  We demonstrate that an interferometer based on modulated magnetic field
pulses enables precise characterization of the energies and lifetimes of Efimov
trimers irrespective of the magnitude and sign of the interactions in 85Rb
thermal gases. Despite thermal effects, interference fringes develop when the
dark time between the pulses is varied. This enables the selective excitation
of coherent superpositions of trimer, dimer and free atom states. The
interference patterns possess two distinct damping timescales at short and long
dark times that are either equal to or twice as long as the lifetime of Efimov
trimers, respectively. Specifically, this behavior at long dark times provides
an interpretation of the unusually large damping timescales reported in a
recent experiment with 7Li thermal gases [Phys. Rev. Lett. 122, 200402 (2019)].
Apart from that, our results constitute a stepping stone towards a high
precision few-body state interferometry for dense quantum gases.
