529 research outputs found
Topological phases of fermions in one dimension
In this paper we show how the classification of topological phases in
insulators and superconductors is changed by interactions, in the case of 1D
systems. We focus on the TR-invariant Majorana chain (BDI symmetry class).
While the band classification yields an integer topological index , it is
known that phases characterized by values of in the same equivalence class
modulo 8 can be adiabatically transformed one to another by adding suitable
interaction terms. Here we show that the eight equivalence classes are distinct
and exhaustive, and provide a physical interpretation for the interacting
invariant modulo 8. The different phases realize different Altland-Zirnbauer
classes of the reduced density matrix for an entanglement bipartition into two
half-chains. We generalize these results to the classification of all one
dimensional gapped phases of fermionic systems with possible anti-unitary
symmetries, utilizing the algebraic framework of central extensions. We use
matrix product state methods to prove our results.Comment: 14 pages, 3 figures, v2: references adde
Engineering Functional Quantum Algorithms
Suppose that a quantum circuit with K elementary gates is known for a unitary
matrix U, and assume that U^m is a scalar matrix for some positive integer m.
We show that a function of U can be realized on a quantum computer with at most
O(mK+m^2log m) elementary gates. The functions of U are realized by a generic
quantum circuit, which has a particularly simple structure. Among other
results, we obtain efficient circuits for the fractional Fourier transform.Comment: 4 pages, 2 figure
Quantum error correction benchmarks for continuous weak parity measurements
We present an experimental procedure to determine the usefulness of a
measurement scheme for quantum error correction (QEC). A QEC scheme typically
requires the ability to prepare entangled states, to carry out multi-qubit
measurements, and to perform certain recovery operations conditioned on
measurement outcomes. As a consequence, the experimental benchmark of a QEC
scheme is a tall order because it requires the conjuncture of many elementary
components. Our scheme opens the path to experimental benchmarks of individual
components of QEC. Our numerical simulations show that certain parity
measurements realized in circuit quantum electrodynamics are on the verge of
being useful for QEC
An Isomonodromy Cluster of Two Regular Singularities
We consider a linear matrix ODE with two coalescing regular
singularities. This coalescence is restricted with an isomonodromy condition
with respect to the distance between the merging singularities in a way
consistent with the ODE. In particular, a zero-distance limit for the ODE
exists. The monodromy group of the limiting ODE is calculated in terms of the
original one. This coalescing process generates a limit for the corresponding
nonlinear systems of isomonodromy deformations. In our main example the latter
limit reads as , where is the -th Painlev\'e equation. We
also discuss some general problems which arise while studying the
above-mentioned limits for the Painlev\'e equations.Comment: 44 pages, 8 figure
correction to free energy in hermitian two-matrix model
Using the loop equations we find an explicit expression for genus 1
correction in hermitian two-matrix model in terms of holomorphic objects
associated to spectral curve arising in large N limit. Our result generalises
known expression for in hermitian one-matrix model. We discuss the
relationship between , Bergmann tau-function on Hurwitz spaces, G-function
of Frobenius manifolds and determinant of Laplacian over spectral curve
Two-dimensional quantum liquids from interacting non-Abelian anyons
A set of localized, non-Abelian anyons - such as vortices in a p_x + i p_y
superconductor or quasiholes in certain quantum Hall states - gives rise to a
macroscopic degeneracy. Such a degeneracy is split in the presence of
interactions between the anyons. Here we show that in two spatial dimensions
this splitting selects a unique collective state as ground state of the
interacting many-body system. This collective state can be a novel gapped
quantum liquid nucleated inside the original parent liquid (of which the anyons
are excitations). This physics is of relevance for any quantum Hall plateau
realizing a non-Abelian quantum Hall state when moving off the center of the
plateau.Comment: 5 pages, 6 figure
Simulating adiabatic evolution of gapped spin systems
We show that adiabatic evolution of a low-dimensional lattice of quantum
spins with a spectral gap can be simulated efficiently. In particular, we show
that as long as the spectral gap \Delta E between the ground state and the
first excited state is any constant independent of n, the total number of
spins, then the ground-state expectation values of local operators, such as
correlation functions, can be computed using polynomial space and time
resources. Our results also imply that the local ground-state properties of any
two spin models in the same quantum phase can be efficiently obtained from each
other. A consequence of these results is that adiabatic quantum algorithms can
be simulated efficiently if the spectral gap doesn't scale with n. The
simulation method we describe takes place in the Heisenberg picture and does
not make use of the finitely correlated state/matrix product state formalism.Comment: 13 pages, 2 figures, minor change
How to perform the most accurate possible phase measurements
We present the theory of how to achieve phase measurements with the minimum
possible variance in ways that are readily implementable with current
experimental techniques. Measurements whose statistics have high-frequency
fringes, such as those obtained from NOON states, have commensurately high
information yield. However this information is also highly ambiguous because it
does not distinguish between phases at the same point on different fringes. We
provide schemes to eliminate this phase ambiguity in a highly efficient way,
providing phase estimates with uncertainty that is within a small constant
factor of the Heisenberg limit, the minimum allowed by the laws of quantum
mechanics. These techniques apply to NOON state and multi-pass interferometry,
as well as phase measurements in quantum computing. We have reported the
experimental implementation of some of these schemes with multi-pass
interferometry elsewhere. Here we present the theoretical foundation, and also
present some new experimental results. There are three key innovations to the
theory in this paper. First, we examine the intrinsic phase properties of the
sequence of states (in multiple time modes) via the equivalent two-mode state.
Second, we identify the key feature of the equivalent state that enables the
optimal scaling of the intrinsic phase uncertainty to be obtained. This enables
us to identify appropriate combinations of states to use. The remaining
difficulty is that the ideal phase measurements to achieve this intrinic phase
uncertainty are often not physically realizable. The third innovation is to
solve this problem by using realizable measurements that closely approximate
the optimal measurements, enabling the optimal scaling to be preserved.Comment: 23 pages, 10 figures; new general definition of resource
A short proof of stability of topological order under local perturbations
Recently, the stability of certain topological phases of matter under weak
perturbations was proven. Here, we present a short, alternate proof of the same
result. We consider models of topological quantum order for which the
unperturbed Hamiltonian can be written as a sum of local pairwise
commuting projectors on a -dimensional lattice. We consider a perturbed
Hamiltonian involving a generic perturbation that can be written
as a sum of short-range bounded-norm interactions. We prove that if the
strength of is below a constant threshold value then has well-defined
spectral bands originating from the low-lying eigenvalues of . These bands
are separated from the rest of the spectrum and from each other by a constant
gap. The width of the band originating from the smallest eigenvalue of
decays faster than any power of the lattice size.Comment: 15 page
Quantum picturalism for topological cluster-state computing
Topological quantum computing is a way of allowing precise quantum
computations to run on noisy and imperfect hardware. One implementation uses
surface codes created by forming defects in a highly-entangled cluster state.
Such a method of computing is a leading candidate for large-scale quantum
computing. However, there has been a lack of sufficiently powerful high-level
languages to describe computing in this form without resorting to single-qubit
operations, which quickly become prohibitively complex as the system size
increases. In this paper we apply the category-theoretic work of Abramsky and
Coecke to the topological cluster-state model of quantum computing to give a
high-level graphical language that enables direct translation between quantum
processes and physical patterns of measurement in a computer - a "compiler
language". We give the equivalence between the graphical and topological
information flows, and show the applicable rewrite algebra for this computing
model. We show that this gives us a native graphical language for the design
and analysis of topological quantum algorithms, and finish by discussing the
possibilities for automating this process on a large scale.Comment: 18 pages, 21 figures. Published in New J. Phys. special issue on
topological quantum computin
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