California Code, Penal Code - PEN § 3702

(a) The prosecuting agency and the incarcerated person under sentence of death may produce witnesses at any hearing held regarding a petition filed pursuant to Section 3701.

(b) When the court concludes that there is reason to believe the incarcerated person is presently or permanently incompetent to be executed, the court shall hear proof produced by either party.  The court may compel the attendance of witnesses, by process of subpoena and attachment, and to perform all other acts necessary to a full and fair hearing and determination of the case.