359 research outputs found
Optimal experiment design revisited: fair, precise and minimal tomography
Given an experimental set-up and a fixed number of measurements, how should
one take data in order to optimally reconstruct the state of a quantum system?
The problem of optimal experiment design (OED) for quantum state tomography was
first broached by Kosut et al. [arXiv:quant-ph/0411093v1]. Here we provide
efficient numerical algorithms for finding the optimal design, and analytic
results for the case of 'minimal tomography'. We also introduce the average
OED, which is independent of the state to be reconstructed, and the optimal
design for tomography (ODT), which minimizes tomographic bias. We find that
these two designs are generally similar. Monte-Carlo simulations confirm the
utility of our results for qubits. Finally, we adapt our approach to deal with
constrained techniques such as maximum likelihood estimation. We find that
these are less amenable to optimization than cruder reconstruction methods,
such as linear inversion.Comment: 16 pages, 7 figure
Comment on "A linear optics implementation of weak values in Hardy's paradox"
A recent experimental proposal by Ahnert and Payne [S.E. Ahnert and M.C.
Payne, Phys. Rev. A 70, 042102 (2004)] outlines a method to measure the weak
value predictions of Aharonov in Hardy's paradox. This proposal contains flaws
such as the state preparation method and the procedure for carrying out the
requisite weak measurements. We identify previously published solutions to some
of the flaws.Comment: To be published in Physical Review
An Contribution to the Hydrogen Lamb Shift from Virtual Light by Light Scattering
The radiative correction to the Lamb shift of order
induced by the light by light scattering insertion in external photons is
obtained. The new contribution turns out to be equal to
. Combining this contribution
with our previous results we obtain the complete correction of order
induced by all diagrams with closed electron loops.
This correction is kHz and kHz for the - and
-states in hydrogen, respectively.Comment: pages, Penn State Preprint PSU/TH/142, February 199
Beam Spin Asymmetry in Electroproduction of Pseudoscalar or Scalar Meson Production off the Scalar Target
We discuss the electroproduction of pseudoscalar () or scalar
() meson production off the scalar target. The most general formulation
of the differential cross section for the or meson production
process involves only one or two hadronic form factors, respectively, on a
scalar target. The Rosenbluth-type separation of the differential cross section
provides the explicit relation between the hadronic form factors and the
different parts of the differential cross section in a completely
model-independent manner. The absence of the beam spin asymmetry for the
pseudoscalar meson production provides a benchmark for the experimental data
analysis. The measurement of the beam spin asymmetry for the scalar meson
production may also provide a unique opportunity not only to explore the
imaginary part of the hadronic amplitude in the general formulation but also to
examine the significance of the chiral-odd generalized parton distribution
(GPD) contribution in the leading-twist GPD formulation.Comment: 8 pages, 1 figur
Maximal Acceleration Corrections to the Lamb Shift of Hydrogen, Deuterium and He
The maximal acceleration corrections to the Lamb shift of one--electron atoms
are calculated in a non--relativistic approximation. They are compatible with
experimental results, are in particularly good agreement with the Lamb
shift in hydrogen and reduce by the experiment--theory discrepancy
for the shift in .Comment: LaTex file, 15 pages, to be published in Phys. Lett.
Photon pair-state preparation with tailored spectral properties by spontaneous four-wave mixing in photonic-crystal fiber
We study theoretically the generation of photon pairs by spontaneous
four-wave mixing (SFWM) in photonic crystal optical fiber. We show that it is
possible to engineer two-photon states with specific spectral correlation
(``entanglement'') properties suitable for quantum information processing
applications. We focus on the case exhibiting no spectral correlations in the
two-photon component of the state, which we call factorability, and which
allows heralding of single-photon pure-state wave packets without the need for
spectral post filtering. We show that spontaneous four wave mixing exhibits a
remarkable flexibility, permitting a wider class of two-photon states,
including ultra-broadband, highly-anticorrelated states.Comment: 17 pages, 7 figures, submitte
Comment on ``Manipulating the frequency entangled states by an acoutic-optical modulator''
A recent theoretical paper [1] proposes a scheme for entanglement swapping
utilizing acousto-optic modulators without requiring a Bell-state measurement.
In this comment, we show that the proposal is flawed and no entanglement
swapping can occur without measurement.Comment: 6 pages, 2 figures submitted to Phys. Rev
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