  We introduce a detector that selectively probes the phononic excitations of a
cold Bose gas. The detector is composed of a single impurity atom confined by a
double-well potential, where the two lowest eigenstates of the impurity form an
effective probe qubit that is coupled to the phonons via density-density
interactions with the bosons. The system is analogous to a two-level atom
coupled to photons of the radiation field. We demonstrate that tracking the
evolution of the qubit populations allows probing both thermal and coherent
excitations in targeted phonon modes. The targeted modes are selected in both
energy and momentum by adjusting the impurity's potential. We show how to use
the detector to observe coherent density waves and to measure temperatures of
the Bose gas down to the nano-Kelvin regime. We analyze how our scheme could be
realized experimentally, including the possibility of using an array of
multiple impurities to achieve greater precision from a single experimental
run.
