3,605 research outputs found
Entangling power of the quantum baker's map
We investigate entanglement production in a class of quantum baker's maps.
The dynamics of these maps is constructed using strings of qubits, providing a
natural tensor-product structure for application of various entanglement
measures. We find that, in general, the quantum baker's maps are good at
generating entanglement, producing multipartite entanglement amongst the qubits
close to that expected in random states. We investigate the evolution of
several entanglement measures: the subsystem linear entropy, the concurrence to
characterize entanglement between pairs of qubits, and two proposals for a
measure of multipartite entanglement. Also derived are some new analytical
formulae describing the levels of entanglement expected in random pure states.Comment: 22 pages, 11 figure
An Analysis of Kinetic Response Variability
Studies evaluating variability of force as a function of absolute force generated are synthesized. Inconsistencies in reported estimates of this relationship are viewed as a function of experimental constraints imposed. Typically, within-subject force variability increases at a negative accelerating rate with equal increments in force produced. Current pulse-step and impulse variability models are unable to accommodate this description, although the notion of efficiency is suggested as a useful construct to explain the description outlined
Information-theoretic approach to quantum error correction and reversible measurement
Quantum operations provide a general description of the state changes allowed
by quantum mechanics. The reversal of quantum operations is important for
quantum error-correcting codes, teleportation, and reversing quantum
measurements. We derive information-theoretic conditions and equivalent
algebraic conditions that are necessary and sufficient for a general quantum
operation to be reversible. We analyze the thermodynamic cost of error
correction and show that error correction can be regarded as a kind of
``Maxwell demon,'' for which there is an entropy cost associated with
information obtained from measurements performed during error correction. A
prescription for thermodynamically efficient error correction is given.Comment: 31 pages, REVTEX, one figure in LaTeX, submitted to Proceedings of
the ITP Conference on Quantum Coherence and Decoherenc
Gleason-Type Derivations of the Quantum Probability Rule for Generalized Measurements
We prove a Gleason-type theorem for the quantum probability rule using frame
functions defined on positive-operator-valued measures (POVMs), as opposed to
the restricted class of orthogonal projection-valued measures used in the
original theorem. The advantage of this method is that it works for
two-dimensional quantum systems (qubits) and even for vector spaces over
rational fields--settings where the standard theorem fails. Furthermore, unlike
the method necessary for proving the original result, the present one is rather
elementary. In the case of a qubit, we investigate similar results for frame
functions defined upon various restricted classes of POVMs. For the so-called
trine measurements, the standard quantum probability rule is again recovered.Comment: 10 pages RevTeX, no figure
On the novelty, efficacy, and significance of weak measurements for quantum tomography
The use of weak measurements for performing quantum tomography is enjoying
increased attention due to several recent proposals. The advertised merits of
using weak measurements in this context are varied, but are generally
represented by novelty, increased efficacy, and foundational significance. We
critically evaluate two proposals that make such claims and find that weak
measurements are not an essential ingredient for most of their advertised
features.Comment: 12 pages, 10 figure
Conditions for compatibility of quantum state assignments
Suppose N parties describe the state of a quantum system by N possibly
different density operators. These N state assignments represent the beliefs of
the parties about the system. We examine conditions for determining whether the
N state assignments are compatible. We distinguish two kinds of procedures for
assessing compatibility, the first based on the compatibility of the prior
beliefs on which the N state assignments are based and the second based on the
compatibility of predictive measurement probabilities they define. The first
procedure leads to a compatibility criterion proposed by Brun, Finkelstein, and
Mermin [BFM, Phys. Rev. A 65, 032315 (2002)]. The second procedure leads to a
hierarchy of measurement-based compatibility criteria which is fundamentally
different from the corresponding classical situation. Quantum mechanically none
of the measurement-based compatibility criteria is equivalent to the BFM
criterion.Comment: REVTEX 4, 19 pages, 1 postscript figur
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