One can encrypt an n-qubit state using a 2n-bit classical secret key. The idea is to use the key to select a random Pauli operator, and apply that operator to the secret as an encryption. (The inverse operator is applied to decrypt.)
The resulting scheme is perfectly secure -- if the key is selected uniformly at random, then even an attacker who know a state entangled with the plaintext cannot distinguish the true ciphertext from an independent random state.
This observation was first made in
A Ambainis, M Mosca, A Tapp, R de Wolf. "Private quantum channels", FOCS 2000.
They also showed that 2n bits of key are necessary for encryption of entangled quantum states.
If you have a classical PRG secure against quantum distinguishers, then you can get away with a much shorter key (as short as seed length required by the generator to generate 2 n output bits).