  Quantum fluctuations impose fundamental limits on measurement and space-time
probing. Although using optimised probe fields can allow to push sensitivity in
a position measurement beyond the "standard quantum limit", quantum
fluctuations of the probe field still result in limitations which are
determined by irreducible dissipation mechanisms. Fluctuation-dissipation
relations in vacuum characterise the mechanical effects of radiation pressure
vacuum fluctuations, which lead to an ultimate quantum noise for positions. For
macroscopic reflectors, the quantum noise on positions is dominated by
gravitational vacuum fluctuations, and takes a universal form deduced from
quantum fluctuations of space-time curvatures in vacuum. These can be
considered as ultimate space-time fluctuations, fixing ultimate quantum limits
in space-time measurements.
