112 research outputs found
Magnetization reversal in mesoscopic Ni80Fe20 wires: A magnetic domain launching device
The magnetization reversal process in mesoscopic permalloy (Ni80Fe20) wire structures has been investigated using scanning Kerr microscopy, magnetic force microscopy (MFM) and micromagnetic calculations. We find that the junction offers a site for reversed domain wall nucleation in the narrow part of the wires. As a consequence, the switching field is dominated by the domain nucleation field and the junction region initiates reversal by the wall motion following the nucleation of domains. Our results suggest the possibility of designing structures that can be used to “launch” reverse domains in narrow wires within a controlled field rang
Adiabatic Elimination in Compound Quantum Systems with Feedback
Feedback in compound quantum systems is effected by using the output from one
sub-system (``the system'') to control the evolution of a second sub-system
(``the ancilla'') which is reversibly coupled to the system. In the limit where
the ancilla responds to fluctuations on a much shorter time scale than does the
system, we show that it can be adiabatically eliminated, yielding a master
equation for the system alone. This is very significant as it decreases the
necessary basis size for numerical simulation and allows the effect of the
ancilla to be understood more easily. We consider two types of ancilla: a
two-level ancilla (e.g. a two-level atom) and an infinite-level ancilla (e.g.
an optical mode). For each, we consider two forms of feedback: coherent (for
which a quantum mechanical description of the feedback loop is required) and
incoherent (for which a classical description is sufficient). We test the
master equations we obtain using numerical simulation of the full dynamics of
the compound system. For the system (a parametric oscillator) and feedback
(intensity-dependent detuning) we choose, good agreement is found in the limit
of heavy damping of the ancilla. We discuss the relation of our work to
previous work on feedback in compound quantum systems, and also to previous
work on adiabatic elimination in general.Comment: 18 pages, 12 figures including two subplots as jpeg attachment
Effects of interdot dipole coupling in mesoscopic epitaxial Fe(100) dot arrays
The domain structure and the coercivity of epitaxial Fe(100) circular dot arrays of different diameters and separations have been studied using magnetic force microscopy (MFM) and focused magneto-optical Kerr effect (MOKE). The MFM images of the 1 µm diameter single domain dot arrays show direct evidence of strong interdot dipole coupling when the separation is reduced down to 0.1 µm. The coercivity of the dots is also found to be dependent on the separation, indicating the effect of the interdot dipole coupling on the magnetization reversal process
Robust unravelings for resonance fluorescence
Monitoring the fluorescent radiation of an atom unravels the master equation
evolution by collapsing the atomic state into a pure state which evolves
stochastically. A robust unraveling is one that gives pure states that, on
average, are relatively unaffected by the master equation evolution (which
applies once the monitoring ceases). The ensemble of pure states arising from
the maximally robust unraveling has been suggested to be the most natural way
of representing the system [H.M. Wiseman and J.A. Vaccaro, Phys. Lett. A {\bf
250}, 241 (1998)]. We find that the maximally robust unraveling of a resonantly
driven atom requires an adaptive interferometric measurement proposed by
Wiseman and Toombes [Phys. Rev. A {\bf 60}, 2474 (1999)]. The resultant
ensemble consists of just two pure states which, in the high driving limit, are
close to the eigenstates of the driving Hamiltonian . This
ensemble is the closest thing to a classical limit for a strongly driven atom.
We also find that it is possible to reasonably approximate this ensemble using
just homodyne detection, an example of a continuous Markovian unraveling. This
has implications for other systems, for which it may be necessary in practice
to consider only continuous Markovian unravelings.Comment: 12 pages including 5 .eps figures, plus one .jpg figur
Quantum state transformation by dispersive and absorbing four-port devices
The recently derived input-output relations for the radiation field at a
dispersive and absorbing four-port device [T. Gruner and D.-G. Welsch, Phys.
Rev. A 54, 1661 (1996)] are used to derive the unitary transformation that
relates the output quantum state to the input quantum state, including
radiation and matter and without placing frequency restrictions. It is shown
that for each frequency the transformation can be regarded as a well-behaved
SU(4) group transformation that can be decomposed into a product of U(2) and
SU(2) group transformations. Each of them may be thought of as being realized
by a particular lossless four-port device. If for narrow-bandwidth radiation
far from the medium resonances the absorption matrix of the four-port device
can be disregarded, the well-known SU(2) group transformation for a lossless
device is recognized. Explicit formulas for the transformation of Fock-states
and coherent states are given.Comment: 24 pages, RevTe
Cooling of a single atom in an optical trap inside a resonator
We present detailed discussions of cooling and trapping mechanisms for an
atom in an optical trap inside an optical cavity, as relevant to recent
experiments. The interference pattern of cavity QED and trapping fields in
space makes the trapping wells distinguishable from one another. This adds
considerable flexibility to creating effective trapping and cooling conditions
and to detection possibilities. Friction and diffusion coefficients are
calculated in and beyond the low excitation limit and full 3-D simulations of
the quasiclassical motion of a Cs atom are performed.Comment: One more figure and one more autho
Adaptive phase estimation is more accurate than non-adaptive phase estimation for continuous beams of light
We consider the task of estimating the randomly fluctuating phase of a
continuous-wave beam of light. Using the theory of quantum parameter
estimation, we show that this can be done more accurately when feedback is used
(adaptive phase estimation) than by any scheme not involving feedback
(non-adaptive phase estimation) in which the beam is measured as it arrives at
the detector. Such schemes not involving feedback include all those based on
heterodyne detection or instantaneous canonical phase measurements. We also
demonstrate that the superior accuracy adaptive phase estimation is present in
a regime conducive to observing it experimentally.Comment: 15 pages, 9 figures, submitted to PR
Grassmann Variables and the Jaynes-Cummings Model
This paper shows that phase space methods using a positive P type
distribution function involving both c-number variables (for the cavity mode)
and Grassmann variables (for the two level atom) can be used to treat the
Jaynes-Cummings model. Although it is a Grassmann function, the distribution
function is equivalent to six c-number functions of the two bosonic variables.
Experimental quantities are given as bosonic phase space integrals involving
the six functions. A Fokker-Planck equation involving both left and right
Grassmann differentiation can be obtained for the distribution function, and is
equivalent to six coupled equations for the six c-number functions.
The approach used involves choosing the canonical form of the (non-unique)
positive P distribution function, where the correspondence rules for bosonic
operators are non-standard and hence the Fokker-Planck equation is also
unusual. Initial conditions, such as for initially uncorrelated states, are
used to determine the initial distribution function. Transformations to new
bosonic variables rotating at the cavity frequency enables the six coupled
equations for the new c-number functions (also equivalent to the canonical
Grassmann distribution function) to be solved analytically, based on an ansatz
from a 1980 paper by Stenholm. It is then shown that the distribution function
is the same as that determined from the well-known solution based on coupled
equations for state vector amplitudes of atomic and n-photon product states.
The treatment of the simple two fermion mode Jaynes-Cummings model is a
useful test case for the future development of phase space Grassmann
distribution functional methods for multi-mode fermionic applications in
quantum-atom optics.Comment: 57 pages, 0 figures. Version
Trapping and cooling single atoms with far-off resonance intracavity doughnut modes
We investigate cooling and trapping of single atoms inside an optical cavity
using a quasi-resonant field and a far-off resonant mode of the Laguerre-Gauss
type. The far-off resonant doughnut mode provides an efficient trapping in the
case when it shifts the atomic internal ground and excited state in the same
way, which is particularly useful for quantum information applications of
cavity quantum electrodynamics (QED) systems. Long trapping times can be
achieved, as shown by full 3-D simulations of the quasi-classical motion inside
the resonator.Comment: 18 pages, 18 figures, RevTe
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Sleepwalking into lock-in? Avoiding wrongs to future people in the governance of solar radiation management research
This paper argues for two ways in which we can avoid the reckless endangerment of future people in the governance of solar radiation management (SRM) research, which could happen through lock-in to SRM deployment from research. SRM research is at an early stage, one at which the mechanisms of lock-in could start to operate. However, lock-in fit to endanger future people could be slowed or stopped through targeted governance. This paper argues that governance of SRM research that does not include provisions to detect, slow, or stop lock-in fails the test of an intergenerationally adequate precautionary principle, and that research governed without these provisions cannot itself be justified as a precaution against the impacts of climate change
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