  A quantum system is usually measured through observations performed on a
second quantum system, or meter, to which it is coupled. In this scenario,
fundamental limitations arise as stated by the celebrated Wigner-Araki-Yanase
theorem and its generalizations, predicting an upper-bound on the measurement's
accuracy (Ozawa's bound). Here, we show it is possible to saturate this
fundamental bound. We propose a simple interferometric setup, arguably within
reach of present technology, in which a flying particle (the quantum meter) is
used to measure the state of a qubit (the target system). We show that the
bound can be saturated and that this happens only if the flying particle is
prepared in a Gaussian wavepacket.
