  We outline the experimental concept and key scientific capabilities of AION
(Atom Interferometer Observatory and Network), a proposed UK-based experimental
programme using cold strontium atoms to search for ultra-light dark matter, to
explore gravitational waves in the mid-frequency range between the peak
sensitivities of the LISA and LIGO/Virgo/ KAGRA/INDIGO/Einstein
Telescope/Cosmic Explorer experiments, and to probe other frontiers in
fundamental physics. AION would complement other planned searches for dark
matter, as well as probe mergers involving intermediate mass black holes and
explore early universe cosmology. AION would share many technical features with
the MAGIS experimental programme in the US, and synergies would flow from
operating AION in a network with this experiment, as well as with other atom
interferometer experiments such as MIGA, ZAIGA and ELGAR. Operating AION in a
network with other gravitational wave detectors such as LIGO, Virgo and LISA
would also offer many synergies.
