82 research outputs found
The noise in gravitational-wave detectors and other classical-force measurements is not influenced by test-mass quantization
It is shown that photon shot noise and radiation-pressure back-action noise
are the sole forms of quantum noise in interferometric gravitational wave
detectors that operate near or below the standard quantum limit, if one filters
the interferometer output appropriately. No additional noise arises from the
test masses' initial quantum state or from reduction of the test-mass state due
to measurement of the interferometer output or from the uncertainty principle
associated with the test-mass state. Two features of interferometers are
central to these conclusions: (i) The interferometer output (the photon number
flux N(t) entering the final photodetector) commutes with itself at different
times in the Heisenberg Picture, [N(t), N(t')] = 0, and thus can be regarded as
classical. (ii) This number flux is linear in the test-mass initial position
and momentum operators x_o and p_o, and those operators influence the measured
photon flux N(t) in manners that can easily be removed by filtering -- e.g., in
most interferometers, by discarding data near the test masses' 1 Hz swinging
freqency. The test-mass operators x_o and p_o contained in the unfiltered
output N(t) make a nonzero contribution to the commutator [N(t), N(t')]. That
contribution is cancelled by a nonzero commutation of the photon shot noise and
radiation-pressure noise, which also are contained in N(t). This cancellation
of commutators is responsible for the fact that it is possible to derive an
interferometer's standard quantum limit from test-mass considerations, and
independently from photon-noise considerations. These conclusions are true for
a far wider class of measurements than just gravitational-wave interferometers.
To elucidate them, this paper presents a series of idealized thought
experiments that are free from the complexities of real measuring systems.Comment: Submitted to Physical Review D; Revtex, no figures, prints to 14
pages. Second Revision 1 December 2002: minor rewording for clarity,
especially in Sec. II.B.3; new footnote 3 and passages before Eq. (2.35) and
at end of Sec. III.B.
Hepatites pós-transfusionais na cidade de Campinas, SP, Brasil: I. Incidência, agentes etiológicos e aspectos clínico-epidemiológicos da hepatite por vírus C
Follow-up study of anti-hepatitis C virus antibodies in blood donors implicated in post-transfusion non-A, non-B hepatitis
Failure of the revised pre-donation guidelines to prevent people with a history of high-risk behaviour from donating blood
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