  We propose a way to measure the momentum p of a nanomechanical oscillator.
The p-detector is based on two tunnel junctions in a Aharonov-Bohm-type setup.
One of the tunneling amplitudes depends on the motion of the oscillator, the
other one not. Although the coupling between the detector and the oscillator is
assumed to be linear in the position x of the oscillator, it turns out that the
finite-frequency noise output of the detector will in general contain a term
proportional to the momentum spectrum of the oscillator. This is a true quantum
phenomenon, which can be realized in practice if the phase of the tunneling
amplitude of the detector is tuned by the Aharonov-Bohm flux Phi to a
p-sensitive value.
