Decoupling with random diagonal-unitary matrices

Date/Time: Mon., 15th June 2015 15:00-16:00 - (room 1320 (3F), Science Building 4, Hongo-campus, University of Tokyo)

Speaker: Dr. Yoshifumi Nakata (The University of Tokyo)

We study decoupling, one of the most important primitives in quantum Shannon theory, with random diagonal-unitary matrices. A decoupling protocol aims at destroying all correlations in an initial state, shared by two parties, by applying an appropriate unitary followed by a quantum channel only on one party. In Ref. [1,2], decouipling has been shown to be achievable using uniformly distributed random unitaries and their approximate versions called unitary 2-designs. In this talk based on Ref. [3,4], we show that decoupling at the same rate is achievable by alternately applying random diagonal-unitaries in the Pauli-Z and -X bases although the resulting unitary is not a very precise unitary 2-design. This implies that, for achieving decoupling, precise unitary 2-designs may not be needed. We also provide a simple quantum circuit achieving decoupling, most of which consists of commuting gates and the non-commuting part is only constant depth. Since the commuting parts can be, in principle, applied simultaneously, this implmentation results in a vast reduction of the execution time and has a practical advantage.