  Single electrons can be conceived as the simplest quantum nodes in a quantum
network. Between electrons, single photons can act as quantum channels to
exchange quantum information. Despite this appealing picture, in conventional
materials, it is extremely difficult to make individual electrons and photons
coherently interact with each other at the visible-infrared wavelengths
suitable for long-distance communication. Here we theoretically demonstrate
that the self-confined single-electron structure in condensed helium-4 can be a
fascinating candidate for single-electron quantum nodes. Each electron in
helium forms a bubble of 1 to 2 nm radius and coherently interacts with
mid-infrared photons. A parametrically amplified femtosecond laser can drive
the electrons into any superposition between the ground and excited states. An
electron inside a slot-waveguide cavity can strongly couple with cavity photons
and exhibits vacuum Rabi oscillations. Two electrons in the cavity naturally
generate entanglement through their respective coupling to the lossy cavity.
The electron-in-helium system offers unique insight in understanding
nonequilibrium quantum dynamics.
