271 research outputs found
Lattice gauge theories simulations in the quantum information era
The many-body problem is ubiquitous in the theoretical description of
physical phenomena, ranging from the behavior of elementary particles to the
physics of electrons in solids. Most of our understanding of many-body systems
comes from analyzing the symmetry properties of Hamiltonian and states: the
most striking example are gauge theories such as quantum electrodynamics, where
a local symmetry strongly constrains the microscopic dynamics. The physics of
such gauge theories is relevant for the understanding of a diverse set of
systems, including frustrated quantum magnets and the collective dynamics of
elementary particles within the standard model. In the last few years, several
approaches have been put forward to tackle the complex dynamics of gauge
theories using quantum information concepts. In particular, quantum simulation
platforms have been put forward for the realization of synthetic gauge
theories, and novel classical simulation algorithms based on quantum
information concepts have been formulated. In this review we present an
introduction to these approaches, illustrating the basics concepts and
highlighting the connections between apparently very different fields, and
report the recent developments in this new thriving field of research.Comment: Pedagogical review article. Originally submitted to Contemporary
Physics, the final version will appear soon on the on-line version of the
journal. 34 page
Persistent Contextual Values as Inter-Process Layers
Mobile applications today often fail to be context aware when they also need
to be customizable and efficient at run-time. Context-oriented programming
allows programmers to develop applications that are more context aware. Its
central construct, the so-called layer, however, is not customizable. We
propose to use novel persistent contextual values for mobile development.
Persistent contextual values automatically adapt their value to the context.
Furthermore they provide access without overhead. Key-value configuration files
contain the specification of contextual values and the persisted contextual
values themselves. By modifying the configuration files, the contextual values
can easily be customized for every context. From the specification, we generate
code to simplify development. Our implementation, called Elektra, permits
development in several languages including C++ and Java. In a benchmark we
compare layer activations between threads and between applications. In a case
study involving a web-server on a mobile embedded device the performance
overhead is minimal, even with many context switches.Comment: 8 pages Mobile! 16, October 31, 2016, Amsterdam, Netherland
Quantum optimal control within the rotating wave approximation
We study the interplay between rotating wave approximation and optimal
control. In particular, we show that for a wide class of optimal control
problems one can choose the control field such that the Hamiltonian becomes
time-independent under the rotating wave approximation. Thus, we show how to
recast the functional minimization defined by the optimal control problem into
a simpler multi-variable function minimization. We provide the analytic
solution to the state-to-state transfer of the paradigmatic two-level system
and to the more general star configuration of an -level system. We
demonstrate numerically the usefulness of this approach in the more general
class of connected acyclic -level systems with random spectra. Finally, we
use it to design a protocol to entangle Rydberg via constant laser pulses atoms
in an experimentally relevant range of parameters.Comment: 8 pages, 5 figure
Dressing the chopped-random-basis optimization: a bandwidth-limited access to the trap-free landscape
In quantum optimal control theory the success of an optimization algorithm is
highly influenced by how the figure of merit to be optimized behaves as a
function of the control field, i.e. by the control landscape. Constraints on
the control field introduce local minima in the landscape --false traps-- which
might prevent an efficient solution of the optimal control problem. Rabitz et
al. [Science 303, 1998 (2004)] showed that local minima occur only rarely for
unconstrained optimization. Here, we extend this result to the case of
bandwidth-limited control pulses showing that in this case one can eliminate
the false traps arising from the constraint. Based on this theoretical
understanding, we modify the Chopped Random Basis (CRAB) optimal control
algorithm and show that this development exploits the advantages of both
(unconstrained) gradient algorithms and of truncated basis methods, allowing to
always follow the gradient of the unconstrained landscape by bandwidth-limited
control functions. We study the effects of additional constraints and show that
for reasonable constraints the convergence properties are still maintained.
Finally, we numerically show that this approach saturates the theoretical bound
on the minimal bandwidth of the control needed to optimally drive the system.Comment: 8 pages, 6 figure
Density Matrix Renormalization Group for Dummies
We describe the Density Matrix Renormalization Group algorithms for time
dependent and time independent Hamiltonians. This paper is a brief but
comprehensive introduction to the subject for anyone willing to enter in the
field or write the program source code from scratch.Comment: 29 pages, 9 figures. Published version. An open source version of the
code can be found at http://qti.sns.it/dmrg/phome.htm
Tensor networks for Lattice Gauge Theories and Atomic Quantum Simulation
We show that gauge invariant quantum link models, Abelian and non-Abelian,
can be exactly described in terms of tensor networks states. Quantum link
models represent an ideal bridge between high-energy to cold atom physics, as
they can be used in cold-atoms in optical lattices to study lattice gauge
theories. In this framework, we characterize the phase diagram of a (1+1)-d
quantum link version of the Schwinger model in an external classical background
electric field: the quantum phase transition from a charge and parity ordered
phase with non-zero electric flux to a disordered one with a net zero electric
flux configuration is described by the Ising universality class.Comment: 9 pages, 9 figures. Published versio
Real-time Dynamics in U(1) Lattice Gauge Theories with Tensor Networks
Tensor network algorithms provide a suitable route for tackling real-time
dependent problems in lattice gauge theories, enabling the investigation of
out-of-equilibrium dynamics. We analyze a U(1) lattice gauge theory in (1+1)
dimensions in the presence of dynamical matter for different mass and electric
field couplings, a theory akin to quantum-electrodynamics in one-dimension,
which displays string-breaking: the confining string between charges can
spontaneously break during quench experiments, giving rise to charge-anticharge
pairs according to the Schwinger mechanism. We study the real-time spreading of
excitations in the system by means of electric field and particle fluctuations:
we determine a dynamical state diagram for string breaking and quantitatively
evaluate the time-scales for mass production. We also show that the time
evolution of the quantum correlations can be detected via bipartite von Neumann
entropies, thus demonstrating that the Schwinger mechanism is tightly linked to
entanglement spreading. To present the variety of possible applications of this
simulation platform, we show how one could follow the real-time scattering
processes between mesons and the creation of entanglement during scattering
processes. Finally, we test the quality of quantum simulations of these
dynamics, quantifying the role of possible imperfections in cold atoms, trapped
ions, and superconducting circuit systems. Our results demonstrate how
entanglement properties can be used to deepen our understanding of basic
phenomena in the real-time dynamics of gauge theories such as string breaking
and collisions.Comment: 15 pages, 25 figures. Published versio
Dynamically localized systems: entanglement exponential sensitivity and efficient quantum simulations
We study the pairwise entanglement present in a quantum computer that
simulates a dynamically localized system. We show that the concurrence is
exponentially sensitive to changes in the Hamiltonian of the simulated system.
Moreover, concurrence is exponentially sensitive to the ``logic'' position of
the qubits chosen. These sensitivities could be experimentally checked
efficiently by means of quantum simulations with less than ten qubits. We also
show that the feasibility of efficient quantum simulations is deeply connected
to the dynamical regime of the simulated system.Comment: 5 pages, 6 figure
Room temperature Rydberg Single Photon Source
We present an optimal protocol to implement a room temperature Rydberg single
photon source within an experimental setup based on micro cells filled with
thermal vapor. The optimization of a pulsed four wave mixing scheme allows to
double the effective Rydberg blockade radius as compared to a simple Gaussian
pulse scheme, releasing some of the constrains on the geometry of the micro
cells. The performance of the optimized protocol is improved by about 70% with
respect to the standard protocol.Comment: 5 pages, 6 figure
Optimal Phonon-to-Spin Mapping in a system of a trapped ion
We propose a protocol for measurement of the phonon number distribution of a
harmonic oscillator based on selective mapping to a discrete spin-1/2 degree of
freedom. We consider a system of a harmonically trapped ion, where a transition
between two long lived states can be driven with resolved motional sidebands.
The required unitary transforms are generated by amplitude-modulated
polychromatic radiation fields, where the time-domain ramps are obtained from
numerical optimization by application of the Chopped RAndom Basis (CRAB)
algorithm. We provide a detailed analysis of the scaling behavior of the
attainable fidelities and required times for the mapping transform with respect
to the size of the Hilbert space. As one application we show how the mapping
can be employed as a building block for experiments which require measurement
of the work distribution of a quantum process
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