The popular conception of the Heisenberg uncertainty principle is that
measurement is unavoidably invasive. We disturb an object when we
observe it, thus introducing error into subsequent measurements.
However, recent experiments (see 6 September 2012 Synopsis) claim to have measurement errors below the Heisenberg limit. To address this apparent contradiction, a paper in Physical Review Letters
reports a new formulation of the uncertainty principle in which
measurement disturbance depends on the performance of the measuring
device, which is quantified as the maximum possible change in the state
of the object.
Paul Busch of the University of York in the UK and his colleagues
believe there is no contradiction here, but only a misunderstanding over
how to characterize the effects of measurement. Previously,
measurement-induced errors have been calculated on a state-by-state
basis, by comparing the state of a system “before” and “after” a
measurement. But Busch et al.
show that defining measurement error in a state-independent way,
through a kind of calibration process of the measuring device, leads to
limits in line with the uncertainty principle.
I expect more of something like this to occur as we probe the more minute detail of QM.