Weak approximate unitary designs and applications to quantum encryption

Unitary $t$-designs are the bread and butter of quantum information theory and beyond. An important issue in practice is that of efficiently constructing good approximations of such unitary $t$-designs. Building on results by Aubrun (Comm. Math. Phys. 2009), we prove that sampling $d^t$\mathrm{p}\mathrm{o}\mathrm{l}\mathrm{y}$(t,\log d, 1/\epsilon)$unitariesfromanexact$t$-design provides with positive probability an $ϵ$-approximate $t$-design, if the error is measured in one-to-one norm distance of the corresponding $t$-twirling channels. As an application, we give a partially derandomized construction of a quantum encryption scheme that has roughly the same key size and security as the quantum one-time pad, but possesses the additional property of being non-malleable against adversaries without quantum side information.