Randomization of quantum states is the quantum analogue of the classical
one-time pad. We present an improved, efficient construction of an
approximately randomizing map that uses O(d/epsilon^2) Pauli operators to map
any d-dimensional state to a state that is within trace distance epsilon of the
completely mixed state. Our bound is a log d factor smaller than that of
Hayden, Leung, Shor, and Winter (2004), and Ambainis and Smith (2004).
Then, we show that a random sequence of essentially the same number of
unitary operators, chosen from an appropriate set, with high probability form
an approximately randomizing map for d-dimensional states. Finally, we discuss
the optimality of these schemes via connections to different notions of
pseudorandomness, and give a new lower bound for small epsilon.Comment: 18 pages, Quantum Computing Back Action, IIT Kanpur, March 2006,
volume 864 of AIP Conference Proceedings, pages 18--36. Springer, New Yor
