in general the Wannier functions obtained by rotating the maximally localized Wannier functions for the original Bloch bands can be sufficiently localized for justifying the coarse graining approximation. We also comment on the relation between the rotation needed to achieve the Dirac form and the standard Foldy-Wouthuysen transformation. Our results provide a solid ground for the interpretation of the experimental results by Salger et al. Phys. Rev. Lett. 107, 240401 (2011) in terms of an effective Dirac dynamics.

cond-mat.quant-gas↗

arXiv2013

Self-consistent tight-binding description of Dirac points moving and merging in two dimensional optical lattices

We present an accurate ab initio tight-binding model, capable of describing the dynamics of Dirac points in tunable honeycomb optical lattices following a recent experimental realization [L. Tarruell et al., Nature 483, 302 (2012)]. Our scheme is based on first-principle maximally localized Wannier functions for composite bands. The tunneling coefficients are calculated for different lattice configurations, and the spectrum properties are well reproduced with high accuracy. In particular, we show which tight binding description is needed in order to accurately reproduce the position of Dirac points and the dispersion law close to their merging, for different laser intensities.

cond-mat.quant-gas↗

arXiv2012

Tight binding models for ultracold atoms in honeycomb optical lattices

We discuss how to construct tight-binding models for ultra cold atoms in honeycomb potentials, by means of the maximally localized Wannier functions (MLWFs) for composite bands introduced by Marzari and Vanderbilt [1]. In particular, we work out the model with up to third-nearest neighbors, and provide explicit calculations of the MLWFs and of the tunneling coefficients for the graphene-lyke potential with two degenerate minima per unit cell. Finally, we discuss the degree of accuracy in reproducing the exact Bloch spectrum of different tight-binding approximations, in a range of typical experimental parameters.

cond-mat.quant-gas↗

arXiv2012

Subdiffusion of nonlinear waves in quasiperiodic potentials

We study the spatio-temporal evolution of wave packets in one-dimensional quasiperiodic lattices which localize linear waves. Nonlinearity (related to two-body interactions) has destructive effect on localization, as recently observed for interacting atomic condensates [Phys. Rev. Lett. 106, 230403 (2011)]. We extend the analysis of the characteristics of the subdiffusive dynamics to large temporal and spatial scales. Our results for the second moment $m_2$ consistently reveal an asymptotic $m_2 \sim t^{1/3}$ and intermediate $m_2 \sim t^{1/2}$ laws. At variance to purely random systems [Europhys. Lett. 91, 30001 (2010)] the fractal gap structure of the linear wave spectrum strongly favors intermediate self-trapping events. Our findings give a new dimension to the theory of wave packet spreading in localizing environments.

cond-mat.dis-nn↗

arXiv2012

Localization in momentum space of ultracold atoms in incommensurate lattices

We characterize the disorder induced localization in momentum space for ultracold atoms in one-dimensional incommensurate lattices, according to the dual Aubry-André model. For low disorder the system is localized in momentum space, and the momentum distribution exhibits time-periodic oscillations of the relative intensity of its components. The behavior of these oscillations is explained by means of a simple three-mode approximation. We predict their frequency and visibility by using typical parameters of feasible experiments. Above the transition the system diffuses in momentum space, and the oscillations vanish when averaged over different realizations, offering a clear signature of the transition.

cond-mat.quant-gas↗

arXiv2012

Fault-Tolerant Exact State Transmission

We show that a category of one-dimensional XY-type models may enable high-fidelity quantum state transmissions, regardless of details of coupling configurations. This observation leads to a fault- tolerant design of a state transmission setup. The setup is fault-tolerant, with specified thresholds, against engineering failures of coupling configurations, fabrication imperfections or defects, and even time-dependent noises. We propose the implementation of the fault-tolerant scheme using hard-core bosons in one-dimensional optical lattices.

quant-ph↗