797 research outputs found
Photon statistics and dynamics of Fluorescence Resonance Energy Transfer
We report high time-resolution measurements of photon statistics from pairs
of dye molecules coupled by fluorescence resonance energy transfer (FRET). In
addition to quantum-optical photon antibunching, we observe photon bunching on
a timescale of several nanoseconds. We show by numerical simulation that
configuration fluctuations in the coupled fluorophore system could account for
minor deviations of our data from predictions of basic Forster theory. With
further characterization we believe that FRET photon statistics could provide a
unique tool for studying DNA mechanics on timescales from 10^-9 to 10^-3 s.Comment: 4 pages, 4 figures, submitted to Physical Review Letter
Tensor Networks with a Twist: Anyon-permuting domain walls and defects in PEPS
We study the realization of anyon-permuting symmetries of topological phases
on the lattice using tensor networks. Working on the virtual level of a
projected entangled pair state, we find matrix product operators (MPOs) that
realize all unitary topological symmetries for the toric and color codes. These
operators act as domain walls that enact the symmetry transformation on anyons
as they cross. By considering open boundary conditions for these domain wall
MPOs, we show how to introduce symmetry twists and defect lines into the state.Comment: 11 pages, 6 figures, 2 appendices, v2 published versio
All bipartite entangled states display some hidden nonlocality
We show that a violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality
can be demonstrated in a certain kind of Bell experiment for all bipartite
entangled states. Our protocol allows local filtering measurements and involves
shared ancilla states that do not themselves violate CHSH. Our result follows
from two main steps. We first provide a simple characterization of the states
that violate the CHSH-inequality after local filtering operations in terms of
witness-like operators. Second, we prove that for each entangled state
, there exists another state not violating CHSH, such that
violates CHSH. Hence, in this scenario, cannot be
substituted by classical correlations without changing the statistics of the
experiment; we say that is not simulable by classical correlations and
our result is that entanglement is equivalent to non-simulability.Comment: 5 pages, 1 figur
A complete family of separability criteria
We introduce a new family of separability criteria that are based on the
existence of extensions of a bipartite quantum state to a larger number
of parties satisfying certain symmetry properties. It can be easily shown that
all separable states have the required extensions, so the non-existence of such
an extension for a particular state implies that the state is entangled. One of
the main advantages of this approach is that searching for the extension can be
cast as a convex optimization problem known as a semidefinite program (SDP).
Whenever an extension does not exist, the dual optimization constructs an
explicit entanglement witness for the particular state. These separability
tests can be ordered in a hierarchical structure whose first step corresponds
to the well-known Positive Partial Transpose (Peres-Horodecki) criterion, and
each test in the hierarchy is at least as powerful as the preceding one. This
hierarchy is complete, in the sense that any entangled state is guaranteed to
fail a test at some finite point in the hierarchy, thus showing it is
entangled. The entanglement witnesses corresponding to each step of the
hierarchy have well-defined and very interesting algebraic properties that in
turn allow for a characterization of the interior of the set of positive maps.
Coupled with some recent results on the computational complexity of the
separability problem, which has been shown to be NP-hard, this hierarchy of
tests gives a complete and also computationally and theoretically appealing
characterization of mixed bipartite entangled states.Comment: 21 pages. Expanded introduction. References added, typos corrected.
Accepted for publication in Physical Review
Perturbative 2-body Parent Hamiltonians for Projected Entangled Pair States
We construct parent Hamiltonians involving only local 2-body interactions for
a broad class of Projected Entangled Pair States (PEPS). Making use of
perturbation gadget techniques, we define a perturbative Hamiltonian acting on
the virtual PEPS space with a finite order low energy effective Hamiltonian
that is a gapped, frustration-free parent Hamiltonian for an encoded version of
a desired PEPS. For topologically ordered PEPS, the ground space of the low
energy effective Hamiltonian is shown to be in the same phase as the desired
state to all orders of perturbation theory. An encoded parent Hamiltonian for
the double semion string net ground state is explicitly constructed as a
concrete example.Comment: 26 pages, 4 figures, v2 published versio
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