11 research outputs found
A Noncommutative Sigma Model
We replaced the classical string theory notions of parameter space and
world-time with noncommutative tori and consider maps between these spaces. The
dynamics of mappings between different noncommutative tori were studied and a
noncommutative generalization of the Polyakov action was derived. The quantum
torus was studied in detail as well as *-homomorphisms between different
quantum tori. A finite dimensional representation of the quantum torus was
studied and the partition function and other path integrals were calculated. At
the end we proved existence theorems for mappings between different
noncommutative tori.Comment: The thesis was based on an article by Vargese Mathai and Jonathan
Rosenberg with the same titl
Interplay of Soundcone and Supersonic Propagation in Lattice Models with Power Law Interactions
We study the spreading of correlations and other physical quantities in
quantum lattice models with interactions or hopping decaying like
with the distance . Our focus is on exponents between 0 and 6,
where the interplay of long- and short-range features gives rise to a complex
phenomenology and interesting physical effects, and which is also the relevant
range for experimental realizations with cold atoms, ions, or molecules. We
present analytical and numerical results, providing a comprehensive picture of
spatio-temporal propagation. Lieb-Robinson-type bounds are extended to strongly
long-range interactions where is smaller than the lattice dimension,
and we report particularly sharp bounds that are capable of reproducing regimes
with soundcone as well as supersonic dynamics. Complementary lower bounds prove
that faster-than-soundcone propagation occurs for in any spatial
dimension, although cone-like features are shown to also occur in that regime.
Our results provide guidance for optimizing experimental efforts to harness
long-range interactions in a variety of quantum information and signaling
tasks.Comment: 20 pages, 8 figure
A noncommutative sigma model.
We replace the classical string theory notions of mapping
between parameter space and world-time with noncommutative tori mapping between these
spaces. The dynamics of mappings between different noncommutative tori are studied and
noncommutative versions of the Polyakov action and the Euler-Lagrange equations are
derived. The quantum torus is studied in detail, as well as C*-homomorphisms between
different quantum tori. A finite dimensional representation of the quantum torus is studied,
and the partition function and other path integrals are calculated. At the end we prove existence
theorems for mappings between different noncommutative tori.This abstract was presented
at the ‘Studentesimposium
in die Natuurwetenskappe
2011’, presented under
the protection of the Suid-
Afrikaanse Akademie vir
Wetenskap en Kuns. The
symposium was held at the
University of South Africa on
27–28 October 2011.http://www.satnt.ac.zaam201
Dynamics of long-range interacting quantum spin systems
Thesis (PhD)--Stellenbosch University, 2015.ENGLISH ABSTRACT: In this thesis we study the time evolution of correlation functions in quantum
lattice models in the presence of long-range interactions or hopping decaying
asymptotically as a power law. For a large class of initial conditions,
exact analytic results are obtained in arbitrary lattice dimension for the longrange
Ising model. In contrast to the nearest-neighbour case, we find that
correlations decay like stretched or compressed exponentials in time. Provided
the long-range character of the interactions is sufficiently strong, pronounced
prethermalization plateaus are observed and relaxation timescales are widely
separated. Starting from uncorrelated states that are easily prepared in experiments,
we show the dynamical emergence of correlations and entanglement
in these far-from-equilibrium interacting quantum systems. We characterize
these correlations by the entanglement entropy, concurrence, and squeezing,
which are inequivalent measures of entanglement corresponding to different
quantum resources.
For interaction exponents larger than the lattice dimensionality, a Lieb-
Robinson-type bound effectively restricts the spreading of correlations to the
interior of a causal region, but allows supersonic (faster than linear) propagation.
Using tools of quantum metrology, for any exponents smaller than the
lattice dimension, we construct Hamiltonians giving rise to quantum channels
with capacities not restricted to any causal region. An analytic analysis of
long-range Ising models illustrates the disappearance of the causal region and
the creation of correlations becoming distance-independent. In all models we
analyzed the spreading of correlations follows a power law, and not the exponential
increase of the long-range Lieb-Robinson bound. Lieb-Robinson-type
bounds are extended to strongly long-range interactions where the interaction
exponent is smaller than the lattice dimension, and we report particularly
sharp bounds that are capable of reproducing regimes with soundcone
as well as supersonic dynamics. Our results provide guidance for optimizing
experimental efforts to harness long-range interactions in a variety of quantum
information and signaling tasks.AFRIKAANSE OPSOMMING: In hierdie tesis studeer ons die tydevolusie van korrelasiefunksies in kwantumroostermodelle
in die teenwoordigheid van lang-afstand interaksies of spronge
wat asimptoties in `n magswet verval. Vir `n groot versameling begintoestande
word presiese analitiese resultate vir die lang-afstand-Ising-model verkry. In
teenstelling met die naaste-naasliggende-interaksie-Ising-model vind ons dat
korrelasies soos uitgerekte of saamgepersde eksponentiele funksies in tyd verval.
Indien die lang-afstand gedrag van die interaksies lank genoeg is word
lanklewende kwasi-stationere toestande gevorm as gevolg van wyd verspreide
tydskale wat in die ontspanningsgedrag van die korrelasiefunksies voorkom.
Wanneer ongekorreleerde begintoestande, wat eenvoudig in die laboratorium
voorberei kan word, gebruik word kan die dinamiese opkoms van verskillende
maatstawe van verstrengeling analities bereken word. Gevolglik kan hierdie
verstrengelde toestande gebruik word om kwantumberekeninge mee uit te voer.
Vir interaksie-eksponente groter as die roosterdimensie bestaan daar sogenaamde
Lieb-Robinson-grense wat die verspreiding van korrelasies beperk tot
die binneruim van `n oorsaaklike-kegel, maar steeds supersoniese verspreiding
(vinniger as line^er) toelaat. Deur gebruik te maak van gereedskap uit kwantummeetkunde
kan ons, vir enige interaksie-eksponent kleiner as die roosterdimensie,
Hamiltoniane konstrueer wat oorsaak gee aan kwantumkanale met
kapasiteite wat nie beperk word deur enige oorsaaklike-kegel nie. Die analitiese
resultate van die lang-afstand-Ising-model illustreer die verdwyning van
die oorsaaklike-kegel sowel as die vorming van afstand onafhanklike korrelasies.
In al die modelle waar ons die verspeiding van korrelasies bestudeer het, het
die speiding magswette gevolg, en nie die eksponentiele toename wat deur die
lang-afstand Lieb-Robinson-grense voorspel word nie. Lieb-Robinson-grense
word uitgebrei na kwantumsisteme met sterk-lang-afstand interaksies waar
die interaksie-eksponent kleiner is as die roosterdimensie. Besonderse skerp
grense word voorgedra wat die oorgang van line^ere na supersoniese verspreiding
vasvang. Die resultate wat in hierdie tesis vasgevang is verskaf riglyne
vir die optimering van eksperimentele pogings on lang-afstnad interaksies in
te span in `n verskeidenheid kwantuminformasie en -seintake