Effect of relativity and vacuum fluctuations on quantum measurement

Vacuum fluctuations can obscure the detection signal of the measurement of the smallest quantum objects like single particles seemingly implying a fundamental limit to measurement accuracy. However, as we show relativistic invariance implies the disappearance of fluctuations for the spacelike spectrum of an observable at zero temperature. This complete absence of noise can be harnessed to perform noiseless measurement of single particles, as we illustrate for electrons or photons. We outline a general scheme to illustrate the noiseless measurement involving the spacelike spectrum of observables based on the self-interference of counterpropagating paths of a single particle in a triangular Sagnac interferometer.