California Code, Penal Code - PEN § 37

(a) Treason against this state consists only in levying war against it, adhering to its enemies, or giving them aid and comfort, and can be committed only by persons owing allegiance to the state. The punishment of treason shall be death or life imprisonment without possibility of parole. The penalty shall be determined pursuant to Sections 190.3 and 190.4.

(b) Upon a trial for treason, the defendant cannot be convicted unless upon the testimony of two witnesses to the same overt act, or upon confession in open court; nor, except as provided in Sections 190.3 and 190.4, can evidence be admitted of an overt act not expressly charged in the indictment or information; nor can the defendant be convicted unless one or more overt acts be expressly alleged therein.