Illustration: Adapted from Guo et al. [1]

Figure 1: Electrons moving through liquid helium, imaged by repeatedly exploding the helium bubbles with the camera shutter open. (a) A single electron in normal helium at $2.4K$ moves upward, carried by convective heat flow in the liquid, and then is repelled to the left by a negative voltage applied to an electrode at the top of the cell. (b) A single electron in the superfluid helium at $1.5K$ moves in a zigzag path from the top to the bottom of the cell, apparently from being trapped on the core of a quantized vortex line that extends from the top of the cell to the bottom. (c) Many electrons are injected into the liquid by immersing a radioactive $β$ source at the bottom of the cell, but only the bubbles directly in the acoustic beam are subject to sufficient negative pressure to “explode” and become visible.