In the following image there are two different styles used for matrix index
l. The first one is the default one. How can one get the second one?
The code is for the first equation is
(Turning comment into answer)
The symbol is called
\ell, but I would recommend not using it, at least in that context where a normal l (italic ell) is not subject to confusion. On older typescripts it was difficult to distinguish between l (italic ell) and 1 (one), so many took the habit of using a different letterform for l (italic ell).