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Related post: quantities, given the amount of physical resources, such as energy, at
hand to perform the measurement.
2) One important consequence of the physical nature of measurement is
the so-called "quantum back action": The extraction of information
from a system can give rise to a feedback effect in which the system
configuration after the measurement is determined by the measurement
outcome. For example, the most extreme case (the so-called von Neumann
or projective measurement) produces a complete determination of the
post-measurement state. When performing successive measurements,
quantum back action can be detrimental, because earlier measurements
can negatively influence successive ones.
3) A common strategy to get around the negative effect of back action
and of Heisenberg uncertainty is to design an experimental apparatus
that monitors only one out of a set of incompatible observables: "less
is more" [3]. This strategy, called "quantum nondemolition
measurement" [3-6], is not as simple Buy Norethindrone Acetate as it sounds. One has to account
for the system's interaction with the external environment, which
tends to extract and disperse information, and for the system
dynamics, which can combine the measured observable with incompatible
ones. Another strategy to get around the Heisenberg uncertainty is to
employ a quantum state in which the uncertainty in the observable to
be monitored is very small (at the cost of a very large uncertainty in
the complementary observable). The research on quantum-enhanced
measurements was spawned by the invention of such techniques [3] and
by the birth of more rigorous treatments of quantum measurements.
4) Most standard measurement techniques do not account for these
quantum subtleties, so that their precision is limited by otherwise
avoidable sources of errors. Typical examples are the
environment-induced noise from vacuum fluctuations (the so-called
"shot noise") that affects the measurement of the electromagnetic
field amplitude, and the dynamically induced noise in the position
measurement of a free mass (the so-called "standard quantum limit").
These sources of imprecision are not as fundamental as the unavoidable
Heisenberg uncertainty relations, because they originate only from a
non-optimal choice of measurement strategy. However, the shot noise
and standard quantum limits set important benchmarks for the quality
of a measurement, and they provide an interesting challenge to devise
quantum strategies that can defeat them.
5) In summary: Quantum mechanics, through the Heisenberg uncertainty
principle, imposes limits on the precision of measurement.
Conventional measurement techniques typically fail to reach these
limits. Conventional bounds to the precision of measurements such as
the shot noise limit or the standard quantum limit are not as
fundamental as the Heisenberg limits and can be overcome using quantum
strategies that employ "quantum tricks" such as squeezing and
entanglement.
References (abridged):
1. H. P. Robertson, Phys. Rev. 34, 163 (1929)
2. N. Margolus, L. B. Levitin, Physica D 120, 188 (1998)
3. C. M. Caves, K. S. Thorne, R. W. P. Drever, V. D. Sandberg, M.
Zimmermann, Rev. Mod. Phys. 52, 341 (1980)
4. K. Bencheikh, J. A. Levenson, P. Grangier, O. Lopez, Phys. Rev.
Lett. 75, 3422 (1995)
5. G. J. Milburn, D. F. Walls, Phys. Rev. A. 28, 2065 (1983)
Science http://www.sciencemag.org
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Related Material:
QUANTUM PHYSICS: ZERO-POINT FLUCTUATIONS
The following points are made by Miles Blencowe (Nature 2003 424:262):
1) In 1927, Werner Heisenberg (1901-1976) introduced his famous
quantum principle, which states that the uncertainties in the position
and the velocity of a particle are inversely proportional to each
other: a particle's position or its velocity can be known precisely,
but not both at once. This principle is one of the cornerstones of
quantum mechanics, and is traditionally relevant to the domain of
subatomic particles. But what about more familiar macroscopic objects,
comprising many atoms, that we think of as possessing simultaneously
well-defined positions and velocities of Norethindrone Acetate Tablets their center-of-mass? If we
could be sufficiently precise in our measurements on such objects,
would we encounter the quantum uncertainty principle at work?
2) If you clamp one end of a wooden ruler to the edge of a table and
then pluck the other, free end, it vibrates with decaying amplitude
and eventually returns to apparent rest. But if you were to look at
the free end of the ruler under a sufficiently powerful microscope, it
would not be at rest at all, but jiggling up and down in a random
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