Crystallization in Ising quantum magnets Schauß, P.; Zeiher, J.; Fukuhara, T. ...
Science (American Association for the Advancement of Science),
03/2015, Letnik:
347, Številka:
6229
Journal Article
Recenzirano
Dominating finite-range interactions in many-body systems can lead to intriguing self-ordered phases of matter. For quantum magnets, Ising models with power-law interactions are among the most ...elementary systems that support such phases. These models can be implemented by laser coupling ensembles of ultracold atoms to Rydberg states. Here, we report on the experimental preparation of crystalline ground states of such spin systems. We observe a magnetization staircase as a function of the system size and show directly the emergence of crystalline states with vanishing susceptibility. Our results demonstrate the precise control of Rydberg many-body systems and may enable future studies of phase transitions and quantum correlations in interacting quantum magnets.
More than 40 years ago, Andreev, Lifshitz and Chester suggested the possible existence of a peculiar solid phase of matter, the microscopic constituents of which can flow superfluidly without ...resistance due to the formation of zero-point defects in the ground state of self-assembled crystals. Yet, a physical system where this mechanism is unambiguously established remains to be found, both experimentally and theoretically. Here we investigate the zero-temperature phase diagram of two-dimensional bosons with finite-range soft-core interactions. For low particle densities, the system is shown to feature a solid phase in which zero-point vacancies emerge spontaneously and give rise to superfluid flow of particles through the crystal. This provides the first example of defect-induced, continuous-space supersolidity consistent with the Andreev-Lifshitz-Chester scenario.
In this paper, a joint segmentation-based classification technique is proposed for fully polarimetric SAR images of suburban areas. This is based on the joint use of the
H/
A/
α polarimetric ...decomposition and multivariate annealed segmentation. The introduction of a segmentation stage before classification allows the exploitation of the information available in the noisy estimate of the anisotropy parameter, retaining at the same time the highest possible spatial resolution. Specifically, in the joint segmentation of entropy, angle and anisotropy images, the latter is exploited when it provides a significant contribution, but the process is mainly driven by the more stable parameters (entropy and angle). When jointly classifying all the pixels belonging to the identified homogeneous segment, the anisotropy parameter can be effectively exploited without introducing any additional noise in the classification output. The impact of the exploitation of the anisotropy parameter is investigated quantitatively through the application of the proposed technique to a AIRSAR C- and L-band image of suburban areas. To characterize the limits that can be achieved, both lower and upper bounds to classification performance are introduced, corresponding respectively to pixel-based classification and joint classification of all the pixels in the regions defined by the ground truth. Results show that a significant performance improvement can be achieved by classifying the homogeneous regions identified by segmentation, instead of single pixels or even small windows of 3×3 pixels. Moreover, it is shown that the exploitation of the anisotropy parameter allows a better classification accuracy, and in particular a better discrimination of built-up areas from other classes.
We investigate the quench dynamics of a two-component Bose mixture and study the onset of modulational instability, which leads the system far from equilibrium. Analogous to the single-component ...counterpart, this phenomenon results in the creation of trains of bright solitons. We provide an analytical estimate of the number of solitons at long times after the quench for each of the two components based on the most unstable mode of the Bogoliubov spectrum, which agrees well with our simulations for quenches to the weak attractive regime when the two components possess equal intraspecies interactions and loss rates. We also explain the significantly different soliton dynamics in a realistic experimental homonuclear potassium mixture in terms of different intraspecies interaction and loss rates. We investigate the quench dynamics of the particle number of each component estimating the characteristic time for the appearance of modulational instability for a variety of interaction strengths and loss rates. Finally, we evaluate the influence of the beyond-mean-field contribution, which is crucial for the ground-state properties of the mixture, in the quench dynamics for both the evolution of the particle number and the radial width of the mixture. In particular, even for quenches to strongly attractive effective interactions, we do not observe the dynamical formation of solitonic droplets.
Experimental platforms based on trapped ions, cold molecules, and Rydberg
atoms have made possible the investigation of highly-nonlocal spin-${1/2}$
Hamiltonians with long-range couplings. Here, we ...study the effects of such
non-local couplings in the long-range spin-${1/2}$ XXZ Heisenberg Hamiltonian.
We calculate explicitly the two-spin energy spectrum, which describes all
possible energetic configurations of two spins pointing in a specific direction
embedded in a background of spins with opposite orientation. For fast decay of
the spin-spin couplings, we find that the two-spin energy spectrum is
characterized by well-defined discrete values, corresponding to bound states,
separated by a set of continuum states describing the scattering region. In the
deep long-range regime instead, the bound states disappear as they get
incorporated by the scattering region. The presence of two-spin bound states
results to be crucial to determine both two- and many-spin dynamics. On one
hand, radically different two-spin spreadings can be observed by tuning the
decay of the spin couplings. On the other hand, two-spin bound states enable
the dynamical stabilization of effective antiferromagnetic states in the
presence of ferromagnetic couplings. Finally, we propose a novel scheme based
on a trapped-ion quantum simulator to experimentally realize the long-range XXZ
model and to study its out-of-equilibrium properties.
We study the emergence of quasicrystal configurations produced purely by quantum fluctuations in the ground-state phase diagram of interacting bosonic systems. By using a variational mean-field ...approach, we determine the relevant features of the pair interaction potential that stabilize such quasicrystalline states in two dimensions. Unlike their classical counterpart, in which the interplay between only two wave vectors determines the resulting symmetries of the solutions, the quantum picture relates in a more complex way to the instabilities of the excitation spectrum. Moreover, the quantum quasicrystal patterns are found to emerge as the ground state with no need of moderate thermal fluctuations. The study extends to the exploration of the excitation properties and the possible existence of super-quasicrystals, i.e. supersolid-like quasicrystalline states in which the long-range non-periodic density profile coexist with a non-zero superfluid fraction. Our calculations show that, in an intermediate region between the homogeneous superfluid and the normal quasicrystal phases, these exotic states indeed exist at zero temperature. Comparison with full numerical simulations provides a solid verification of the variational approach adopted in this work.
Deals with multitemporal sequences of synthetic aperture radar (SAR) images with regions possibly affected by step reflectivity patterns of change. Specifically, it addresses the problems of ...detecting a temporal step pattern with small reflectivity change against a constant pattern and of estimating the transition instant for the step. The statistically optimized signal processing techniques proposed in this work are most appropriate for the new generation of SAR systems with high revisit time that are currently under development. We propose two different techniques, based on the maximum likelihood (ML) approach, that make different use of prior knowledge on the radar cross section (RCS) levels of the searched pattern. They process the whole sequence to achieve optimal discrimination capability between regions affected and not affected by a step change and optimal estimation accuracy for the step transition instant. The first technique (known step pattern (KSP)-detector) assumes complete knowledge of the RCS levels of the searched pattern of change, while the second one (USP-detector) is based on the assumption of totally unknown step pattern.
PDF
