Propagation of near-zone-boundary, acoustic phonons in solid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ with the use of superconducting tunnel junctions

We demonstrate the long lifetime, against spontaneous decay ($\hslash \omega \gg \mathrm{kT}$), of transverse phonons in the highly anharmonic solid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. This is proved through the quantum generation and detection of the propagating phonons using superconducting tunnel junctions of Sn and Al. Phonons of wavelength 10-20 Å are shown to propagate macroscopic distances ∼1 mm at $T\lesssim 0.2$ K.