arXiv subjects

Alex Zucca

Publications and source records attributed to Alex Zucca.

13 recordsLinked to original sources

arXiv2022

Quantum critical dynamics in a 5000-qubit programmable spin glass

Experiments on disordered alloys suggest that spin glasses can be brought into low-energy states faster by annealing quantum fluctuations than by conventional thermal annealing. Due to the importance of spin glasses as a paradigmatic computational testbed, reproducing this phenomenon in a programmable system has remained a central challenge in quantum optimization. Here we achieve this goal by realizing quantum critical spin-glass dynamics on thousands of qubits with a superconducting quantum annealer. We first demonstrate quantitative agreement between quantum annealing and time-evolution of the Schr\"odinger equation in small spin glasses. We then measure dynamics in 3D spin glasses on thousands of qubits, where simulation of many-body quantum dynamics is intractable. We extract critical exponents that clearly distinguish quantum annealing from the slower stochastic dynamics of analogous Monte Carlo algorithms, providing both theoretical and experimental support for a scaling advantage in reducing energy as a function of annealing time.

quant-ph↗

arXiv2022

Coherent quantum annealing in a programmable 2000-qubit Ising chain

Quantum simulation has emerged as a valuable arena for demonstrating and understanding the capabilities of near-term quantum computers. Quantum annealing has been used successfully in simulating a range of open quantum systems, both at equilibrium and out of equilibrium. However, in all previous experiments, annealing has been too slow to simulate a closed quantum system coherently, due to the onset of thermal effects from the environment. Here we demonstrate coherent evolution through a quantum phase transition in the paradigmatic setting of the 1D transverse-field Ising chain, using up to 2000 superconducting flux qubits in a programmable quantum annealer. In large systems we observe the quantum Kibble-Zurek mechanism with theoretically predicted kink statistics, as well as characteristic positive kink-kink correlations, independent of system temperature. In small chains, excitation statistics validate the picture of a Landau-Zener transition at a minimum gap. In both cases, results are in quantitative agreement with analytical solutions to the closed-system quantum model. For slower anneals we observe anti-Kibble-Zurek scaling in a crossover to the open quantum regime. These experiments demonstrate that large-scale quantum annealers can be operated coherently, paving the way to exploiting coherent dynamics in quantum optimization, machine learning, and simulation tasks.

quant-ph↗

arXiv2021

Diversity metric for evaluation of quantum annealing

Solving discrete NP-hard problems is an important part of scientific discoveries and operations research as well as many commercial applications. A commonly used metric to compare meta-heuristic solvers is the time required to obtain an optimal solution, known as time to solution. However, for some applications it is desirable to have a set of high-quality and diverse solutions, instead of a single optimal one. For these applications, time to solution may not be informative of the performance of a solver, and another metric would be necessary. In particular, it is not known how well quantum solvers sample the configuration space in comparison to their classical counterparts. Here, we apply a recently introduced collective distance measure in solution space to quantify diversity by Mohseni et. al. and, based on that, we employ time-to-diversity as a metric for evaluation of meta-heuristics solvers. We use this measure to compare the performance of the D-Wave quantum annealing processor with a few classical heuristic solvers on a set of synthetic problems and show that D-Wave quantum annealing processor is indeed a competitive heuristic, and on many instances outperforms state-of-the-art classical solvers, while it remains on par on other instances. This suggests that a portfolio solver that combines quantum and classical solutions may win over all solvers.

quant-ph↗

arXiv2021

Imprints of cosmological tensions in reconstructed gravity

There has been a significant interest in modifications of the standard $\Lambda$ Cold Dark Matter ($\Lambda$CDM) cosmological model prompted by tensions between certain datasets, most notably the Hubble tension. The late-time modifications of the $\Lambda$CDM model can be parametrized by three time-dependent functions describing the expansion history of the Universe and gravitational effects on light and matter in the Large Scale Structure. We perform the first joint Bayesian reconstruction of these three functions from a combination of recent cosmological observations, utilizing a theory-informed prior built on the general Horndeski class of scalar-tensor theories. This reconstruction is interpreted in light of the well-known $H_0$, the $S_8$ and the $A_L$ tensions. We identify the phenomenological features that alternative theories would need to have in order to ease some of the tensions, and deduce important constraints on broad classes of modified gravity models. Among other things, our findings suggest that late-time dynamical dark energy and modifications of gravity are not likely to offer a solution to the Hubble tension, or simultaneously solve the $A_L$ and $S_8$ tensions.

astro-ph.CO↗

arXiv2020

Improving nonstoquastic quantum annealing with spin-reversal transformations

Nonstoquastic Hamiltonians are hard to simulate due to the sign problem in quantum Monte Carlo simulation. It is however unclear whether nonstoquasticity can lead to advantage in quantum annealing. Here we show that YY-interaction between the qubits makes the adiabatic path during quantum annealing, and therefore the performance, dependent on spin-reversal transformations. With the right choice of spin-reversal transformation, a nonstoquastic Hamiltonian with YY-interaction can outperform stoquastic Hamiltonians with similar parameters. We introduce an optimization protocol to determine the optimal transformation and discuss the effect of suboptimality.

quant-ph↗

arXiv2019

Generalized Brans-Dicke theories in light of evolving dark energy

The expansion history of the Universe reconstructed from a combination of recent data indicates a preference for a changing Dark Energy (DE) density. Moreover, the DE density appears to be increasing with cosmic time, with its equation of state being below -1 on average, and possibly crossing the so-called phantom divide. Scalar-tensor theories, in which the scalar field mediates a force between matter particles, offer a natural framework in which the effective DE equation of state can be less than -1 and cross the phantom barrier. We consider the generalized Brans-Dicke (GBD) class of scalar-tensor theories and reconstruct their Lagrangian given the effective DE density extracted from recent data. Then, given the reconstructed Lagrangian, we solve for the linear perturbations and investigate the characteristic signatures of these reconstructed GBD in the cosmological observables, such as the cosmic microwave background (CMB) anisotropy, the galaxy number counts, and their cross-correlations. In particular, we demonstrate that the Integrated Sachs-Wolfe (ISW) effect probed by the cross-correlation of CMB with the matter distribution can rule out scalar-tensor theories as the explanation of the observed DE dynamics independently from the laboratory and solar system fifth force constraints.

astro-ph.CO↗

arXiv2019

Observational Constraints on Constant Roll Inflation

Constant-roll inflation was recently introduced by Motohashi, Starobinsky and Yokoyama as a phenomenological way to parametrize deviations from the slow-roll scenarios. In this paper, we investigate the dynamics of both the background and the perturbations in this model, without making any slow-roll assumptions. The perturbation spectra are computed with an efficient and accurate novel method that allowed us to quickly scan the parameter space of constant-roll inflation. We derive the constraints on the model parameters from the cosmic microwave background anisotropy measurements provided by the joint analysis of the Planck Collaboration and the BICEP2/Keck Array data.

astro-ph.CO↗

arXiv2019

MGCAMB with massive neutrinos and dynamical dark energy

We present a major upgrade of MGCAMB, a patch for the Einstein-Boltzmann solver CAMB used for phenomenological tests of general relativity against cosmological datasets. This new version is compatible with the latest CosmoMC code and includes a consistent implementation of massive neutrinos and dynamical dark energy. The code has been restructured to make it easier to modify to fit the custom needs of specific users. To illustrate the capabilities of the code, we present joint constraints on the modified growth, massive neutrinos and the dark energy equation of state from the latest cosmological observations, including the recent galaxy counts and weak lensing measurements from the Dark Energy Survey, and find a good consistency with the ΛCDM model.

astro-ph.CO↗

arXiv2018

Evolution of dark energy reconstructed from the latest observations

We reconstruct evolution of the dark energy (DE) density using a nonparametric Bayesian approach from a combination of latest observational data. We caution against parameterizing DE in terms of its equation of state as it can be singular in modified gravity models, and using it introduces a bias preventing negative effective DE densities. We find a $3.7σ$ preference for an evolving effective DE density with interesting features. For example, it oscillates around the $Λ$CDM prediction at $z\lesssim0.7$, and could be negative at $z\gtrsim2.3$; dark energy can be pressure-less at multiple redshifts, and a short period of cosmic deceleration is allowed at $0.1 \lesssim z\lesssim 0.2$. We perform the reconstruction for several choices of the prior, as well as a evidence-weighted reconstruction. We find that some of the dynamical features, such as the oscillatory behaviour of the DE density, are supported by the Bayesian evidence, which is a first detection of a dynamical DE with a positive Bayesian evidence. The evidence-weighted reconstruction prefers a dynamical DE at a $(2.5\pm0.06)σ$ significance level.

astro-ph.CO↗

arXiv2018

Searching for Primordial Magnetic Fields with CMB B-modes

Was the primordial universe magnetized? The answer to this question would help explain the origin of micro-Gauss strength magnetic fields observed in galaxies. It is also of fundamental importance in developing a complete theory of the early universe. While there can be other signatures of cosmological magnetic fields, a signature in the cosmic microwave background (CMB) would prove their primordial origin. The B-mode polarization of CMB is particularly promising in this regard because there are relatively few other sources of B-modes, and because the vortical modes sourced by the primordial magnetic field (PMF) survive diffusion damping up to a small fraction of the Silk length. At present, the Planck temperature and polarization spectra combined with the B-mode spectrum measured by the South Pole Telescope (SPT) constrain the PMF strength to be no more than $\sim 1$ nano-Gauss (nG). Because of the quartic scaling of the CMB anisotropy spectra with the PMF strength, this bound will not change by much even with the significantly better measurements of the B-mode spectrum by the Stage III and Stage IV CMB experiments. On the other hand, tightening the bound well below the $1$ nG threshold would rule out the purely primordial origin (requiring no dynamo action) of galactic fields. Considering Faraday rotation, which converts some of the E-modes into B-modes and scales linearly with the field strength, will help to achieve this goal. As we demonstrate, the upcoming experiments, such as SPT-3G and the Simons Observatory, will be sensitive to fields of $\sim 0.5$ nG strength thanks to the mode-coupling signature induced by Faraday rotation. A future Stage IV ground based experiment or a Space Probe will be capable of probing fields below $0.1$ nG, and would detect a scale-invariant PMF of $0.2$ nG strength without de-lensing or subtracting the galactic rotation measure.

astro-ph.CO↗

arXiv2017

Unscreening scalarons with a black hole

It is typically believed that the additional degrees of freedom in any modification of gravity are completely suppressed by the large energy densities coexisting with an astrophysical black hole. In this paper, we find that this might not always be the case. This belief holds for black holes formed via gravitational collapse in very dense environments, whereas the black holes with sufficiently low accretion rates that have low matter densities inside innermost stable circular orbit will generally unscreen chameleons. We develop a novel technique to study the dynamics of accretion of a scalar field onto a Schwarzschild-like black hole which is accurate on both short and long time scales. In particular, we study the behavior of the extra scalar degree of freedom in the Starobinsky and Hu-Sawicki $f(R)$ theories, for the symmetron model, and for the Ratra-Peebles model. Aside from calculating non-trivial static field profiles outside the black hole, we provide the tools to study the (in)stability and evolution towards the equilibrium solution for any generic well behaved set of parameters and initial conditions. Our code is made publicly available for further research and modifications to study other models.

gr-qc↗

arXiv2016

Constraints on Primordial Magnetic Fields from Planck combined with the South Pole Telescope CMB B-mode polarization measurements

A primordial magnetic field (PMF) present before recombination can leave specific signatures on the cosmic microwave background (CMB) fluctuations. Of particular importance is its contribution to the B-mode polarization power spectrum. Indeed, vortical modes sourced by the PMF can dominate the B-mode power spectrum on small scales, as they survive damping up to a small fraction of the Silk length. Therefore, measurements of the B-mode polarization at high-$\ell$ , such as the one recently performed by the South Pole Telescope (SPT), have the potential to provide stringent constraints on the PMF. We use the publicly released SPT B-mode polarization spectrum, along with the temperature and polarization data from the Planck satellite, to derive constraints on the magnitude, the spectral index and the energy scale at which the PMF was generated. We find that, while Planck data constrains the magnetic amplitude to $B_{1 \, \text{Mpc}} < 3.3$ nG at 95\% confidence level (CL), the SPT measurement improves the constraint to $B_{1 \, \text{Mpc}} < 1.5$ nG. The magnetic spectral index, $n_B$, and the time of the generation of the PMF are unconstrained. For a nearly scale-invariant PMF, predicted by simplest inflationary magnetogenesis models, the bound from Planck+SPT is $B_{1 \, \text{Mpc}} < 1.2$ nG at 95% CL. For PMF with $n_B=2$, expected for fields generated in post-inflationary phase transitions, the 95% CL bound is $B_{1 \, \text{Mpc}} < 0.002$ nG, corresponding to the magnetic fraction of the radiation density $Ω_{Bγ} < 10^{-3}$ or the effective field $B_{\rm eff} < 100$ nG. The patches for the Boltzmann code CAMB and the Markov Chain Monte Carlo engine CosmoMC, incorporating the PMF effects on CMB, are made publicly available.

astro-ph.CO↗

arXiv2015

Searching for scalar gravitational interactions in current and future cosmological data

Modified gravity theories often contain a scalar field of gravitational strength which interacts with matter. We examine constraints on the range and the coupling strength of a scalar gravitational degree of freedom using a subset of current data that can be safely analyzed within the linear perturbation theory. Using a model-independent implementation of scalar-tensor theories in MGCAMB in terms of two functions of the scale factor describing the mass and the coupling of the scalar degree of freedom, we derive constraints on the $f(R)$, generalized chameleon, Symmetron and Dilaton models. Since most of the large scale structure data available today is from relatively low redshifts, only a limited range of observed scales is in the linear regime, leading to relatively weak constraints. We then perform a forecast for a future large scale structure survey, such as Large Synoptic Survey Telescope (LSST), which will map a significant volume at higher redshifts, and show that it will produce much stronger constraints on scalar interactions in specific models. We also perform a principal component analysis and find that future surveys should be able to provide tight constraints on several eigenmodes of the scalar mass evolution.

astro-ph.CO↗