Cal. Pen. Code § 2690
The Secretary of the Department of Corrections and Rehabilitation may authorize the temporary removal of an inmate from prison or any other institution for the detention of adults under the jurisdiction of the Department of Corrections and Rehabilitation, including removal for the purpose of attending college classes or permitting the inmate to participate in, or assist with, the gathering of evidence relating to crimes. The secretary may require that the temporary removal be under custody. Unless the inmate is removed for medical treatment, the removal shall not be for a period longer than three days. The secretary may require, except when the removal is for medical treatment or to assist with the gathering of evidence related to crimes, the inmate to reimburse the state, in whole or in part, for expenses incurred by the state in connection with the temporary removal.
Ca. Pen. Code § 2690