Caja Negación

is an item given to the Espada by Sōsuke Aizen.

Overview
Caja Negación is a special cube-shaped device that is intended to simulate the effects of Negación fields on the target. When used, the cube isolates and traps the subject in a separate, alternate dimension from which an Arrancar of average strength cannot escape. A Caja Negación is utilized by an Espada in order to punish their Fracción if they deem that the situation requires it. The device was not intended to be used on other Espada and as they are much stronger than the average Arrancar, an Espada would be able to escape from the alternate dimension within a certain amount of time.

Grimmjow Jaegerjaquez uses this device on Ulquiorra Cifer in order to prevent him from interfering in his planned battle with Ichigo Kurosaki. As Ulquiorra has a very high amount of Reiatsu, Grimmjow states that Ulquiorra would not be trapped permanently and should manage to free himself in two or three hours. Evidently, the small cube device must be placed in the Arrancar's Hollow hole, as during their battle, Grimmjow waited to use it until he could place it in Ulquiorra's chest.

Trivia

 * It is called the in the English dub.