11,251 research outputs found
The Dirac Quantum Cellular Automaton in one dimension: Zitterbewegung and scattering from potential
We study the dynamical behaviour of the quantum cellular automaton of Refs.
[1, 2], which reproduces the Dirac dynamics in the limit of small wavevectors
and masses. We present analytical evaluations along with computer simulations,
showing how the automaton exhibits typical Dirac dynamical features, as the
Zitterbewegung and the scattering behaviour from potential that gives rise to
the so-called Klein paradox. The motivation is to show concretely how pure
processing of quantum information can lead to particle mechanics as an emergent
feature, an issue that has been the focus of solid-state, optical and
atomic-physics quantum simulator.Comment: 8 pages, 7 figure
Quantum Field as a quantum cellular automaton: the Dirac free evolution in one dimension
We present a quantum cellular automaton model in one space-dimension which
has the Dirac equation as emergent. This model, a discrete-time and causal
unitary evolution of a lattice of quantum systems, is derived from the
assumptions of homogeneity, parity and time-reversal invariance. The comparison
between the automaton and the Dirac evolutions is rigorously set as a
discrimination problem between unitary channels. We derive an exact lower bound
for the probability of error in the discrimination as an explicit function of
the mass, the number and the momentum of the particles, and the duration of the
evolution. Computing this bound with experimentally achievable values, we see
that in that regime the QCA model cannot be discriminated from the usual Dirac
evolution. Finally, we show that the evolution of one-particle states with
narrow-band in momentum can be effi- ciently simulated by a dispersive
differential equation for any regime. This analysis allows for a comparison
with the dynamics of wave-packets as it is described by the usual Dirac
equation. This paper is a first step in exploring the idea that quantum field
theory could be grounded on a more fundamental quantum cellular automaton model
and that physical dynamics could emerge from quantum information processing. In
this framework, the discretization is a central ingredient and not only a tool
for performing non-perturbative calculation as in lattice gauge theory. The
automaton model, endowed with a precise notion of local observables and a full
probabilistic interpretation, could lead to a coherent unification of an
hypothetical discrete Planck scale with the usual Fermi scale of high-energy
physics.Comment: 21 pages, 4 figure
A Wait-free Multi-word Atomic (1,N) Register for Large-scale Data Sharing on Multi-core Machines
We present a multi-word atomic (1,N) register for multi-core machines
exploiting Read-Modify-Write (RMW) instructions to coordinate the writer and
the readers in a wait-free manner. Our proposal, called Anonymous Readers
Counting (ARC), enables large-scale data sharing by admitting up to
concurrent readers on off-the-shelf 64-bits machines, as opposed to the most
advanced RMW-based approach which is limited to 58 readers. Further, ARC avoids
multiple copies of the register content when accessing it---this affects
classical register's algorithms based on atomic read/write operations on single
words. Thus it allows for higher scalability with respect to the register size.
Moreover, ARC explicitly reduces improves performance via a proper limitation
of RMW instructions in case of read operations, and by supporting constant time
for read operations and amortized constant time for write operations. A proof
of correctness of our register algorithm is also provided, together with
experimental data for a comparison with literature proposals. Beyond assessing
ARC on physical platforms, we carry out as well an experimentation on
virtualized infrastructures, which shows the resilience of wait-free
synchronization as provided by ARC with respect to CPU-steal times, proper of
more modern paradigms such as cloud computing.Comment: non
Statistical Learning Theory for Location Fingerprinting in Wireless LANs
In this paper, techniques and algorithms developed in the framework of statistical learning theory are analyzed and applied to the problem of determining the location of a wireless device by measuring the signal strengths from a set of access points (location fingerprinting). Statistical Learning Theory provides a rich theoretical basis for the development of models starting from a set of examples. Signal strength measurement is part of the normal operating mode of wireless equipment, in particular Wi-Fi, so that no custom hardware is required. The proposed techniques, based on the Support Vector Machine paradigm, have been implemented and compared, on the same data set, with other approaches considered in the literature. Tests performed in a real-world environment show that results are comparable, with the advantage of a low algorithmic complexity in the normal operating phase. Moreover, the algorithm is particularly suitable for classification, where it outperforms the other techniques
Solutions of a two-particle interacting quantum walk
We study the solutions of the interacting Fermionic cellular automaton
introduced in Ref. [Phys Rev A 97, 032132 (2018)]. The automaton is the
analogue of the Thirring model with both space and time discrete. We present a
derivation of the two-particles solutions of the automaton, which exploits the
symmetries of the evolution operator. In the two-particles sector, the
evolution operator is given by the sequence of two steps, the first one
corresponding to a unitary interaction activated by two-particle excitation at
the same site, and the second one to two independent one-dimensional Dirac
quantum walks. The interaction step can be regarded as the discrete-time
version of the interacting term of some Hamiltonian integrable system, such as
the Hubbard or the Thirring model. The present automaton exhibits scattering
solutions with nontrivial momentum transfer, jumping between different regions
of the Brillouin zone that can be interpreted as Fermion-doubled particles, in
stark contrast with the customary momentum-exchange of the one dimensional
Hamiltonian systems. A further difference compared to the Hamiltonian model is
that there exist bound states for every value of the total momentum, and even
for vanishing coupling constant. As a complement to the analytical derivations
we show numerical simulations of the interacting evolution.Comment: 16 pages, 6 figure
Planning through Automatic Portfolio Configuration: The PbP Approach
In the field of domain-independent planning, several powerful planners implementing different techniques have been developed. However, no one of these systems outperforms all others in every known benchmark domain. In this work, we propose a multi-planner approach that automatically configures a portfolio of planning techniques for each given domain. The configuration process for a given domain uses a set of training instances to: (i) compute and analyze some alternative sets of macro-actions for each planner in the portfolio identifying a (possibly empty) useful set, (ii) select a cluster of planners, each one with the identified useful set of macro-actions, that is expected to perform best, and (iii) derive some additional information for configuring the execution scheduling of the selected planners at planning time. The resulting planning system, called PbP (Portfolio- based Planner), has two variants focusing on speed and plan quality. Different versions of PbP entered and won the learning track of the sixth and seventh International Planning Competitions. In this paper, we experimentally analyze PbP considering planning speed and plan quality in depth. We provide a collection of results that help to understand PbP�s behavior, and demonstrate the effectiveness of our approach to configuring a portfolio of planners with macro-actions
Quantum Walks, Weyl equation and the Lorentz group
Quantum cellular automata and quantum walks provide a framework for the
foundations of quantum field theory, since the equations of motion of free
relativistic quantum fields can be derived as the small wave-vector limit of
quantum automata and walks starting from very general principles. The intrinsic
discreteness of this framework is reconciled with the continuous Lorentz
symmetry by reformulating the notion of inertial reference frame in terms of
the constants of motion of the quantum walk dynamics. In particular, among the
symmetries of the quantum walk which recovers the Weyl equation--the so called
Weyl walk--one finds a non linear realisation of the Poincar\'e group, which
recovers the usual linear representation in the small wave-vector limit. In
this paper we characterise the full symmetry group of the Weyl walk which is
shown to be a non linear realization of a group which is the semidirect product
of the Poincar\'e group and the group of dilations.Comment: 9 pages, 2 figure
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