// A balanced interferometer: phase changes where the light exits.
let light = input(0) in
unpack power_split(0.5, light) as (upper, lower) in
let shifted = phase(pi / 2, upper) in
unpack unitary([[0.7071067811865476, 0.7071067811865476],
                [0.7071067811865476, -0.7071067811865476]],
               (shifted, lower)) as (bright, dark) in
output(1) <- bright;
output(2) <- dark;
()