330,912 research outputs found
The power of quantum systems on a line
We study the computational strength of quantum particles (each of finite
dimensionality) arranged on a line. First, we prove that it is possible to
perform universal adiabatic quantum computation using a one-dimensional quantum
system (with 9 states per particle). This might have practical implications for
experimentalists interested in constructing an adiabatic quantum computer.
Building on the same construction, but with some additional technical effort
and 12 states per particle, we show that the problem of approximating the
ground state energy of a system composed of a line of quantum particles is
QMA-complete; QMA is a quantum analogue of NP. This is in striking contrast to
the fact that the analogous classical problem, namely, one-dimensional
MAX-2-SAT with nearest neighbor constraints, is in P. The proof of the
QMA-completeness result requires an additional idea beyond the usual techniques
in the area: Not all illegal configurations can be ruled out by local checks,
so instead we rule out such illegal configurations because they would, in the
future, evolve into a state which can be seen locally to be illegal. Our
construction implies (assuming the quantum Church-Turing thesis and that
quantum computers cannot efficiently solve QMA-complete problems) that there
are one-dimensional systems which take an exponential time to relax to their
ground states at any temperature, making them candidates for being
one-dimensional spin glasses.Comment: 21 pages. v2 has numerous corrections and clarifications, and most
importantly a new author, merged from arXiv:0705.4067. v3 is the published
version, with additional clarifications, publisher's version available at
http://www.springerlink.co
Non-equilibrium dynamics of quantum systems: order parameter evolution, defect generation, and qubit transfer
In this review, we study some aspects of the non-equilibrium dynamics of
quantum systems. In particular, we consider the effect of varying a parameter
in the Hamiltonian of a quantum system which takes it across a quantum critical
point or line. We study both sudden and slow quenches in a variety of systems
including one-dimensional ultracold atoms in an optical lattice, an infinite
range ferromagnetic Ising model, and some exactly solvable spin models in one
and two dimensions (such as the Kitaev model). We show that quenching leads to
the formation of defects whose density has a power-law dependence on the
quenching rate; the power depends on the dimensionalities of the system and of
the critical surface and on some of the exponents associated with the critical
point which is being crossed. We also study the effect of non-linear quenching;
the power law of the defects then depends on the degree of non-linearity.
Finally, we study some spin-1/2 models to discuss how a qubit can be
transferred across a system.Comment: 36 pages, 14 figures; an updated version will be published in
"Quantum Quenching, Annealing and Computation", Eds. A. Das, A. Chandra and
B. K. Chakrabarti, Lect. Notes in Phys., Springer, Heidelberg (2009, to be
published
Simulating 0+1 Dimensional Quantum Gravity on Quantum Computers: Mini-Superspace Quantum Cosmology and the World Line Approach in Quantum Field Theory
Quantum computers are a promising candidate to radically expand computational
science through increased computing power and more effective algorithms. In
particular quantum computing could have a tremendous impact in the field of
quantum cosmology. The goal of quantum cosmology is to describe the evolution
of the Universe through the Wheeler-DeWitt equation or path integral methods
without having to first formulate a full theory of quantum gravity. The quantum
computer provides an advantage in this endeavor because it can perform path
integrals in Lorentzian space and does not require constructing contour
integrations in Euclidean gravity. Also quantum computers can provide
advantages in systems with fermions which are difficult to analyze on classical
computers. In this study, we first employed classical computational methods to
analyze a Friedmann-Robertson-Walker mini-superspace with a scalar field and
visualize the calculated wave function of the Universe for a variety of
different values of the spatial curvature and cosmological constant. We them
used IBM's Quantum Information Science Kit Python library and the variational
quantum eigensolver to study the same systems on a quantum computer. The
framework can also be extended to the world line approach to quantum field
theory.Comment: 5 pages, 4 figure
Line junction in a quantum Hall system with two filling fractions
We present a microscopic model for a line junction formed by counter or
co-propagating single mode quantum Hall edges corresponding to different
filling factors. The ends of the line junction can be described by two possible
current splitting matrices which are dictated by the conditions of both lack of
dissipation and the existence of a linear relation between the bosonic fields.
Tunneling between the two edges of the line junction then leads to a
microscopic understanding of a phenomenological description of line junctions
introduced some time ago. The effect of density-density interactions between
the two edges is considered, and renormalization group ideas are used to study
how the tunneling parameter changes with the length scale. This leads to a
power law variation of the conductance of the line junction with the
temperature. Depending on the strength of the interactions the line junction
can exhibit two quite different behaviors. Our results can be tested in bent
quantum Hall systems fabricated recently.Comment: 9 pages including 4 figure
Critical Dynamics of Singlet Excitations in a Frustrated Spin System
We construct and analyze a two-dimensional frustrated quantum spin model with
plaquette order, in which the low-energy dynamics is controlled by spin
singlets. At a critical value of frustration the singlet spectrum becomes
gapless, indicating a quantum transition to a phase with dimer order. This T=0
transition belongs to the 3D Ising universality class, while at finite
temperature a 2D Ising critical line separates the plaquette and dimerized
phases.
The magnetic susceptibility has an activated form throughout the phase
diagram, whereas the specific heat exhibits a rich structure and a power law
dependence on temperature at the quantum critical point.
We argue that the novel quantum critical behavior associated with singlet
criticality discussed in this work can be relevant to a wide class of quantum
spin systems, such as antiferromagnets on Kagome and pyrochlore lattices, where
the low-energy excitations are known to be spin singlets, as well as to the
CAVO lattice and several recently discovered strongly frustrated square-lattice
antiferromagnets.Comment: 5 pages, 5 figures, additional discussion and figure added, to appear
in Phys. Rev.
Abelian Conformal Field theories and Determinant Bundles
The present paper is the first in a series of papers, in which we shall
construct modular functors and Topological Quantum Field Theories from the
conformal field theory developed in [TUY].
The basic idea is that the covariant constant sections of the sheaf of vacua
associated to a simple Lie algebra over Teichm\"uller space of an oriented
pointed surface gives the vectorspace the modular functor associates to the
oriented pointed surface. However the connection on the sheaf of vacua is only
projectively flat, so we need to find a suitable line bundle with a connection,
such that the tensor product of the two has a flat connection.
We shall construct a line bundle with a connection on any family of pointed
curves with formal coordinates. By computing the curvature of this line bundle,
we conclude that we actually need a fractional power of this line bundle so as
to obtain a flat connection after tensoring. In order to functorially extract
this fractional power, we need to construct a preferred section of the line
bundle.
We shall construct the line bundle by the use of the so-called -ghost
systems (Faddeev-Popov ghosts) first introduced in covariant quantization [FP].
We follow the ideas of [KNTY], but decribe it from the point of view of [TUY].Comment: A couple of typos correcte
Theory of defect production in nonlinear quench across a quantum critical point
We study defect production in a quantum system subjected to a nonlinear power
law quench which takes it either through a quantum critical or multicritical
point or along a quantum critical line. We elaborate on our earlier work [D.
Sen, K. Sengupta, S. Mondal, \prl 101, 016806 (2008)] and present a detailed
analysis of the scaling of the defect density with the quench rate
and exponent \al for each of the above-mentioned cases. We also compute the
correlation functions for defects generated in nonlinear quenches through a
quantum critical point and discuss the dependence of the amplitudes of such
correlation functions on the exponent \al. We discuss several experimental
systems where these theoretical predictions can be tested.Comment: 9+ pages, v
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