A bit underwhelming IMO.
Either increase the amount reduced, or lower the cost to like... free...
I originally had the normal card reducing the targets attack by 2, but would that be too overpowered?
No way it would be Overpowered.
There are cards that can potentially kill a creature for 1 quanta. Cards that can delay a creature for many turns for 1 quanta.
Heck, even skull buckler costs 1 quanta and it effectively decreases the attack of ALL your opponent's creatures by 1, AND it's permanent, and it's considered one of the most underwhelming cards in the game.
You really need to increase the amount of attack lowered.
I would suggest something like reducing the attack to 0 for a certain amount of turns.
Unless, actually, you meant that it loses 1 attack per turn. Then it would actually be pretty good, but you need to change the card text to something like "Target creature gains -1/-0 each turn."
Or perhaps the upped card would decrease the attack by 1 plus the number of light quantum divided by 4?
If that doesn't sound good, then I could run with the 'lowers to 0 for x turns'.
EDIT: Modified both cards- better, worse?