Two photon pairs (aihbH +air bit +a2hb2H+a2pb2v )o No errors i26 i26 IHlH +a ae ae +(a+b)0)ae i26 20 ae +2(b+a1b mh人u2Hba,b+ 202 )0 ae i(6+6) ae i(+6)Two photon pairs： 2 1 1 1 1 2 2 2 2 ( ) 0 a b a b a b a b H H V V H H V V + + + + + + + + + + + 2 2 2 1 1 1 1 2 2 2 2 2 2 2 ( ) ( ) 1 1 1 1 2 2 2 2 ( ) 0 ( ) 0 2( )( ) 0 i i H H V V a b i i H H V V a b i i H H V V H H V V a b a b a b e e a b a b e e a b a b a b a b e e               + + + + + + + +   + + + + + + + + + +   + + + + + + No errors: