19,759 research outputs found
Sensing Small Changes in a Wave Chaotic Scattering System
Classical analogs of the quantum mechanical concepts of the Loschmidt Echo
and quantum fidelity are developed with the goal of detecting small
perturbations in a closed wave chaotic region. Sensing techniques that employ a
one-recording-channel time-reversal-mirror, which in turn relies on time
reversal invariance and spatial reciprocity of the classical wave equation, are
introduced. In analogy with quantum fidelity, we employ Scattering Fidelity
techniques which work by comparing response signals of the scattering region,
by means of cross correlation and mutual information of signals. The
performance of the sensing techniques is compared for various perturbations
induced experimentally in an acoustic resonant cavity. The acoustic signals are
parametrically processed to mitigate the effect of dissipation and to vary the
spatial diversity of the sensing schemes. In addition to static boundary
condition perturbations at specified locations, perturbations to the medium of
wave propagation are shown to be detectable, opening up various real world
sensing applications in which a false negative cannot be tolerated.Comment: 14 pages, 11 figures, as published on J. Appl. Phy
Information-theoretic limitations on approximate quantum cloning and broadcasting
We prove new quantitative limitations on any approximate simultaneous cloning
or broadcasting of mixed states. The results are based on information-theoretic
(entropic) considerations and generalize the well known no-cloning and
no-broadcasting theorems. We also observe and exploit the fact that the
universal cloning machine on the symmetric subspace of qudits and
symmetrized partial trace channels are dual to each other. This duality
manifests itself both in the algebraic sense of adjointness of quantum channels
and in the operational sense that a universal cloning machine can be used as an
approximate recovery channel for a symmetrized partial trace channel and vice
versa. The duality extends to give control on the performance of generalized
UQCMs on subspaces more general than the symmetric subspace. This gives a way
to quantify the usefulness of a-priori information in the context of cloning.
For example, we can control the performance of an antisymmetric analogue of the
UQCM in recovering from the loss of fermionic particles.Comment: 13 pages; new results on approximate cloning between general
subspaces, e.g., cloning of fermion
Sensor Based on Extending the Concept of Fidelity to Classical Waves
We propose and demonstrate a remote sensor scheme by applying the quantum
mechanical concept of fidelity loss to classical waves. The sensor makes
explicit use of time-reversal invariance and spatial reciprocity in a wave
chaotic system to sensitively and remotely measure the presence of small
perturbations. The loss of fidelity is measured through a classical wave-analog
of the Loschmidt echo by employing a single-channel time-reversal mirror to
rebroadcast a probe signal into the perturbed system. We also introduce the use
of exponential amplification of the probe signal to partially overcome the
effects of propagation losses and to vary the sensitivity.Comment: 4 pages, 2 figure
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