357 research outputs found
Higgs Mode and Magnon Interactions in 2D Quantum Antiferromagnets from Raman Scattering
We present a theory for Raman scattering on 2D quantum antiferromagnets. The
microscopic Fleury-Loudon Hamiltonian is expressed in terms of an effective
- model. Well within the N\'eel ordered phase, the Raman spectrum
contains a two-magnon and a two-Higgs contribution, which are calculated
diagramatically. The vertex functions for both the Higgs and magnon
contributions are determined from a numerical solution of the corresponding
Bethe-Salpeter equation. Due to the momentum dependence of the Raman vertex in
the relevant symmetry, the contribution from the Higgs mode is
strongly suppressed. Except for intermediate values of the Higgs mass, it does
not show up as separate peak in the spectrum but gives rise to a broad
continuum above the dominant contribution from two-magnon excitations. The
latter give rise to a broad, asymmetric peak at , which
is a result of magnon-magnon interactions mediated by the Higgs mode. The full
Raman spectrum is determined completely by the antiferromagnetic exchange
coupling and a dimensionless Higgs mass. Experimental Raman spectra of
undoped cuprates turn out to be in very good agreement with the theory only
with inclusion of the Higgs contribution. They thus provide a clear signature
of the presence of a Higgs mode in spin one-half 2D quantum antiferromagnets.Comment: 12 pages, 15 figure
Exploring the grand-canonical phase diagram of interacting bosons in optical lattices by trap squeezing
In this paper we theoretically discuss how quantum simulators based on
trapped cold bosons in optical lattices can explore the grand-canonical phase
diagram of homogeneous lattice boson models, via control of the trapping
potential independently of all other experimental parameters (trap squeezing).
Based on quantum Monte Carlo, we establish the general scaling relation linking
the global chemical potential to the Hamiltonian parameters of the Bose-Hubbard
model in a parabolic trap, describing cold bosons in optical lattices; we find
that this scaling relation is well captured by a modified Thomas-Fermi scaling
behavior - corrected for quantum fluctuations - in the case of high enough
density and/or weak enough interactions, and by a mean-field Gutzwiller Ansatz
over a much larger parameter range. The above scaling relation allows to
control experimentally the chemical potential, independently of all other
Hamiltonian parameters, via trap squeezing; given that the global chemical
potential coincides with the local chemical potential in the trap center,
measurements of the central density as a function of the chemical potential
gives access to the information on the bulk compressibility of the Bose-Hubbard
model. Supplemented with time-of-flight measurements of the coherence
properties, the measurement of compressibility enables one to discern among the
various possible phases realized by bosons in an optical lattice with or
without external (periodic or random) potentials -- e.g. superfluid, Mott
insulator, band insulator, and Bose glass. We theoretically demonstrate the
trap-squeezing investigation of the above phases in the case of bosons in a
one-dimensional optical lattice, and in a one-dimensional incommensurate
superlattice.Comment: 27 pages, 26 figures. v2: added references and further discussion of
the local-density approximation
Anomalous fluctuations in phases with a broken continuous symmetry
It is shown that the Goldstone modes associated with a broken continuous
symmetry lead to anomalously large fluctuations of the zero field order
parameter at any temperature below T_c. In dimensions 2<d<4, the variance of
the extensive spontaneous magnetization scales as L^4 with the system size L,
independent of the order parameter dynamics. The anomalous scaling is a
consequence of the 1/q^{4-d} divergence of the longitudinal susceptibility. For
ground states in two dimensions with Goldstone modes vanishing linearly with
momentum, the dynamical susceptibility contains a singular contribution
(q^2-\omega^2/c^2)^{-1/2}. The dynamic structure factor thus exhibits a
critical continuum above the undamped spin wave pole, which may be detected by
neutron scattering in the N\'eel-phase of 2D quantum antiferromagnets.Comment: final version, minor change
Long-range big quantum-data transmission
We introduce an alternative type of quantum repeater for long-range quantum
communication with improved scaling with the distance. We show that by
employing hashing, a deterministic entanglement distillation protocol with
one-way communication, one obtains a scalable scheme that allows one to reach
arbitrary distances, with constant overhead in resources per repeater station,
and ultrahigh rates. In practical terms, we show that also with moderate
resources of a few hundred qubits at each repeater station, one can reach
intercontinental distances. At the same time, a measurement-based
implementation allows one to tolerate high loss, but also operational and
memory errors of the order of several percent per qubit. This opens the way for
long-distance communication of big quantum data.Comment: revised manuscript including new result
Atomic quantum dots coupled to BEC reservoirs
We study the dynamics of an atomic quantum dot, i.e. a single atom in a tight
optical trap which is coupled to a superfluid reservoir via laser transitions.
Quantum interference between the collisional interactions and the laser induced
coupling to the phase fluctuations of the condensate results in a tunable
coupling of the dot to a dissipative phonon bath, allowing an essentially
complete decoupling from the environment. Quantum dots embedded in a 1D
Luttinger liquid of cold bosonic atoms realize a spin-Boson model with ohmic
coupling, which exhibits a dissipative phase transition and allows to directly
measure atomic Luttinger parameters.Comment: 5 pages, 2 figures. Submitted version. For the particular 1D case and
its relation with Kondo physics see cond-mat/021241
Simple proof of confidentiality for private quantum channels in noisy environments
Complete security proofs for quantum communication protocols can be
notoriously involved, which convolutes their verification, and obfuscates the
key physical insights the security finally relies on. In such cases, for the
majority of the community, the utility of such proofs may be restricted. Here
we provide a simple proof of confidentiality for parallel quantum channels
established via entanglement distillation based on hashing, in the presence of
noise, and a malicious eavesdropper who is restricted only by the laws of
quantum mechanics. The direct contribution lies in improving the linear
confidentiality levels of recurrence-type entanglement distillation protocols
to exponential levels for hashing protocols. The proof directly exploits the
security relevant physical properties: measurement-based quantum computation
with resource states and the separation of Bell-pairs from an eavesdropper. The
proof also holds for situations where Eve has full control over the input
states, and obtains all information about the operations and noise applied by
the parties. The resulting state after hashing is private, i.e., disentangled
from the eavesdropper. Moreover, the noise regimes for entanglement
distillation and confidentiality do not coincide: Confidentiality can be
guaranteed even in situation where entanglement distillation fails. We extend
our results to multiparty situations which are of special interest for secure
quantum networks.Comment: 5 + 11 pages, 0 + 4 figures, A. Pirker and M. Zwerger contributed
equally to this work, replaced with accepted versio
Backflow in a Fermi Liquid
We calculate the backflow current around a fixed impurity in a Fermi liquid.
The leading contribution at long distances is radial and proportional to 1/r^2.
It is caused by the current induced density modulation first discussed by
Landauer. The familiar 1/r^3 dipolar backflow obtained in linear response by
Pines and Nozieres is only the next to leading term, whose strength is
calculated here to all orders in the scattering. In the charged case the
condition of perfect screening gives rise to a novel sum rule for the phase
shifts. Similar to the behavior in a classical viscous liquid, the friction
force is due only to the leading contribution in the backflow while the dipolar
term does not contribute.Comment: 4 pages, 1 postscript figure, uses ReVTeX and epsfig macro, submitted
to Physical Review Letter
Oscillating Casimir force between impurities in one-dimensional Fermi liquids
We study the interaction of two localized impurities in a repulsive
one-dimensional Fermi liquid via bosonization. In a previous paper [Phys. Rev.
A 72, 023616 (2005)], it was shown that at distances much larger than the
interparticle spacing the impurities interact through a Casimir-type force
mediated by the zero sound phonons of the underlying quantum liquid. Here we
extend these results and show that the strength and sign of this Casimir
interaction depend sensitively on the impurities separation. These oscillations
in the Casimir interaction have the same period as Friedel oscillations. Their
maxima correspond to tunneling resonances tuned by the impurities separation.Comment: This paper is a continuation of Phys. Rev. A 72, 023616 (2005). v2:
two appendix adde
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