The Quantum Cube (量子キューブ Ryōshi Kyūbu) is a mystical object of great power used by Aigami in Yu-Gi-Oh! The Dark Side of Dimensions. It previously belonged to Shadi, and is the eighth Millennium Item, only usable when the other seven have been placed in the Millennium Stone and the spirit of the Pharaoh has departed to the afterlife.
Aigami later used it to transport Scud and his gang to another dimension, and used it to teleport from the mall in Domino City to the Valley of the Kings in Egypt, where he disintegrated a KaibaCorp employee, and attempted to kill Kaiba with it, albeit unsuccessfully, then attempted to kill Ryo Bakura and Joey Wheeler with it. In the latter instance he was foiled by Atem. While in the Valley of Kings, he transformed it into a Duel Disk to Duel Kaiba, and later to Duel against Yugi.
After Aigami's defeat, Kaiba took possession of the Cube, using it to enter the afterlife in order to Duel Atem.
Using the cube, Aigami can transport people to an alternate dimension, where they will disintegrate and die if not rescued. It can also directly disintegrate people.
The world he sent Joey to was made up of and sustained by his memories, and as soon as he forgot something, it would disappear. However, Joey could also remember things to recreate a place, such as the street he rode his bike down earlier.