Evidence for a Bose–Einstein condensate in liquid 4He from quantum evaporation

Bose–Einstein condensation (BEC) is a purely quantum phenomenon whereby a macroscopic number of identical atoms occupy the same single-particle state1. Interest in this phenomenon has grown considerably following the direct demonstration of BEC in low-density gases of alkali metal atoms2,3,4. It is therefore worth reconsidering the case of liquid 4He, which is generally accepted to have such a condensate5, but for which similarly direct evidence is lacking6. Nevertheless, theoretical models that depend on the existence of a condensate have proved successful at explaining many of the properties of this system7,8,9, and BEC is considered to underlie the striking phenomena of superfluidity and quantized vorticity observed in liquid 4He. So the current issue is not whether there is a condensate in this system, but how to demonstrate its existence in a clear and simple way. Here I argue that an earlier measurement10 of evaporation from liquid 4He caused by a collimated beam of phonons provides such a demonstration. The calculated angular distribution of evaporated atoms agrees well with that measured if it is assumed that the atoms initially had zero momentum parallel to the surface of the liquid—this is to be expected if the atoms originate from a condensate. This process of quantum evaporation also opens the possibility for creating beams of phase-coherent atoms of short wavelength.