152 research outputs found
Deterministic creation of stationary entangled states by dissipation
We propose a practical physical system for creation of a stationary
entanglement by dissipation without employing the environment engineering
techniques. The system proposed is composed of two perfectly distinguishable
atoms, through their significantly different transition frequencies, with only
one atom addressed by an external laser field. We show that the arrangement
would easily be realized in practice by trapping the atoms at the distance
equal to the quarter-wavelength of a standing-wave laser field and locating one
of the atoms at a node and the other at the successive antinode of the wave.
The undesirable dipole-dipole interaction between the atoms, that could be
large at this small distance, is adjusted to zero by a specific initial
preparation of the atoms or by a specific polarization of the atomic dipole
moments. Following this arrangement, we show that the dissipative relaxation
can create a stationary entanglement on demand by tuning the Rabi frequency of
the laser field to the difference between the atomic transition frequencies.
The laser field dresses the atom and we identify that the entangled state
occurs when the frequency of one of the Rabi sidebands of the driven atom tunes
to frequency of the undriven atom. It is also found that this system behaves as
a cascade open system where the fluorescence from the dressed atom drives the
other atom with no feedback.Comment: Published versio
Quantum interference in optical fields and atomic radiation
We discuss the connection between quantum interference effects in optical
beams and radiation fields emitted from atomic systems. We illustrate this
connection by a study of the first- and second-order correlation functions of
optical fields and atomic dipole moments. We explore the role of correlations
between the emitting systems and present examples of practical methods to
implement two systems with non-orthogonal dipole moments. We also derive
general conditions for quantum interference in a two-atom system and for a
control of spontaneous emission. The relation between population trapping and
dark states is also discussed. Moreover, we present quantum dressed-atom models
of cancellation of spontaneous emission, amplification on dark transitions,
fluorescence quenching and coherent population trapping.Comment: To be published in Journal of Modern Optics Special Issue on Quantum
Interferenc
Initial-Phase Spectroscopy as a Control of Entangled Systems
We introduce the concept of initial-phase spectroscopy as a control of the
dynamics of entangled states encoded into a two-atom system interacting with a
broadband squeezed vacuum field. We illustrate our considerations by examining
the transient spectrum of the field emitted by two systems, the small sample
(Dicke) and the spatially extended (non-Dicke) models. It is found that the
shape of the spectral components depends crucially on the relative phase
between the initial entangled state and the squeezed field. We follow the
temporal evolution of the spectrum and show that depending on the relative
phase a hole burning can occur in one of the two spectral lines. We compare the
transient behavior of the spectrum with the time evolution of the initial
entanglement and find that the hole burning can be interpreted as a
manifestation of the phenomenon of entanglement sudden death. In addition, we
find that in the case of the non-Dicke model, the collective damping rate may
act like an artificial tweezer that rotates the phase of the squeezed field.Comment: 20 pages, 9 figure
Cavity-induced giant Kerr nonlinearities in a driven V-type atom
We discuss a simple and experimentally realizable model for creation of
enhanced Kerr nonlinearities accompanied by vanishing absorption. The model
involves a V-type atom subjected to a strong drive laser, a weak probe laser
and coupled to a single-mode cavity field. Working in the bad-cavity limit, we
find that the simultaneous coupling of the cavity field to both atomic
transitions creates a coherence between the transitions and thus can lead to
quantum interference effects. We investigate the influences of the cavity field
frequency, the cavity field-atom coupling constants and the atomic decay
constants on the linear and the third-order (Kerr) nonlinear susceptibilities.
We predict giant Kerr nonlinearities with vanishing absorption and attribute
this effect to the combination of the Purcell effect and the cavity-induced
quantum interference.Comment: 17 pages 5 figure
Transition from antibunching to bunching for two dipole-interacting atoms
It is known that there is a transition from photon antibunching to bunching
in the resonance fluorescence of a driven system of two two-level atoms with
dipole-dipole interaction when the atomic distance decreases and the other
parameters are kept fixed. We give a simple explanation for the underlying
mechanism which in principle can also be applied to other systems. PACS numbers
42.50.Ar, 42.50FxComment: Submitted to Phys. Rev. A; 15 pages Latex + 4 figure
Squashed States of Light: Theory and Applications to Quantum Spectroscopy
Using a feedback loop it is possible to reduce the fluctuations in one
quadrature of an in-loop field without increasing the fluctuations in the
other. This effect has been known for a long time, and has recently been called
``squashing'' [B.C. Buchler et al., Optics Letters {\bf 24}, 259 (1999)], as
opposed to the ``squeezing'' of a free field in which the conjugate
fluctuations are increased. In this paper I present a general theory of
squashing, including simultaneous squashing of both quadratures and
simultaneous squeezing and squashing. I show that a two-level atom coupled to
the in-loop light feels the effect of the fluctuations as calculated by the
theory. In the ideal limit of light squeezed in one quadrature and squashed in
the other, the atomic decay can be completely suppressed.Comment: 8 pages plus one figure. Submitted to JEOS-B for Dan Walls Special
Issu
The process, outcomes and context of the sanitation change induced by the Swachh Bharat Mission in rural Jharkhand, India
Background
The Indian Swachh Bharat Mission (SBM) was launched in 2014 with the goal to make India open defecation (OD) free by October 2019. Although it is known that the ambitious goal was not achieved, the nature of the sanitation change brought about by the SBM in different parts of India is poorly understood. One reason is a dearth of case studies that would shed light on the performance of the SBM simultaneously across its different domains. This article provides an example of such study. Employing a Process, Outcomes, Context approach, the objective is to understand the process and outcomes of the SBM-induced sanitation change in a specific context of rural Jharkhand.
Methods
The study utilizes data collected through field research conducted in the rural areas of Ranchi district, Jharkhand, a state in east-central India. This data was obtained via repeated cross-sectional household surveys conducted at the beginning and at the end of the SBM, supplemented by key informant interviews with SBM stakeholders.
Findings
We identified political support of SBM implementation and its acceptance amongst the population. Female community workers became key agents of SBM implementation at local level. The SBM increased toilet coverage in the study area from 15% to 85% and lowered the OD rate from 93% to 26%. It substantially reduced structural inequalities in access to toilets, furthered social sanitation norms, improved some of the attitudes towards toilet use, but impacted less on hygiene and sanitation knowledge. The implementation mainly concentrated on the construction of subsidized toilets but less on improving public understanding of safe sanitation practices.
Conclusions
Although the SBM reduced sanitation inequalities in access to toilets in the study area, the behaviour change component was underplayed, focusing more on spreading normative sanitation messages and less on public education. Sustainability of the observed sanitation change remains a key question for the future. This article calls for more systematic production of geographically situated knowledge on the performance of sanitation interventions
Theory of quantum fluctuations of optical dissipative structures and its application to the squeezing properties of bright cavity solitons
We present a method for the study of quantum fluctuations of dissipative
structures forming in nonlinear optical cavities, which we illustrate in the
case of a degenerate, type I optical parametric oscillator. The method consists
in (i) taking into account explicitly, through a collective variable
description, the drift of the dissipative structure caused by the quantum
noise, and (ii) expanding the remaining -internal- fluctuations in the
biorthonormal basis associated to the linear operator governing the evolution
of fluctuations in the linearized Langevin equations. We obtain general
expressions for the squeezing and intensity fluctuations spectra. Then we
theoretically study the squeezing properties of a special dissipative
structure, namely, the bright cavity soliton. After reviewing our previous
result that in the linear approximation there is a perfectly squeezed mode
irrespectively of the values of the system parameters, we consider squeezing at
the bifurcation points, and the squeezing detection with a plane--wave local
oscillator field, taking also into account the effect of the detector size on
the level of detectable squeezing.Comment: 10 figure
Quantum entanglement and disentanglement of multi-atom systems
We present a review of recent research on quantum entanglement, with special
emphasis on entanglement between single atoms, processing of an encoded
entanglement and its temporary evolution. Analysis based on the density matrix
formalism are described. We give a simple description of the entangling
procedure and explore the role of the environment in creation of entanglement
and in disentanglement of atomic systems. A particular process we will focus on
is spontaneous emission, usually recognized as an irreversible loss of
information and entanglement encoded in the internal states of the system. We
illustrate some certain circumstances where this irreversible process can in
fact induce entanglement between separated systems. We also show how
spontaneous emission reveals a competition between the Bell states of a two
qubit system that leads to the recently discovered "sudden" features in the
temporal evolution of entanglement. An another problem illustrated in details
is a deterministic preparation of atoms and atomic ensembles in long-lived
stationary squeezed states and entangled cluster states. We then determine how
to trigger the evolution of the stable entanglement and also address the issue
of a steered evolution of entanglement between desired pairs of qubits that can
be achieved simply by varying the parameters of a given system.Comment: Review articl
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