QUANTUM LIMITS IN SPACETIME MEASUREMENTS

Quantum fluctuations impose fundamental limits on measurement and space-time probing. Although using optimized probe fields can allow sensitivity in a position measurement to be pushed 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 characterize 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 spacetime curvatures in vacuum. These can be considered as ultimate spacetime fluctuations, fixing ultimate quantum limits in spacetime measurements.