1,443 research outputs found
Efficient solvability of Hamiltonians and limits on the power of some quantum computational models
We consider quantum computational models defined via a Lie-algebraic theory.
In these models, specified initial states are acted on by Lie-algebraic quantum
gates and the expectation values of Lie algebra elements are measured at the
end. We show that these models can be efficiently simulated on a classical
computer in time polynomial in the dimension of the algebra, regardless of the
dimension of the Hilbert space where the algebra acts. Similar results hold for
the computation of the expectation value of operators implemented by a
gate-sequence. We introduce a Lie-algebraic notion of generalized mean-field
Hamiltonians and show that they are efficiently ("exactly") solvable by means
of a Jacobi-like diagonalization method. Our results generalize earlier ones on
fermionic linear optics computation and provide insight into the source of the
power of the conventional model of quantum computation.Comment: 6 pages; no figure
Density matrix numerical renormalization group for non-Abelian symmetries
We generalize the spectral sum rule preserving density matrix numerical
renormalization group (DM-NRG) method in such a way that it can make use of an
arbitrary number of not necessarily Abelian, local symmetries present in the
quantum impurity system. We illustrate the benefits of using non-Abelian
symmetries by the example of calculations for the T-matrix of the two-channel
Kondo model in the presence of magnetic field, for which conventional NRG
methods produce large errors and/or take a long run-time.Comment: 12 pages, 6 figures, PRB forma
Stigma resistance at the personal, peer, and public levels: A new conceptual model.
Stigma resistance is consistently linked with key recovery outcomes, yet theoretical work is limited. This study explored stigma resistance from the perspective of individuals with serious mental illness (SMI). Twenty-four individuals with SMI who were either peer-service providers (those with lived experience providing services; N = 14) or consumers of mental health services (N = 10) engaged in semistructured interviews regarding experiences with stigma, self-stigma, and stigma resistance, including key elements of this process and examples of situations in which they resisted stigma. Stigma resistance is an ongoing, active process that involves using one’s experiences, knowledge, and sets of skills at the (1) personal, (2) peer, and (3) public levels. Stigma resistance at the personal level involves (a) not believing stigma or catching and challenging stigmatizing thoughts, (b) empowering oneself by learning about mental health and recovery, (c) maintaining one’s recovery and proving stigma wrong, and (d) developing a meaningful identity apart from mental illness. Stigma resistance at the peer level involves using one’s experiences to help others fight stigma and at the public level, resistance involved (a) education, (b) challenging stigma, (c) disclosing one’s lived experience, and (d) advocacy work. Findings present a more nuanced conceptualization of resisting stigma, grounded in the experiences of people with SMI. Stigma resistance is an ongoing, active process of using one’s experiences, skills, and knowledge to develop a positive identity. Interventions should consider focusing on personal stigma resistance early on and increasing the incorporation of peers into services
Implications of Qudit Superselection rules for the Theory of Decoherence-free Subsystems
The use of d-state systems, or qudits, in quantum information processing is
discussed. Three-state and higher dimensional quantum systems are known to have
very different properties from two-state systems, i.e., qubits. In particular
there exist qudit states which are not equivalent under local unitary
transformations unless a selection rule is violated. This observation is shown
to be an important factor in the theory of decoherence-free, or noiseless,
subsystems. Experimentally observable consequences and methods for
distinguishing these states are also provided, including the explicit
construction of new decoherence-free or noiseless subsystems from qutrits.
Implications for simulating quantum systems with quantum systems are also
discussed.Comment: 13 pages, 1 figures, Version 2: Typos corrected, references fixed and
new ones added, also includes referees suggested changes and a new exampl
Development of a unified tensor calculus for the exceptional Lie algebras
The uniformity of the decomposition law, for a family F of Lie algebras which
includes the exceptional Lie algebras, of the tensor powers ad^n of their
adjoint representations ad is now well-known. This paper uses it to embark on
the development of a unified tensor calculus for the exceptional Lie algebras.
It deals explicitly with all the tensors that arise at the n=2 stage, obtaining
a large body of systematic information about their properties and identities
satisfied by them. Some results at the n=3 level are obtained, including a
simple derivation of the the dimension and Casimir eigenvalue data for all the
constituents of ad^3. This is vital input data for treating the set of all
tensors that enter the picture at the n=3 level, following a path already known
to be viable for a_1. The special way in which the Lie algebra d_4 conforms to
its place in the family F alongside the exceptional Lie algebras is described.Comment: 27 pages, LaTeX 2
Decoherence Free Subspaces for Quantum Computation
Decoherence in quantum computers is formulated within the Semigroup approach.
The error generators are identified with the generators of a Lie algebra. This
allows for a comprehensive description which includes as a special case the
frequently assumed spin-boson model. A generic condition is presented for
error-less quantum computation: decoherence-free subspaces are spanned by those
states which are annihilated by all the generators. It is shown that these
subspaces are stable to perturbations and moreover, that universal quantum
computation is possible within them.Comment: 4 pages, no figures. Conditions for decoherence-free subspaces made
more explicit, updated references. To appear in PR
Low-Frequency Radio Transients in the Galactic Center
We report the detection of a new radio transient source, GCRT J1746-2757,
located only 1.1 degrees north of the Galactic center. Consistent with other
radio transients toward the Galactic center, this source brightened and faded
on a time scale of a few months. No X-ray counterpart was detected. We also
report new 0.33 GHz measurements of the radio counterpart to the X-ray
transient source, XTE J1748-288, previously detected and monitored at higher
radio frequencies. We show that the spectrum of XTE J1748-288 steepened
considerably during a period of a few months after its peak. We also discuss
the need for a more efficient means of finding additional radio transients
Loop Quantization of the Supersymmetric Two-Dimensional BF Model
In this paper we consider the quantization of the 2d BF model coupled to
topological matter. Guided by the rigid supersymmetry this system can be viewed
as a super-BF model, where the field content is expressed in terms of
superfields. A canonical analysis is done and the constraints are then
implemented at the quantum level in order to construct the Hilbert space of the
theory under the perspective of Loop Quantum Gravity methods.Comment: 17 pages, Late
The Spin Density Matrix II: Application to a system of two quantum dots
This work is a sequel to our work "The Spin Density Matrix I: General Theory
and Exact Master Equations" (eprint arXiv:0708.0644 [cond-mat]). Here we
compare pure- and pseudo-spin dynamics using as an example a system of two
quantum dots, a pair of localized conduction-band electrons in an n-doped GaAs
semiconductor. Pure-spin dynamics is obtained by tracing out the orbital
degrees of freedom, whereas pseudo-spin dynamics retains (as is conventional)
an implicit coordinate dependence. We show that magnetic field inhomogeneity
and spin-orbit interaction result in a non-unitary evolution in pure-spin
dynamics, whereas these interactions contribute to the effective pseudo-spin
Hamiltonian via terms that are asymmetric in spin permutations, in particular,
the Dzyaloshinskii-Moriya (DM) spin-orbit interaction. We numerically
investigate the non-unitary effects in the dynamics of the triplet states
population, purity, and Lamb energy shift, as a function of interdot distance
and magnetic field difference. The spin-orbit interaction is found to produce
effects of roughly four orders of magnitude smaller than those due to magnetic
field difference in the pure-spin model. We estimate the spin-orbit interaction
magnitude in the DM-interaction term. Our estimate gives a smaller value than
that recently obtained by Kavokin [Phys. Rev. B 64, 075305 (2001)], who did not
include double occupancy effects. We show that a necessary and sufficient
condition for obtaining a universal set of quantum logic gates, involving only
two spins, in both pure- and pseudo-spin models is that the magnetic field
inhomogeneity and the Heisenberg interaction are both non-vanishing. We also
briefly analyze pure-spin dynamics in the electron on liquid helium system
recently proposed by Lyon [Phys. Rev. A 74, 052338 (2006)].Comment: 16 pages including 12 figures. Sequel to "The Spin Density Matrix I:
General Theory and Exact Master Equations", arXiv:0708.064
Deformation of orthosymplectic Lie superalgebra osp(1|2)
Triangular deformation of the orthosymplectic Lie superalgebra osp(1|4) is
defined by chains of twists. Corresponding classical r-matrix is obtained by a
contraction procedure from the trigonometric r-matrix. The carrier space of the
constant r-matrix is the Borel subalgebra.Comment: LaTeX, 8 page
- …