272 research outputs found
Collective Uncertainty in Partially-Polarized and Partially-Decohered Spin-1/2 Systems
It has become common practice to model large spin ensembles as an effective
pseudospin with total angular momentum J = N x j, where j is the spin per
particle. Such approaches (at least implicitly) restrict the quantum state of
the ensemble to the so-called symmetric Hilbert space. Here, we argue that
symmetric states are not generally well-preserved under the type of decoherence
typical of experiments involving large clouds of atoms or ions. In particular,
symmetric states are rapidly degraded under models of decoherence that act
identically but locally on the different members of the ensemble. Using an
approach [Phys. Rev. A 78, 052101 (2008)] that is not limited to the symmetric
Hilbert space, we explore potential pitfalls in the design and interpretation
of experiments on spin-squeezing and collective atomic phenomena when the
properties of the symmetric states are extended to systems where they do not
apply.Comment: 13 pages, 7 figure
Collective processes of an ensemble of spin-1/2 particles
When the dynamics of a spin ensemble are expressible solely in terms of
symmetric processes and collective spin operators, the symmetric collective
states of the ensemble are preserved. These many-body states, which are
invariant under particle relabeling, can be efficiently simulated since they
span a subspace whose dimension is linear in the number of spins. However, many
open system dynamics break this symmetry, most notably when ensemble members
undergo identical, but local, decoherence. In this paper, we extend the
definition of symmetric collective states of an ensemble of spin-1/2 particles
in order to efficiently describe these more general collective processes. The
corresponding collective states span a subspace which grows quadratically with
the number of spins. We also derive explicit formulae for expressing arbitrary
identical, local decoherence in terms of these states.Comment: 12 pages, see 0805.2910 for simulations using these method
Efficient feedback controllers for continuous-time quantum error correction
We present an efficient approach to continuous-time quantum error correction
that extends the low-dimensional quantum filtering methodology developed by van
Handel and Mabuchi [quant-ph/0511221 (2005)] to include error recovery
operations in the form of real-time quantum feedback. We expect this paradigm
to be useful for systems in which error recovery operations cannot be applied
instantaneously. While we could not find an exact low-dimensional filter that
combined both continuous syndrome measurement and a feedback Hamiltonian
appropriate for error recovery, we developed an approximate reduced-dimensional
model to do so. Simulations of the five-qubit code subjected to the symmetric
depolarizing channel suggests that error correction based on our approximate
filter performs essentially identically to correction based on an exact quantum
dynamical model
Using Workplace Personality to Guide Improvement of Law Enforcement Selection
Recurrent police-public conflict suggests misalignment in desired police behavior between police and the public. We explored differences in desired police characteristics between police and members of the American public. Although racial minorities endorsed more negative attitudes of police overall, we found no meaningful differences in desired police characteristics between police and the public or between racial minority and majority participants. Second, we combined multiple criterion-related validation studies in similar jobs via meta-analyses and synthetic validity analyses to identify personality predictors of police performance dimensions. Third, we assessed base rates and adverse impact of these personality characteristics in police. Incumbent officers scored significantly lower on desired characteristics and higher on undesired characteristics than applicants. Overall, scales measuring Emotional Stability, Agreeableness, Conscientiousness, Excitability, and Skepticism seem job-relevant across samples, predictive of performance, and unlikely to cause adverse impact. Focusing on these characteristics in hiring could contribute to positive changes in police performance
Single shot parameter estimation via continuous quantum measurement
We present filtering equations for single shot parameter estimation using
continuous quantum measurement. By embedding parameter estimation in the
standard quantum filtering formalism, we derive the optimal Bayesian filter for
cases when the parameter takes on a finite range of values. Leveraging recent
convergence results [van Handel, arXiv:0709.2216 (2008)], we give a condition
which determines the asymptotic convergence of the estimator. For cases when
the parameter is continuous valued, we develop quantum particle filters as a
practical computational method for quantum parameter estimation.Comment: 9 pages, 5 image
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