666 research outputs found
A generalized multi-polaron expansion for the spin-boson model: Environmental entanglement and the biased two-state system
We develop a systematic variational coherent state expansion for the
many-body ground state of the spin-boson model, in which a quantum two-level
system is coupled to a continuum of harmonic oscillators. Energetic constraints
at the heart of this technique are rationalized in terms of polarons
(displacements of the bath states in agreement with classical expectations) and
antipolarons (counter-displacements due to quantum tunneling effects). We
present a comprehensive study of the ground state two-level system population
and coherence as a function of tunneling amplitude, dissipation strength, and
bias (akin to asymmetry of the double well potential defining the two-state
system). The entanglement among the different environmental modes is
investigated by looking at spectroscopic signatures of the bipartite
entanglement entropy between a given environmental mode and all the other
modes. We observe a drastic change in behavior of this entropy for increasing
dissipation, indicative of the entangled nature of the environmental states. In
addition, the entropy spreads over a large energy range at strong dissipation,
a testimony to the wide entanglement window characterizing the underlying Kondo
state. Finally, comparisons to accurate numerical renormalization group
calculations and to the exact Bethe Ansatz solution of the model demonstrate
the rapid convergence of our variationally-optimized multi-polaron expansion,
suggesting that it should also be a useful tool for dissipative models of
greater complexity, as relevant for numerous systems of interest in quantum
physics and chemistry.Comment: 17 pages, 14 figure
Weak-Localization in Chaotic Versus Non-Chaotic Cavities: A Striking Difference in the Line Shape
We report experimental evidence that chaotic and non-chaotic scattering
through ballistic cavities display distinct signatures in quantum transport. In
the case of non-chaotic cavities, we observe a linear decrease in the average
resistance with magnetic field which contrasts markedly with a Lorentzian
behavior for a chaotic cavity. This difference in line-shape of the
weak-localization peak is related to the differing distribution of areas
enclosed by electron trajectories. In addition, periodic oscillations are
observed which are probably associated with the Aharonov-Bohm effect through a
periodic orbit within the cavities.Comment: 4 pages revtex + 4 figures on request; amc.hub.94.
Stabilizing Spin Coherence Through Environmental Entanglement in Strongly Dissipative Quantum Systems
The key feature of a quantum spin coupled to a harmonic bath---a model
dissipative quantum system---is competition between oscillator potential energy
and spin tunneling rate. We show that these opposing tendencies cause
environmental entanglement through superpositions of adiabatic and
antiadiabatic oscillator states, which then stabilizes the spin coherence
against strong dissipation. This insight motivates a fast-converging
variational coherent-state expansion for the many-body ground state of the
spin-boson model, which we substantiate via numerical quantum tomography.Comment: 5 pages, 3 figures, supplementary file attached. This article
supersedes arXiv:1301.743
Waveguide QED: Many-Body Bound State Effects on Coherent and Fock State Scattering from a Two-Level System
Strong coupling between a two-level system (TLS) and bosonic modes produces
dramatic quantum optics effects. We consider a one-dimensional continuum of
bosons coupled to a single localized TLS, a system which may be realized in a
variety of plasmonic, photonic, or electronic contexts. We present the exact
many-body scattering eigenstate obtained by imposing open boundary conditions.
Multi-photon bound states appear in the scattering of two or more photons due
to the coupling between the photons and the TLS. Such bound states are shown to
have a large effect on scattering of both Fock and coherent state wavepackets,
especially in the intermediate coupling strength regime. We compare the
statistics of the transmitted light with a coherent state having the same mean
photon number: as the interaction strength increases, the one-photon
probability is suppressed rapidly, and the two- and three-photon probabilities
are greatly enhanced due to the many-body bound states. This results in
non-Poissonian light.Comment: 10 page
Detecting Photon-Photon Interactions in a Superconducting Circuit
A local interaction between photons can be engineered by coupling a nonlinear
system to a transmission line. The required high impedance transmission line
can be conveniently formed from a chain of Josephson junctions. The
nonlinearity is generated by side-coupling this chain to a Cooper pair box. We
propose to probe the resulting photon-photon interactions via their effect on
the current-voltage characteristic of a voltage-biased Josephson junction
connected to the transmission line. Considering the Cooper pair box to be in
the weakly anharmonic regime, we find that the dc current through the probe
junction yields features around the voltages , where
is the plasma frequency of the superconducting circuit. The features
at are a direct signature of the photon-photon interaction in the
system.Comment: 10 pages, 7 figure
Semiclassical Propagation of Wavepackets with Real and Complex Trajectories
We consider a semiclassical approximation for the time evolution of an
originally gaussian wave packet in terms of complex trajectories. We also
derive additional approximations replacing the complex trajectories by real
ones. These yield three different semiclassical formulae involving different
real trajectories. One of these formulae is Heller's thawed gaussian
approximation. The other approximations are non-gaussian and may involve
several trajectories determined by mixed initial-final conditions. These
different formulae are tested for the cases of scattering by a hard wall,
scattering by an attractive gaussian potential, and bound motion in a quartic
oscillator. The formula with complex trajectories gives good results in all
cases. The non-gaussian approximations with real trajectories work well in some
cases, whereas the thawed gaussian works only in very simple situations.Comment: revised text, 24 pages, 6 figure
The Thermopower of Quantum Chaos
The thermovoltage of a chaotic quantum dot is measured using a current
heating technique. The fluctuations in the thermopower as a function of
magnetic field and dot shape display a non-Gaussian distribution, in agreement
with simulations using Random Matrix Theory. We observe no contributions from
weak localization or short trajectories in the thermopower.Comment: 4 pages, 3 figures, corrected: accidently omitted author in the
Authors list, here (not in the article
Mesoscopic Transport Through Ballistic Cavities: A Random S-Matrix Theory Approach
We deduce the effects of quantum interference on the conductance of chaotic
cavities by using a statistical ansatz for the S matrix. Assuming that the
circular ensembles describe the S matrix of a chaotic cavity, we find that the
conductance fluctuation and weak-localization magnitudes are universal: they
are independent of the size and shape of the cavity if the number of incoming
modes, N, is large. The limit of small N is more relevant experimentally; here
we calculate the full distribution of the conductance and find striking
differences as N changes or a magnetic field is applied.Comment: 4 pages revtex 3.0 (2-column) plus 2 postscript figures (appended),
hub.pam.94.
Strongly correlated photons generated by coupling a three- or four-level system to a waveguide
We study the generation of strongly correlated photons by coupling an atom to
photonic quantum fields in a one-dimensional waveguide. Specifically, we
consider a three-level or four-level system for the atom. Photon-photon bound
states emerge as a manifestation of the strong photon-photon correlation
mediated by the atom. Effective repulsive or attractive interaction between
photons can be produced, causing either suppressed multiphoton transmission
(photon blockade) or enhanced multiphoton transmission (photon-induced
tunneling). As a result, nonclassical light sources can be generated on demand
by sending coherent states into the proposed system. We calculate the
second-order correlation function of the transmitted field and observe bunching
and antibunching caused by the bound states. Furthermore, we demonstrate that
the proposed system can produce photon pairs with a high degree of spectral
entanglement, which have a large capacity for carrying information and are
important for large-alphabet quantum communication.Comment: 13+ pages, 7 figure
Metabotropic glutamate receptor 2/3 (mGluR2/3) activation suppresses TRPV1 sensitization in mouse, but not human sensory neurons
AbstractThe use of human tissue to validate putative analgesic targets identified in rodents is a promising strategy for improving the historically poor translational record of preclinical pain research. We recently demonstrated that in mouse and human sensory neurons, agonists for metabotropic glutamate receptors 2 and 3 (mGluR2/3) reduce membrane hyperexcitability produced by the inflammatory mediator prostaglandin E2(PGE2). Previous rodent studies indicate that mGluR2/3 can also reduce peripheral sensitization by suppressing inflammation-induced sensitization of TRPV1. Whether this observation similarly translates to human sensory neurons has not yet been tested. We found that activation of mGluR2/3 with the agonist APDC suppressed PGE2-induced sensitization of TRPV1 in mouse, but not human, sensory neurons. We also evaluated sensory neuron expression of the gene transcripts for mGluR2 (Grm2), mGluR3 (Grm3), and TRPV1 (Trpv1). The majority ofTrpv1+mouse and human sensory neurons expressedGrm2and/orGrm3, and in both mice and humans,Grm2was expressed in a greater percentage of sensory neurons thanGrm3. Although we demonstrated a functional difference in the modulation of TRPV1 sensitization by mGluR2/3 activation between mouse and human, there were no species differences in the gene transcript colocalization of mGluR2 or mGluR3 with TRPV1 that might explain this functional difference. Taken together with our previous work, these results suggest that mGluR2/3 activation suppresses only some aspects of human sensory neuron sensitization caused by PGE2. These differences have implications for potential healthy human voluntary studies or clinical trials evaluating the analgesic efficacy of mGluR2/3 agonists or positive allosteric modulators.</jats:p
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