59 research outputs found
String order in dipole-blockaded quantum liquids
We study the quantum melting of quasi-one-dimensional lattice models in which
the dominant energy scale is given by a repulsive dipolar interaction. By
constructing an effective low-energy theory, we show that the melting of
crystalline phases can occur into two distinct liquid phases, having the same
algebraic decay of density-density correlations, but showing a different
non-local correlation function expressing string order. We present possible
experimental realizations using ultracold atoms and molecules, introducing an
implementation based on resonantly driven Rydberg atoms that offers additional
benefits compared to a weak admixture of the Rydberg state.Comment: 6 pages, 4 figure
An Operational Definition of Topological Order
The unrivaled robustness of topologically ordered states of matter against
perturbations has immediate applications in quantum computing and quantum
metrology, yet their very existence poses a challenge to our understanding of
phase transitions. In particular, topological phase transitions cannot be
characterized in terms of local order parameters, as it is the case with
conventional symmetry-breaking phase transitions. Currently, topological order
is mostly discussed in the context of nonlocal topological invariants or
indirect signatures like the topological entanglement entropy. However, a
comprehensive understanding of what actually constitutes topological order is
still lacking. Here we show that one can interpret topological order as the
ability of a system to perform topological error correction. We find that this
operational approach corresponding to a measurable observable does not only lay
the conceptual foundations for previous classifications of topological order,
but it can also be applied to hitherto inaccessible problems, such as the
question of topological order for mixed quantum states arising in open quantum
systems. We demonstrate the existence of topological order in open systems and
their phase transitions to topologically trivial states, including topological
criticality. Our results demonstrate the viability of topological order in
nonequilibrium quantum systems and thus substantially broaden the scope of
possible technological applications. We therefore expect our work to be a
starting point for many future theoretical and experimental investigations,
such as the application of our approach to fracton or Floquet topological
order, or the direct experimental realization of the error correction protocol
presented in our work for the development of future quantum technological
devices.Comment: 7 pages, 5 figure
Time evolution of open quantum many-body systems
We establish a generic method to analyze the time evolution of open quantum
many-body systems. Our approach is based on a variational integration of the
quantum master equation describing the dynamics and naturally connects to a
variational principle for its nonequilibrium steady state. We successfully
apply our variational method to study dissipative Rydberg gases, finding
excellent quantitative agreement with small-scale simulations of the full
quantum master equation. We observe that correlations related to non-Markovian
behavior play a significant role during the relaxation dynamics towards the
steady state. We further quantify this non-Markovianity and find it to be
closely connected to an information-theoretical measure of quantum and
classical correlations.Comment: 5+3 pages, 8 figure
Dissipative Preparation of Antiferromagnetic Order in the Fermi-Hubbard Model
The Fermi-Hubbard model is one of the key models of condensed matter physics,
which holds a potential for explaining the mystery of high-temperature
superconductivity. Recent progress in ultracold atoms in optical lattices has
paved the way to studying the model's phase diagram using the tools of quantum
simulation, which emerged as a promising alternative to the numerical
calculations plagued by the infamous sign problem. However, the temperatures
achieved using elaborate laser cooling protocols so far have been too high to
show the appearance of antiferromagnetic and superconducting quantum phases
directly. In this work, we demonstrate that using the machinery of dissipative
quantum state engineering, one can efficiently prepare antiferromagnetic order
in present-day experiments with ultracold fermions. The core of the approach is
to add incoherent laser scattering in such a way that the antiferromagnetic
state emerges as the dark state of the driven-dissipative dynamics. In order to
elucidate the development of the antiferromagnetic order we employ two
complementary techniques: Monte Carlo wave function simulations for small
systems and a recently proposed variational method for open quantum systems,
operating in the thermodynamic limit. The controlled dissipation channels
described in this work are straightforward to add to already existing
experimental setups.Comment: 9 pages, 5 figure
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