California Code, Code of Civil Procedure - CCP § 2034.290

(a) A demand for an exchange of information concerning expert trial witnesses, and any expert witness lists and declarations exchanged shall not be filed with the court.

(b) The party demanding the exchange shall retain both the original of the demand, with the original proof of service affixed, and the original of all expert witness lists and declarations exchanged in response to the demand until six months after final disposition of the action. At that time, all originals may be destroyed unless the court, on motion of any party and for good cause shown, orders that the originals be preserved for a longer period.

(c) Notwithstanding subdivisions (a) and (b), a demand for exchange of information concerning expert trial witnesses, and all expert witness lists and declarations exchanged in response to it, shall be lodged with the court when their contents become relevant to an issue in any pending matter in the action.