  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 space-like
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 space-like spectrum of observables
based on the self-interference of counter-propagating paths of a single
particle in a triangular Sagnac interferometer.
