Commercial Fixed Wing
Air Traffic Control Time Certified In Pos 1 (yrs) 21
Communication Breakdown; Situational Awareness; Confusion
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance
Airspace Structure; Chart Or Publication; Human Factors; Procedure
Two similar sounding callsigns on frequency: Flight ABCD and Flight AECD.  Both aircraft were being vectored to the final approach course into SNA airport.  Flight ABCD was in front of Flight AECD.  I issued and altitude of 4;300 feet to Flight ABCD and Flight AECD responded at the same exact time.  All I heard was Flight ABCD reading it back.  I then observed Flight AECD descending below the 5;200 foot Minimum Vectoring Altitude and issued a low altitude alert; stop climb; and climb back up.   Need to be more observant of similar sounding call signs and should have issued the phraseology to warn both aircraft.  Lesson learned.
SCT TRACON Controller reported they did not detect the wrong aircraft read a descent clearance back to them due to similar sounding call signs. The aircraft descended below the Minimum Vectoring Altitude.