A quantum stasis field was a type of energy field capable of preventing a Changeling from altering their form and fixing them into their solid state.
A prototype of this device was first constructed by the Obsidian Order by 2371. During the joint Obsidian Order-Tal Shiar strike force into the Gamma Quadrant, Enabran Tain gave Elim Garak the task of interrogating the Changeling Odo and learn any secrets about the Dominion or about his people. Without the ability to revert to his natural form, Odo experienced intense agony until Garak finally deactivated the device allowing him to return to his dormant state. (DS9 episode: "The Die is Cast")
After Odo and Garak returned to Deep Space 9, they were debriefed by Starfleet Intelligence and the Tal Shiar on their experiences, including the quantum stasis field device. In 2374, a similar device was used, probably by Section 31, to stop Odo from using his abilities. (DS9 novel: Hollow Men)
In 2378, Crell Moset suggested that a Changeling who had posed as Andrew Ellis and Arak Katal could have been held in one form by using a quantum stasis field. (VOY - Spirit Walk novel: Enemy of My Enemy)
In the alternate future timeline caused by the War of the Prophets, Weyoun, Kai of the Bajoran Ascendancy in the timeline, made use of a smaller version of the quantum stasis field generator to freeze Odo in his solid state after which he mercilessly beat the Changeling. He also informed him that the device was miniaturized under orders from Damar and Weyoun believed that he may have been planning to betray the Founders at the time. (DS9 novel: The War of the Prophets)