15,808 research outputs found
Coherent States in Field Theory
Coherent states have three main properties: coherence, overcompleteness and
intrinsic geometrization. These unique properties play fundamental roles in
field theory, especially, in the description of classical domains and quantum
fluctuations of physical fields, in the calculations of physical processes
involving infinite number of virtual particles, in the derivation of functional
integrals and various effective field theories, also in the determination of
long-range orders and collective excitations, and finally in the exploration of
origins of topologically nontrivial gauge fields and associated gauge degrees
of freedom.Comment: 33 page, Invited Article for a forthcoming Indian National Science
Academy publicatio
Light-Front QCD and Heavy Quark Systems
In this series of lectures, I shall begin with the current investigations on
phenomenology of hadron dynamics to demonstrate the importance of solving
hadronic bound states within the framework of light-front (LF) QCD. Then, I
will describe the basic procedure how to formulate the canonical theory of
LFQCD, including light-front quantization of QCD, light-front gauge
singularity, and light-front two-component formalism. I will also present a
complete one-loop QCD calculation in terms of the light-front time-ordering
perturbation theory, in comparison with the usual covariant perturbative QCD
calculation. Following thereby I will discuss the development of heavy-quark
effective theory and the manifestation of heavy quark symmetry on the
light-front. Finally, by applying recently developed similarity renormalization
group approach to light-front heavy quark effective theory, I will show a
rigorous derivation of quark confinement interaction from LFQCD and its
application to solve heavy hadron bound states.Comment: lecture notes, 45 pages. Based on lectures delivered at the First
International School on Light-Front Quantization and Non-Perturbative QCD,
May 6 - June 2, 1996, International Institute of Theoretical and Applied
Physics, Iowa State University, Ames, IA, U.S.
Quantum Heisenberg Antiferromagnets versus Nonlinear Model Without the Large S Limit
In this letter, I develop a new topologically invariant coherent state path
integral for spin systems, and apply it to the quantum Heisenberg model on a
square lattice. As a result, the quantum nonlinear model for arbitrary
values of spin can be directly obtained. The effective coupling constant and
spin wave velocity are modified by and , where is a
natural temperature scale for the reliability of the theory. The formulation
can also be extended to other generalized coherent state path integrals.Comment: RevTex, 4page, no figur
Quantum Nonlinear Sigma Model for Arbitrary Spin Heisenberg Antiferromagnets
In this Letter, we derive a quantum nonlinear sigma model (QNLSM) for quantum
Heisenberg antiferromagnets (QHA) with arbitrary S (spin) values. A upper limit
of the low temperature is naturally carried out for the reliability of the
QNLSM. The S dependence of the effective coupling constant and the spin wave
velocity in the QNLSM are also obtained explicitly. The resulting spin wave
velocity for 2-dim spin-1/2 QHA highly concurs with the experimental data of
high compound La2CuO4. The predicted correlation lengths for 2d QHA and
spin-gap magnitudes for 1d QHA also agrees with the accurate numerical results.Comment: 4 pages, 1 figure, minor revision of the tex
Decoherence suppression of open quantum systems through a strong coupling to non-Markovian reservoirs
In this paper, we provide a mechanism of decoherence suppression for open
quantum systems in general, and that for "Schrodinger cat-like" state in
particular, through the strong couplings to non-Markovian reservoirs. Different
from the usual strategies of suppressing decoherence by decoupling the system
from the environment in the literatures, here the decoherence suppression
employs the strong back-reaction from non-Markovian reservoirs. The mechanism
relies on the existence of the singularities (bound states) of the
nonequilibrium retarded Green function which completely determines the
dissipation and decoherence dynamics of open systems. As an application, we
examine the decoherence dynamics of a photonic crystal nanocavity that is
coupled to a waveguide. The strong non-Markovian suppression of decoherence for
the optical cat state is attained.Comment: 5 pages, 4 figure
Decoherence dynamics of Majorana qubits under braiding operations
We study the decoherence dynamics of Majorana qubit braiding operations in a
topological superconducting chain (TSC) system, in which the braiding is
performed by controlling the electron-chemical potentials of the TSCs and the
couplings between them. By solving rigorously the Majorana qubit dynamics, we
show how the Majorana qubit coherence is generated through bogoliubon
correlations formed by exchanging Majorana zero modes (MZMs) between two TSCs
in braiding operations. Using the exact master equation, we demonstrate how
MZMs and also the bogoliubon correlations dissipate due to charge fluctuations
of the controlling gates at both the zero and finite temperatures. As a result,
Majorana qubit coherence and the fermion parity conservation cannot be immune
from local perturbations during braiding operations.Comment: 6 pages, 3 figures, Supplementary material
Exact homogeneous master equation for open quantum systems incorporating initial correlations
We show that the exact master equation incorporating initial correlations for
open quantum systems, within the Nakajima-Zwanzig operator-projection method,
is a homegenous master equation for the reduced density matrix. We also derive
explicitly the exact master equation for a large class of bosonic and fermionic
open quantum systems incorporating initial correlations, the resulting master
equation is homogenous. We find that the effects of the initial correlations
can be fully embedded into the fluctuation dynamics through the exact
homogenous master equation. Also a generalized nonequilibrium
fluctuation-dissipation theorem incorporating the initial correlations is
obtained.Comment: 5 page
Non-Markovian entanglement dynamics of noisy continuous variable quantum channels
We investigate the entanglement dynamics of continuous-variable quantum
channels in terms of an entangled squeezed state of two cavity fields in a
general non-Markovian environment. Using the Feynman-Vernon influence
functional theory in the coherent-state representation, we derive an exact
master equation with time-dependent coefficients reflecting the non-Markovian
influence of the environment. The influence of environments with different
spectral densities, e.g., Ohmic, sub-Ohmic, and super-Ohmic, is numerically
studied. The non-Markovian process shows its remarkable influences on the
entanglement dynamics due to the sensitive time-dependence of the dissipation
and noise functions within the typical time scale of the environment. The Ohmic
environment shows a weak dissipation-noise effect on the entanglement dynamics,
while the sub-Ohmic and super-Ohmic environments induce much more severe noise.
In particular, the memory of the system interacting with the environment
contributes a strong decoherence effect to the entanglement dynamics in the
super-Ohmic case.Comment: The final versio
Non-equilibrium theory of charge qubit decoherence in the quantum point contact measurement
A non-equilibrium theory describing the charge qubit dynamics measured by a
quantum point contact is developed based on Schwinger-Keldysh's approach. Using
the real-time diagram technique, we derive the master equation to all orders in
perturbation expansions. The non-Markovian processes in the qubit dynamics is
naturally taken into account. The qubit decoherence, in particular, the
influence of the tunneling-electron fluctuation in the quantum point contact
with a longer time correlation, is studied in the framework. We consider the
Lorentzian-type spectral density to characterize the channel mixture of the
electron tunneling processes induced by the measurement and determine the
correlation time scale of the tunneling-electron fluctuation. The result shows
that as the quantum point contact is casted with a narrower profile of the
spectral density, tunneling electrons can propagate with a longer time
correlation and lead to the non-Markovian processes of the qubit dynamics. The
qubit electron in the charge qubit will be driven coherently. The quantum point
contact measurement with the minimum deviation of the electron tunneling
processes prevents the qubit state from the decoherence.Comment: 14 pages, 7 figure
Quantum Transport Theory for Photonic Networks
In this paper, we develop a quantum transport theory to describe photonic
transport in photonic networks. The photonic networks concerned in the paper
consist of all-optical circuits incorporating photonic bandgap waveguides and
driven resonators. The photonic transport flowing through waveguides are
entirely determined from the exact master equation of the driven resonators.
The master equation of the driven resonators is obtained by explicitly
eliminating all the waveguide degrees of freedom while the back-reactions
between resonators and waveguides are fully taken into account. The relations
between the driven photonic dynamics and photocurrents are obtained. The
non-Markovian memory structure and quantum coherence and decoherence effects in
photonic transport are also fully included. This quantum transport theory
unifies two fundamental nonequilibrium approaches, the Keldysh's nonequilibrium
Green function technique and the Feynman-Vernon influence functional approach,
together to make the investigation of the transient quantum photonic transport
become more powerful. As an illustration, the theory is applied to the
transport phenomena of a driven nanocavity coupled to two waveguides in
photonic crystals. The controllability of photonic transport through the driven
resonator is demonstrated.Comment: 19 pages, 6 figures (the paper is extended, more references are
added
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