Critical behavior of dissipative two-dimensional spin lattices

We explore critical properties of two-dimensional lattices of spins interacting via an anisotropic Heisenberg Hamiltonian and subject to incoherent spin flips. We determine the steady-state solution of the master equation for the density matrix via the corner-space renormalization method. We investigate the finite-size scaling and critical exponent of the magnetic linear susceptibility associated to a dissipative ferromagnetic transition. We show that the Von Neumann entropy increases across the critical point, revealing a strongly mixed character of the ferromagnetic phase. Entanglement is witnessed by the quantum Fisher information which exhibits a critical behavior at the transition point, showing that quantum correlations play a crucial role in the transition even though the system is in a mixed state.Comment: Accepted for publication on Phys. Rev. B (6 pages, 5 figures

High-order Time Expansion Path Integral Ground State

The feasibility of path integral Monte Carlo ground state calculations with very few beads using a high-order short-time Green's function expansion is discussed. An explicit expression of the evolution operator which provides dramatic enhancements in the quality of ground-state wave-functions is examined. The efficiency of the method makes possible to remove the trial wave function and thus obtain completely model-independent results still with a very small number of beads. If a single iteration of the method is used to improve a given model wave function, the result is invariably a shadow-type wave function, whose precise content is provided by the high-order algorithm employed.Comment: 4 page

Il buon andamento della PA al “tempo degli ossimori”: “proroga e revisione delle procedure”

La Nota, muovendo dal principio del buon andamento nell'ottica dell'amministrazione "di risultato", si sofferma su alcune questioni problematiche offerte da decisioni amministrative che appaiono non rispondenti a coerenza sistematica

Condensate fraction in liquid 4He at zero temperature

We present results of the one-body density matrix (OBDM) and the condensate fraction n_0 of liquid 4He calculated at zero temperature by means of the Path Integral Ground State Monte Carlo method. This technique allows to generate a highly accurate approximation for the ground state wave function Psi_0 in a totally model-independent way, that depends only on the Hamiltonian of the system and on the symmetry properties of Psi_0. With this unbiased estimation of the OBDM, we obtain precise results for the condensate fraction n_0 and the kinetic energy K of the system. The dependence of n_0 with the pressure shows an excellent agreement of our results with recent experimental measurements. Above the melting pressure, overpressurized liquid 4He shows a small condensate fraction that has dropped to 0.8% at the highest pressure of p = 87 bar.Comment: 12 pages. 4 figures. Accepted for publication on "Journal of Low Temperature Physics

Superfluidity of metastable bulk glass para-hydrogen at low temperature

Molecular para-hydrogen has been proposed theoretically as a possible candidate for superfluidity, but the eventual superfluid transition is hindered by its crystallization. In this work, we study a metastable non crystalline phase of bulk p-H2 by means of the Path Integral Monte Carlo method in order to investigate at which temperature this system can support superfluidity. By choosing accurately the initial configuration and using a non commensurate simulation box, we have been able to frustrate the formation of the crystal in the simulated system and to calculate the temperature dependence of the one-body density matrix and of the superfluid fraction. We observe a transition to a superfluid phase at temperatures around 1 K. The limit of zero temperature is also studied using the diffusion Monte Carlo method. Results for the energy, condensate fraction, and structure of the metastable liquid phase at T=0 are reported and compared with the ones obtained for the stable solid phase.Comment: 10 pages, accepted for publication in Phys. Rev.

Can the Mystery of the Speed of Light be Unveiled by Kinetic Waves?

An in-depth analysis of the dynamics connected to the Doppler-effect brings clear light to elements of contradiction with the original ground on which the axiom of the constancy of lightspeed was based. Thereby, with regard to electromagnetic phenomenology, the duality waves/particles and the wavy dynamic of light-propagation suggest the existence of a natural kind of waves, which differently from the classic ones, are originating by kinetic thrust and propagating, also though vacuum, by inertial force. The model taken into consideration, to which has been given the name of “kinetic waves” is, like the classic one, a concretely existing natural phenomenon which can also be visually perceived if produced on molecular scale. Moreover, kinetic waves seem to offer many more points of similarity, in dynamic and behavior, with the electromagnetic waves, than the classic model. Applying the obtained results to astrophysical field, taking as example the quasar 3C-273, the recently found, most far galaxy GN-z11 and the galaxy NGC 224 (better known as Andromeda), can mathematically and concretely be sustained that none of the energy sources we optically perceive, showing a Doppler-shift, is regressing nor approaching. In the appendix, a suggested and accurately described experiment on base of Radar Astronomy to possibly confirm the validity of the model presented by this article

Quantum Phases of Attractive Matter Waves in a Toroidal Trap

Investigating the quantum phase transition in a ring from a uniform attractive Bose-Einstein condensate to a localized bright soliton we find that the soliton undergoes transverse collapse at a critical interaction strength, which depends on the ring dimensions. In addition, we predict the existence of other soliton configurations with many peaks, showing that they have a limited stability domain. Finally, we show that the phase diagram displays several new features when the toroidal trap is set in rotation.Comment: 6 pages, 5 figures. To be published in Phys. Rev.

Efficient Quantum Tensor Product Expanders and k-designs

Quantum expanders are a quantum analogue of expanders, and k-tensor product expanders are a generalisation to graphs that randomise k correlated walkers. Here we give an efficient construction of constant-degree, constant-gap quantum k-tensor product expanders. The key ingredients are an efficient classical tensor product expander and the quantum Fourier transform. Our construction works whenever k=O(n/log n), where n is the number of qubits. An immediate corollary of this result is an efficient construction of an approximate unitary k-design, which is a quantum analogue of an approximate k-wise independent function, on n qubits for any k=O(n/log n). Previously, no efficient constructions were known for k>2, while state designs, of which unitary designs are a generalisation, were constructed efficiently in [Ambainis, Emerson 2007].Comment: 16 pages, typo in references fixe

Definition of a shortcut methodology for assessing flood-related Na-Tech risk

Abstract. In this paper a qualitative methodology for the initial assessment of flood-related Na-Tech risk was developed as a screening tool to identify which situations require a much more expensive quantitative risk analysis (QRA). Through the definition of some suitable key hazard indicators (KHIs), the proposed methodology allows the identification of the Na-Tech risk level associated with a given situation; the analytical hierarchy process (AHP) was used as a multi-criteria decision tool for the evaluation of such qualitative KHIs. The developed methodology was validated through two case studies by comparing the predicted risk levels with the results of much more detailed QRAs previously presented in literature and then applied to the real flood happened at Spolana a.s., Neratovice, Czech Republic in August 2002.</p