25 research outputs found
From one-dimensional charge conserving superconductors to the gapless Haldane phase
We develop a framework to analyze one-dimensional topological superconductors
with charge conservation. In particular, we consider models with flavors of
fermions and symmetry, associated with the conservation of
the fermionic parity of each flavor. For a single flavor, we recover the result
that a distinct topological phase with exponentially localized zero modes does
not exist due to absence of a gap to single particles in the bulk. For ,
however, we show that the ends of the system can host low-energy,
exponentially-localized modes. The analysis can readily be generalized to
systems in other symmetry classes. To illustrate these ideas, we focus on
lattice models with symmetric interactions, and study the
phase transition between the trivial and the topological gapless phases using
bosonization and a weak-coupling renormalization group analysis. As a concrete
example, we study in detail the case of . We show that in this case, the
topologically non-trivial superconducting phase corresponds to a gapless
analogue of the Haldane phase in spin-1 chains. In this phase, although the
bulk is gapless to single particle excitations, the ends host spin-
degrees of freedom which are exponentially localized and protected by the spin
gap in the bulk. We obtain the full phase diagram of the model numerically,
using density matrix renormalization group calculations. Within this model, we
identify the self-dual line studied by Andrei and Destri [Nucl. Phys. B,
231(3), 445-480 (1984)], as a first-order transition line between the gapless
Haldane phase and a trivial gapless phase. This allows us to identify the
propagating spin- kinks in the Andrei-Destri model as the topological
end-modes present at the domain walls between the two phases
Matrix Product Operators, Matrix Product States, and ab initio Density Matrix Renormalization Group algorithms
Current descriptions of the ab initio DMRG algorithm use two superficially
different languages: an older language of the renormalization group and
renormalized operators, and a more recent language of matrix product states and
matrix product operators. The same algorithm can appear dramatically different
when written in the two different vocabularies. In this work, we carefully
describe the translation between the two languages in several contexts. First,
we describe how to efficiently implement the ab-initio DMRG sweep using a
matrix product operator based code, and the equivalence to the original
renormalized operator implementation. Next we describe how to implement the
general matrix product operator/matrix product state algebra within a pure
renormalized operator-based DMRG code. Finally, we discuss two improvements of
the ab initio DMRG sweep algorithm motivated by matrix product operator
language: Hamiltonian compression, and a sum over operators representation that
allows for perfect computational parallelism. The connections and
correspondences described here serve to link the future developments with the
past, and are important in the efficient implementation of continuing advances
in ab initio DMRG and related algorithms.Comment: 35 pages, 10 figure
Criticality and entanglement in non-unitary quantum circuits and tensor networks of non-interacting fermions
Models for non-unitary quantum dynamics, such as quantum circuits that
include projective measurements, have been shown to exhibit rich quantum
critical behavior. There are many complementary perspectives on this behavior.
For example, there is a known correspondence between d-dimensional local
non-unitary quantum circuits and tensor networks on a D=(d+1)-dimensional
lattice. Here, we show that in the case of systems of non-interacting fermions,
there is furthermore a full correspondence between non-unitary circuits in d
spatial dimensions and unitary non-interacting fermion problems with static
Hermitian Hamiltonians in D=(d+1) spatial dimensions. This provides a powerful
new perspective for understanding entanglement phases and critical behavior
exhibited by non-interacting circuits. Classifying the symmetries of the
corresponding non-interacting Hamiltonian, we show that a large class of random
circuits, including the most generic circuits with randomness in space and
time, are in correspondence with Hamiltonians with static spatial disorder in
the ten Altland-Zirnbauer symmetry classes. We find the criticality that is
known to occur in all of these classes to be the origin of the critical
entanglement properties of the corresponding random non-unitary circuit. To
exemplify this, we numerically study the quantum states at the boundary of
Haar-random Gaussian fermionic tensor networks of dimension D=2 and D=3. We
show that the most general such tensor network ensemble corresponds to a
unitary problem of non-interacting fermions with static disorder in
Altland-Zirnbauer symmetry class DIII, which for both D=2 and D=3 is known to
exhibit a stable critical metallic phase. Tensor networks and corresponding
random non-unitary circuits in the other nine Altland-Zirnbauer symmetry
classes can be obtained from the DIII case by implementing Clifford algebra
extensions for classifying spaces.Comment: (25+14) pages, 19 figure
Evidence of topological superconductivity in planar Josephson junctions
Majorana zero modes are quasiparticle states localized at the boundaries of
topological superconductors that are expected to be ideal building blocks for
fault-tolerant quantum computing. Several observations of zero-bias conductance
peaks measured in tunneling spectroscopy above a critical magnetic field have
been reported as experimental indications of Majorana zero modes in
superconductor/semiconductor nanowires. On the other hand, two dimensional
systems offer the alternative approach to confine Ma jorana channels within
planar Josephson junctions, in which the phase difference {\phi} between the
superconducting leads represents an additional tuning knob predicted to drive
the system into the topological phase at lower magnetic fields. Here, we report
the observation of phase-dependent zero-bias conductance peaks measured by
tunneling spectroscopy at the end of Josephson junctions realized on a InAs/Al
heterostructure. Biasing the junction to {\phi} ~ {\pi} significantly reduces
the critical field at which the zero-bias peak appears, with respect to {\phi}
= 0. The phase and magnetic field dependence of the zero-energy states is
consistent with a model of Majorana zero modes in finite-size Josephson
junctions. Besides providing experimental evidence of phase-tuned topological
superconductivity, our devices are compatible with superconducting quantum
electrodynamics architectures and scalable to complex geometries needed for
topological quantum computing.Comment: main text and extended dat
Matrix product operators, matrix product states, and ab initio density matrix renormalization group algorithms
Current descriptions of the ab initio density matrix renormalization group (DMRG) algorithm use two superficially different languages: an older language of the renormalization group and renormalized operators, and a more recent language of matrix product states and matrix product operators. The same algorithm can appear dramatically different when written in the two different vocabularies. In this work, we carefully describe the translation between the two languages in several contexts. First, we describe how to efficiently implement the ab initio DMRG sweep using a matrix product operator based code, and the equivalence to the original renormalized operator implementation. Next we describe how to implement the general matrix product operator/matrix product state algebra within a pure renormalized operator-based DMRG code. Finally, we discuss two improvements of the ab initio DMRG sweep algorithm motivated by matrix product operator language: Hamiltonian compression, and a sum over operators representation that allows for perfect computational parallelism. The connections and correspondences described here serve to link the future developments with the past and are important in the efficient implementation of continuing advances in ab initio DMRG and related algorithms
Comparison of user groups' perspectives of barriers and facilitators to implementing electronic health records: a systematic review
<p>Abstract</p> <p>Background</p> <p>Electronic health record (EHR) implementation is currently underway in Canada, as in many other countries. These ambitious projects involve many stakeholders with unique perceptions of the implementation process. EHR users have an important role to play as they must integrate the EHR system into their work environments and use it in their everyday activities. Users hold valuable, first-hand knowledge of what can limit or contribute to the success of EHR implementation projects. A comprehensive synthesis of EHR users' perceptions is key to successful future implementation. This systematic literature review was aimed to synthesize current knowledge of the barriers and facilitators influencing shared EHR implementation among its various users.</p> <p>Methods</p> <p>Covering a period from 1999 to 2009, a literature search was conducted on nine electronic databases. Studies were included if they reported on users' perceived barriers and facilitators to shared EHR implementation, in healthcare settings comparable to Canada. Studies in all languages with an empirical study design were included. Quality and relevance of the studies were assessed. Four EHR user groups were targeted: physicians, other health care professionals, managers, and patients/public. Content analysis was performed independently by two authors using a validated extraction grid with pre-established categorization of barriers and facilitators for each group of EHR users.</p> <p>Results</p> <p>Of a total of 5,695 potentially relevant publications identified, 117 full text publications were obtained after screening titles and abstracts. After review of the full articles, 60 publications, corresponding to 52 studies, met the inclusion criteria. The most frequent adoption factors common to all user groups were design and technical concerns, ease of use, interoperability, privacy and security, costs, productivity, familiarity and ability with EHR, motivation to use EHR, patient and health professional interaction, and lack of time and workload. Each user group also identified factors specific to their professional and individual priorities.</p> <p>Conclusions</p> <p>This systematic review presents innovative research on the barriers and facilitators to EHR implementation. While important similarities between user groups are highlighted, differences between them demonstrate that each user group also has a unique perspective of the implementation process that should be taken into account.</p