Relative resilience to noise of standard and sequential approaches to measurement-based quantum computation

A possible alternative to the standard model of measurement-based quantum computation (MBQC) is offered by the sequential model of MBQC -- a particular class of quantum computation via ancillae. Although these two models are equivalent under ideal conditions, their relative resilience to noise in practical conditions is not yet known. We analyze this relationship for various noise models in the ancilla preparation and in the entangling-gate implementation. The comparison of the two models is performed utilizing both the gate infidelity and the diamond distance as figures of merit. Our results show that in the majority of instances the sequential model outperforms the standard one in regard to a universal set of operations for quantum computation. Further investigation is made into the performance of sequential MBQC in experimental scenarios, thus setting benchmarks for possible cavity-QED implementations.Comment: 11 pages, 11 figures; close to published versio

A double molecular disc in the triple-barred starburst galaxy NGC 6946: structure and stability

The late-type spiral galaxy NGC 6946 is a prime example of molecular gas dynamics driven by "bars within bars". Here we use data from the BIMA SONG and HERACLES surveys to analyse the structure and stability of its molecular disc. Our radial profiles exhibit a clear transition at distance R ~ 1 kpc from the galaxy centre. In particular, the surface density profile breaks at R ~ 0.8 kpc and is well fitted by a double exponential distribution with scale lengths R_1 ~ 200 pc and R_2 ~ 3 kpc, while the 1D velocity dispersion sigma decreases steeply in the central kpc and is approximately constant at larger radii. The fact that we derive and use the full radial profile of sigma rather than a constant value is perhaps the most novel feature of our stability analysis. We show that the profile of the Q stability parameter traced by CO emission is remarkably flat and well above unity, while the characteristic instability wavelength exhibits clear signatures of the nuclear starburst and inner bar within bar. We also show that CO-dark molecular gas, stars and other factors can play a significant role in the stability scenario of NGC 6946. Our results provide strong evidence that gravitational instability, radial inflow and disc heating have driven the formation of the inner structures and the dynamics of molecular gas in the central kpc.Comment: MNRAS, in pres

MY LIFE AS TUTOR: Reflections on Two Recent Experiences

In this final report, I briefly reflect on two parallel teaching experiences as tutor. Besides, I briefly view such experiences in interaction with my research work, private life and new teaching position. In harmony with my conception of teaching, I avoid the standard formal style of reports and try an interactive dialogue with the reader.Comment: 9 pages (tex

Astrophysical Dynamics 1999/2000: Merging Research and Education

The workshop `Astrophysical Dynamics 1999/2000' followed a homonymous advanced research course, and both activities were organized by me. In this opening paper of the proceedings book, I describe them and document their strong impact on the academic life of the local institutions. The advanced research course was open to graduate students, senior researchers, and motivated under-graduate students with good background in physics and mathematics. The course covered several multi-disciplinary issues of modern research on astrophysical dynamics, and thus also of interest to physicists, mathematicians and engineers. The major topic was gas dynamics, viewed in context with stellar dynamics and plasma physics. The course was complemented by parallel seminars on hot topics given by experts in such fields, and open to a wide scientific audience. In particular, I gave a friendly introduction to wavelets, which are becoming an increasingly powerful tool not only for processing signals and images but also for analysing fractals and turbulence, and which promise to have important applications to dynamical modelling of disc galaxies. The workshop was open to a wide scientific audience. The workshop with published proceedings book was, as a matter of fact, the innovative form of exam that I proposed for the advanced research course. The contributions were refereed and their quality is high on average, exceptionally high in a few cases. The advanced research course and the workshop all together produced great enthusiasm in the students and welcomed the participation of a hundred different people, which means an order of magnitude more than an average graduate course at Chalmers University of Technology and G\"oteborg University.Comment: opening paper; the proceedings book is in http://www.oso.chalmers.se/~romeo/PROCEEDINGS_BOOK_

Chemodynamic evolution of dwarf galaxies in tidal fields

The mass-metallicity relation shows that the galaxies with the lowest mass have the lowest metallicities. As most dwarf galaxies are in group environments, interaction effects such as tides could contribute to this trend. We perform a series of smoothed particle hydrodynamics (SPH) simulations of dwarf galaxies in external tidal fields to examine the effects of tides on their metallicities and metallicity gradients. In our simulated galaxies, gravitational instabilities drive gas inwards and produce centralized star formation and a significant metallicity gradient. Strong tides can contribute to these instabilities, but their primary effect is to strip the outer low-metallicity gas, producing a truncated gas disk with a large metallicity. This suggests that the role of tides on the mass-metallicity relation is to move dwarf galaxies to higher metallicities.Comment: Accepted to Ap

Characterizing gravitational instability in turbulent multi-component galactic discs

Gravitational instabilities play an important role in galaxy evolution and in shaping the interstellar medium (ISM). The ISM is observed to be highly turbulent, meaning that observables like the gas surface density and velocity dispersion depend on the size of the region over which they are measured. In this work we investigate, using simulations of Milky Way-like disc galaxies with a resolution of $\sim 9$ pc, the nature of turbulence in the ISM and how this affects the gravitational stability of galaxies. By accounting for the measured average turbulent scalings of the density and velocity fields in the stability analysis, we can more robustly characterize the average level of stability of the galaxies as a function of scale, and in a straightforward manner identify scales prone to fragmentation. Furthermore, we find that the stability of a disc with feedback-driven turbulence can be well described by a "Toomre-like" $Q$ stability criterion on all scales, whereas the classical $Q$ can formally lose its meaning on small scales if violent disc instabilities occur in models lacking pressure support from stellar feedback.Comment: 11 pages, 5 figures, submitted to MNRA

Reply to Melott's Comment on ``Discreteness Effects in Lambda Cold Dark Matter Simulations: A Wavelet-Statistical View'' by Romeo et al

Melott has made pioneering studies of the effects of particle discreteness in N-body simulations, a fundamental point that needs careful thought and analysis since all such simulations suffer from numerical noise arising from the use of finite-mass particles. Melott (arXiv:0804.0589) claims that the conclusions of our paper (arXiv:0804.0294) are essentially equivalent to those of his earlier work. Melott is wrong: he has jumped onto one of our conclusions and interpreted that in his own way. Here we point out the whys and the wherefores

Characterizing and modeling the dynamics of online popularity

Online popularity has enormous impact on opinions, culture, policy, and profits. We provide a quantitative, large scale, temporal analysis of the dynamics of online content popularity in two massive model systems, the Wikipedia and an entire country's Web space. We find that the dynamics of popularity are characterized by bursts, displaying characteristic features of critical systems such as fat-tailed distributions of magnitude and inter-event time. We propose a minimal model combining the classic preferential popularity increase mechanism with the occurrence of random popularity shifts due to exogenous factors. The model recovers the critical features observed in the empirical analysis of the systems analyzed here, highlighting the key factors needed in the description of popularity dynamics.Comment: 5 pages, 4 figures. Modeling part detailed. Final version published in Physical Review Letter

On totally geodesic submanifolds in the Jacobian locus

We study submanifolds of A_g that are totally geodesic for the locally symmetric metric and which are contained in the closure of the Jacobian locus but not in its boundary. In the first section we recall a formula for the second fundamental form of the period map due to Pirola, Tortora and the first author. We show that this result can be stated quite neatly using a line bundle over the product of the curve with itself. We give an upper bound for the dimension of a germ of a totally geodesic submanifold passing through [C] in M_g in terms of the gonality of C. This yields an upper bound for the dimension of a germ of a totally geodesic submanifold contained in the Jacobian locus, which only depends on the genus. We also study the submanifolds of A_g obtained from cyclic covers of the projective line. These have been studied by various authors. Moonen determined which of them are Shimura varieties using deep results in positive characteristic. Using our methods we show that many of the submanifolds which are not Shimura varieties are not even totally geodesic.Comment: To appear on International Journal of Mathematic