4,024 research outputs found
Selective linear or quadratic optomechanical coupling via measurement
The ability to engineer both linear and non-linear coupling with a mechanical
resonator is an important goal for the preparation and investigation of
macroscopic mechanical quantum behavior. In this work, a measurement based
scheme is presented where linear or square mechanical displacement coupling can
be achieved using the optomechanical interaction linearly proportional to the
mechanical position. The resulting square displacement measurement strength is
compared to that attainable in the dispersive case using the direct interaction
to the mechanical displacement squared. An experimental protocol and parameter
set are discussed for the generation and observation of non-Gaussian states of
motion of the mechanical element.Comment: 7 pages, 2 figures, (accepted in Physical Review X
Entanglement under restricted operations: Analogy to mixed-state entanglement
We show that the classification of bi-partite pure entangled states when
local quantum operations are restricted yields a structure that is analogous in
many respects to that of mixed-state entanglement. Specifically, we develop
this analogy by restricting operations through local superselection rules, and
show that such exotic phenomena as bound entanglement and activation arise
using pure states in this setting. This analogy aids in resolving several
conceptual puzzles in the study of entanglement under restricted operations. In
particular, we demonstrate that several types of quantum optical states that
possess confusing entanglement properties are analogous to bound entangled
states. Also, the classification of pure-state entanglement under restricted
operations can be much simpler than for mixed-state entanglement. For instance,
in the case of local Abelian superselection rules all questions concerning
distillability can be resolved.Comment: 10 pages, 2 figures; published versio
Measuring measurement--disturbance relationships with weak values
Using formal definitions for measurement precision {\epsilon} and disturbance
(measurement backaction) {\eta}, Ozawa [Phys. Rev. A 67, 042105 (2003)] has
shown that Heisenberg's claimed relation between these quantities is false in
general. Here we show that the quantities introduced by Ozawa can be determined
experimentally, using no prior knowledge of the measurement under investigation
--- both quantities correspond to the root-mean-squared difference given by a
weak-valued probability distribution. We propose a simple three-qubit
experiment which would illustrate the failure of Heisenberg's
measurement--disturbance relation, and the validity of an alternative relation
proposed by Ozawa
Modal dynamics for positive operator measures
The modal interpretation of quantum mechanics allows one to keep the standard
classical definition of realism intact. That is, variables have a definite
status for all time and a measurement only tells us which value it had.
However, at present modal dynamics are only applicable to situations that are
described in the orthodox theory by projective measures. In this paper we
extend modal dynamics to include positive operator measures (POMs). That is,
for example, rather than using a complete set of orthogonal projectors, we can
use an overcomplete set of nonorthogonal projectors. We derive the conditions
under which Bell's stochastic modal dynamics for projective measures reduce to
deterministic dynamics, showing (incidentally) that Brown and Hiley's
generalization of Bohmian mechanics [quant-ph/0005026, (2000)] cannot be thus
derived. We then show how {\em deterministic} dynamics for positive operators
can also be derived. As a simple case, we consider a Harmonic oscillator, and
the overcomplete set of coherent state projectors (i.e. the Husimi POM). We
show that the modal dynamics for this POM in the classical limit correspond to
the classical dynamics, even for the nonclassical number state . This
is in contrast to the Bohmian dynamics, which for energy eigenstates, the
dynamics are always non-classical.Comment: 14 page
Feedback-stabilization of an arbitrary pure state of a two-level atom
Unit-efficiency homodyne detection of the resonance fluorescence of a
two-level atom collapses the quantum state of the atom to a stochastically
moving point on the Bloch sphere. Recently,Hofmann, Mahler, and Hess [Phys.
Rev. A {\bf 57}, 4877 (1998)] showed that by making part of the coherent
driving proportional to the homodyne photocurrent can stabilize the state to
any point on the bottom half of the sphere. Here we reanalyze their proposal
using the technique of stochastic master equations, allowing their results to
be generalized in two ways. First, we show that any point on the upper or lower
half, but not the equator, of the sphere may be stabilized. Second, we consider
non-unit-efficiency detection, and quantify the effectiveness of the feedback
by calculating the maximal purity obtainable in any particular direction in
Bloch space.Comment: 9 pages, 7 figures, Physical Review
Optomechanical tailoring of quantum fluctuations
We propose the use of feedback mechanism to control the level of quantum
noise in a radiation field emerging from a pendular Fabry-Perot cavity. It is
based on the possibility to perform quantum nondemolition measurements by means
of optomechanical coupling.Comment: ReVTeX file, 8 pages, 1 Postscript figure. to appear in J. Opt. B:
Quant. Semiclass. Op
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