At once and Here and now

At once adverb - Without delay or hesitation; with no time intervening.

Here and now and at once are semantically related. in presently topic. In some cases you can use "Here and now" instead an adverb phrase "At once".

Here and now noun - The time currently existing or in progress.

At once and here and now are semantically related. You can use "At once" instead an adverb phrase "Here and now".