565 research outputs found
Engineering correlation and entanglement dynamics in spin systems
We show that the correlation and entanglement dynamics of spin systems can be
understood in terms of propagation of spin waves. This gives a simple, physical
explanation of the behaviour seen in a number of recent works, in which a
localised, low-energy excitation is created and allowed to evolve. But it also
extends to the scenario of translationally invariant systems in states far from
equilibrium, which require less local control to prepare. Spin-wave evolution
is completely determined by the system's dispersion relation, and the latter
typically depends on a small number of external, physical parameters.
Therefore, this new insight into correlation dynamics opens up the possibility
not only of predicting but also of controlling the propagation velocity and
dispersion rate, by manipulating these parameters. We demonstrate this
analytically in a simple, example system.Comment: 4 pages, 4 figures, REVTeX4 forma
Entanglement flow in multipartite systems
We investigate entanglement dynamics in multipartite systems, establishing a
quantitative concept of entanglement flow: both flow through individual
particles, and flow along general networks of interacting particles. In the
former case, the rate at which a particle can transmit entanglement is shown to
depend on that particle's entanglement with the rest of the system. In the
latter, we derive a set of entanglement rate equations, relating the rate of
entanglement generation between two subsets of particles to the entanglement
already present further back along the network. We use the rate equations to
derive a lower bound on entanglement generation in qubit chains, and compare
this to existing entanglement creation protocols.Comment: 13 pages, 5 figures, REVTeX format. Proof of lemma 3 corrected.
Restructured and expande
Separable states can be used to distribute entanglement
We show that no entanglement is necessary to distribute entanglement; that
is, two distant particles can be entangled by sending a third particle that is
never entangled with the other two. Similarly, two particles can become
entangled by continuous interaction with a highly mixed mediating particle that
never itself becomes entangled. We also consider analogous properties of
completely positive maps, in which the composition of two separable maps can
create entanglement.Comment: 4 pages, 2 figures. Slight modification
Translationally Invariant Universal Quantum Hamiltonians in 1D
. Recent work has characterized rigorously what it means for one
quantum system to simulate another and demonstrated the existence of
universal Hamiltonians—simple spin lattice Hamiltonians that can replicate the entire physics of any other quantum many-body system. Previous
universality results have required proofs involving complicated ‘chains’ of
perturbative ‘gadgets.’ In this paper, we derive a significantly simpler
and more powerful method of proving universality of Hamiltonians, directly leveraging the ability to encode quantum computation into ground
states. This provides new insight into the origins of universal models and
suggests a deep connection between universality and complexity. We apply this new approach to show that there are universal models even in
translationally invariant spin chains in 1D. This gives as a corollary a
new Hamiltonian complexity result that the local Hamiltonian problem
for translationally invariant spin chains in one dimension with an exponentially small promise gap is PSPACE-complete. Finally, we use these
new universal models to construct the first known toy model of 2D–1D
holographic duality between local Hamiltonians
Magnetic field control of cycloidal domains and electric polarization in multiferroic BiFeO
The magnetic field induced rearrangement of the cycloidal spin structure in
ferroelectric mono-domain single crystals of the room-temperature multiferroic
BiFeO is studied using small-angle neutron scattering (SANS). The cycloid
propagation vectors are observed to rotate when magnetic fields applied
perpendicular to the rhombohedral (polar) axis exceed a pinning threshold value
of 5\,T. In light of these experimental results, a phenomenological model
is proposed that captures the rearrangement of the cycloidal domains, and we
revisit the microscopic origin of the magnetoelectric effect. A new coupling
between the magnetic anisotropy and the polarization is proposed that explains
the recently discovered magnetoelectric polarization to the rhombohedral axis
Fundamental limitations in the purifications of tensor networks
We show a fundamental limitation in the description of quantum many-body
mixed states with tensor networks in purification form. Namely, we show that
there exist mixed states which can be represented as a translationally
invariant (TI) matrix product density operator (MPDO) valid for all system
sizes, but for which there does not exist a TI purification valid for all
system sizes. The proof is based on an undecidable problem and on the
uniqueness of canonical forms of matrix product states. The result also holds
for classical states.Comment: v1: 11 pages, 1 figure. v2: very minor changes. About to appear in
Journal of Mathematical Physic
Assessing non-Markovian dynamics
We investigate what a snapshot of a quantum evolution - a quantum channel
reflecting open system dynamics - reveals about the underlying continuous time
evolution. Remarkably, from such a snapshot, and without imposing additional
assumptions, it can be decided whether or not a channel is consistent with a
time (in)dependent Markovian evolution, for which we provide computable
necessary and sufficient criteria. Based on these, a computable measure of
`Markovianity' is introduced. We discuss how the consistency with Markovian
dynamics can be checked in quantum process tomography. The results also clarify
the geometry of the set of quantum channels with respect to being solutions of
time (in)dependent master equations.Comment: 5 pages, RevTex, 2 figures. (Except from typesetting) version to be
published in the Physical Review Letter
Effect of an electric field on a floating lipid bilayer: a neutron reflectivity study
We present here a neutron reflectivity study of the influence of an
alternative electric field on a supported phospholipid double bilayer. We
report for the first time a reproducible increase of the fluctuation amplitude
leading to the complete unbinding of the floating bilayer. Results are in good
agreement with a semi-quantitative interpretation in terms of negative
electrostatic surface tension.Comment: 12 pages, 7 figures, 1 table accepted for publication in European
Physical Journal E Replaced with with correct bibliograph
Square vortex lattice at anomalously low magnetic fields in electron-doped NdCeCuO
We report here on the first direct observations of the vortex lattice in the
bulk of electron-doped NdCeCuO single crystals. Using
small angle neutron scattering, we have observed a square vortex lattice with
the nearest-neighbors oriented at 45 from the Cu-O bond direction,
which is consistent with theories based on the d-wave superconducting gap.
However, the square symmetry persists down to unusually low magnetic fields.
Moreover, the diffracted intensity from the vortex lattice is found to decrease
rapidly with increasing magnetic field.Comment: 4 pages, 4 Figures, accepted for publication in Phys. Rev. Let
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