81 research outputs found
Topological R\'enyi entropy after a quantum quench
We present an analytical study on the resilience of topological order after a
quantum quench. The system is initially prepared in the ground state of the
toric-code model, and then quenched by switching on an external magnetic field.
During the subsequent time evolution, the variation in topological order is
detected via the topological Renyi entropy of order 2. We consider two
different quenches: the first one has an exact solution, while the second one
requires perturbation theory. In both cases, we find that the long-term time
average of the topological Renyi entropy in the thermodynamic limit is the same
as its initial value. Based on our results, we argue that topological order is
resilient against a wide range of quenches.Comment: 5 pages, 4 figures, published version with structural changes, see
supplemental material at
http://link.aps.org/supplemental/10.1103/PhysRevLett.110.17060
Quantum geometric tensor away from Equilibrium
The manifold of ground states of a family of quantum Hamiltonians can be
endowed with a quantum geometric tensor whose singularities signal quantum
phase transitions and give a general way to define quantum phases. In this
paper, we show that the same information-theoretic and geometrical approach can
be used to describe the geometry of quantum states away from equilibrium. We
construct the quantum geometric tensor for ensembles of states
that evolve in time and study its phase diagram and equilibration properties.
If the initial ensemble is the manifold of ground states, we show that the
phase diagram is conserved, that the geometric tensor equilibrates after a
quantum quench, and that its time behavior is governed by out-of-time-order
commutators (OTOCs). We finally demonstrate our results in the exactly solvable
Cluster-XY model
Thermalization of Topological Entropy after a Quantum Quench
In two spatial dimensions, topological order is robust for static
deformations at zero temperature, while it is fragile at any finite
temperature. How robust is topological order after a quantum quench? In this
paper we show that topological order thermalizes under the unitary evolution
after a quantum quench. If the quench preserves gauge symmetry, there is a
residual topological entropy exactly like in the finite temperature case.
We obtain this result by studying the time evolution of the topological
2-R\'enyi entropy in a fully analytical, exact way. These techniques can be
then applied to systems with strong disorder to show whether a many-body
localization phenomenon appears in topologically ordered systems.Comment: new plot with finite time results added; typos fixe
Bipartite entanglement and entropic boundary law in lattice spin systems
We investigate bipartite entanglement in spin-1/2 systems on a generic
lattice. For states that are an equal superposition of elements of a group
of spin flips acting on the fully polarized state , we
find that the von Neumann entropy depends only on the boundary between the two
subsystems and . These states are stabilized by the group . A
physical realization of such states is given by the ground state manifold of
the Kitaev's model on a Riemann surface of genus . For a square
lattice, we find that the entropy of entanglement is bounded from above and
below by functions linear in the perimeter of the subsystem and is equal to
the perimeter (up to an additive constant) when is convex. The entropy of
entanglement is shown to be related to the topological order of this model.
Finally, we find that some of the ground states are absolutely entangled, i.e.,
no partition has zero entanglement. We also provide several examples for the
square lattice.Comment: 10 pages, figs, RevTeX
Adiabatic Preparation of Topological Order
Topological order characterizes those phases of matter that defy a
description in terms of symmetry and cannot be distinguished in terms local
order parameters. This type of order plays a key role in the theory of the
fractional quantum Hall effect, as well as in topological quantum information
processing. Here we show that a system of n spins forming a lattice on a
Riemann surface can undergo a second order quantum phase transition between a
spin-polarized phase and a string-net condensed phase. This is an example of a
phase transition between magnetic and topological order. We furthermore show
how to prepare the topologically ordered phase through adiabatic evolution in a
time that is upper bounded by O(\sqrt{n}). This provides a physically plausible
method for constructing a topological quantum memory. We discuss applications
to topological and adiabatic quantum computing.Comment: 4 pages, one figure. v4: includes new error estimates for the
adiabatic evolutio
Transitions in Entanglement Complexity in Random Circuits
Entanglement is the defining characteristic of quantum mechanics. Bipartite
entanglement is characterized by the von Neumann entropy. Entanglement is not
just described by a number, however; it is also characterized by its level of
complexity. The complexity of entanglement is at the root of the onset of
quantum chaos, universal distribution of entanglement spectrum statistics,
hardness of a disentangling algorithm and of the quantum machine learning of an
unknown random circuit, and universal temporal entanglement fluctuations. In
this paper, we numerically show how a crossover from a simple pattern of
entanglement to a universal, complex pattern can be driven by doping a random
Clifford circuit with gates. This work shows that quantum complexity and
complex entanglement stem from the conjunction of entanglement and
non-stabilizer resources, also known as magic
Entanglement of Random hypergraph states
Random quantum states and operations are of fundamental and practical interests. In this paper, we investigate the entanglement properties of random hypergraph states, which generalize the notion of graph states by applying generalized controlled-phase gates on an initial reference product state. In particular, we study the two ensembles generated by random controlled-Z (CZ) and controlled-controlled-Z (CCZ) gates, respectively. By applying tensor network representation and combinational counting, we analytically show that the average subsystem purity and entanglement entropy for the two ensembles feature the same volume law but greatly differ in typicality, namely, the purity fluctuation is small and universal for the CCZ ensemble while it is large for the CZ ensemble. We discuss the implications of these results for the onset of entanglement complexity and quantum chaos
Universality and robustness of revivals in the transverse field XY model
We study the structure of the revivals in an integrable quantum many-body
system, the transverse field XY spin chain, after a quantum quench. The time
evolutions of the Loschmidt echo, the magnetization, and the single spin
entanglement entropy are calculated. We find that the revival times for all of
these observables are given by integer multiples of T_rev \sim L / v_max where
L is the linear size of the system and v_max is the maximal group velocity of
quasiparticles. This revival structure is universal in the sense that it does
not depend on the initial state and the size of the quench. Applying
non-integrable perturbations to the XY model, we observe that the revivals are
robust against such perturbations: they are still visible at time scales much
larger than the quasiparticle lifetime. We therefore propose a generic
connection between the revival structure and the locality of the dynamics,
where the quasiparticle speed v_max generalizes into the LiebRobinson speed
v_LR.Comment: completely overhauled version including results on non integrable
model
- …