This is a very nice puzzle game! The game mechanics fit's to the theme, are also solid and well introduced. The difficulty of the levels builds up slowly but make you think. The presentation is great and feels good. Well, overall it's a great entry.
But I think there are few small things, which could benefit from small changes. Like, it would be helpful if the portals would have numbers or some kind of indicators to see how they are related. Without this information it was just try and error, which felt like work than playing a puzzle game.
The movement it self felt not quite precise. An indicator on which tile the cube would stop at releasing the key, would help a lot.
But, however this is a great game! Well done and keep it up!