143 research outputs found
Quantum nondemolition measurements on two-level atomic systems and temporal Bell inequalities
The evolution of a two-level system subjected to stimulated transitions which
is undergoing a sequence of measurements of the level occupation probability is
evaluated. Its time correlation function is compared to the one obtained
through the pure Schroedinger evolution. Systems of this kind have been
recently proposed for testing the quantum mechanical predictions against those
of macrorealistic theories, by means of temporal Bell inequalities. The
classical requirement of noninvasivity, needed to define correlation functions
in the realistic case, finds a quantum counterpart in the quantum nondemolition
condition. The consequences on the observability of quantum mechanically
predicted violations to temporal Bell inequalities are drawn and compared to
the already dealt case of the rf-SQUID dynamics.Comment: 7 pages, 2 figures, to appear in Appl. Phys.
Lattice Gauge Tensor Networks
We present a unified framework to describe lattice gauge theories by means of
tensor networks: this framework is efficient as it exploits the high amount of
local symmetry content native of these systems describing only the gauge
invariant subspace. Compared to a standard tensor network description, the
gauge invariant one allows to speed-up real and imaginary time evolution of a
factor that is up to the square of the dimension of the link variable. The
gauge invariant tensor network description is based on the quantum link
formulation, a compact and intuitive formulation for gauge theories on the
lattice, and it is alternative to and can be combined with the global symmetric
tensor network description. We present some paradigmatic examples that show how
this architecture might be used to describe the physics of condensed matter and
high-energy physics systems. Finally, we present a cellular automata analysis
which estimates the gauge invariant Hilbert space dimension as a function of
the number of lattice sites and that might guide the search for effective
simplified models of complex theories.Comment: 28 pages, 9 figure
Ab-initio characterization of the quantum linear-zigzag transition using DMRG
Ions of the same charge inside confining potentials can form crystalline
structures which can be controlled by means of the ions density and of the
external trap parameters. In particular, a linear chain of trapped ions
exhibits a transition to a zigzag equilibrium configuration, which is
controlled by the strength of the transverse confinement. Studying this phase
transition in the quantum regime is a challenging problem, even when employing
numerical methods to simulate microscopically quantum many-body systems. Here
we present a compact analytical treatment to map the original long-range
problem into a short-range quantum field theory on a lattice. We provide a
complete numerical architecture, based on Density Matrix Renormalization Group,
to address the effective quantum phi-four model. This technique is instrumental
in giving a complete characterization of the phase diagram, as well as pinpoint
the universality class of the criticality.Comment: 13 pages, 10 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
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
Optimal control of Rydberg lattice gases
We present optimal control protocols to prepare different many-body quantum
states of Rydberg atoms in optical lattices. Specifically, we show how to
prepare highly ordered many-body ground states, GHZ states as well as some
superposition of symmetric excitation number Fock states, that inherit the
translational symmetry from the Hamiltonian, within sufficiently short
excitation times minimizing detrimental decoherence effects. For the GHZ
states, we propose a two-step detection protocol to experimentally verify the
optimal preparation of the target state based only on standard measurement
techniques. Realistic experimental constraints and imperfections are taken into
account by our optimization procedure making it applicable to ongoing
experiments.Comment: Accepted versio
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