According to this:
nnoremap Q @='n.'<CR>: The @ key tells Vim to execute a macro. Rather than using a named register we use the expression register, which lets us specify the contents in place. The characters inside of the quotes are interpreted as keystrokes, and the carriage return enters the string into the expression register.
But I tried this:
nnoremap K @='k<C-Y>'<CR>
and it doesn't work. Edit: nevermind, this one works, but this one doesn't:
noremap J @="j<C-E>"<CR>
Edit: I was also trying to make
noremap J @="j<C-E>"<CR> work. But one of the comments below links to a question and explains why this one doesn't work: this