Error in plotting a quantity involving integral!

With my limited knowledge of Mathematica, I tried but could not figure out the issues in the following few-line code:

   mat1 = 1/2 ( {      {1, -I},      {I, 1}     } ); mat2[x_, t_] = {{1/2 - 1/2 Sqrt[1 - 4 x^2] - Sqrt[      1 - 4 x^2]/(-1 + E^(t Sqrt[1 - 4 x^2])), I x}, {-I x,      1/2 (1 + Sqrt[1 - 4 x^2] + (        2 Sqrt[1 - 4 x^2])/(-1 + E^(t Sqrt[1 - 4 x^2])))}};  intx = Integrate[mat2[x, t], {t, 0, 50}]; int0 = Integrate[mat2[0, t], {t, 0, 50}]; quantity[x_] := (Tr[ mat1.intx] - Tr[mat1.int0] )^2;  Plot[quantity[x], {x, 0, 1}] 

Any help would be greatly appreciated. Thanks.