The Quantum Brain is a brain made of light which works like a real brain. It was created by Aida using the Laser-Coupling Gloves and knowledge from the Darkhold. It gives LMDs full human consciousness, as well as any memories of their human counterpart. This allowed Holden Radcliffe to infiltrate S.H.I.E.L.D. in order to obtain the Darkhold and attempt to upload S.H.I.E.L.D. agents into the Framework.
Having taken the Laser-Coupling Gloves back to the Radcliffe Residence after rescuing Phil Coulson and Leo Fitz from another dimension, Aida used them to create a brain using the information she obtained from the Darkhold.
Quantum Brains were built into the Life-Model Decoys of Melinda May and Holden Radcliffe in order to store all their memories and make them act and feel like their templates. After Radcliffe LMD was captured, Leo Fitz discovered the Quantum Brain and was amazed by the advanced technology and the operating capability.
Quantum Brains work exactly like real human brains, being capable of storing all the data contained in their original templates. When used in a Life-Model Decoy, it will make the LMD nearly indistinguishable from the person they are based on, even making them believe they are them.