Beyond classical friction between two macroscopic bodies in contact, there are predictions regarding friction in the temperature regime where excitations are described by quantum mechanics. In the classical limit, the friction between two bodies moving relative to each other along a perfectly smooth and incommensurate interface is zero. In the quantum picture, where the elementary excitations are phonons, their populations in two bodies moving at a relative speed V relative to each other would be Doppler shifted. One friction mechanism originates in the exchange of phonons between two such bodies across the interface[1]. The Doppler shift leads to a anisotropic rate of the Umklapp processes, generating a net flow of momentum from one of the bodies into the other. This momentum transfer is effectively a friction force which operates even if the interface is perfectly smooth. We  found a way to realize a situation where a relative motion exists between two macroscopic masses of hcp solid ⁴He inside a torsional oscillator. In-situ detection of the relative motion[2] shows that the interface between these two masses is the basal plane. To test the friction model, we carried out measurements of the internal dissipation of the torsional oscillator as function of temperature and tangential speed. Our results are in good agreement with the prediction of the phonon friction model[1].

[1] V. L. Popov, Phys. Rev. Lett. 83, 1632(1999).

[2] E. Livne, A. Eyal, O. Scaly, and E. Polturak, J. Low Temp. Phys. 180,185 (2015).