533,960 research outputs found
Initial states and decoherence of histories
We study decoherence properties of arbitrarily long histories constructed
from a fixed projective partition of a finite dimensional Hilbert space. We
show that decoherence of such histories for all initial states that are
naturally induced by the projective partition implies decoherence for arbitrary
initial states. In addition we generalize the simple necessary decoherence
condition [Scherer et al., Phys. Lett. A (2004)] for such histories to the case
of arbitrary coarse-graining.Comment: 10 page
A review of the decoherent histories approach to the arrival time problem in quantum theory
We review recent progress in understanding the arrival time problem in
quantum mechanics, from the point of view of the decoherent histories approach
to quantum theory. We begin by discussing the arrival time problem, focussing
in particular on the role of the probability current in the expected classical
solution. After a brief introduction to decoherent histories we review the use
of complex potentials in the construction of appropriate class operators. We
then discuss the arrival time problem for a particle coupled to an environment,
and review how the arrival time probability can be expressed in terms of a POVM
in this case. We turn finally to the question of decoherence of the
corresponding histories, and we show that this can be achieved for simple
states in the case of a free particle, and for general states for a particle
coupled to an environment.Comment: 10 pages. To appear in DICE 2010 conference proceeding
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Education y Justicia: A Living History of the Latinx Experience in U.T Austin
Social policy in the United States has historically and systematically prevented Latinx students from attaining educational equity. The resistance to this inequality in Austin is located in institutional spaces, the creation of cultural spaces, and the reclamation of erased histories. This has lead to the need for the creation of cultural spaces and legislation that encourage the equity, power, and culture of the Latinx community. We are mapping these living histories of resistance to help form frameworks for rethinking educational social policy. In this case we used U.T. and the greater Austin area to explore the living histories of the struggle for educational justice in our city and propose frameworks for rethinking educational social policy.Mexican American Studie
Consistent probabilities in loop quantum cosmology
A fundamental issue for any quantum cosmological theory is to specify how
probabilities can be assigned to various quantum events or sequences of events
such as the occurrence of singularities or bounces. In previous work, we have
demonstrated how this issue can be successfully addressed within the consistent
histories approach to quantum theory for Wheeler-DeWitt-quantized cosmological
models. In this work, we generalize that analysis to the exactly solvable loop
quantization of a spatially flat, homogeneous and isotropic cosmology sourced
with a massless, minimally coupled scalar field known as sLQC. We provide an
explicit, rigorous and complete decoherent histories formulation for this model
and compute the probabilities for the occurrence of a quantum bounce vs. a
singularity. Using the scalar field as an emergent internal time, we show for
generic states that the probability for a singularity to occur in this model is
zero, and that of a bounce is unity, complementing earlier studies of the
expectation values of the volume and matter density in this theory. We also
show from the consistent histories point of view that all states in this model,
whether quantum or classical, achieve arbitrarily large volume in the limit of
infinite `past' or `future' scalar `time', in the sense that the wave function
evaluated at any arbitrary fixed value of the volume vanishes in that limit.
Finally, we briefly discuss certain misconceptions concerning the utility of
the consistent histories approach in these models.Comment: 22 pages, 3 figures. Matches published versio
Somewhere in the Universe: Where is the Information Stored When Histories Decohere?
We investigate the idea that decoherence is connected with the storage of
information about the decohering system somewhere in the universe. The known
connection between decoherence of histories and the existence of records is
extended from the case of pure initial states to mixed states. Records may
still exist but are necessarily imperfect. We formulate an
information-theoretic conjecture about decoherence due to an environment: the
number of bits required to describe a set of decoherent histories is
approximately equal to the number of bits of information thrown away to the
environment in the coarse-graining process. This idea is verified in a simple
model consisting of a particle coupled to an environment that can store only
one bit of information. We explore the decoherence and information storage in
the quantum Brownian motion model. It is shown that the variables that the
environment naturally measures and stores information about are the Fourier
components of the function (describing the particle trajectory). The
records storing the information about the Fourier modes are the positions and
momenta of the environmental oscillators at the final time. Decoherence is
possible even if there is only one oscillator in the environment. The
information count of the histories and records in the environment add up
according to our conjecture. These results give quantitative content to the
idea that decoherence is related to ``information lost''.Comment: 48 pages, plain Tex. Second revisio
Deccoherent Histories and Measurement of Temporal Correlation Functions for Leggett-Garg Inequalities
We consider two protocols for the measurement of the temporal correlation
functions of a dichotomic variable Q appearing in Leggett-Garg type
inequalities. The protocols measure solely whether Q has the same or different
sign at the ends of a given time interval. They are inspired, in part, by a
decoherent histories analysis of the two-time histories of Q although the
protocols are ultimately expressed in macrorealistic form independent of
quantum theory. The first type involves an ancilla coupled to the system with
two sequential CNOT gates, and the two-time histories of the system are
determined in a single final time measurement of the ancilla. It is
non-invasive for special choices of initial system states and partially
invasive for more general choices. Modified Leggett-Garg type inequalities
which accommodate the partial invasiveness are discussed. The quantum picture
of the protocol shows that for certain choices of primary system initial state
the protocol is undetectable with respect to final system state measurements,
although it is still invasive at intermediate times. This invasiveness can be
reduced with different choices of ancilla states and the protocol is then
similar in flavour to a weak measurement. The second type of protocol is based
on the fact that the behaviour of Q over a time interval can be determined from
knowledge of the dynamics together with a measurement of certain initial (or
final) data. Its quantum version corresponds to the known fact that when sets
of histories are decoherent, their probabilities may be expressed in terms of a
record projector, hence the two-time histories in which Q has the same or
different sign can be determined by a single projective measurement. The
resulting protocol resembles the decay-type protocol proposed by Huelga and
collaborators (which is non-invasive but requires a stationarity assumption).Comment: 33 pages. Revised appendix on LG inequalities for partially invasive
measurements. Accepted for publication in Physical Review
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